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.

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.
1. [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.
2. [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:

@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);
    }
  }
}

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.

Propose rollback implementation changes to guard against concurrent jobs

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