tree("key") as a Side Effect

This page lists where tree("key") differs from Apache TinkerPop 3.8.2. Everything not listed follows Tree.feature.

tree() without a key is a reducing barrier that emits the tree. tree("a") is a side effect: every traverser passes through unchanged and its path is added to the tree stored under a. A second tree("a") adds to the same tree, cap("a") and select("a") read it, and by() works on both forms:

g.V().out().tree("a").select("a").count(Scope.local)   // six rows of 3
g.V("1").out().out().tree("a").by("name").both().both().cap("a")

Deviations

• A later step does not see a half-built tree. The engine runs each step over the whole batch before the next one starts, so g.V().out().order().by("name").local(__.tree("a")).select("a") gives the finished tree on every row, where TinkerPop's streaming execution gives the growing tree (1, 1, 1, 2, 2, 3). Everything inside one local() child is evaluated per traverser and is progressive: g.V().out().local(__.tree("a").select("a").count(Scope.local)) matches TinkerPop. Reading the tree before the traversers are done needs a streaming executor.
• Bulk is ignored, as in TinkerPop: a path reached many times is one branch.
• A key that already holds something other than a tree is an error (aggregate("a").tree("a")). TinkerPop fails with its own cast error.
• An empty stream still leaves an empty tree that cap("a") returns as {}.