How should I do multi-threaded insertion in Neo4j? - multithreading

Question

I am trying to solve a problem that occurs when inserting related nodes in Neo4j. Nodes are inserted by several threads using the standard save method of org.springframework.data.neo4j.repository.GraphRepository.
Sometimes the insertion fails when fetching a related node in order to define a relationship. The exception messages are like this: org.neo4j.graphdb.NotFoundException: '__type__' on http://neo4j:7474/db/data/relationship/105550.
Calling this URL from curl returns a JSON object which appears to have __type__ correctly defined, which suggests that the exception is caused by a race between inserting threads.
The method that originates the calls to the repository is annotated #Neo4jTransactional. What atomicity and transaction isolation does #Neo4jTransactional guarantee? And how should I use it for multi-threaded insertions?
Update:
I have now been able to see this happening in the debugger. The code is trying to fetch the node at one end of this relationship, together with all its relationships. It throws an exception because the type attribute is missing. This is the JSON initially returned.
{
"extensions" : { },
"start" : "http://localhost:7474/db/data/node/617",
"property" : "http://localhost:7474/db/data/relationship/533/properties/{key}",
"self" : "http://localhost:7474/db/data/relationship/533",
"properties" : "http://localhost:7474/db/data/relationship/533/properties",
"type" : "CONTAINS",
"end" : "http://localhost:7474/db/data/node/650",
"metadata" : {
"id" : 533,
"type" : "CONTAINS"
},
"data" : { }
}
A few seconds later, the same REST call returns this JSON:
{
"extensions" : { },
"start" : "http://localhost:7474/db/data/node/617",
"property" : "http://localhost:7474/db/data/relationship/533/properties/{key}",
"self" : "http://localhost:7474/db/data/relationship/533",
"properties" : "http://localhost:7474/db/data/relationship/533/properties",
"type" : "CONTAINS",
"end" : "http://localhost:7474/db/data/node/650",
"metadata" : {
"id" : 533,
"type" : "CONTAINS"
},
"data" : {
"__type__" : "ProductRelationship"
}
}
I can't understand why there is such a long delay between inserting the relationship and specifying the type. Why doesn't it all happen at once?