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Zookeeper, mail # user - Dynamic reconfiguration


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Re: Dynamic reconfiguration
Jared Cantwell 2012-07-28, 17:57
No that you would want to do this, but simply overwriting a config file would "uncommit" a configuration and make that server think the last committed configuration was whatever is in the file?

Jared

On Jul 28, 2012, at 11:33 AM, Alexander Shraer <[EMAIL PROTECTED]> wrote:

> No problem!
>
> The way it works is that before a server acks a reconfig operation it writes a special tmp file to disk (dynamicConfigFilename + ".tmp"). Servers look for this file during recovery, they don't look for the configuration in the log as for normal data, because we found it to be difficult to extract the right info from the log exactly at the stage we needed it in the recovery. When a commit message is received a server renames the tmp file to dynamicConfigFilename.
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> Recently there was a change committed by someone to start using atomic file operations for different files in ZooKeeper. At some point we'll probably change the renaming above to use these atomic operations.
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> Alex
>  
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> On Sat, Jul 28, 2012 at 10:17 AM, Jared Cantwell <[EMAIL PROTECTED]> wrote:
> Thanks Alex for the detailed explanations--  it really helps to fill in my understanding of the implementation left open by the papers/presentations I've read (without having to read the code yet :-) ).  #2 is what I was unsure of, but makes perfect sense.
>
> Obviously committing the new configuration to the internal database is a prerequisite to committing on a server, but is writing the new configuration file to disk also a prerequisite for committing the new configuration?  I'm curious about this so I can match it with my observations, since reading the configuration file is much easier than getting the database state.
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> ~Jared
>
>
> On Sat, Jul 28, 2012 at 11:02 AM, Alexander Shraer <[EMAIL PROTECTED]> wrote:
> Hi Jared,
>
> figuring out what happened and how to recover is part of the reconfiguration protocol. I don't think that this is something you as a user should do, unless I missunderstand what you're trying to do. This should be handled by ZooKeeper just like it handles other failures without admin intervention.
>
> In your scenario, D-F come up and one of them is elected leader (since you said they know about the commit), so they start running the new config normally. When A-C come up, several things may happen:
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> 1. During the preliminary FastLeaderElection, A-C will try to connect to D and E, and in fact they'll also try to connect with the new config members that they know was proposed. So most chances are that someone in the new config will send them the new config file and they'll store it and act accordingly (connect as non-voting followers in the new config). To make this happen, I changed FastLeaderElection to talk with proposed configs (if known) and to piggiback the last active config you know of on all messages.
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> 2. Its possible that somehow A-C complete FastLeaderElection without talking to D-F. But since a reconfiguration was committed, it was acked by a quorum of the old config (and a quorum of the new one). Therefore, whoever is "elected" in the old config, knows about the reconfig proposal (this is guaranteed by normal ZooKeeper leader recovery). Before doing anything else, the new leader among A-C will try to complete the reconfiguration, which involves getting enough acks from a quorum of the new config. But in your scenario the servers in the new config will not connect to it because they moved on, so the candidate-leader will just give up and go back to (1) above.
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> 3. In the remote chance that someone who heard about the reconfig commit connects to a candidate-leader who didn't hear about it, the first thing it does  is to tell that candidate-leader that its not up to date, and the leader just updates its config file, gives up on being a leader and returns to (1). This was done by changing the first message that a follower/observer sends to a leader it is connecting to, even before the synchronization starts.
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