Thomas Mueller created OAK-858:
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Summary: NodeBuilder.getChildNodeCount performance and scalability
Key: OAK-858
URL: https://issues.apache.org/jira/browse/OAK-858
Project: Jackrabbit Oak
Issue Type: Improvement
Reporter: Thomas Mueller
The method NodeBuilder.getChildNodeCount() is currently supposed to return the
exact number of child nodes of a node. If there are no or only few child nodes
(which is the most common case), this isn't a problem.
However, if there are many nodes (thousands, maybe millions), then keeping an
accurate and up-to-date count is tricky. It is specially tricky in a cluster,
if you want to allow concurrent add node / delete node. This is for example
needed for the UUID index currently. There would be a way to avoid concurrent
add/remove: by using some hierarchy, that is, _avoid_ using many child nodes.
But efficient, scalable support for many child nodes is one of the goals of Oak
in my view.
I think it's not worth the effort to support efficient, accurate child node
*counts* if there are many child nodes. Instead, I suggest to change the
contract, and possibly even change the method.
The current usages of the method NodeBuilder.getChildNodeCount(), excluding
usage within getChildNodeCount itself, toString(), and tests:
* AbstractNodeState.equals, where it's used to avoid iterating over all child
nodes if possible. But it doesn't always avoid iterating over all child nodes,
so this method anyway is problematic. I even suggest to remove it (or throw an
exception) because of the potential performance problem if there are many child
nodes.
* Template constructor, where there are only 3 cases: 0, 1, and many child
nodes.
* EmptyNodeState.equals, where there are only 2 cases: 0 and non-0.
* SecureNodeState.WrapChildEntryFunction.apply, where there are only 2 cases: 0
and non-0.
Because of that, in theory we could simply change the contract of the method to
return only "0, 1, Long.MAX_VALUE". However this seems dangerous.
Instead, I see two options:
* change the method to return an enum: NO, ONE, MANY.
* change the method to NodeBuilder.getChildNodeCount(long max), where max is
the maximum returned value. So that a typical method call would be
getChildNodeCount(1) if you only care about 0 or non-0.
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