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https://issues.apache.org/jira/browse/ZOOKEEPER-2163?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14516139#comment-14516139
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Raul Gutierrez Segales commented on ZOOKEEPER-2163:
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I want to do another — more detailed — pass but I think it generally looks
good. Overloading the ephemeralOwner field is something that probably needs to
be documented carefully and we need to think about how to make container znodes
discoverable (i.e.: how can operators know which znodes are container ones?).
Thoughts?
cc: [~michim], [~phunt], [~fpj], [~hdeng], [~rakeshr]
> Introduce new ZNode type: container
> -----------------------------------
>
> Key: ZOOKEEPER-2163
> URL: https://issues.apache.org/jira/browse/ZOOKEEPER-2163
> Project: ZooKeeper
> Issue Type: New Feature
> Components: c client, java client, server
> Affects Versions: 3.5.0
> Reporter: Jordan Zimmerman
> Assignee: Jordan Zimmerman
> Attachments: zookeeper-2163.patch
>
>
> BACKGROUND
> ============
> A recurring problem for ZooKeeper users is garbage collection of parent
> nodes. Many recipes (e.g. locks, leaders, etc.) call for the creation of a
> parent node under which participants create sequential nodes. When the
> participant is done, it deletes its node. In practice, the ZooKeeper tree
> begins to fill up with orphaned parent nodes that are no longer needed. The
> ZooKeeper APIs don’t provide a way to clean these. Over time, ZooKeeper can
> become unstable due to the number of these nodes.
> CURRENT SOLUTIONS
> ===================
> Apache Curator has a workaround solution for this by providing the Reaper
> class which runs in the background looking for orphaned parent nodes and
> deleting them. This isn’t ideal and it would be better if ZooKeeper supported
> this directly.
> PROPOSAL
> =========
> ZOOKEEPER-723 and ZOOKEEPER-834 have been proposed to allow EPHEMERAL nodes
> to contain child nodes. This is not optimum as EPHEMERALs are tied to a
> session and the general use case of parent nodes is for PERSISTENT nodes.
> This proposal adds a new node type, CONTAINER. A CONTAINER node is the same
> as a PERSISTENT node with the additional property that when its last child is
> deleted, it is deleted (and CONTAINER nodes recursively up the tree are
> deleted if empty).
> CANONICAL USAGE
> ================
> {code}
> while ( true) { // or some reasonable limit
> try {
> zk.create(path, ...);
> break;
> } catch ( KeeperException.NoNodeException e ) {
> try {
> zk.createContainer(containerPath, ...);
> } catch ( KeeperException.NodeExistsException ignore) {
> }
> }
> }
> {code}
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