Guaranteed message delivery until session timeout?
Hello, I am wondering what guarantees (if any) zookeeper provides for reliable messaging for operation return codes up to a session timeout. Basically, I would like to know whether a zookeeper client can rely on observing the return code for a successful operation which creates an ephemeral (or ephemeral sequential) znode -or- have a guarantee that its session was timed out and the ephemeral znode destroyed. That is, does zookeeper provide guaranteed delivery of the operation return code unless the session is invalidated by a timeout? Thanks, Bryan
Re: Guaranteed message delivery until session timeout?
Which API are you talking about? C? I think that the difference between connection loss and session expiration might mess you up slightly in your disjunction here. On Wed, Jun 30, 2010 at 7:45 AM, Bryan Thompson br...@systap.com wrote: Hello, I am wondering what guarantees (if any) zookeeper provides for reliable messaging for operation return codes up to a session timeout. Basically, I would like to know whether a zookeeper client can rely on observing the return code for a successful operation which creates an ephemeral (or ephemeral sequential) znode -or- have a guarantee that its session was timed out and the ephemeral znode destroyed. That is, does zookeeper provide guaranteed delivery of the operation return code unless the session is invalidated by a timeout? Thanks, Bryan
Re: Guaranteed message delivery until session timeout?
Isn't this the same question that you sent this morning? On Wed, Jun 30, 2010 at 3:36 PM, Bryan Thompson br...@systap.com wrote: Hello, I am wondering what guarantees (if any) zookeeper provides for reliable messaging for operation return codes up to a session timeout. Basically, I would like to know whether a zookeeper client can rely on observing the return code for a successful operation which creates an ephemeral (or ephemeral sequential) znode -or- have a guarantee that its session was timed out and the ephemeral znode destroyed. That is, does zookeeper provide guaranteed delivery of the operation return code unless the session is invalidated by a timeout? Thanks, Bryan
Re: Guaranteed message delivery until session timeout?
On 06/30/2010 09:37 AM, Ted Dunning wrote: Which API are you talking about? C? I think that the difference between connection loss and session expiration might mess you up slightly in your disjunction here. On Wed, Jun 30, 2010 at 7:45 AM, Bryan Thompsonbr...@systap.com wrote: I am wondering what guarantees (if any) zookeeper provides for reliable messaging for operation return codes up to a session timeout. Basically, I in particular see timeliness http://hadoop.apache.org/zookeeper/docs/current/zookeeperProgrammers.html#ch_zkGuarantees would like to know whether a zookeeper client can rely on observing the return code for a successful operation which creates an ephemeral (or ephemeral sequential) znode -or- have a guarantee that its session was timed out and the ephemeral znode destroyed. That is, does zookeeper provide Any ephemeral node(s) associated with a session will be deleted when the session is invalidated (session expiration or client close request). Patrick
Re: Guaranteed message delivery until session timeout?
Also this: Once an update has been applied, it will persist from that time forward until a client overwrites the update. This guarantee has two corollaries: If a client gets a successful return code, the update will have been applied. On some failures (communication errors, timeouts, etc) the client will not know if the update has applied or not. We take steps to minimize the failures, but the only guarantee is only present with successful return codes. (This is called the *monotonicity condition* in Paxos.) Any updates that are seen by the client, through a read request or successful update, will never be rolled back when recovering from server failures. I think that the clear implications here are: a) if you get a successful return code and no session expiration, your ephemeral file is there b) if the ephemeral files is created, you might not get the successful return code (due to connection loss), but the ephemeral file might continue to exist (because connection loss != session loss) c) if you get a failure return code, your ephemeral file was not created On Wed, Jun 30, 2010 at 4:33 PM, Patrick Hunt ph...@apache.org wrote: in particular see timeliness http://hadoop.apache.org/zookeeper/docs/current/zookeeperProgrammers.html#ch_zkGuarantees
Re: Guaranteed message delivery until session timeout?
Yes. That is true. In particular, your link to a server (or the server itself) can fail causing your client to switch to a different ZK server and retry there. This can and often does happen without you knowing. On Wed, Jun 30, 2010 at 4:48 PM, Bryan Thompson br...@systap.com wrote: With regard to timeliness:The clients view of the system is guaranteed to be up-to-date within a certain time bound. (On the order of tens of seconds.) Either system changes will be seen by a client within this bound, or the client will detect a service outage. This seems to imply that there are retries for transient communication failures. Is that true?
Re: Guaranteed message delivery until session timeout?
I think that you are correct, but a real ZK person should answer this. On Wed, Jun 30, 2010 at 4:48 PM, Bryan Thompson br...@systap.com wrote: For example, if a client registers a watch, and a state change which would trigger that watch occurs _after_ the client has successfuly registered the watch with the zookeeper quorum, is it possible that the client would not observe the watch trigger due to communication failure, etc., even while the clients session remains valid? It sounds like the answer is no per the timeliness guarantee. Is that correct?
