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From John Laban <j...@pagerduty.com>
Subject best practices for simulating transactions in Cassandra
Date Mon, 05 Dec 2011 19:40:27 GMT
Hello,

I'm building a system using Cassandra as a datastore and I have a few
places where I am need of transactions.

I'm using ZooKeeper to provide locking when I'm in need of some concurrency
control or isolation, so that solves that half of the puzzle.

What I need now is to sometimes be able to get atomicity across multiple
writes by simulating the "begin/rollback/commit" abilities of a relational
DB.  In other words, there are places where I need to perform multiple
updates/inserts, and if I fail partway through, I would ideally be able to
rollback the partially-applied updates.

Now, I *know* this isn't possible with Cassandra.  What I'm looking for are
all the best practices, or at least tips and tricks, so that I can get
around this limitation in Cassandra and still maintain a consistent
datastore.  (I am using quorum reads/writes so that eventual consistency
doesn't kick my ass here as well.)

Below are some ideas I've been able to dig up.  Please let me know if any
of them don't make sense, or if there are better approaches:


1) Updates to a row in a column family are atomic.  So try to model your
data so that you would only ever need to update a single row in a single CF
at once.  Essentially, you model your data around transactions.  This is
tricky but can certainly be done in some situations.

2) If you are only dealing with multiple row *inserts* (and not updates),
have one of the rows act as a 'commit' by essentially validating the
presence of the other rows.  For example, say you were performing an
operation where you wanted to create an Account row and 5 User rows all at
once (this is an unlikely example, but bear with me).  You could insert 5
rows into the Users CF, and then the 1 row into the Accounts CF, which acts
as the commit.  If something went wrong before the Account could be
created, any Users that had been created so far would be orphaned and
unusable, as your business logic can ensure that they can't exist without
an Account.  You could also have an offline cleanup process that swept away
orphans.

3) Try to model your updates as idempotent column inserts instead.  How do
you model updates as inserts?  Instead of munging the value directly, you
could insert a column containing the operation you want to perform (like
"+5").  It would work kind of like the Consistent Vote Counting
implementation: ( https://gist.github.com/416666 ).  How do you make the
inserts idempotent?  Make sure the column names correspond to a request ID
or some other identifier that would be identical across re-drives of a
given (perhaps originally failed) request.  This could leave your datastore
in a temporarily inconsistent state, but would eventually become consistent
after a successful re-drive of the original request.

4) You could take an approach like Dominic Williams proposed with Cages:
http://ria101.wordpress.com/2010/05/12/locking-and-transactions-over-cassandra-using-cages/
  The gist is that you snapshot all the original values that you're
about
to munge somewhere else (in his case, ZooKeeper), make your updates, and
then delete the snapshot (and that delete needs to be atomic).  If the
snapshot data was never deleted, then subsequent accessors (even readers)
of the data rows need to do the rollback of the previous transaction
themselves before they can read/write this data.  They do the rollback by
just overwriting the current values with what is in the snapshot.  It
offloads the work of the rollback to the next worker that accesses the
data.  This approach probably needs an generic/high-level programming layer
to handle all of the details and complexity, and it doesn't seem like it
was ever added to Cages.


Are there other approaches or best practices that I missed?  I would be
very interested in hearing any opinions from those who have tackled these
problems before.

Thanks!
John

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