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. |
