The problem with using Cassandra for locking is that if you ever have read consistency issues (which you will unless you use ALL) you will have inconsistent values.
In general I would avoid doing a read-before-write with Cassandra. I would come up with another way to update my data. Regards, Oleg Dulin Please note my new office #: 732-917-0159 On Jan 31, 2013, at 9:19 AM, Daniel Godás <dgo...@gmail.com> wrote: > Ok, I've done some reading. If I understood it correctly the idea > would be to send messages to the queue that contain a transaction i.e. > a list of CQL commands to be run atomically. When one of the consumers > gets the message it can run the transaction atomically before allowing > another consumer to get the next message. If this is correct then in > order to handle cases in which I need to interleave code with the CQL > statements e.g. to check retrieved values, I need to implement a > protocol that uses the message queue as a locking mechanism. How is > this better than using cassandra for locking? (using the algorithm I > proposed or another one). > > 2013/1/31 Oleg Dulin <oleg.du...@liquidanalytics.com>: >> This may help: >> >> http://activemq.apache.org/how-do-distributed-queues-work.html >> >> http://activemq.apache.org/topologies.html >> >> http://activemq.apache.org/how-do-i-embed-a-broker-inside-a-connection.html >> >> Although I would use ActiveMQ spring configuration, not write code. But the >> point is -- you can have multiple processes participating in an ActiveMQ >> federation; you can configure AMQ's fault tolerance profiles to your liking >> without having to set up a yet another server with a single point of failure. >> >> You have a single distributed queue. Each process has a writer consumer on >> that queue. AMQ knows to load balance, only one consumer at a time gets to >> write. Instead of writing to cassandra, you send your data item to the >> queue. The next available consumer gets the message and writes it -- all in >> the order of messages on the queue, and only one consumer writer at a time. >> >> Regards, >> Oleg Dulin >> Please note my new office #: 732-917-0159 >> >> On Jan 31, 2013, at 8:11 AM, Daniel Godás <dgo...@gmail.com> wrote: >> >>> Sounds good, I'll try it out. Thanks for the help. >>> >>> 2013/1/31 Oleg Dulin <oleg.du...@liquidanalytics.com>: >>>> Use embedded amq brokers , no need set up any servers . It literally is >>>> one line of code to turn it on, and 5 lines of code to implement what you >>>> want. >>>> >>>> We have a cluster of servers writing to Cassandra this way and we are not >>>> using any j2ee containers. >>>> >>>> On Thursday, January 31, 2013, Daniel Godás wrote: >>>> >>>>> 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 <javascript:;>>: >>>>>> 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<javascript:;>> >>>>> 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 >>>>>> >>>>> >>>> >>>> >>>> -- >>>> Sent from Gmail Mobile >>
