keith-turner opened a new issue, #4183:
URL: https://github.com/apache/accumulo/issues/4183
For automatic splits the following happens.
* Tablet group watcher periodically scans the metadata table looking for
tablets to split
* When a tablet is found to split, the manager tracks in memory that the
tablet is in the process of splitting
* A Fate operation is started to split the tablet.
In #4178 the above tracking that is done in manager memory is moved to FATE.
However the FATE implementation is not complete, its only stubbed out in
#4178. To complete #4178, need the following capabilities in FATE.
* The ability to specify a key when creating a FATE operation. For the
split this case this key would be the extent. The fate operation should only
be created if the key does not exists.
* The ability to handle failures in the case where the FATE operation is
created with a key, but is not seeded.
The following is an example of one way this could work.
```java
FateStore store = ...;
OptionalLong txid = store.create("split", createKey(extent));
if(!txid.isPresent()){
// check to see if it was created but not seeded
txid = store.findByKey("split", createKey(extent));
if(txid.isPresent()){
// TODO reserve and if in the new state, then seed fate op
}
} else {
// TODO seed the fate op
}
```
Maybe the above code would be more efficient as something like
`Optional<Pair<Long, TStatus>> createOrFind(key)`.
The implementation of the create(key) function could hash the key to obtain
a 64 bit fate id. It would need to handle collisions in a similar way to a
hashmap.
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