Created TabletMetadataCache

[dlmarion]

TabletMetadataCache (TMC) caches TabletMetadata for a KeyExtent. TMC loads TabletMetadata via Ample.readTablet with all columns specified and removes TabletMetadata when it has been changed. TMC uses ZooKeeper for signaling TabletMetadata changes. TMC will reduce the load on the metadata and root table where server processes load/unload Tablets frequently for on-demand processing.

[EdColeman]

Did you happen to check how many additional watchers this will add?

[dlmarion]

Did you happen to check how many additional watchers this will add?

Did you mean the persistent recursive watch? I'm assuming that it will be one per TabletMetadataCache (where and where it's used). I have not seen anything that says a recursive watch adds a watcher to each child node. I did consider the impact of watches. I was thinking of introducing etcd for this very reason, but I didn't think that it would be well received as it would add an additional runtime dependency. IIRC etcd watches are tied to the client connection, so when the connection is closed, the watches are gone.

[EdColeman]

There are two dimension to my question about the number of watchers.

1 - there is a test with expected number of watchers and the number may need to be adjusted. (and of minor importance)
2 - if there is a watcher added for each extent, then on large systems that could be an incredible load placed on ZooKeeper that needs to be considered.

[dlmarion]

if there is a watcher added for each extent, then on large systems that could be an incredible load placed on ZooKeeper that needs to be considered.

Looking at the implementation notes, it appears that there is only one watcher created on the server-side. It says:

PathParentIterator is the "meat" of the implementation. Rather than set watchers on every ZNode implied by a recursive watcher. WatchManager passes any paths it processes through PathParentIterator which iterates up each parent znode looking for watchers.

[dlmarion]

Automated build that is failing is because it's forcing a zookeeper version of 3.5.10. I think we can remove that for 3.0

[dlmarion]

Closing in favor of #3563