QuickGraph#6 Building the Wikipedia Knowledge Graph in Neo4j (QG#2 revisited)

After last week’s Neo4j online meetup, I thought I’d revisit QuickGraph#2 and update it a bit to include a couple new things:

  • How to load not only categories but also pages (as in Wikipedia articles) and enrich the graph by querying DBpedia. In doing this I’ll describe some advanced usage of APOC procedures.
  • How to batch load the whole Wikipedia hierarchy of categories into Neo4j

Everything I explain here will also go into an interactive guide that you can easily run from your Neo4j instance. Or why not giving it a try in the Neo4j Sandbox?

All you have to do is run this on your Neo4j browser:

:play https://guides.neo4j.com/wiki

For a description of the Wikipedia data and the MediaWiki API, check QuickGraph#2.

Loading the data into Neo4j

First, let’s prepare the DB with a few indexes to accelerate the ingestion and querying of the data:

CREATE INDEX ON :Category(catId)
CREATE INDEX ON :Category(catName)
CREATE INDEX ON :Page(pageTitle)

Approach 1: Loading a reduced subset incrementally through the MediaWiki API

This approach uses the WikiMedia API and is adequate if all you want is a portion of the category hierarchy around a particular topic. Let’s say we want to create the Wikipedia Knowledge Graph about Databases.

The first thing we’ll do is create the root category: Databases.

CREATE (c:Category:RootCategory {catId: 0, catName: 'Databases', subcatsFetched : false, pagesFetched : false, level: 0 })

Now we’ll iteratively load the next level of subcategories to a depth of our choice. I’ve selected only three levels down from the root.

UNWIND range(0,3) as level 
CALL apoc.cypher.doit("
MATCH (c:Category { subcatsFetched: false, level: $level})
CALL apoc.load.json('https://en.wikipedia.org/w/api.php?format=json&action=query&list=categorymembers&cmtype=subcat&cmtitle=Category:' + apoc.text.urlencode(c.catName) + '&cmprop=ids%7Ctitle&cmlimit=500')
YIELD value as results
UNWIND results.query.categorymembers AS subcat
MERGE (sc:Category {catId: subcat.pageid})
ON CREATE SET sc.catName = substring(subcat.title,9),
 sc.subcatsFetched = false,
 sc.pagesFetched = false,
 sc.level = $level + 1
WITH sc,c
CALL apoc.create.addLabels(sc,['Level' + ($level + 1) + 'Category']) YIELD node
MERGE (sc)-[:SUBCAT_OF]->(c)
SET c.subcatsFetched = true", { level: level }) YIELD value
RETURN value

Once we have the categories, we can load the pages in a similar way:

UNWIND range(0,4) as level 
CALL apoc.cypher.doit("
MATCH (c:Category { pagesFetched: false, level: $level })
CALL apoc.load.json('https://en.wikipedia.org/w/api.php?format=json&action=query&list=categorymembers&cmtype=page&cmtitle=Category:' + apoc.text.urlencode(c.catName) + '&cmprop=ids%7Ctitle&cmlimit=500')
YIELD value as results
UNWIND results.query.categorymembers AS page
MERGE (p:Page {pageId: page.pageid})
ON CREATE SET p.pageTitle = page.title, p.pageUrl = 'http://en.wikipedia.org/wiki/' + apoc.text.urlencode(replace(page.title, ' ', '_'))
WITH p,c
SET c.pagesFetched = true", { level: level }) yield value
return value

Notice that we are only loading the id and the title for each page. This is because the MediaWiki API only exposes metadata about pages, but we can get some extra information on them from the DBpedia. DBpedia is a crowd-sourced community effort to extract structured information from Wikipedia and make this information available on the Web.
There is a public instance of the DBpedia that exposes an SPARQL endpoint that we can query to get a short description of a given Wikipedia page. The Cypher fragment below embeds the SPARQL query that’s sent to the endpoint.

?x <http://xmlns.com/foaf/0.1/isPrimaryTopicOf> <@wikiurl@> ;
<http://dbpedia.org/ontology/abstract> ?label .
FILTER(LANG(?label) = '' || LANGMATCHES(LANG(?label), 'en')) } LIMIT 1
" AS sparqlPattern
UNWIND range(0,3) as level
CALL apoc.cypher.doit("
MATCH (c:Category { level: $level })<-[:IN_CATEGORY]-(p:Page)
WHERE NOT exists(p.abstract) 
WITH DISTINCT p, apoc.text.replace(sparqlPattern,'@wikiurl@',p.pageUrl) as runnableSparql LIMIT 100
CALL apoc.load.json('http://dbpedia.org/sparql/?query=' + apoc.text.urlencode(runnableSparql) + '&format=application%2Fsparql-results%2Bjson') YIELD value
SET p.abstract = value.results.bindings[0].label.value
", { level: level, sparqlPattern: sparqlPattern }) yield value
return value

I’ve limited to 100 pages per level because we are generating an HTTP request to the DBpedia endpoint for each Page node in our graph. Feel free to remove this limit but keep in mind that this can take a while.

Ok, so we have our Wikipedia Knowledge Graph on Databases and we can start querying it.

Querying the graph

We can list categories by the number of sub/super categories or by the number of pages. We can also create custom indexes like the balanceIndex below that tells us how ‘balanced’ (ratio between supercategories and subcategories) a category is. Closer to zero are the more balanced categories and closer to one are the more unbalanced.

MATCH (c:Category)
WITH c.catName AS category, 
size((c)<-[:SUBCAT_OF]-()) AS subCatCount,  size((c)-[:SUBCAT_OF]->()) AS superCatCount,
size((c)<-[:IN_CATEGORY]-()) AS pageCount WHERE subCatCount > 0 AND superCatCount > 0
RETURN category, 
ABS(toFloat(superCatCount - subCatCount)/(superCatCount + subCatCount)) as balanceIndex
ORDER BY subCatCount DESC 

We can also aggregate these values to produce stats on our Knowledge Graph

MATCH (c:Category)
WITH c.catName AS category,
size((c)<-[:SUBCAT_OF]-()) AS subCatCount, size((c)-[:SUBCAT_OF]->()) AS superCatCount,
size((c)<-[:IN_CATEGORY]-()) AS pageCount,
size((c)-[:SUBCAT_OF]-()) AS total
RETURN AVG(subCatCount) AS `AVG #subcats`,
MIN(subCatCount) AS `MIN #subcats`,
MAX(subCatCount) AS `MAX #subcats`,
percentileCont(subCatCount,0.9) AS `.9p #subcats`,
AVG(pageCount) AS `AVG #pages`,
MIN(pageCount) AS `MIN #pages`,
MAX(pageCount) AS `MAX #pages`,
percentileCont(pageCount,0.95) AS `.9p #pages`,
AVG(superCatCount) AS `AVG #supercats`,
MIN(superCatCount) AS `MIN #supercats`,
MAX(superCatCount) AS `MAX #supercats`,
percentileCont(superCatCount,0.95) AS `.9p #supercats`

Screen Shot 2017-04-26 at 01.53.16

Approach 2: Batch loading the data with LOAD CSV from an SQL dump

There is a snapshot of the Wikipedia categories and their hierarchical relationships (as of mid-April 2017) here. It contains 1.4 million categories and 4 million hierarchical relationships. They can both be loaded into Neo4j using LOAD CSV. You can run the queries as they are or download the files to your Neo4j’s instance import directory and use LOAD CSV FROM "file:///..." instead.

First the categories. Notice that we are loading a couple of extra properties in the Category nodes: the pageCount and the subcatCount. These numbers are a precomputed in the data dump and not always accurate.

LOAD CSV FROM "https://github.com/jbarrasa/datasets/blob/master/wikipedia/data/cats.csv?raw=true" as row
CREATE (c:Category { catId: row[0]}) 
SET c.catName = row[2], c.pageCount = toInt(row[3]), c.subcatCount = toInt(row[4])

And then the subcategory relationships

LOAD CSV FROM "https://github.com/jbarrasa/datasets/blob/master/wikipedia/data/rels.csv?raw=true" as row
MATCH (from:Category { catId: row[0]}) 
MATCH (to:Category { catId: row[1]})
CREATE (from)-[:SUBCAT_OF]->(to)

If you’re interested in regenerating fresh CSV files, here’s how:

  • Start by downloading the latest DB dumps from the Wikipedia downloads page.
    For the category hierarchy, you’ll only need the following tables: category, categorylinks and page.
  • Load them in your DBMS.
  • Generate the categories CSV file by running the following SQL
select p.page_id as PAGE_ID, c.cat_id as CAT_ID, cast(c.cat_title as nCHAR) as CAT_TITLE , c.cat_pages as CAT_PAGES_COUNT, c.cat_subcats as CAT_SUBCAT_COUNT
into outfile '/Users/jbarrasa/Applications/neo4j-enterprise-3.1.2/import/wiki/cats.csv' fields terminated by ',' enclosed by '"' escaped by '\\' lines terminated by '\n' 
from test.category c, test.page p
where c.cat_title = p.page_title
and p.page_namespace = 14
  • Generate the relationships file by running the following SQL
select p.page_id as FROM_PAGE_ID, p2.page_id as TO_PAGE_ID
into outfile '/Users/jbarrasa/Applications/neo4j-enterprise-3.1.2/import/wiki/rels.csv' fields terminated by ',' enclosed by '"' escaped by '\\' lines terminated by '\n' 
from test.category c, test.page p , test.categorylinks l, test.category c2, test.page p2
where l.cl_type = 'subcat'
and c.cat_title = p.page_title
and p.page_namespace = 14
and l.cl_from = p.page_id
and l.cl_to = c2.cat_title
and c2.cat_title = p2.cat_title
and p2.page_namespace = 14

What’s interesting about this QuickGraph?

It showcases interesting usages of procedures like apoc.cypher.doit to run Cypher fragments within our query or apoc.load.json to interact with APIs producing JSON results.

Rich category hierarchies like the one in Wikipedia are graphs and extremely useful for recommendation or  graph-enhanced search. Have a look at the queries in QG#2 and the ones in the interactive guide for some ideas.

:play https://guides.neo4j.com/wiki

QuickGraph#5 Learning a taxonomy from your tagged data

The Objective

Say we have a dataset of multi-tagged items: books with multiple genres, articles with multiple topics, products with multiple categories… We want to organise logically these tags -the genres, the topics, the categories…- in a descriptive but also actionable way. A typical organisation will be hierarchical, like a taxonomy.

But rather than building it manually, we are going to learn it from the data in an automated way. This means that the quality of the results will totally depend on the quality and distribution of the tagging in your data, so sometimes we’ll produce a rich taxonomy but sometimes the data will only yield a set of rules describing how tags relate to each other.

Finally, we’ll want to show how this taxonomy can be used and I’ll do it with an example on content recommendation / enhanced search.

The dataset

We’ll use data from Goodreads on books and how they’ve been categorised by readers. In Goodreads, there is a notion of “shelf” which is a user created public category or tag that can be added to books and reused by other readers. Here is a page from Goodreads on a book along with the graph view of the data that we’ll extract from the page for this experiment.

Each book has a few shelves (genres) and an author, although I will not use the author information in this case.

Here is the data load script that you can try on your local Neo4j instance:

CREATE INDEX ON :Author(name)
CREATE INDEX ON :Genre(name)
LOAD CSV WITH HEADERS FROM "https://raw.githubusercontent.com/jbarrasa/datasets/master/goodreads/booklist.csv" AS row
MERGE (b:Book { id : row.itemUrl})
SET b.description = row.description, b.title = row.itemTitle
WITH b, row
UNWIND split(row.genres,';') AS genre
MERGE (g:Genre { name: substring(genre,8)})
MERGE (b)-[:HAS_GENRE]->(g)
WITH b, row
UNWIND split(row.author,';') AS author
MERGE (a:Author { name: author})
MERGE (b)-[:HAS_AUTHOR]->(a)

The data model is pretty simple as we’ve seen, and it only models three types of entities, the books, their authors and the genres. Here is the db.schema:

Screen Shot 2017-03-31 at 02.50.34.png

And some metrics on the tagging:

  • AVG number of genres per book: 5.4
  • MAX number of genres per book: 10
  • MIN number of genres per book: 1

You can get them by running this query:

MATCH (n:Book) 
WITH id(n) AS bookid, size((n)-[:HAS_GENRE]->()) AS genreCount
RETURN AVG(genreCount) AS avgNumGenres, MAX(genreCount) AS maxNumGenres, MIN(genreCount) AS minNumGenres

The Taxonomy Learning Algorithm

The algorithm is based on tag co-occurrence. The items in our dataset have multiple tags each, which means that tags will co-occur (will appear together) in a number of items. This algorithm will analyse the sets of items where tags co-occur and apply some pretty straightforward logic: If every item tagged as A is also tagged as B, we can derive that A implies B or in other words, the category defined by tag A is “narrower-than” the category defined by tag B. Easy, right?

Let’s look at the algo step by set using the simple model described before on books and genres. Books are our tagged items and the genres are the tags.


STEP1: Compute co-occurrence

Co-occurrence is the basic building block for the algorithm and is in itself a quite useful relationship because it indicates some degree of overlap between tags and therefore a certain degree of similarity which is something that can be exploited for query expansion or recommendation.

The co-occurrence index between two categories A and B is computed as the portion of items in category A that are also in category B, this is a simple division of the number of items tagged as both A and B divided by the number of items in tagged as A.

COOC(A,B) = #items tagged as both A and B /  #items tagged as A

Screen Shot 2017-03-31 at 00.56.10.png

Notice that while the co-occurrence relationship is not directional, the co-occurrence index is so we will persist in our graph the cooccurrence of two tags as two relationships one on each direction containing the co-occurrence index.

In cypher:

MATCH (g:Genre) WHERE SIZE((g)<-[:HAS_GENRE]-()) > 5 //Threshold 
WITH g, size((g)<-[:HAS_GENRE]-()) as totalCount
MATCH (g)<-[:HAS_GENRE]-(book)-[:HAS_GENRE]->(relatedGenre)
WITH g, relatedGenre, toFloat(count(book)) / totalCount AS coocIndex
CREATE (g)-[:CO_OCCURS {index: coocIndex }]->(relatedGenre)

I’ve also included in the Cypher implementation a WHERE clause (marked red) to exclude categories that contain fewer items than a given threshold. This is an optional adjustment that you may want to apply and like this one there are a number of optimisations that can be applied to the basic co-occurrence computation to make it produce higher quality results.

STEP2: Infer same-as relationships

Once we have co-occurrence in the graph, we want to detect equivalent genres. Two genres are equivalent if the co-occurrence index in both directions is 1 or in other words, if every item having genre g1 has also genre g2 and vice versa.

Here is the Cypher that does the job. Equivalent categories (genres) are linked through the SAME_AS relationship

MATCH (g1)-[co1:CO_OCCURS {index : 1}]->(g2),
      (g2)-[co2:CO_OCCURS { index: 1}]->(g1)
WHERE ID(g1) > ID(g2)
MERGE (g1)-[:SAME_AS]-(g2)

STEP3: Infer narrower-than relationships

Here is where we infer the hierarchical relationship between two categories (genres). Very similar to the previous rule, we check now if one of the co-occurrence indexes is 1 and the other is less than 1. Or in English, if every item having genre g1 has also genre g2 but the opposite is not true.

The Cypher that creates the NARROWER_THAN hierarchy is as follows.

MATCH (g1)-[co1:CO_OCCURS]->(g2), 
WHERE ID(g1) > ID(g2) 
      AND co1.index = 1 and co2.index < 1 MERGE (g1)-[:NARROWER_THAN]->(g2)

STEP4: Reduce transitive narrower-than relationships

Finally, this computation may have produced more NARROWER_THAN relationships than needed so we want to remove transitive ones. If (X)-[:NARROWER_THAN]->(Y) and (Y)-[:NARROWER_THAN]->(Z), then we may want to get rid of any (X)-[:NARROWER_THAN]->(Z) as it is kind of redundant.  But this is an optional step that you may or may not want to include.

MATCH (g1)-[:NARROWER_THAN*2..]->(g3), 

So that’s it! Here is the taxonomy:

A couple of interesting examples:

  • military-science-fiction << space-opera << science-fiction
  • nutrition << health << non-fiction

Worth mentioning that while these are true in our dataset, as our graph grows and evolves over time, we may find contradictions to them that would invalidate the taxonomy so as data evolves it will make sense to re-compute the taxonomy creation.

Using the taxonomy

The objective of learning how a set of tags relate was to then be able to use it in some meaningful way. The CO_OCCURS relationship in itself is a useful one as it indicates some degree of overlap between tags and therefore a certain degree of similarity. But NARROWER_THAN has stronger semantics, let’s see how could we use it:

One possible use would be to recommend books based on the taxonomy we’ve just learned. This first query lists available subcategories with the number of items in them

MATCH (b:Book) WHERE b.title = 'Slow Bullets'
MATCH (b)-[:HAS_GENRE]->(g)<-[:NARROWER_THAN]-(childGenre)
WHERE size((g)<-[:HAS_GENRE]-()) < 500       AND NOT (b)-[:HAS_GENRE]->()-[:NARROWER_THAN]->(g) 
     AND NOT (b)-[:HAS_GENRE]->(childGenre)
RETURN "Hello! '" + b.title + "' is tagged as '" + g.name + "', and we have " + size((childGenre)<-[:HAS_GENRE]-()) + " books on '" + childGenre.name + "' which is a narrower category. Want to have a look? " AS recommendationQuestion

When run on ‘Pride and Prejudice’ it produces this output:

Screen Shot 2017-03-31 at 12.22.46.png

Actually it does a bit more than just that, if we analyse the Cypher, we can see that we start from a selected book and for each genre with less than 500 books in it -we want to exclude large ones like ‘fiction’ as they are too generic to provide relevant recommendations- we get the sub-categories that the current book is not tagged as. We also stop the generation of sub-category based recommendations if there is already a sibling subcategory in the tags of the selected book. Basically, if a book is tagged as ‘sports’ and ‘tennis’, tennis being a subcategory of sports, we will not recommend other subcategories of sports like ‘hockey’ or ‘football’. Yes, all that in 5 lines of cypher! Anyway, this is just one possible query that uses the hierarchy and that makes sense in my data set but you may want to tune it to yours.

And this second query, very similar to the previous one, lists the actual items in the subcategory:

MATCH (b:Book) WHERE b.title = {title}
MATCH (b)-[:HAS_GENRE]->(g)<-[:NARROWER_THAN]-(childGenre)
WHERE size((g)<-[:HAS_GENRE]-()) < 500 AND NOT (b)-[:HAS_GENRE]->()-[:NARROWER_THAN]->(g) AND NOT (b)-[:HAS_GENRE]->(childGenre)
WITH childGenre
MATCH (booksInChildCategory)-[:HAS_GENRE]->(childGenre)
RETURN booksInChildCategory.title AS bookTitle, substring(booksInChildCategory.description,1, 70) + '...' AS description

We can run it this time on ‘Slow Bullets’ and it will produce:

Screen Shot 2017-03-31 at 12.17.55.png

Notice that both queries are neutral from the point of view of what’s in the taxonomy, as the taxonomy evolves over time, the results will be different.

Of course, recommendation can get a lot more complicated but this is just a basic suggestion on how the taxonomy could be used. Another option is to use the NARROWER_THAN in combination with the CO_OCCURS for richer recommendations in case there are no NARROWER_THAN alternatives. More on this in future blog posts.

What’s interesting about this QuickGraph?

This is a basic attempt at analysing tag co-occurrence using a graph. The algorithm can be refined in a number of ways but I thought it would be interesting to share it in its basic form and maybe blog later on on how to improve it.

I think the most interesting is the fact that the approach is generic and can be used in many contexts to build a purely dynamic and automated solution. The taxonomy creation algorithm can be re-run on a regular basis as new tagged data is added to the graph and the logic (like the one described in the “using the taxonomy” section) will produce results adapted to the fresh version of the taxonomy.

It’s worth mentioning that the quality of the hierarchy will directly depend on the quality of your data tagging! We are not creating a formal ontology here but rather building a pragmatic and actionable taxonomy derived in an automated way from your data.

Watch this space for other examples of use of this approach and some suggested refinements.

I’d also love to get your feedback!

Quick note on getting the data

To get some data from Goodreads your best option is to write some code using their API. Other alternatives are for instance import.io (click to try the import.io ‘lightning’ scraper on a GoodReads list) or HTML scraping libraries for your favourite programming language, rvest if you’re an R fan or Beautiful Soup or lxml if you prefer python.

If you want to test the algorithm on your Neo4j instance with the same dataset I used you just need to run the data load scripts above, they include the link to the data.

Neo4j is your RDF store (part 2)

As in previous posts, for those of you less familiar with the differences and similarities between RDF and the Property Graph, I recommend you watch this talk I gave at Graph Connect San Francisco in October 2016.

In the previous post on this series, I showed the most basic way in which a portion of your graph can be exposed as RDF. That was identifying a node by ID or URI if your data was imported from an RDF dataset. In this one, I’ll explore a more interesting way by running Cypher queries and serialising the resulting subgraph as RDF.

The dataset

For this example I’ll use the Nortwind database that you can easily load in your Neo4j instance by running the following in your Neo4j browswer.

:play northwind graph

If you follow the step by step instructions you should get the graph built in no time. You’re ready then to run queries like “Get the detail of the orders by Rita Müller containing at least a dairy product”. Here is the cypher for it:

MATCH (cust:Customer {contactName : "Rita Müller"})-[p:PURCHASED]->(o:Order)-[or:ORDERS]->(pr:Product)
WHERE (o)-[:ORDERS]->()-[:PART_OF]->(:Category {categoryName:"Dairy Products"})

And this the resulting graph:

Screen Shot 2016-12-16 at 12.46.40.png

Serialising the output of a cypher query as RDF

The result of the previous query is a portion of the Nortwhind graph, a set of nodes and relationships that can be serialised as RDF using the neosemantics neo4j extension.

Once installed on your Neo4j instance, you’ll notice that the neosemantics extension includes a cypher endpoint /rdf/cypher (described here) that takes a cypher queryas input and returns the results serialised as RDF with the usual choice of serialisation format in the HTTP request.

The endpoint can be tested directly from the browser and will produce JSON-LD by default.

Screen Shot 2016-12-16 at 12.58.39.png

The uris of the resources in RDF are generated from the node ids in neo4j and in this first version of the LPG-to-RDF endpoint, all elements in the graph -RDF properties and types- share the same generic vocabulary namespace (It will be different if your graph has been imported from an RDF dataset as we’ll see in the final section).

Validating the RDF output on the W3C RDF Validation Service

A simple way of validating the output of the serialisation could be to load it into the W3C RDF validation service. It takes two simple steps:

Step one: Run your Cypher query on the rdf/cypyher endpoint selecting application/rdf+xml as serialization format on the Accept header of the http request. This is what the curl expresion would look like:

curl http://localhost:7474/rdf/cypher -H Accept:application/rdf+xml 
     -d "MATCH (cust:Customer {contactName : 'Rita Müller'})-[p:PURCHASED]->(o:OrdeERS]->(pr:Product) WHERE (o)-[:ORDERS]->()-[:PART_OF]->(:Category {categoryName:'Dairy Products'}) RETURN *"

This should produce something like this (showing only the first few rows):

<?xml version="1.0" encoding="UTF-8"?>

<rdf:RDF xmlns:neovoc="neo4j://vocabulary#"

<rdf:Description rdf:about="neo4j://indiv#77511">
    <rdf:type rdf:resource="neo4j://vocabulary#Customer"/>
    <neovoc:country rdf:datatype="http://www.w3.org/2001/XMLSchema#string">Germany</neovoc:country>
    <neovoc:address rdf:datatype="http://www.w3.org/2001/XMLSchema#string">Adenauerallee 900</neovoc:address>
    <neovoc:contactTitle rdf:datatype="http://www.w3.org/2001/XMLSchema#string">Sales Representative</neovoc:contactTitle>
    <neovoc:city rdf:datatype="http://www.w3.org/2001/XMLSchema#string">Stuttgart</neovoc:city>
    <neovoc:phone rdf:datatype="http://www.w3.org/2001/XMLSchema#string">0711-020361</neovoc:phone>
    <neovoc:contactName rdf:datatype="http://www.w3.org/2001/XMLSchema#string">Rita Müller</neovoc:contactName>
    <neovoc:companyName rdf:datatype="http://www.w3.org/2001/XMLSchema#string">Die Wandernde Kuh</neovoc:companyName>
    <neovoc:postalCode rdf:datatype="http://www.w3.org/2001/XMLSchema#string">70563</neovoc:postalCode>
    <neovoc:customerID rdf:datatype="http://www.w3.org/2001/XMLSchema#string">WANDK</neovoc:customerID>
    <neovoc:fax rdf:datatype="http://www.w3.org/2001/XMLSchema#string">0711-035428</neovoc:fax>
    <neovoc:region rdf:datatype="http://www.w3.org/2001/XMLSchema#string">NULL</neovoc:region>

<rdf:Description rdf:about="neo4j://indiv#77937">
    <neovoc:ORDERS rdf:resource="neo4j://indiv#76432"/>

I know the XML based format is pretty horrible but we need it because it’s the only one that the RDF validator accetps 😦

Step two:  Go to the W3C RDF validation service page (https://www.w3.org/RDF/Validator/) and copy the xml from the previous step in the text box and select triples and graph in the display options. Hit Parse RDF and… you should get the list of 266 parsed triples plus a graphical representation of the RDF graph like this one:


Yes, I know, huge if we compare it to the original property graph but this is normal. RDF makes an atomic decomposition of every single statement in your data. In an RDF graph not only entities but also every single property produce a new vertex, leading to this explosion in the size of the graph.

Screen Shot 2016-12-16 at 15.58.33.png

That’s a slide from this talk at Graph Connect SF in Oct 2016 where I discussed that it’s normal that the number of triples in an RDF dataset is an order of magnitude bigger than the number of nodes in a LPG.

The portion of the Northwind graph returned by our example query is not an exception 19 nodes => 266 triples.

If the graph was imported from RDF…

So if your graph in Neo4j had been imported using the semantics.importRDF procedure (described in previous blog posts and with some examples) then you want to use the rdf/cypheronrdf endpoint (described here) instead. It works exactly in the same way, but uses the uris as unique identifiers for nodes instead of the ids.

If you’re interested on what this would look like, watch this space for part three of this series.


As in the previous post, the main takeaway is that it is pretty straightforward to offer an RDF “open standards compliant” API for publishing your graph while still getting the benefits of native graph storage and Cypher querying in Neo4j.




Neo4j is your RDF store (part 1)

If you want to understand the differences and similarities between RDF and the Labeled Property Graph implemented by Neo4j, I’d recommend you watch this talk I gave at Graph Connect San Francisco in October 2016.


Let me start with some basics: RDF is a standard for data exchange, but it does not impose any particular way of storing data.

What do I mean by that? I mean that data can be persisted in many ways: tables, documents, key-value pairs, property graphs, triple graphs… and still be published/exchanged as RDF.

It is true though that the bigger the paradigm impedance mismatch -the difference between RDF’s modelling paradigm (a graph) and the underlying store’s one-, the more complicated and inefficient the translation for both ingestion and publishing will be.

I’ve been blogging over the last few months about how Neo4j can easily import RDF data and in this post I’ll focus on the opposite: How can a Neo4j graph be published/exposed as RDF.

Because in case you didn’t know, you can work with Neo4j getting the benefits of native graph storage and processing -best performance, data integrity and scalability- while being totally ‘open standards‘ to the eyes of any RDF aware application.

Oh! hang on… and your store will also be fully open source!

A “Turing style” test of RDFness

In this first section I’ll show the simplest way in which data from a graph in Neo4j can be published as RDF but I’ll also demonstrate that it is possible to import an RDF dataset into Neo without loss of information in a way that the RDF produced when querying Neo4j is identical to that produced by the original triple store.

Screen Shot 2016-11-17 at 01.18.36.png

You’ll probably be familiar with the Turing test where a human evaluator tests a machine’s ability to exhibit intelligent behaviour, to the point where it’s indistinguishable from that of a human. Well, my test aims to prove Neo4j’s ability to exhibit “RDF behaviour” to an RDF consuming application, making it indistinguishable from that of a triple store. To do this I’ll use the neosemantics neo4j extension.

The simplest test one can think of, could be something like this:

Starting from an RDF dataset living in a triple store, we migrate it (all or partially) into Neo4j. Now if we run a Given a SPARQL DESCRIBE <uri> query on the triple store and its equivalent rdf/describe/uri<uri> in Neo4j, do they return the same set of triples? If that is the case -and if we also want to be pompous- we could say that the results are semantically equivalent, and therefore indistinguishable to a consumer application.

We are going to run this test step by step on data from the British National Bibliography dataset:

Get an RDF node description from the triple store

To do that, we’ll run the following SPARQL DESCRIBE query in the British National Bibliography public SPARQL endpoint, or alternatively in the more user friendly SPARQL editor.

DESCRIBE <http://bnb.data.bl.uk/id/person/BulgakovMikhail1891-1940>

The request returns an RDF fragment containing all information about Mikhail Bulgakov in the BNB. A pretty cool author, by the way, which I strongly recommend. The fragment actually contains 86 triples, the first of which are these:

<http://bnb.data.bl.uk/id/person/BulgakovMikhail1891-1940> <http://xmlns.com/foaf/0.1/givenName> "Mikhail" .
<http://bnb.data.bl.uk/id/person/BulgakovMikhail1891-1940> <http://www.w3.org/2000/01/rdf-schema#label> "Bulgakov, Mikhail, 1891-1940" .
<http://bnb.data.bl.uk/id/person/BulgakovMikhail1891-1940> <http://xmlns.com/foaf/0.1/familyName> "Bulgakov" .
<http://bnb.data.bl.uk/id/person/BulgakovMikhail1891-1940> <http://xmlns.com/foaf/0.1/name> "Mikhail Bulgakov" .
<http://bnb.data.bl.uk/id/person/BulgakovMikhail1891-1940> <http://www.bl.uk/schemas/bibliographic/blterms#hasCreated> <http://bnb.data.bl.uk/id/resource/010535795> .
<http://bnb.data.bl.uk/id/person/BulgakovMikhail1891-1940> <http://www.bl.uk/schemas/bibliographic/blterms#hasCreated> <http://bnb.data.bl.uk/id/resource/008720599> .

You can get the whole set running the query in the SPARQL editor I mentioned before or sending an  HTTP request with the query to the SPARQL endpoint:

curl -i http://bnb.data.bl.uk/sparql?query=DESCRIBE+%3Chttp%3A%2F%2Fbnb.data.bl.uk%2Fid%2Fperson%2FBulgakovMikhail1891-1940%3E -H Accept:text/plain

Ok, so that’s our base line,  exactly the output we want to get from Neo4j to be able to affirm that they are indistinguishable to an RDF consuming application.

Move the data from the triple store to Neo4j

We need to load the RDF data into Neo4j. We could load the whole British National Bibliography since it’s available for download as RDF, but for this example we are going to load just the portion of data that we need.

I will not go into the details of how this happens as it’s been described in previous blog posts and with some examples. The semantics.importRDF procedure runs a straightforward and lossless import of RDF data into Neo4j. The procedure is part of the neosemantics extension. If you want to run the test with me on your Neo4j instance, now is the moment when you need to install it (instructions in the README).

Once the extension ins installed, the migration could not be simpler, just run the following stored procedure:

CALL semantics.importRDF("http://bnb.data.bl.uk/sparql?query=DESCRIBE+%3Chttp%3A%2F%2Fbnb.data.bl.uk%2Fid%2Fperson%2FBulgakovMikhail1891-1940%3E",

We are passing as parameter the url of the BNB SPARQL endpoint returning the RDF data needed for our test, along with some import configuration options. The output of the execution shows that the 86 triples have been correctly imported into Neo4j:

Screen Shot 2016-11-16 at 03.01.52.png

Now that the data is in Neo4j and you can query it with Cypher and visualise it in the browser. Here is a query example returning Bulgakov and all the nodes he’s connected to:

MATCH (a)-[b]-(c:Resource { uri: "http://bnb.data.bl.uk/id/person/BulgakovMikhail1891-1940"})

Screen Shot 2016-11-16 at 02.54.34.png

There is actually not much information in the graph yet, just the node representing good old Mikhail with a few properties (name, uri, etc…) and connections to the works he created or contributed to, the events of his birth and death and a couple more. But let’s not worry about size for now, well deal with that later. The question was: can we now query our Neo4j graph and produce the original set of RDF triples? Let’s see.

Get an RDF description of the same node, now from Neo4j

The neosemantics repo also includes an extensions (http endpoints) that provide precisely this capability. The equivalent in Neo4j of the SPARQL DESCRIBE on Mikhail Bulgakov would be the following:

:GET /rdf/describe/uri?nodeuri=http://bnb.data.bl.uk/id/person/BulgakovMikhail1891-1940

If you run it in the browser, you will get the default serialisation which is JSON-LD, something like this:

Screen Shot 2016-11-16 at 16.40.23.png

But if you set in the request header the serialisation format of your choice -for example using curl again- you can get the RDF fragment in any of the available formats.

curl -i http://localhost:7474/rdf/describe/uri?nodeuri=http://bnb.data.bl.uk/id/person/BulgakovMikhail1891-1940 -H accept:text/plain

Well, you should not be surprised to know that it return 86 triples, exactly the same set that the original query on the triple store returned.

So mission accomplished. At least for the basic case.

RDF out Neo4j’s movie database

I thought it could be interesting to prove that an RDF dataset can be imported into Neo4j and then published without loss of information but OK, most of you may not care much about existing RDF datasets, that’s fair enough. You have a graph in Neo4j and you just want to publish it as RDF. This means that in your graph, the nodes don’t necessarily have a property for the uri (why would they?) or are labelled as Resources. Not a problem.

Ok, so if your graph is not the result of some RDF import, the service you want to use instead of the uri based one, is the nodeid based equivalent.

:GET /rdf/describe/id?nodeid=<nodeid>

We’ll use for this example Neo4j’s movie database. You can get it loaded in your Neo4j instance by running

:play movies

You can get the ID of a node either directly by clicking on it on the browser or by running a simple query like this one:

MATCH (x:Movie {title: "Unforgiven"}) 

In my Neo4j instance, the returned ID is 97 so the GET request would pass this ID and return in the browser the JSON-LD serialisation of the node representing the movie “Unforgiven” with its attributes and the set of nodes connected to it (both inbound and outbound connections):


But as in the previous case, the endpoint can also produce your favourite serialisation just by setting it in the accept parameter in the request header.

curl -i http://localhost:7474/rdf/describe/id?nodeid=97 -H accept:text/plain

When setting the serialisation to N-Triples forma the previous request gets you these triples:

<neo4j://indiv#97> <http://www.w3.org/1999/02/22-rdf-syntax-ns#type> <neo4j://vocabulary#Movie> .
<neo4j://indiv#97> <neo4j://vocabulary#tagline> "It's a hell of a thing, killing a man" .
<neo4j://indiv#97> <neo4j://vocabulary#title> "Unforgiven" .
<neo4j://indiv#97> <neo4j://vocabulary#released> "1992"^^<http://www.w3.org/2001/XMLSchema#long> .
<neo4j://indiv#167> <neo4j://vocabulary#REVIEWED> <neo4j://indiv#97> .
<neo4j://indiv#89> <neo4j://vocabulary#ACTED_IN> <neo4j://indiv#97> .
<neo4j://indiv#99> <neo4j://vocabulary#DIRECTED> <neo4j://indiv#97> .
<neo4j://indiv#98> <neo4j://vocabulary#ACTED_IN> <neo4j://indiv#97> .
<neo4j://indiv#99> <neo4j://vocabulary#ACTED_IN> <neo4j://indiv#97> .

The sharpest of you may notice when you run it that there is  a bit missing. There are relationship properties in the movie database that are lost in the RDF fragment. Yes, that is because there is no way of expressing that in RDF. At least not without recurring to horribly complicated patterns like reification or singleton property that are effectively unusable in any practical real world use case. But we’ll get to that too in future posts.



I guess the main one is that if you want to get the benefits of native graph storage and be able to query your graph with Cypher in Neo4j but also want to:

  •  be able to easily import RDF data into your graph and/or
  •  offer an RDF “open standards compliant” API for publishing your graph

Well, that’s absolutely fine, because we’ve just seen how Neo4j does a great job at producing and consuming RDF.

Remember: RDF is about data exchange, not about storage.

There is more to come on producing RDF from Neo4j than what I’ve shown in this post. For instance, publishing the results of a Cypher query as RDF. Does it sound interesting?Watch this space.

Also I’d love to hear your feedback!




QuickGraph#4 Explore your browser history in Neo4j

The dataset

For this example I am going to use my browser history data. Most browsers store this data in SQLite. This means relational data, easy to access from Neo4j using the apoc.load.jdbc  stored procedure. I’m a Chrome user, and in my Mac, Chrome stores the history db at

~/Library/Application Support/Google/Chrome/Default/History

There are two main tables in the History DB: urls and visits. I’m going to explore them directly from Neo4j’s browser using the same apoc.load.jdbc procedure. In order to do that, you’ll have to download first a jdbc driver for SQLite, and copy it in the plugins directory of your Neo4j instance. Also keep in mind that Chrome locks the History DB when the browser is open so if you want to play with it(even read only acces) you will have to either close the browser or as I did, copy the DB (a single file) somewhere else and work from that snapshot.

This Cypher fragment will return the first few records of the urls table and we see on them things like an unique identifier for the page, its url, title of the page and some counters with the number of visits and the number of times the url has been typed as opposed to reached by following a hyperlink.

CALL apoc.load.jdbc("jdbc:sqlite:/Users/jbarrasa/Documents/Data/History",
                    "urls") yield row 

The results look like this on my browser history.


The visits table contain information about the page visit event, a timestamp (visit_time), a unique identifier (id) for each visit and most interesting, whether the visit follows a previous one (from_visit). This would mean that there was a click on a hyperlink that lead from page A to page B.


A bit of SQL manipulation using the date and time functions on the SQLite side will filter out the columns from the visits table that we don’t care about for this experiment and also format the timestamp in a user friendly date and time.

SELECT id, url, time(((visit_time/1000000)-11644473600), 'unixepoch') as visit_time, 
date(((visit_time/1000000)-11644473600), 'unixepoch') as visit_date,
visit_time as visit_time_raw 
FROM visits

Here’s what records look like using this query. Nice and ready to be imported into Neo4j.


Loading the data into Neo4j

The model I’m planning to build is quite simple: I’ll use a node to represent a web page and a separate one to represent each individual visit to a page. Each visit event is linked to the page through the :VISIT_TO_PAGE relationship, and chained page visits (hyperlink navigation) are linked through the :NAVIGATION_TO relationship. Here is what that looks visually on an example navigation from a post on the Neo4j blog to a page with some code on Github:


Ok, so let’s go with the import scripts.  First the creation of Page nodes out of every record in the urls table:

CALL apoc.load.jdbc("jdbc:sqlite:/Users/jbarrasa/Documents/Data/History",
                    "urls") yield row 
WITH row 
CREATE (p:Page {page_id: row.id, 
                page_url: row.url, 
                page_title: row.title, 
                page_visit_count: row.visit_count, 
                page_typed_count: row.typed_count})

And I’ll do the same with the visits, but linking them to the pages we’ve just loaded. Actually, to accelerate the page lookup I’ll create an index on page ids first.

CREATE INDEX ON :Page(page_id)

And here’s the Cypher running the visit data load.

WITH "SELECT id, url, visit_time as visit_time_raw, 
 time(((visit_time/1000000)-11644473600), 'unixepoch') as visit_time, 
 date(((visit_time/1000000)-11644473600), 'unixepoch') as visit_date 
 FROM visits" AS sqlstring

CALL apoc.load.jdbc("jdbc:sqlite:/Users/jbarrasa/Documents/Data/History",
                    sqlstring ) yield row
WITH row 
MATCH (p:Page {page_id: row.url}) 
CREATE (v:PageVisit { visit_id: row.id, 
                      visit_time: row.visit_time, 
                      visit_date: row.visit_date, 
                      visit_timestamp: row.visit_time_raw}) 

And finally, I’ll load the transitions between visits but as we did before with the pages, let’s create first an index on visit ids:

CREATE INDEX ON :PageVisit(visit_id)
WITH "SELECT id, from_visit, transition, segment_id, visit_duration 
      FROM visits" AS sqlstring
CALL apoc.load.jdbc("jdbc:sqlite:/Users/jbarrasa/Documents/Data/History",
                    ) yield row 
WITH row 
MATCH (v1:PageVisit {visit_id: row.from_visit}),
      (v2:PageVisit {visit_id: row.id}) 

So we are ready to start querying our graph!

Querying the graph

Let’s look for a direct navigation in the graph that goes for instance from a page in the Neo4j web site to Twitter.

MATCH (v1)-[:VISIT_TO_PAGE]->(p1),
WHERE p1.page_url CONTAINS 'neo4j.com' 
      AND p2.page_url CONTAINS 'twitter.com'

In my browser history data, this produces the following output. Notice that I’ve extended it to include an extra navigation step. I’ve done that just by clicking on the graph visualisation in the Neo4j browser to make the example more understandable:


It actually corresponds to a visit to the Neo4j blog, followed by me tweeting how cool was what I just read. The proof that I’m working with real data is the actual tweet (!)

Ok, so while this basic model is good to analyse individual journeys, I think extracting a Site node by aggregating all pages in the same site can give us interesting insights. Let’s go for it.

Extending the model

This could be done in different ways, for example we could write a stored procedure and call it from a Cypher script. Having the full power of java, we could do a proper parsing of the url string to extract the domain.

I will do it differently though, I’ll run a SQL query on the History SQLite DB including string transformations to substring the urls and extract the domain name (sort of). The SQL that extracts the root of the url could be the following one:

SELECT id, substr(url,9,instr(substr(url,9),'/')-1) as site_root 
FROM urls 
WHERE instr(url, 'https://')=1 
SELECT id, substr(url,8,instr(substr(url,8),'/')-1) as site_root 
FROM urls
WHERE instr(url, 'http://')=1

Quite horrible, I know. But my intention is to show how the graph can be extended with new data without having to recreate it. Quite a common scenario when you work with graphs, but relax, graphs are good at accommodating change, nothing to do with RDBMS migrations when having to change your schema.

So this new query produces rows containing just the domain (the root of the url) and the page id that I will use to match to previously loaded pages. Something like this:


And the Cypher that loads it and adds the extra information in our graph would be this:

WITH "select substr(url,9,instr(substr(url,9),'/')-1) as site_root, id 
      from urls where instr(url, 'https://')=1 
      select substr(url,8,instr(substr(url,8),'/')-1) as site_root, id 
      from urls where instr(url, 'http://')=1"  AS query
CALL apoc.load.jdbc("jdbc:sqlite:/Users/jbarrasa/Documents/Data/History",
                     query) yield row 
WITH row 
MATCH (p:Page {page_id: row.id})
MERGE (s:Site {site_root: row.site_root})

And once we have the sites we can include weighted site level navigation. The weight is simply calculated by summing the number of transitions between pages belonging to each site. Here is the Cypher that does the job:

MATCH (s:Site)<-[:PAGE_IN_SITE]-()<-[:VISIT_TO_PAGE]-()<-[inbound:NAVIGATION_TO]-()-[:VISIT_TO_PAGE]->()-[:PAGE_IN_SITE]->(otherSite) 
WHERE otherSite <> s 
WITH otherSite, s, count(inbound) as weight 
CREATE (otherSite)-[sn:SITE_DIRECT_NAVIGATION{weight:weight}]->(s)

This is a much richer graph, where we can traverse not only individual journeys, but also Site level connections. In the following visualisation we can see that there are some transitions between the http://www.theguardian.co.uk and the http://www.bbc.co.uk sites (indicated in green), also to other sites like en.wikipedia.org. In the same capture we can see one of the individual navigations that explain the existence of  a :SITE_DIRECT_NAVIGATION relationship between the Guardian node and the BBC one. It actually represents a hyperlink I clicked on the Guardian’s article that connected it to a BBC one. The purple sequence of events (page visits) details my journey and the yellow nodes represent the pages, pretty much the same we saw on the previous example from neo4j.com to twitter.com.


We can also have a bird’s eye view of a few thousand of the nodes on the graph and notice some interesting facts:

Screen Shot 2016-09-29 at 21.41.11.png

I’ve highlighted some interesting Site nodes. We can se that the most highly connected (more central in the visualization) are the googles and the URL shortening services (t.co, bit.ly, etc.). It makes sense because you typically navigate in and out of them, they are kind of bridge nodes in your navigation. This is confirmed if we run the betweenness centrality algorithm on the sites and their connections. Briefly, betweenness centrality is an indicator of a node’s centrality in a graph and is equal to the number of shortest paths from all nodes to all others that pass through that node.

Here is the Cypher script, again invoking the graph algo implementation as a stored procedure that you can find in the amazing APOC library:

MATCH (s:Site)
WITH collect(s) AS nodes
CALL apoc.algo.betweenness(['SITE_DIRECT_NAVIGATION'],nodes,'BOTH') 
  YIELD node, score
RETURN node.site_root, score

And these are the top five results of the computation on my browser history.


I’m sure you can think of many other interesting queries on your own navigation, what’s the average length of a journey, how many different sites it traverses, is it mostly intra-site? Are there any isolated clusters? An example of this in my browser history are the Amazon sites (amazon.co.uk and music.amazon.co.uk). There seem to be loads of transitions (navigation) between them but none in or out to/from other sites. You can visually see this on the bottom left part of the previous bird’s eye view. I’m sure you will come up with many more but I’ll finish this QuickGraph with a query involving some serious path exploration.

The question is: Which sites have I navigated to from LinkedIn pages, how many times have I reached them and how long (as in how many hyperlink clicks) did it take me to get to them? You may be asking yourself how on earth would you even express that in SQL(?!?!). Well, not to worry, you’ll be pleased to see that it takes less writing expressing the query in Cypher than it takes to do it in English. Here it is:

MATCH (v1)-[:VISIT_TO_PAGE]->(p1)-[:PAGE_IN_SITE]-(s1:Site {site_root: "www.linkedin.com"}) 
MATCH p = (v1)-[:NAVIGATION_TO*]->(v2)-[:VISIT_TO_PAGE]->(p2)-[:PAGE_IN_SITE]-(s2)
WHERE s2 <> s1
WITH length(p) AS pathlen, s2.site_root AS site 
RETURN AVG(pathlen) AS avglen, count(*) AS count, site ORDER BY avglen

And my results, 21 milliseconds later…


What’s interesting about this QuickGraph?

This experiment shows several interesting things, the first being how straightforward it can be to load relational data into Neo4j using the apoc.load.jdbc  stored procedure. As a matter of fact, the same applies to other types of data sources for example Web Services as I described in previous posts.

The second takeaway is how modelling and storing as a graph data that is naturally a graph (sequences of page visits) as opposed to shoehorning it into relational tables opens a lot of opportunities for querying and exploration that would be unthinkable in SQL.

Finally I’ve also shown how some graph algorithms (betweenness centrality) can be applied easily to your graph using stored procedures in Cypher. Worth mentioning that you can extend the list of available ones by writing your own and easily deploying it on your Neo4j instance.

QuickGraph#3 A step-by-step example of RDF to Property Graph transformation

The dataset

For this example I am going to use a sample movie dataset from the Cayley project. It’s a set of half a million triples about actors, directors and movies that can be downloaded here. Here is what the dataset looks like:

</en/meet_the_parents> <name> "Meet the Parents" .
</en/meet_the_parents> <type> </film/film> .
</en/meet_the_parents> </film/film/directed_by> </en/jay_roach> .
</en/meet_the_parents> </film/film/starring> _:28754 . 
_:28754 </film/performance/actor> </en/ben_stiller> .
_:28754 </film/performance/character> "Gaylord Focker" .
</en/meet_the_parents> </film/film/starring> _:28755 .

One could argue whether this dataset is actual RDF or just a triple based graph since it does not use valid URIs or even the RDF vocabulary (note for example that instead of  http://www.w3.org/1999/02/22-rdf-syntax-ns#type we find just type). But this would be a rather pointless discussion in my opinion. For what it’s worth, the graph is parseable with standard RDF parsers which is enough and as we’ll see the problems derived from this can be fixed, which is the point of this post.


Loading the data into Neo4j

I’ll use the RDF Importer described here for the data load. Now, there is something to take into account, even though the data set is called ‘30kmoviedata.nq’ it does not contain quads but triples, so I tried the parser setting the serialization format to ‘N-Triples’. The parser threw an error complaining about the structure of the URIs:

Not a valid (absolute) IRI: /film/performance/actor [line 1]

However, funnily enough the file parses as Turtle format. So if you want to give it a try, remember to set the second parameter of the importRDF stored procedure to ‘Turtle’ and run the import in the usual way. It took only 39 seconds to load the 471K triples on my laptop.


Fixing the model

Fixing dense nodes representing categories

First thing we notice is that because the data set does not use the RDF vocabulary, the a <type> b statements are not transformed into labeled nodes as would have happened if rdf:type was used instead. So there are a couple of unusually dense nodes representing the categories (person and movie) because most of the nodes in the dataset are either actors or movies and are therefore linked to either one or the other category node. The two dense nodes are immediately visible in a small sample of 1000 nodes:


We can get counts on the number of nodes connected to each of them by running this query:

MATCH (x)-[:ns1_type]->(t) RETURN t.uri, count (x)


The natural way of representing categories in the Label Property Graph model is by using labels so let’s fix this!  Here is the Cypher fragment that does the job:

MATCH (x)-[:ns1_type]->({uri : 'file:/film/film'}) 
SET x:Film

And once we have the nodes labeled with their categories we can get rid of the dense nodes and the links that connect the rest of the nodes to them.

MATCH (f {uri : 'file:/film/film'}) DETACH DELETE f

Exactly the same applies to the other category: ‘file:/film/person’

MATCH (x)-[:ns1_type]->({uri : 'file:/people/person'}) 
SET x:Person 

MATCH (p {uri : 'file:/people/person'}) DETACH DELETE p

Fixing unneeded intermediate nodes holding relationship properties

In the tiny fragment that I copied at the beginning of the post, we can already see that the data set suffers from one of the known limitations of triple based graph models which is the impossibility of adding attributes to relationships. To do that, intermediate nodes need to be created. Let’s have a look at the example in the previous data fragment graphically.

Ben Stiller plays the role of Gaylord Focker in the movie Meet the Parents and when modelling this (think how would you draw that in a whiteboard) our intuition says something like this:


Screen Shot 2016-09-09 at 21.11.20.png

But in a triple based model you will need to introduce an intermediate node to hold the role played by an actor in a movie. Something like this.


This obviously creates a gap between what you conceive when modelling a domain and what is stored in disk and ultimately queried. You will have to map what’s in your head, what you drew in the whiteboard when sketching the model to what the triple based formalism forces you to actually create. Does this ring a bell? Join tables in the relational model maybe? In your head it’s a many-to-many relationship but in the relational model it has to be modelled in a separate join table, an artificial construct imposed by the modelling paradigm that inevitably builds a gap between the conceptual model and the physical one. This ultimately makes your model harder to understand and maintain and your SQL queries looooooonger and less performant. But not to worry, we’ll fix this by using the property graph model, the one that is closer to the way we as humans understand and model domains.

But before we do that, let’s look at another problem derived from this. This complex model introduces the possibility of data quality problems in the form of broken links. What if we have the first leg connecting our intermediate node with the movie but no connection with the actor?  It would be a totally meaningless piece of information. The pattern I’m describing would be expressed like this:

()-[r:ns2_starring]->(x) WHERE NOT (x)-[:ns0_actor]->()

And a query producing a ‘Data Quality’ report on this particular issue could look something like this:

MATCH ()-[r:ns2_starring]->(x) WHERE NOT (x)-[:ns0_actor]->() 
WITH COUNT(r) as brokenLinks
MATCH ()-[r:ns2_starring]->(x)-[:ns0_actor]->() 
WITH COUNT(r) as linked, brokenLinks
RETURN linked + brokenLinks as total, linked, brokenLinks,  
     toFloat(brokenLinks)* 100/(linked + brokenLinks) as percentageBroken

Screen Shot 2016-09-09 at 17.43.59.png

So 0.03% does not seem to be significant, probably the dataset was truncated in a bad way, which would explain the missing bits. Anyway, we can get rid of these broken links that don’t add any value to our graph. Here’s how:

MATCH ()-[r:ns2_starring]->(x) WHERE NOT (x)-[:ns0_actor]->() 

Ok, so now we are in a position to get rid of the ugly and unintuitive intermediate nodes that I described before and replace them with relationships containing attributes on them.

MATCH (film)-[r:ns2_starring]->(x)-[:ns0_actor]->(actor)
CREATE (actor)-[:ACTS_IN { character: x.ns0_character}]->(film)
Deleted 136694 nodes, set 15043 properties, created 136694 relationships, statement executed in 7029 ms.

And voilà! Here is the final model zooming on the ‘Gaylord Focker’ area:

MATCH (actor)-[:ACTS_IN { character : 'Gaylord Focker' }]->(movie) 


Screen Shot 2016-09-09 at 18.37.38.png

And to finish, one of our favourites at Neo4j, a recommendation engine for Hollywood actors. Who should Ben Stiller work with? We’ll base this in the concept of friend-of-a-friend. If Ben has worked several times with actor X and actor X has worked several times with actor Y then there is a good chance that Ben might be interested in working with actor Y.

Here is the Cypher query that returns our best recommendations for Ben Stiller:

MATCH (ben:Person {ns1_name: 'Ben Stiller'})-[:ACTS_IN]->(movie)<-[:ACTS_IN]-(friend) 
WITH ben, friend, count(movie) AS timesWorkedWithBen ORDER BY timesWorkedWithBen DESC LIMIT 3 //limit to top 3 
MATCH (friend)-[:ACTS_IN]->(movie)<-[:ACTS_IN]-(friendOfFriend)
WHERE NOT (ben)-[:ACTS_IN]->(movie)<-[:ACTS_IN]-(friendOfFriend) AND friendOfFriend <> ben
RETURN friend.ns1_name AS friendOfBen, timesWorkedWithBen, friendOfFriend.ns1_name AS recommendationForBen, count(movie) AS timesWorkedWithFriend ORDER BY timesWorkedWithFriend DESC limit 50

Easy, right? And here are the recommendations:

Screen Shot 2016-09-09 at 20.46.32.png

The following two visualisations give an idea of the portion of the graph explored with our recommendation query. This first one shows Ben’s friends and the movies where they worked together (~400 nodes in total):

Screen Shot 2016-09-09 at 19.26.35.png

And the next shows Ben’s friends’ friends, again with the movies that connect them (~1800 nodes):

Screen Shot 2016-09-09 at 19.34.10.png

You can try to write something similar on the original triple based graph using SPARQL, Gremlin or any other language but I bet you it will be less compact, less intuitive and certainly less performant than the Cypher I wrote. Prove me wrong if you can 😉

What’s interesting about this QuickGraph?

The example highlights some of the modelling limitations of triple based graph models like RDF and how it is possible to transform a model originally created as RDF into a more intuitive and easier to query and explore using the Labeled Property Graph in Neo4j.






QuickGraph#2 How is Wikipedia’s knowledge organised

The dataset

For this QuickGraph I’ll use data about Wikipedia Categories. You may have noticed at the bottom of every Wikipedia article a section listing the categories it’s classified under. Every Wikipedia article will have at least one category, and categories branch into subcategories forming overlapping trees. It is sometimes possible for a category (and the Wikipedia hierarchy is an example of this) to be a subcategory of more than one parent category, so the hierarchy is effectively a graph.

I guess an example will be helpful at this point: If you open the Wikipedia page on the mathematician Leonard Euler you will find at the bottom of it the following list of categories:

Screen Shot 2016-08-03 at 09.43.02

If you click on any of them, for instance ‘Swiss mathematicians‘ you will be able to navigate up and down the hierarchy of categories from the selected one. I’ll go a couple of steps ahead here and show the final product before explaining how to build it: The structure you are navigating is a graph and looks something like this:

Screen Shot 2016-08-03 at 09.57.45

In the diagram the color coding of the Category nodes indicates the (shortest) distance to the root category so we will have Level1 (green) to Level4 (pink) categories. We can see that the ‘Swiss Mathematician’ category can be reached following many different paths from the root. A couple of examples would be:

People > People by occupation> People in STEM fields > Mathematicians > Mathematicians by nationality > Swiss mathematicians 


Science > Academic disciplines> STEM > Mathematics > Mathematicians > Mathematicians by nationality > Swiss mathematicians 

So this is the graph that we are going to build and explore/query in Neo4j in this blog post.

The data source

Wikipedia data can be accessed through the MediaWiki action API, and the specific request that returns information for a given category is described here and looks as follows:


The request returns a simple JSON structure containing all subcategories of the one passed as parameter (cmtitle). Here is an example of the web service output:

Screen Shot 2016-08-02 at 04.15.33

Loading the data into Neo4j

Since the Wikipedia categorymembers web service takes the name of a category as input parameter, we’ll have to seed the graph with a root node in order to start loading categories. This node will be the starting point for our data load. For this example we will use the category “Main Topic Classification” which groupsWikipedia’s major topic classifications.

CREATE (c:Category:RootCategory {catId: '0000000', catName: 'Main_topic_classifications', fetched : false})

An index on category Ids will help speeding up the merges, so let’s add it.

CREATE INDEX ON :Category(catId)

Now from the top level category we’ll iteratively call the categorymembers web service and process the JSON fragment returned to create nodes and relationships in the graph. We will do this in Neo4j using the apoc.load.json procedure in the APOC library. Here is the Cypher script that does the job. It first invokes the web service with the name of the category, and then process the JSON that the web service returns by creating new instances of Category nodes connected to the parent trhough :SUBCAT_OF relationships.

MATCH (c:Category { fetched: false}) 
call apoc.load.json('https://en.wikipedia.org/w/api.php?format=json&action=query&list=categorymembers&cmtype=subcat&cmtitle=Category:' + replace(c.catName,' ', '%20') + '&cmprop=ids|title&cmlimit=500') 
YIELD value as results 
UNWIND results.query.categorymembers AS subcat 
MERGE (sc:Category:Level1Category {catId: subcat.pageid}) 
ON CREATE SET sc.catName = substring(subcat.title,9), 
              sc.fetched = false
MERGE (sc)-[:SUBCAT_OF]->(c)
SET c.fetched = true

Each iteration of the loader adds a new level to the category hierarchy by picking up all ‘unexpanded’ nodes and fetching for each of them all the subcategories. Nodes are marked on creation with the fetched : false property and once they have been expanded (subcategories retrieved and added to the graph) the property fetched is set to true.

So a first iteration of the loader after creating the top level category ” instantiates 13 new categories

Screen Shot 2016-08-02 at 11.47.20

Which are obviously the 13 main topic classifications in Wikipedia.

Screen Shot 2016-08-02 at 11.54.11

Screen Shot 2016-08-03 at 11.50.49

The second iteration of the loader picks each one of these 13 and instantiates its subcategories producing 423 new next level nodes.

Screen Shot 2016-08-02 at 11.56.51

A third iteration brings in 5815 new nodes and a fourth one takes the final number close to 50K categories by adding 42840 more. It’s important to keep in mind that every category expansion involves a request being sent to the wikipedia API so I would not recommend this approach for hierarchies more than 4 levels deep.  If you’re interested in loading the whole set of categories probably the best would be to download them as an RDB dump (the files enwiki-latest-category.sql.gz and enwiki-latest-categorylinks.sql.gz from the downloads page) and then use the superfast batch importer to create your graph in Neo4j. Maybe an idea for a future graphgist?

Ok, we have now a graph of 50K odd categories from Wikipedia organised hierarchically via :SUBCAT_OF relationships, so let’s try and run some interesting queries on them. Here’s how a few thousand categories (up to level three) look like in the Neo4j browser: the root node is the red one towards the center of the image, Level 1 categories are the green ones, followed by Level 2 in blue and Level 3 in yellow.


Screen Shot 2016-08-02 at 14.42.05

An alternative graph based on a smaller number of initial thematic classifications can be built using “Fundamental_categories” as root. In Wikipedia’s terms, this alternative classification “is intended to contain all and only the few most fundamental ontological categories which can reasonably be expected to contain every possible Wikipedia article under their category trees”. I leave this for the interested reader to try.

Querying the graph

What is the shortest full hierarchy for a given category?

Let’s use for this example the ‘Swiss Mathematicians’ example from the introduction.

MATCH shortestFullHierarchy = shortestPath((swissMatem:Category {catName : 'Swiss mathematicians'})-[:SUBCAT_OF*]->(root:RootCategory)) 
RETURN shortestFullHierarchy

The shortestPath function returns the expected depth four hierarchy:

Screen Shot 2016-08-03 at 10.48.23

There will be cases where more than one shortest path exist, and these can be picked up with the allShortestPaths variant of the shortest path function:

MATCH shortestFullHierarchies = allShortestPaths((swissMatem:Category {catName : 'Philosophers by period'})-[:SUBCAT_OF*]->(root:RootCategory)) 
RETURN shortestFullHierarchies

The query returns all shortest full hierarchies (all with max depth of 4) for the ‘Ancient philosophers’ category.

Screen Shot 2016-08-03 at 11.15.05

What are the richest categories in the top four levels of the Wikipedia?

What is a rich category? Well, this is a QuickGraph so I’ll come up with my own quick (although I hope still minimally reasoned) definition but let’s keep in mind that I’m not trying to build a theory about hierarchy relevance but rather show how the graph can be easily queried in interesting ways.

Right, so a rich category is one that has loads of subcategories because that means it’s complex enough to be broken down with such a high level of detail. Similarly, a rich category is also one that can be categorised from different perspectives, or in other words, one that has multiple parent categories. But we will want to discard unbalanced ones (many subcategories but very few super-categories or vice versa) to avoid enumeration-style categories like “Religion by country”, “Culture by nationality” or “People by ethnic or national descent”. With these three criteria we can build a graph query that returns our top ten. Here is the Cypher for our richest categories:

MATCH (cat:Category)
WITH cat, 
 size((cat)-[:SUBCAT_OF]->()) as superCatDegree, 
 size(()-[:SUBCAT_OF]->(cat)) as subCatDegree
WHERE ABS(toFloat(superCatDegree - subCatDegree)/(superCatDegree + subCatDegree)) < 0.4
RETURN cat.catName, (superCatDegree + subCatDegree)/2 AS richness 
ORDER BY richness DESC 

And the results are quite interesting…


Screen Shot 2016-08-02 at 20.47.04

If now we want to explore visually the Marxism category…

Screen Shot 2016-08-02 at 21.43.07

Unexpectedly long hierarchies!

One thing that caught my eye from the point of view of the structure of the graph was the fact that even though I had only loaded 4 levels, I was able to find way longer chains of parent-child categories. This is because not all :SUBCAT_OF relationships go from level n to level n-1. That is actually what makes the structure a graph and not a tree. The following query returns :SUBCAT_OF chains longer than 12 ( path =()-[r:SUBCAT_OF*12..]->() ), but checks that every node in the chain is at a maximum distance of 4 from the root ( shortestPath((node)-[:SUBCAT_OF*..4]->(root:Category {catId:’0000000′})) ).

MATCH path =()-[r:SUBCAT_OF*12..]->() WITH path LIMIT 1
UNWIND nodes(path) AS node
MATCH shortest = shortestPath((node)-[:SUBCAT_OF*..4]->(root:Category {catId:'0000000'}))
RETURN path, shortest

And the first result shows how effectively, even though there are chains of length 6 (and longer than that, actually) we can easily see that all the nodes in the chain are never more than 4 hops away from the root node. This is consistent with the fact that we’ve only loaded 4 levels of the category hierarchy.

Screen Shot 2016-08-02 at 19.20.14



Yes, there are some. So if one day you decide to read the whole Wikipedia (as you do) and you choose to do it by category, then be careful not to enter into an infinite loop of knowledge 🙂

Looops are easy to pick up with this simple Cypher query:

MATCH loop = (cat)-[r:SUBCAT_OF*]->(cat) 

The caption on the left shows a loop and the one on the right shows the same loop but keeping the hierarchical order. Green nodes are level 1 categories, blue ones are level 2 and finally yellow ones represent the level 3 categories. The interesting thing is that Geography, even though it’s level one, it’s also a subcategory of Earth Sciences that is level 3. The same thing happens with Nature, being both level 1 and a subcategory of Environmental social science concepts, which is level 3. These two extra relationships create the loop that produces the following chain of :SUBCAT_OF.

MATCH loop = (cat)-[r:SUBCAT_OF*]->(cat) 
RETURN reduce(s = "", x IN nodes(loop) | s + ' > '+ x.catName) AS categoryChain limit 1

Screen Shot 2016-08-02 at 17.23.52

You can also try to find longer loops by just specifying the minimum length of the path

loop = (cat)-[r:SUBCAT_OF*12..]->(cat)

And get chains like:

Screen Shot 2016-08-02 at 18.19.21

What’s interesting about this QuickGraph?

The graph is built directly from querying a web service from the MediaWiki action API. The apoc.load.json procedure in the APOC library can invoke this service and ingest the JSON structure returned all within your Cypher script. Find how to install and use (and extend!) the APOC library of stored procedures for Neo4j.

Rich category hierarchies are graphs and extremely useful tool for scenarios like recommendation systems or  graph-enhanced search.