Cole Greer created TINKERPOP-3207:
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Summary: Add Barriers to GroupSideEffect, GroupCountSideEffect,
TreeSideEffect, and Subgraph Steps
Key: TINKERPOP-3207
URL: https://issues.apache.org/jira/browse/TINKERPOP-3207
Project: TinkerPop
Issue Type: Improvement
Components: process
Affects Versions: 3.7.4
Reporter: Cole Greer
The gremlin language is mostly agnostic as to if a traversal should be executed
lazily (DFS), or eagerly (BFS). Our documentation does not specify a preference
for either method, and most traversals will give the same results regardless of
evaluation order.
There are currently 5 steps which may lead to different results if evaluated in
different orders: AggregateLocalStep, GroupSideEffectStep,
GroupCountSideEffectStep, TreeSideEffectStep, and SubgraphStep.
Consider the following example:
{code:java}
// Lazy evaluation
gremlin> g.V().groupCount(“x”).select(“x”)
==>[v[1]:1]
==>[v[1]:1,v[2]:1]
==>[v[1]:1,v[2]:1,v[3]:1]
==>[v[1]:1,v[2]:1,v[3]:1,v[4]:1]
==>[v[1]:1,v[2]:1,v[3]:1,v[4]:1,v[5]:1]
==>[v[1]:1,v[2]:1,v[3]:1,v[4]:1,v[5]:1,v[6]:1]
//Eager evaluation
gremlin> g.V().groupCount(“x”).select(“x”)
==>[v[1]:1,v[2]:1,v[3]:1,v[4]:1,v[5]:1,v[6]:1]
==>[v[1]:1,v[2]:1,v[3]:1,v[4]:1,v[5]:1,v[6]:1]
==>[v[1]:1,v[2]:1,v[3]:1,v[4]:1,v[5]:1,v[6]:1]
==>[v[1]:1,v[2]:1,v[3]:1,v[4]:1,v[5]:1,v[6]:1]
==>[v[1]:1,v[2]:1,v[3]:1,v[4]:1,v[5]:1,v[6]:1]
==>[v[1]:1,v[2]:1,v[3]:1,v[4]:1,v[5]:1,v[6]:1]
{code}
This inconsistency can be resolved by pulling the solution from
AggregateGlobalStep and making each of these steps LocalBarrier's. If there are
ever cases where users need the old "one-at-a-time" accumulation, this will
still be achievable by embedding the step inside local().
{code:java}
gremlin> g.V().local(groupCount("x")).select("x")
==>[v[1]:1]
==>[v[1]:1,v[2]:1]
==>[v[1]:1,v[2]:1,v[3]:1]
==>[v[1]:1,v[2]:1,v[3]:1,v[4]:1]
==>[v[1]:1,v[2]:1,v[3]:1,v[4]:1,v[5]:1]
==>[v[1]:1,v[2]:1,v[3]:1,v[4]:1,v[5]:1,v[6]:1]
{code}
This will ensure that all traversals will return consistent results, regardless
of if is is run in a lazy or eager traversal engine.
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