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https://issues.apache.org/jira/browse/STORM-677?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14327661#comment-14327661
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Robert Joseph Evans commented on STORM-677:
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Periodically, every 10 seconds at least, the current compiled topology is
downloaded, if it changed, by all workers from zookeeper and the
routing/connections are updated. Connections not needed any longer are closed,
and new connections are created. If a worker receives any tuples that are not
for it, they are discarded and an error message is logged. If a connection is
to the wrong worker, in my experience, it has always been because workers were
reassigned and the remote supervisor launched new workers faster then the
current worker could update the routing. This is also why it is important that
the send method in the client not bock, because of the way locking is handled
in storm if it does block, it can block the routing/connections from being
updated and incorrect connections can persist longer than the typical 10 second
maximum.
Also secure versions of storm optionally use SASL when creating connections, so
any connection to a worker of a different topology will never be established in
the first place.
> Maximum retries strategy may cause data loss
> --------------------------------------------
>
> Key: STORM-677
> URL: https://issues.apache.org/jira/browse/STORM-677
> Project: Apache Storm
> Issue Type: Bug
> Affects Versions: 0.9.3, 0.10.0
> Reporter: Michael Noll
> Priority: Minor
> Labels: Netty
>
> h3. Background
> Storm currently supports the configuration setting
> storm.messaging.netty.max_retries. This setting is supposed to limit the
> number of reconnection attempts a Netty client will perform in case of a
> connection loss.
> Unfortunately users have run into situations where this behavior will result
> in data loss:
> {quote}
> https://github.com/apache/storm/pull/429/files#r24681006
> This could be a separate JIRA, but we ran into a situation where we hit the
> maximum number of reconnection attempts, and the exception was eaten because
> it was thrown from a background thread and it just killed the background
> thread. This code appears to do the same thing.
> {quote}
> The problem can be summarized by the following example: Once a Netty client
> hits the maximum number of connection retries, it will stop trying to
> reconnect (as intended) but will also continue to run forever without being
> able to send any messages to its designated remote targets. At this point
> data will be lost because any messages that the Netty client is supposed to
> send will be dropped (by design). And since the Netty client is still alive
> and thus considered "functional", Storm is not able to do something about
> this data loss situation.
> For a more detailed description please take a look at the discussion in
> https://github.com/apache/storm/pull/429/files#r24742354.
> h3. Possible solutions
> (Most of this section is copy-pasted from an [earlier discussion on this
> problem|https://github.com/apache/storm/pull/429/files#r24742354].)
> There are at least three approaches we may consider:
> # Let the Netty client die if max retries is reached, so that the Storm task
> has the chance to re-create a client and thus break out of the client's
> discard-messages-forever state.
> # Let the "parent" Storm task die if (one of its possibly many) Netty clients
> dies, so that by restarting the task we'll also get a new Netty client.
> # Remove the max retries semantics as well as the corresponding setting from
> Storm's configuration. Here, a Netty client will continue to reconnect to a
> remote destination forever. The possible negative impact of these reconnects
> (e.g. number of TCP connection attempts in a cluster) are kept in check by
> our exponential backoff policy for such connection retries.
> My personal opinion on these three approaches:
> - I do not like (1) because I feel it introduces potentially confusing
> semantics: We keep having a max retries setting, but it is not really a hard
> limit anymore. It rather becomes a "max retries until we recreate a Netty
> client", and would also reset any exponential backoff strategy of the
> "previous" Netty client instance (cf. StormBoundedExponentialBackoffRetry).
> If we do want such resets (but I don't think we do at this point), then a
> cleaner approach would be to implement such resetting inside the retry policy
> (again, cf. StormBoundedExponentialBackoffRetry).
> - I do not like (2) because a single "bad" Netty client would be able to take
> down a Storm task, which among other things would also impact any other,
> working Netty clients of the Storm task.
> - Option (3) seems a reasonable approach, although it breaks backwards
> compatibility with regard to Storm's configuration (because we'd now ignore
> storm.messaging.netty.max_retries).
> Here's initial feedback from other developers:
> {quote}
> https://github.com/apache/storm/pull/429/files#r24824540
> revans2: I personally prefer option 3, no maximum number of reconnection
> attempts. Having the client decide that it is done, before nimbus does feels
> like it is asking for trouble.
> {quote}
> {quote}
> https://github.com/ptgoetz
> ptgoetz: I'm in favor of option 3 as well. I'm not that concerned about
> storm.messaging.netty.max_retries being ignored. We could probably just log a
> warning that that configuration option is deprecated and will be ignored if
> the value is set.
> {quote}
> {quote}
> https://github.com/apache/storm/pull/429#issuecomment-74914806
> nathanmarz: Nimbus only knows a worker is having trouble when it stops
> sending heartbeats. If a worker gets into a bad state, the worst thing to do
> is have it continue trying to limp along in that bad state. It should instead
> suicide as quickly as possible. It seems counterintuitive, but this
> aggressive suiciding behavior actually makes things more robust as it
> prevents processes from getting into weird, potentially undefined states.
> This has been a crucial design principle in Storm from the beginning. One
> consequence of it is that any crucial system thread that receives an
> unrecoverable exception must suicide the process rather than die quietly.
> For the connection retry problem, it's a tricky situation since it may not be
> able to connect because the other worker is still getting set up. So the
> retry policy should be somehow related to the launch timeouts for worker
> processes specified in the configuration. Not being able to connect after the
> launch timeout + a certain number of attempts + a buffer period would
> certainly qualify as a weird state, so the process should suicide in that
> case. Suiciding and restarting gets the worker back to a known state.
> So in this case, I am heavily in favor of Option 2. I don't care about
> killing the other tasks in the worker because this is a rare situation. It is
> infinitely more important to get the worker back to a known, robust state
> than risk leaving it in a weird state permanently.
> {quote}
> If we decide to go with option 3, then the essence of the fix is the
> following modification of Client.java:
> {code}
> private boolean reconnectingAllowed() {
> // BEFORE:
> // return !closing && connectionAttempts.get() <=
> (maxReconnectionAttempts + 1);
> return !closing;
> }
> {code}
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