EdColeman commented on pull request #1962:
URL: https://github.com/apache/accumulo/pull/1962#issuecomment-793181218
@dlmarion - thinking about this, there is only one edge case where a
sequential, ephemeral node would ever find a node with a lower sequential
number that was created "after" the current node. The sequential numbers are
assigned based on the parent, so each child node is created with a 1-up number.
For a lowered numbered node to wink into existence it would take something
like, (and hopefully zookeeper prevents this):
```
process A creates a sequential node - this reserves the number, but the node
is not "created" yet.
process B creates a sequential node - this reserves the "next, higher"
number`, the node IS created.
time passes...
the node created by A comes into existence.
```
If zookeeper protects against this by atomically assigning a sequential
number and creating the node, then if you create a sequential node, sequential
nodes with lower numbers either exist - or if they don't they can never be
recreated using the zookeeper sequential API. Any subsequent sequential nodes
created will always have a higher sequential number, never lower.
The sequential numbers always increasing is baked into zookeeper and
fundamental to the leader election recipe. So, if you have a sequential node
and look for nodes with lower numbers, they either exist or they never will.
If you checked for existence and then added the watcher the only edge case
would be if the node was deleted between the check and the watch add - in which
case we'd probably want to remove the watcher.
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