Monthly Archives: October 2021

Let’s build something Outrageous – Part 15: Connected

Checking if two nodes are directly connected is something you often have to do in a graph. There are a few different ways to implement this feature depending on how the database keeps track of relationships. In Neo4j a double linked list of relationships is kept per node grouped by the relationship type in both the incoming and outgoing directions. To check if two nodes are directly connected, one has to traverse one of the lists (preferably the shortest one) and checking to see if the other node id is included in that list. If we don’t know the relationship type, we have to check all the groups (for dense nodes, or light nodes there are no groups and we check them all anyway).

In Amazon Neptune the SPOG index can be queried twice. Once with the first node in the S position and the second node in the O position, then again with the positions reversed (with the P position being the relationship type). If we don’t know the relationship type we can query the indexes twice per relationship type.

Checking if two nodes are directly connected is similar to checking for set membership, and one trick we could use is a bloom filter and variant data structures. Long time readers will remember this blog post outlining exactly how to do that and achieve 100x faster checks including a “double check” to get around the probabilistic nature of these data structures.

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