On Feb 7, 2009, at 11:02 AM, Geir Magnusson Jr. wrote:
Thanks for the info. Is there a third mode possible? Namely all or
nothing with conflict check, with the understanimg that the
conflict guarantee is only at commit, and all bets are off after
that when replicated?
That's what we currently have. It's possible to keep supporting it,
but it doesn't work with any of CouchDB's distributed features. It's
only appropriate for a single node instance, even a hot standby slave
will have inconsistent states.
-Damien
On Feb 7, 2009, at 10:47 AM, Damien Katz <[email protected]>
wrote:
I'm working on a branch that implements couchdb the security
features with replication. It not done yet, but anyone is welcome
to look at the branch in /branches/rep_security.
In this patch I am attempting to implement new transactions models.
The old transaction model has you all or nothing commits for a
group of docs, along with conflict checking. If any document was in
conflict, the transaction as a whole doesn't save.
The problems with this are:
1. Transactions don't work with replication. Replication doesn't
repeat the bulk single transaction, it just copies the documents
individually to the target replica. This means any downstream
replica can and will sees inconsistent states until replication
fully completes, not "all or nothing" states. With bidirectional
replication is even worse, as you can get edit conflicts that must
be resolved by an external process, .
2. Transactions don't work in a partitioned database without a huge
performance hit (locking + 2 phase commits).
So I propose supporting 2 different transaction models:
This first is to support "All or nothing commits", but without
guaranteed conflict checking. So you can save bunch of documents to
the database and be sure they are all safely stored, or none are
safely stored, but you can't be guarantee you don't have any
conflicts when you do.
The second is support non-acid bulk transactions, where some
document fail and some succeed. If the db crashes in the middle of
the transaction, some documents may have made it to disk
(completely intact), while others have not. The client will need to
check to be sure.
With these 2 transactions models, it's possible to deploy the same
apps on a single machine or a huge partitioned cluster. To support
the current model, it's only possible to deploy apps on a single
machine. I propose we drop the current model as bulk transactions
are not supportable in clustered or replicated set ups.
-Damien
