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Neo4j: Traversal query timeout

I’ve been spending some of my spare time over the last few weeks creating an application that generates running routes from Open Roads data – transformed and imported into Neo4j of course!

I’ve created a user defined procedure which combines several shortest path queries, but I wanted to exit any of these shortest path searches if they were taking too long. My code without a timeout looks like this:

StandardExpander orderedExpander = new OrderedByTypeExpander()
    .add( RelationshipType.withName( "CONNECTS" ), Direction.BOTH );
 
PathFinder<Path> shortestPathFinder = GraphAlgoFactory.shortestPath( expander, 250 );
 
...

There are several places where we could check the time elapsed, but the expand method in the Expander seemed like an obvious one to me. I wrote my own Expander class which looks like this:

public class TimeConstrainedExpander implements PathExpander
{
    private final StandardExpander expander;
    private final long startTime;
    private final Clock clock;
    private int pathsExpanded = 0;
    private long timeLimitInMillis;
 
    public TimeConstrainedExpander( StandardExpander expander, Clock clock, long timeLimitInMillis )
    {
        this.expander = expander;
        this.clock = clock;
        this.startTime = clock.instant().toEpochMilli();
        this.timeLimitInMillis = timeLimitInMillis;
    }
 
    @Override
    public Iterable<Relationship> expand( Path path, BranchState state )
    {
        long timeSoFar = clock.instant().toEpochMilli() - startTime;
        if ( timeSoFar > timeLimitInMillis )
        {
            return Collections.emptyList();
        }
 
        return expander.expand( path, state );
    }
 
    @Override
    public PathExpander reverse()
    {
        return expander.reverse();
    }
}

The code snippet from earlier now needs to be updated to use our new class, which isn’t too tricky:

StandardExpander orderedExpander = new OrderedByTypeExpander()
    .add( RelationshipType.withName( "CONNECTS" ), Direction.BOTH );
 
TimeConstrainedExpander expander = new TimeConstrainedExpander(orderedExpander, 
    Clock.systemUTC(), 200);
 
PathFinder<Path> shortestPathFinder = GraphAlgoFactory.shortestPath( expander, 250 );
...

I’m not sure if this is the best way to achieve what I want but after failing with several other approaches at least this one actually works!

Published on Java Code Geeks with permission by Mark Needham, partner at our JCG program. See the original article here: Neo4j: Traversal query timeout

Opinions expressed by Java Code Geeks contributors are their own.

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