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From "Scott Green" <smallbad...@gmail.com>
Subject Re: HBase Design Ideas, Part II
Date Thu, 30 Nov 2006 03:24:07 GMT
Hi Mike

Are you still working on HBase? and could you please introduce the
project status to us? I have strong interesting in your idea :)

- Scott

On 5/16/06, Michael Cafarella <michael.cafarella@gmail.com> wrote:
> Hi everyone,
>
> My previous mail mentioned a bunch of design ideas that were mainly
> lifted from Jeff Dean's BigTable talk.  BigTable seems like a useful
> way to do large-scale row storage, and their decisions largely seem
> like the right ones.
>
> However, BigTable still leaves some things on the table.  Items to
> improve include a query language and multi-row locking, among
> other things.
>
> Dean said explicitly in his talk that they wanted to avoid multirow
> locking because it's complicated, error-prone, and maybe not necessary.
> He's right on at least the first two, and maybe the third.
>
> Multiple row locks are useful when you're making a change to
> several rows that should be atomic; you want all the changes
> or none of the changes.  It's also used in traditional databases
> if you want to perform an expensive read operation (like a
> multiway join) and you want to make sure the results don't
> get modified while you're reading.
>
> Distributed lock acquisition is very hard to do.  It's bug-prone
> and often has very weird performance ramifications.  It's
> difficult to get working, difficult to tune, difficult to everything.
>
> Here are a few ideas on what to do:
> 1)  Suck it up and have the client acquire locks on multiple
> HRegionServers simultaneously.  All clients would have to
> agree to acquire locks according to some global ordering to
> avoid deadlock.  HRegions would not be allowed to migrate
> to a new server if locked.
>
> If this is a rare circumstance, a better approach would be
> to have a dedicated "lock acquirer" through which clients
> make requests.  It doesn't help the theoretical problem here,
> but it would make debugging an awful lot easier.
>
> 2)  In the case of long-lasting read operations, we can
> use versioning to guarantee consistency.  If each row is
> annotated with an edit timestamp, and we know that there
> is sufficient version history available, the long-lasting job
> can run over a specific version only.
>
> Edits can continue to be made to the database while the
> read-only job is ongoing.  The operation is performed over
> the database as of the time the task was submitted.
>
> 3) In the case of multiple row updates, we may be able to
> use different edit semantics to avoid locking.  For example,
> consider that we want to add a single column/value pair to
> multiple rows.  We want this to happen atomically, so that
> both rows get the value or neither of them do so.
>
> If it's just an add, then we don't need to lock the rows at
> all; the add will always succeed, even if other writes
> intervene. Traditionally there's been no difference between
> among data "updates", so they all require locking.  If we
> can get a client to adjust the update semantics slightly,
> then the locking can be much more relaxed.
>
> I'd say that "add" or "append" semantics are likely to be
> at least as common as "edit" semantics.
>
> Can you think of the family of edit semantics you'd like
> to see offered here?
>
> Also, how useful do you think a general-purpose query language
> would be for HBase?  It would be fairly straightforward to implement,
> for example, a poor man's version of SQL that has different locking
> and update behavior (and which chucks out the more exotic elements).
> This might be compiled into a piece of code that is executed
> immediately, or it might be transformed into a long-lasting mapreduce
> job.
>
> I have a few ideas for such a language, but I'm worried it's getting
> a little far afield from what we're interested in for Hadoop.
>
> --Mike
>
>

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