Doesn't that require you to set up a server for the message queue and know it's address? That sort of defeats the purpose of having a database like cassandra in which all nodes are equal and there's no single point of failure.
2013/1/31 Oleg Dulin <oleg.du...@liquidanalytics.com>: > Use a JMS message queue to send objects you want to write. Your writer > process then will listen on this queue and write to Cassandra. This ensures > that all writes happen in an orderly fashion, one batch at a time. > > I suggest ActiveMQ. It is easy to set up. This is what we use for this type > of a use case. No need to overcomplicate this with Cassandra. > > > Regards, > Oleg Dulin > Please note my new office #: 732-917-0159 > > On Jan 31, 2013, at 6:35 AM, Daniel Godás <dgo...@gmail.com> wrote: > >> Hi all, >> >> I need a locking mechanism on top of cassandra so that multiple >> clients can protect a critical section. I've seen some attempts, >> including Dominic Williams' wait chain algorithm but I think it can be >> simplified. This is the procedure I wrote to implement a simple mutex. >> Note that it hasn't been thoroughly tested and I have been using >> cassandra for a very short time so I'd appreciate any comments on >> obvious errors or things I'm doing plain wrong and will never work. >> >> The assumptions and requirements for the algorithm are the same as >> Dominic Williams' >> (http://media.fightmymonster.com/Shared/docs/Wait%20Chain%20Algorithm.pdf). >> >> We will create a column family for the locks referred to as "locks" >> throughout this procedure. The column family contains two columns; an >> identifier for the lock which will also be the column key ("id") and >> a counter ("c"). Throughout the procedure "my_lock_id" will be used as >> the lock identifier. An arbitrary time-to-live value is required by >> the algorithm. This value will be referred to as "t". Choosing an >> appropriate value for "t" will be postponed until the algorithm is >> deemed good. >> >> === begin procedure === >> >> (A) When a client needs to access the critical section the following >> steps are taken: >> >> --- begin --- >> >> 1) SELECT c FROM locks WHERE id = my_lock_id >> 2) if c = 0 try to acquire the lock (B), else don't try (C) >> >> --- end --- >> >> (B) Try to acquire the lock: >> >> --- begin --- >> >> 1) UPDATE locks USING TTL t SET c = c + 1 WHERE id = my_lock_id >> 2) SELECT c FROM locks WHERE id = my_lock_id >> 3) if c = 1 we acquired the lock (D), else we didn't (C) >> >> --- end --- >> >> (C) Wait before re-trying: >> >> --- begin --- >> >> 1) sleep for a random time higher than t and start at (A) again >> >> --- end --- >> >> (D) Execute the critical section and release the lock: >> >> --- begin --- >> >> 1) start background thread that increments c with TTL = t every t / 2 >> interval (UPDATE locks USING TTL t SET c = c + 1 WHERE id = >> my_lock_id) >> 2) execute the critical section >> 3) kill background thread >> 4) DELETE * FROM locks WHERE id = my_lock_id >> >> --- end --- >> >> === end procedure === >> >> Looking forward to read your comments, >> Dan >
