[jira] [Updated] (OAK-858) NodeBuilder.getChildNodeCount performance and scalability

["Thomas Mueller (JIRA)" <ji...@apache.org>]

     [ https://issues.apache.org/jira/browse/OAK-858?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel ]

Thomas Mueller updated OAK-858:
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    Component/s:     (was: mk)
    
> NodeBuilder.getChildNodeCount performance and scalability
> ---------------------------------------------------------
>
>                 Key: OAK-858
>                 URL: https://issues.apache.org/jira/browse/OAK-858
>             Project: Jackrabbit Oak
>          Issue Type: Improvement
>          Components: core
>            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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