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From Oleg Dulin <oleg.du...@liquidanalytics.com>
Subject Re: Implementing a locking mechanism - RFC
Date Thu, 31 Jan 2013 12:46:50 GMT
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.dulin@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 <dgodas@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
> >
>


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