Type: New Feature
Affects Version/s: None
Fix Version/s: None
Tl;dr My team at Indeed has developed ZooKeeper functionality to handle stateful retrying of connectionloss for write operations, and we wanted to reach out to discuss if this is something the Curator team may be interested in incorporating.
We initially reached out to the Zookeeper team (https://issues.apache.org/jira/browse/ZOOKEEPER-3927) but were redirected to Curator as the better place to contribute them. The changes could be relatively easily added as additional parameters and/or extensions of the existing retry behavior in Curator's write operations.
Hi Curator Devs,
My team uses zookeeper extensively as part of a distributed key-value store we've built at Indeed (think HBase replacement). Due to our deployment setup co-locating our database daemons with our large hadoop cluster, and the network-intensive nature of a lot of our compute jobs, we were experiencing a large amount of transient ConnectionLoss issues. This was especially problematic on important write operations, such as the creation deletion of distributed locks/leases or updating distributed state in the cluster.
We saw that some existing zookeeper client wrappers handled retrying in the presence of ConnectionLoss, but all of the ones we looked at (including Curator) didn't allow for retrying writes wiith all of the proper state. Consider the case of retrying a create. If the initial create had succeeded on the server, but the client got connectionloss, the client would get a NodeExists exception on the retried request, even though the znode was created. This resulted in many issues. For the distributed lock/lease example, to other nodes, it looked like the calling node had been successful acquiring the "lock", and to the calling node, it appeared that it was not able to acquire the "lock", which results in a deadlock.
Curator has parameters that can modify the behavior upon retry, but those were not sufficient. For example, create() has orSetData(), and delete() has guaranteed().
To solve this, we implemented a set of "connection-loss tolerant primitives" for the main types of write operations. They handle a connection loss by retrying the operation in a loop, but upon error cases in the retry, inspect the current state to see if it matches the case where a previous round that got connectionloss actually succeeded.
- createRetriable(String path, byte data)
- setDataRetriable(String path, byte newData, int currentVersion)
- deleteRetriable(String path, int currentVersion)
- compareAndDeleteRetriable(String path, byte currentData, int currentVersion)
For example, in createRetriable, it will retry the create again on connection loss. If the retried call gets a NodeExists exception, it will check to see if (getData(path) == data and dataVersion == 0). If it does, it assumes the first create succeeded and returns success, otherwise it propagates the NodeExists exception.
These primitives have allowed us to program our ZooKeeper layer as if ConnectionLoss isn't a transient state we have to worry about, since they have essentially the same guarantees as the non-retriable functions in the zookeeper api do (with a slight difference in semantics).
Because these behaviors could be relatively easily added to Curator as additional parameters to the existing mechanisms, and (to my knowledge) aren't implemented anywhere else, we think it could be a useful contribution to the Curator project. If this isn't something that Curator is interested in incorporating, Indeed may also consider open sourcing it as a standalone library.