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From "Todd Lipcon (JIRA)" <j...@apache.org>
Subject [jira] Commented: (HBASE-2238) Review all transitions -- compactions, splits, region opens, log splitting -- for crash-proofyness and atomicity
Date Fri, 19 Feb 2010 06:27:28 GMT

    [ https://issues.apache.org/jira/browse/HBASE-2238?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12835607#action_12835607

Todd Lipcon commented on HBASE-2238:

I think the general pattern that all transitions need to follow is:

1) HLog that the RS intends to do some operation
2) Perform the operation in a way that is still undoable (eg create compacted HFile but don't
yet remove old ones)
3) HLog that the RS has finished the action
4) Clean up from part 2 (eg remove the pre-compaction HFiles)

We assume:
- Whenever a RS has failed, the master will open its HLog for append.
- This steals the write lease and increases the generation stamp on its last block.
- Thus the next time the RS attempts to hflush(), it will receive an IOException (I think
a LeaseExpiredException to be specific?)

Failure cases at each step:

Fail before 1) no problem, data isn't touched
Fail after 1 but before 3) the transition is an indeterminate state. When the master recovers,
it can roll back to the pre-transition state
Fail after 3) when the master recovers, it can complete the "cleanup" transition for the regionserver
(even if the regionserver got halfway through cleanup)

This pattern relies on cleanup being idempotent, and state transitions being undoable.

The above examples are for the compaction case, but I think the same general ideas apply elsewhere.

> Review all transitions -- compactions, splits, region opens, log splitting -- for crash-proofyness
and atomicity
> ----------------------------------------------------------------------------------------------------------------
>                 Key: HBASE-2238
>                 URL: https://issues.apache.org/jira/browse/HBASE-2238
>             Project: Hadoop HBase
>          Issue Type: Bug
>            Reporter: stack
> This issue is about reviewing state transitions in hbase to ensure we're sufficently
hardened against crashes.  This issue I see as an umbrella issue under which we'd look at
compactions, splits, log splits, region opens -- what else is there?  We'd look at each in
turn to see how we survive crash at any time during the transition.  For example, we think
compactions idempotent but we need to prove it so.  Splits are for sure not, not at the moment
(Witness disabled parents but daughters missing or only one of them available).
> Part of this issue would be writing tests that aim to break transitions.
> In light of above, here is recent off-list note from Todd Lipcon (and "another"):
> {code}
> I thought a bit more last night about the discussion we were having
> regarding various HBase components doing operations on the HDFS data,
> and ensuring that in various racy scenarios that we don't have two
> region servers or masters overlapping.
> I came to the conclusion that ZK data can't be used to actually have
> effective locks on HDFS directories, since we can never know that we
> still have a ZK lock when we do an operation. Thus the operations
> themselves have to be idempotent, or recoverable in the case of
> multiple nodes trying to do the same thing. Or, we have to use HDFS
> itself as a locking mechanism - this is what we discussed using write
> leases essentially as locks.
> Since I didn't really trust myself, I ran my thoughts by "Another"
> and he concurs (see
> below). Figured this is food for thought for designing HBase data
> management to be completely safe/correct.
> ...
> ---------- Forwarded message ----------
> From: Another <another@XXXXXX.com>
> Date: Wed, Feb 17, 2010 at 10:50 AM
> Subject: locks
> To: Todd Lipcon <todd@XXXXXXX.com>
> Short answer is no, you're right.
> Because HDFS and ZK are partitioned (in the sense that there's no
> communication between them) and there may be an unknown delay between
> acquiring the lock and performing the operation on HDFS you have no
> way of knowing that you still own the lock, like you say.
> If the lock cannot be revoked while you have it (no timeouts) then you
> can atomically check that you still have the lock and do the operation
> on HDFS, because checking is a no-op. Designing a system with no lock
> revocation in the face of failures is an exercise for the reader :)
> The right way is for HDFS and ZK to communicate to construct an atomic
> operation. ZK could give a token to the client which it also gives to
> HDFS, and HDFS uses that token to do admission control. There's
> probably some neat theorem about causality and the impossibility of
> doing distributed locking without a sufficiently strong atomic
> primitive here.
> Another
> {code}

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