[
https://issues.apache.org/jira/browse/HUDI-6596?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17750879#comment-17750879
]
Krishen Bhan commented on HUDI-6596:
------------------------------------
Thanks for the reply!
> The table lock could become a bottleneck, potentially leading to performance
> issues as other operations might be blocked too. It might be useful to
> consider how frequently you expect concurrent rollbacks to occur and whether
> this might create a performance problem.
Based on our use case at least, I would expect attempted concurrent rollbacks
to be uncommon / an edge case for now, and more rare as my organization
internally makes more fixes/changes to our orchestration process.
But you bring up a good concern that because this locks around the
scheduledRollback call , it will block other hudi writers from progressing
while it generates the rollback plan, which might be relatively time-consuming
(since in spark HUDI 0.10 can lead to launching a new spark stage) at least
compared to other places where HUDI holds the table lock. If this becomes a
concern for other HUDI users (and we want HUDI OCC locking to avoid holding
table lock while reading/creating an arbitrary/unbounded number of
instants/data files in parallel), one solution I can think of would be to
refactor HoodieTable scheduleRollback call such that we can first create the
rollback plan before acquiring the table lock (in my proposed approach), and
then later if we actually call HoodieTable scheduleRollback we just pass in
this existing rollback plan. This way the actual "work" needed to generate
rollback plan is done beforehand before locking, and we only check and update
instant file(s) while under lock (and since in my proposed approach we anyway
only schedule rollback if none has ever been scheduled before, I think that we
shouldn't have to worry about the rollback plan we created becoming
invalid/stale). Though I'm not sure how feasible this is since it since it may
affect public APIs.
> Can we ensure rollbacks are idempotent in case of repeated failures or
> retries?
Yes making sure rollbacks are idempotent ( in the sense that a pending rollback
can keep on being retried until success) is a must. Both the current/original
implementation and the proposed approach should address this. Both
approaches/implementations handle the case where rollback is pending but the
instant to rollback is gone (where we need to re-materialize the instant info
and tell the rollback execution to not actually delete instants). Also, in the
proposed approach here in step 3 we are directly using the pending rollback
instant that we observe in the timeline, if one exists. Unfortunately the logic
and my phrasing for step 3 is a bit awkward, since because the caller can pass
in a
pendingRollbackInfo
that it expects to be executed, I decided that we couldn't just ignore it, but
rather we needed to make validate that this pendingRollbackInfo is the same as
the pending rollback instant we just saw in the timeline, and abort the
rollback if not.
> Worth considering edge cases where heartbeats could become stale or be missed
> (e.g., if a job crashes without properly closing its heartbeat). Handling
> these scenarios gracefully will help ensure that rollbacks can still proceed
> when needed.
Yeah, if a failed rollback job doesn't clean up the heartbeat after failure,
any rollback attempt right after (within the interval of `[heartbeat timeout *
(allowed heartbeat misses + 1)]` I think) will fail. The alternative (that I
can think of) would be to simply allow for the chance of 2+ rollback jobs to
work on the same rollback instant. The issue though is that even if the
HoodieTable executeRollback implementation prevents the dataset from being
corrupted and will just produce a retry-able failure, I thought that it would
be noisy/tricky for a user to understand/debug. So I decided to add Step 4,
since from my perspective/understanding it was a tradeoff between "always
failing with an easy to understand retry-able exception" versus "rarely failing
with a hard to understand retry-able exception".
Though I think the chance of "stale heartbeat" can be lowered by updating the
commit code path to clean the heartbeat if an exception is raised (this isn't
done in 0.10 I think, but not sure if its still like this in 0.14). In fact my
organization internally does this with our internal forked/modified version of
clustering replacecommit (we have modified replacecommit to not have an
immutable plan, similar to commit). Though of course this isn't a guarantee see
a HUDI spark job writer might fail with a lower level runtime error.
> Propose rollback implementation changes to guard against concurrent jobs
> -------------------------------------------------------------------------
>
> Key: HUDI-6596
> URL: https://issues.apache.org/jira/browse/HUDI-6596
> Project: Apache Hudi
> Issue Type: Wish
> Reporter: Krishen Bhan
> Priority: Trivial
>
> h1. Issue
> The existing rollback API in 0.14
> [https://github.com/apache/hudi/blob/master/hudi-client/hudi-client-common/src/main/java/org/apache/hudi/client/BaseHoodieTableServiceClient.java#L877]
> executes a rollback plan, either taking in an existing rollback plan
> provided by the caller for a previous rollback or attempt, or scheduling a
> new rollback instant if none is provided. Currently it is not safe for two
> concurrent jobs to call this API (when skipLocking=False and the callers
> aren't already holding a lock), as this can lead to an issue where multiple
> rollback requested plans are created or two jobs are executing the same
> rollback instant at the same time.
> h1. Proposed change
> One way to resolve this issue is to refactor this rollback function such that
> if skipLocking=false, the following steps are followed
> # Acquire the table lock
> # Reload the active timeline
> # Look at the active timeline to see if there is a inflight rollback instant
> from a previous rollback attempt, if it exists then assign this is as the
> rollback plan to execute. Also, check if a pending rollback plan was passed
> in by caller. Then it executes the following steps depending on whether the
> caller passed a pending rollback instant plan.
> ## [a] If a pending inflight rollback plan was passed in by caller, then
> check that there is a previous attempted rollback instant on timeline (and
> that the instant times match) and continue to use this rollback plan. If that
> isn't the case, then raise a rollback exception since this means another job
> has concurrently already executed this plan. Note that in a valid HUDI
> dataset there can be at most one rollback instant for a corresponding commit
> instant, which is why if we no longer see a pending rollback in timeline in
> this phase we can safely assume that it had already been executed to
> completion.
> ## [b] If no pending inflight rollback plan was passed in by caller and no
> pending rollback instant was found in timeline earlier, then schedule a new
> rollback plan
> # Now that a rollback plan and requested rollback instant time has been
> assigned, check for an active heartbeat for the rollback instant time. If
> there is one, then abort the rollback as that means there is a concurrent job
> executing that rollback. If not, then start a heartbeat for that rollback
> instant time.
> # Release the table lock
> # Execute the rollback plan and complete the rollback instant. Regardless of
> whether this succeeds or fails with an exception, close the heartbeat. This
> increases the chance that the next job that tries to call this rollback API
> will follow through with the rollback and not abort due to an active previous
> heartbeat
>
> * These steps will only be enforced for skipLocking=false, since if
> skipLocking=true then that means the caller may already be explicitly holding
> a table lock. In this case, acquiring the lock again in step (1) will fail.
> * Acquiring a lock and reloading timeline for (1-3) will guard against data
> race conditions where another job calls this rollback API at same time and
> schedules its own rollback plan and instant. This is since if no rollback has
> been attempted before for this instant, then before step (1), there is a
> window of time where another concurrent rollback job could have scheduled a
> rollback plan, failed execution, and cleaned up heartbeat, all while the
> current rollback job is running. As a result, even if the current job was
> passed in an empty pending rollback plan, it still needs to check the active
> timeline to ensure that no new rollback pending instant has been created.
> * Using a heartbeat will signal to other callers in other jobs that there is
> another job already executing this rollback. Checking for expired heartbeat
> and (re)-starting the heartbeat has to be done under a lock, so that multiple
> jobs don't each start it at the same time and assume that they are the only
> ones that are heartbeating.
> * The table lock is no longer needed after (5), since it can now be safely
> assumed that no other job (calling this rollback API) will execute this
> rollback instant.
> One example implementation to achieve this:
>
> {code:java}
> @Deprecated
> public boolean rollback(final String commitInstantTime,
> Option<HoodiePendingRollbackInfo> pendingRollbackInfo, boolean skipLocking,
> Option<String> rollbackInstantTimeOpt) throws HoodieRollbackException {
> final Timer.Context timerContext = this.metrics.getRollbackCtx();
> final Option<HoodieInstant> commitInstantOpt;
> final HoodieTable<T, I, K, O> table;
> try {
> table = createTable(config, hadoopConf);
> } catch (Exception e) {
> throw new HoodieRollbackException("Failed to initalize table for rollback
> " + config.getBasePath() + " commits " + commitInstantTime, e);
> }
> final String rollbackInstantTime;
> final boolean deleteInstantsDuringRollback;
> final HoodieInstant instantToRollback;
> try {
> if (!skipLocking) {
> // Do step 1 and 2
> txnManager.beginTransaction();
> table.getMetaClient().reloadActiveTimeline();
> }
> final Option<HoodiePendingRollbackInfo> previousAttemptedRollback;
> if (skipLocking) {
> // If skipLocking = true, then there directly use pendingRollbackInfo
> without checking the status of this rollback instant on active timeline
> // This is since the caller is responsible for ensuring there is no
> concurrent rollback
> previousAttemptedRollback = pendingRollbackInfo;
> } else {
> // step 3
> // If skipLocking = false, we need to check the timeline for the latest
> pending rollback, in case a concurrent rollback before
> // step 1 has already executed pendingRollbackInfo
> previousAttemptedRollback =
> getPendingRollbackInfo(table.getMetaClient(), commitInstantTime, false);
> if (pendingRollbackInfo.isPresent()) {
> // step 3a If a pendingRollbackInfo was passed in, verify that it is
> the same as the pending rollback that was just observed. If not, then
> // abort the rollback
> previousAttemptedRollback.orElseThrow(
> () -> new HoodieRollbackException(
> String.format("Pending rollback instant %s no longer
> inflight", pendingRollbackInfo.get().getRollbackInstant().getTimestamp())
> )
> );
> // This will only fail if the table is in an illegal state, where
> there are 2+ rollback plans for one instant. This
> // check shouldn't be necessary, but just keeping it here for now to
> demonstrate
>
> ValidationUtils.checkArgument(previousAttemptedRollback.get().getRollbackInstant().getTimestamp().equals(
> pendingRollbackInfo.get().getRollbackInstant().getTimestamp())
> );
> }
> }
> rollbackInstantTime = previousAttemptedRollback
> .map(pendingRollback ->
> pendingRollback.getRollbackInstant().getTimestamp())
> .orElse(rollbackInstantTimeOpt.orElseGet(() ->
> HoodieActiveTimeline.createNewInstantTime()));
> commitInstantOpt =
> Option.fromJavaOptional(table.getActiveTimeline().getCommitsTimeline().getInstants()
> .filter(instant ->
> HoodieActiveTimeline.EQUALS.test(instant.getTimestamp(), commitInstantTime))
> .findFirst());
> LOG.info(String.format("Begin rollback of instant %s at instantTime %s",
> commitInstantTime, rollbackInstantTime));
> LOG.info(String.format("Scheduling Rollback at instant time : %s "
> + "(exists in active timeline: %s), with rollback plan: %s",
> rollbackInstantTime, commitInstantOpt.isPresent(),
> previousAttemptedRollback.isPresent()));
> if (previousAttemptedRollback.isPresent()) {
> if (commitInstantOpt.isPresent()) {
> instantToRollback = commitInstantOpt.get();
> deleteInstantsDuringRollback = true;
> } else {
> // A previous pending rollback plan still needs to be executed and
> completed even if the instant to rollback
> // is no longer in active timeline. This can be safely done by
> re-creating the instant to rollback and
> // configuring the rollback execution later on to not delete the
> instants during rollback.
> instantToRollback = new HoodieInstant(
> true,
> previousAttemptedRollback.get().getRollbackPlan().getInstantToRollback().getAction(),
> commitInstantTime);
> deleteInstantsDuringRollback = false;
> }
> } else {
> // Step 3b
> // A new rollback can only be scheduled if the commit to rollback is
> still in the active timeline
> if (!commitInstantOpt.isPresent()) {
> LOG.warn("Cannot find instant " + commitInstantTime + " in the
> timeline, for rollback");
> return false;
> }
> instantToRollback = commitInstantOpt.get();
> deleteInstantsDuringRollback = true;
> Option<HoodieRollbackPlan> newRollbackPlanOption =
> table.scheduleRollback(context, rollbackInstantTime,
> commitInstantOpt.get(), false, config.shouldRollbackUsingMarkers());
> newRollbackPlanOption.orElseThrow(() -> new HoodieRollbackException(
> String.format("Failed to schedule rollback of %s at instant time
> %s", commitInstantTime, rollbackInstantTime))
> );
> }
> // Step 4
> // This heartbeating logic should/will only be triggered if skipLocking =
> false. If
> // the rollback instant time has just been newly scheduled these
> heartbeat checks will still correctly
> // show the (non-existent) heartbeat as expired
> if (!skipLocking) {
> try {
> if (heartbeatClient.isHeartbeatExpired(rollbackInstantTime)) {
> heartbeatClient.stop(rollbackInstantTime);
> } else {
> throw new HoodieRollbackException(String.format("Cannot execute
> rollback instant %s due to active heartbeat", rollbackInstantTime);
> }
> heartbeatClient.start(rollbackInstantTime);
> } catch (IOException e) {
> throw new HoodieRollbackException(String.format("Could not access
> last heartbeat for %s", rollbackInstantTime);
> }
> }
> } catch (Exception e) {
> throw new HoodieRollbackException("Failed to use/create rollback plan
> for" + config.getBasePath() + " commits " + commitInstantTime, e);
> } finally {
> // Step 5
> if (!skipLocking) {
> txnManager.endTransaction();
> }
> }
> // Step 6
> try {
> HoodieRollbackMetadata rollbackMetadata = table.rollback(context,
> rollbackInstantTime, instantToRollback, deleteInstantsDuringRollback,
> skipLocking);
> if (timerContext != null) {
> long durationInMs = metrics.getDurationInMs(timerContext.stop());
> metrics.updateRollbackMetrics(durationInMs,
> rollbackMetadata.getTotalFilesDeleted());
> }
> return true;
> } catch (Exception e) {
> throw new HoodieRollbackException("Failed to execute rollback " +
> config.getBasePath() + " commits " + commitInstantTime, e);
> } finally {
> if (!skipLocking) {
> heartbeatClient.stop(rollbackInstantTime);
> }
> }
> }{code}
>
>
> h2. Why might this change be useful?
> Although these scenarios can be resolved at the application/orchestration
> level rather than HUDI, we are still working on this fix in our internal
> deployment of HUDI since we want to avoid edge cases where 2+ jobs can call
> this rollback API for the same instant at the same time.
>
--
This message was sent by Atlassian Jira
(v8.20.10#820010)
