In march we will restart our bigdata.ro sessions, workshops mainly aimed to help participants navigate easier through big data architectures and get a basic understanding in some of the possible components of such architectures. We have discussed in the past Cassandra, HDFS, Hive, Impala, Elasticsearch, Solr, Spark & Spark SQL, generic big data architectures and on March 16th we will continue our journey with one of the unusual children of noSQL: the graph database Neo4j. Not quite similar with the other noSQL siblings, this database is not derived from the likes of DynamoDB or BigTable like others do, but instead addresses relationship between data not just the data itself. The result is amazing, the use cases are incredible and Calin Constantinov will guide us through the basics of this interesting solution.
See below a few questions and answers in advance of the workshop, hopefully these will increase your curiosity towards Neo4j.
Valentina Crisan – bigdata.ro
Calin Constantinov – trainer “Intro to Neo4j” workshop, March 16th
What is a graph database and which are the possible use cases that favour such a database?
Calin: They say “everything is a graph”. Indeed, even the good old entity-relationship diagram is no exception to this rule. And graphs come with a great “feature” which us humans tend to value very much: they are visual! Graphs can easily be represented on a whiteboard and immediately understood by a wide audience.
Moreover, in a traditional database, explicit relationships are destroyed the very moment we store data and need to be recreated on-demand using JOIN operations. A native graph database has preferential treatment for relationships meaning that there are actual pointers linking an entity to all its neighbors.
I remember the first time I needed to implement a rather simple Access Control List solution that needed to support various inheritable groups, permissions and exceptions. Writing this in SQL can quickly become a nightmare.
But of course, the most popular example is social data similar to what Facebook generates. For some wild reason, imagine you need to figure out the year with the most events attended by at least 5 of your 2rd degree connections (friends-of-friends), with an additional restriction that none of these 5 are friends between them. I wouldn’t really enjoy implementing that with anything other than Neo4j!
However, not all graphs are meant to be stored in a graph database. For instance, while a set of unrelated documents can be represented as a graph with no edges, please don’t rush to using Neo4j for this use-case. I think a Document store is a better persistence choice.
In terms of adoption, 75% of the Fortune 100 companies are already using Neo4j. As for concrete use-case examples, Neo4j is behind eBay’s ShopBot for Graph-Powered Conversational Commerce while NBC News used it for uncovering 200K tweets tied to Russian trolls. My personal favourite is the “Panama Papers” where 2.6TB of spaghetti data, made up of 11.5M heterogeneous documents, was fed to Neo4j. And I think we all know the story that led the investigative team to win the Pulitzer Prize.
What graph databases exist out there and how is Neo4j different from those?
Calin: Full disclosure, given the wonderful community around it (and partially because it happened to be the top result of my Google search), it was love at first sight with me and Neo4j. So, while I’m only using Neo4j in my work, I do closely follow what’s happening in the graph world.
JanusGraph, “a graph database that carries forward the legacy of TitanDB” is one of the most well-known alternatives. A major difference is that JanusGraph is more of a “graph abstraction layer” meaning that it requires a storage backend instead of it being a native graph.
OrientDB is also popular do its Multi-Model, Polyglot Persistence implementation. This means that it’s capable of storing graph, document and key/value data, while maintaining direct connections between records. The only drawback is that it might have not yet reached the maturity and stability required by the most data-intensive tasks out there.
More recently, TigerGraph showed impressive preliminary results, so I might need to check that out soon.
Is the Neo4j architecture a distributed one? Does it scale horizontally like other noSQL databases?
Calin: The short answer is that Neo4j can store graphs of any sizes in an ACID-compliant, distributed, Highly-Available, Causal Clustering architecture, where data replication is based on the state-of-the-art Raft protocol.
In order to achieve best performance, we would probably need to partition the graph in some way. Unfortunately, this is typically a NP-hard problem and, more often than not, our graphs are densely connected which can really make some form of clustering quite challenging. To make matters worse, coming back to the Facebook example, we need to understand that this graph is constantly changing, with each tap of the “Like” button. This means that our graph database can easily end up spending more time finding a (sub-)optimal partition than actually responding to queries. Moreover, when combining a complex query with a bad partitioning of the data, you wind up with requiring a lot of network transfers within the cluster, which will most likely cost more than a cache miss. In turn, this could also have a negative effect on query predictability. Sorry to disappoint you, but this is the reason for which Neo4j doesn’t yet support data distribution. And it’s a good reason too!
So, a limitation in the way Neo4j scales is that every database instance has a complete replica of the graph. Ideally, for best performance, all instances need to have enough resources to keep the whole graph in memory. If this is not the case, in some scenarios, we can at least attempt to achieve cache sharding by identifying all queries hitting a given region of the graph and always routing them to the same instance. As a starting point, there is a built-in load-balancer which can potentially be extended to do that. Additionally, we can easily direct I/O requests intelligently in a heterogeneous environment, designating some Read Replicas for handling read queries while only writing to instances packing the most power. This is a good thing for read operations which can easily scale horizontally. Write operations are however the bottleneck. Nevertheless, the guys over at Neo4j are always coming up with clever ways to significantly improve write performance with every new release.
Does Neo4j work with unstructured/flexible structured data?
Calin: A graph is composed of nodes and relationships. We are able to group similar nodes together by attaching a label, such as “User” and “Post”. Similarly, a relationship can have a type, such as “LIKED” and “TAGGED”. Neo4j is a property graph meaning that multiple name-value pairs can be added both to relationships and nodes. While it is mandatory in Neo4j for relationships to have exactly one type, labels and properties are optional. New ones can be defined on-the-fly and nodes and relationships of the same type don’t all necessarily need to have the same properties. If needed, Neo4j does support indexes and constraints, which can, for instance, improve query performance, but this is as close as you get to an actual schema.
In regards to the question, the whole point of a graph database is to structure your data in some way. Keep in mind that the true value of your data lies within uncovering relationships between entities. If you feel like this doesn’t fit your use-case, coming back to the example of having a number of unrelated free-text documents or even some form of semi-structured data, while Neo4j now supports full text indexing and search, there are clearly better alternatives out there, such as key-value and document stores.
How is it best and easiest to get started with Neo4j?
Calin: Apart from attending my workshop, right? I think the best way to get up to speed with Neo4j is to use the Neo4j Sandbox. It’s a cloud-based trial environment which only requires a browser to work and which comes preloaded with a bunch of interesting datasets. If you’re into a more academic approach, I highly recommend grabbing a copy of “Graph Databases” or “Neo4j in Action“.
Can you detail how can users interact with Neo4j? What about developers, are there pre-built drivers or interfaces?
Calin: Neo4j packs a very nice Browser that enables users to quite easily query and visualize the graph. This comes with syntax highlighting and autocompletion for Cypher, the query language used by Neo4j. It also features a very handy way to interact with query execution plans.
Developers can “talk” with the database using a REST API or, better yet, the proprietary binary protocol called Bolt, which is already uniformly encapsulated by a number of official drivers, covering the most popular programming languages out there.
However, as I don’t want to spoil the fun, that’s all you’re getting out of me today. But do come and join us on the 16th of March. Please.