make DataTree immutable
-----------------------
Key: ZOOKEEPER-1285
URL: https://issues.apache.org/jira/browse/ZOOKEEPER-1285
Project: ZooKeeper
Issue Type: Improvement
Reporter: Thomas Koch
Assignee: Thomas Koch
Having an immutable DataTree structure in the ZooKeeper server is an ambitious
goal but is possible. Advantages would be:
- No synchronization needed when accessing the DataTree.
- The snapshotter thread gets an immutable datatree and will write a consistent
DataTree to the disk.
- No headaches whether multi transactions could lead to issues with
(de)serialization.
- Much better testability.
- No concurrency - No headaches.
- I hope for considerable speed improvements. Maybe also some memory savings,
at least from refactorings possible after this step.
- Statistical Data about the tree can be updated on every tree modification and
is always consistent. The need to save statistical data in extra nodes for the
quota feature goes away.
Possible further improvements:
Read requests actually don't need to enter the processor pipeline. Instead
each server connection could get a reference to a (zxid, tree) tuple. Updates
are delivered as (zxid, newTree, triggerWatchesCallback) to the server
connections.
The watches could be managed at each server connection instead of centrally at
the DataTree.
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