Row-level db locking is an internal mechanism for database to implement ACID transactions. It's not supposed to be exposed to users and implementation specific. For example, in MySQL you can only have it with InnoDB. With MyISAM you can have table-level locking only. There is no SQL standard that I'm aware of for you to explicitly grab a lock on a row, but you can use statement like "SELECT xxx FOR UPDATE" to block other transactions. I think Nova uses this trick to allocate fixed IP addresses from the pool.
ZooKeeper is different from a database. One typical usage is to use it as a lock service only. That is the state does not live in ZooKeeper, but elsewhere. So you can grab the lock, do whatever processing and release the lock later without race condition. For instance you can spawn a VM, take snapshot, etc.. Another usage is that if the state is relatively small (e.g. within 1MB), you can put it inside on a ZooKeeper node. ZooKeeper supports atomic node update in the spirit of compare and swap. ZooKeeper also differs from a DB from a HA perspective. It is designed to be distributed and fault tolerant. With 2f+1 nodes running, you can tolerate f node failures. Databases like MySQL is designed for a single node. You can use DRBD to replicate to another node but once there is a network disconnection it's getting tricky to recover. Thanks, Yun On Tue, Nov 8, 2011 at 8:56 AM, Sandy Walsh <sandy.wa...@rackspace.com> wrote: > Hi Yun, > > Thanks again for the pointer to ZooKeeper. I think it's a very interesting > project and looks like a great way to solve the worker sync problem we face > with Orchestration. > > One thing that wasn't clear to me immediately was the benefit of using > something like ZooKeeper vs doing row-level db locking (where the row is the > transaction state)? > > Perhaps you could elaborate on that a little? > > -S > -- Mailing list: https://launchpad.net/~nova-orchestration Post to : nova-orchestration@lists.launchpad.net Unsubscribe : https://launchpad.net/~nova-orchestration More help : https://help.launchpad.net/ListHelp
