Write blocking behavior in neo4j cypher over trans-national REST api
I am using CYPHER as shown below (with Neo4j 2.2.3, above transactional kef REST endpoint) to atomically read and increment the counter property on a node:
MATCH node WHERE id(node)={nodeId}
SET node.counter=COALESCE(node.counter,0) + 1
RETURN node.counter
I have multiple threads creating REST records like this and now I've seen a case where two threads got the same counter value.
This behavior is similar to what is described as a possible regression from 2.1 to 2.2: Neo4j 2.2 Cypher Lock Regression?
However, it has previously been suggested that this is not possible in CYPHER (see comments here: https://stackoverflow.com/a/1499298/ ... ) ... even before, it seems like it was possible (see: https://stackoverflow.com/a/2996999/) 2250118 / ... )
Is there a CYPHER request similar to the one above that will reliably generate unique (monotonically increasing) counter values when there are multiple concurrent REST requests?
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In Neo4j, you need to acquire a write lock before reading and updating a value. We do this as two statements in one transaction (one REST call). We are using MERGE / ON CREATE / ON MATCH, but I don't think this is necessary or even will work in your case. I believe this should work for you.
expression 1:
MATCH node WHERE id(node)={nodeId}
SET node.__lock=true
RETURN node.__lock
expression 2:
MATCH node WHERE id(node)={nodeId}
SET node.counter=COALESCE(node.counter,0) + 1
SET node.__lock=false
RETURN node.counter
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Adding to @SeanTimm's answer, you can explicitly block nodes via Cypher, if you are using the popular (and open source) APOC plugin for Neo4j, APOC has a choice of explicit blocking routines that can be called via Cypher such ascall apoc.lock.nodes([nodes])
If this option is available to you, it is probably preferable to some other hacky solutions.
More on neo4j-contrib.github.io/neo4j-apoc-procedures/#_locking
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