Representing (and incrementing) relationship strength in Neo4j

Question

I would like to represent the changing strength of relationships between nodes in a Neo4j graph.

For a static graph, this is easily done by setting a "strength" property on the relationship:

  A --knows--> B
       |
     strength
       |
       3

However, for a graph that needs updating over time, there is a problem, since incrementing the value of the property can't be done atomically (via the REST interface) since a read-before-write is required. Incrementing (rather than merely updating) is necessary if the graph is being updated in response to incoming streamed data.

I would need to either ensure that only one REST client reads and writes at once (external synchronization), or stick to only the embedded API so I can use the built-in transactions. This may be workable but seems awkward.

One other solution might be to record multiple relationships, without any properties, so that the "strength" is actually the count of relationships, i.e.

A knows B
A knows B
A knows B

means a relationship of strength 3.

• Disadvantage: only integer strengths can be recorded
• Advantage: no read-before-write is required
• Disadvantage: (probably) more storage required
• Disadvantage: (probably) much slower to extract the value since multiple relationships must be extracted and counted

Has anyone tried this approach, and is it likely to run into performance issues, particularly when reading?

Is there a better way to model this?

Answer 1

Nice idea. To reduce storage and multi-reads those relationships could be aggregated to one in a batch job which runs transactionally.

Each rel could also carry an individual weight value, whose aggregated value is used as weight. It doesn't have to be integer based and could also be negative to represent decrements.

You could also write a small server-extension for updating a weight value on a single relationship transactionally. Would probably even make sense for the REST API (as addition to the "set single value" operation have a modify single value operation.

PUT http://localhost:7474/db/data/node/15/properties/mod/foo 

The body contains the delta value (1.5, -10). Another idea would be to replace the mode keyword by the actual operation.

PUT http://localhost:7474/db/data/node/15/properties/add/foo 
PUT http://localhost:7474/db/data/node/15/properties/or/foo 
PUT http://localhost:7474/db/data/node/15/properties/concat/foo 

What would "increment" mean in a non integer case?