Can threads see each other's changes in multi-threaded transactions?

Question

I'm employing multi-threaded transactions as described by JanusGraph docs. Each of my threads contributes to building a directory tree. Before inserting a new vertex for a specific directory, each thread first checks if such a vertex already exists within the same query. Vertexes are only inserted with .orElseGet if no existing one can be found.

Vertex vertex = graph.traversal().V()
    .hasLabel(VertexLabels.DIRECTORY)
    .has(PropertyKeys.PATH, directory.path())
    .tryNext()
    .orElseGet(() -> {
        return graph.addVertex(
            T.label, VertexLabels.DIRECTORY,
            PropertyKeys.PATH, directory.path());
    });

Technically, this should prevent duplicates assuming that all threads operate within the same transactional scope. I do however encounter duplicates. The docs don't seem to give any answers regarding this issue. Can you confirm whether multi-threaded transactions operate within the same scope?

score 3 · Accepted Answer · answered Nov 20 '19 at 11:27

Multi-threaded transactions operate in the same scope, but I suppose it remains possible for the threads to race if you haven't configured a unique constraint on PropertyKeys.PATH. Doing so does mean that locking would be enabled which might slow down your ingestion rate but will ensure uniqueness.

As a side note please consider avoiding use of the Graph API (graph.addVertex()) and sticking to pure Gremlin - the "get or create" pattern is described here.

Can threads see each other's changes in multi-threaded transactions?

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