[rabbitmq-discuss] Dropping out messages from a queue

[Alexis Richardson]

Gagan,

Hi - thank-you for sending the diagram and information which was very
useful.  There are several ways to implement the 'twitter style'
pattern that you have described.  See below for one way to do it that
illustrates the AMQP model.  It may not be the most efficient solution
in terms of performance as I shall point out.

Two quick questions:

1. Are you using fanout exchanges for this (if you want one exchange
per user, then the answer should be 'yes').

2. Where is the RabbitMQ server running?  Is it on Server 1 or 2, or
is it on a 3rd server?  It may not be that important but it would be
good to know.

OK - to your cases -

Case1: User B sending his message to UserB Exchange which will route
message to User A queue but User A is not connected to any of servers.
At this point of  all messages sent to Queue A should be dropped.

>>> One way to do this is to identify the lifecycle of Queue A with the presence of User A on servers 1 and 2.  In other words, when User A is not connected to server 1, and also not connected to server 2, then Queue A does not exist.  Note - This is not the only way to achieve what you want but it may be the least fiddly :-)

Case2: User A connected to server 1 but UserA consumer is not yet
subscribed to UserA queue. If UserB sends any message to its exchange
which is routed to Queue of UserA. At this moment of time I want all
messages to be stored in queue.

>>> One way to do this (continuing from the above) is to create Queue A immediately after UserA connects to server 1.  This is an action that server 1 can initiate on behalf of UserA.  After Queue A has been created, then it should be bound to all exchanges from which it will receive messages, such as UserB's exchange.  This will then have the stated effect you seek "If UserB sends any message to its exchange [it] is routed to Queue of UserA.".  Also, because UserA's consumer has not yet started consuming messages from Queue A, the messages will all remain stored in the queue.

>>> In addition, if instead of proceeding to Case 3, we revert to Case 1, for example User A disconnects from Server 1, then Queue A can simply be deleted (along with its contents).  (See also Case 5 below)

>>> However - while simple, the above is possibly not the most performant solution if (a) you are running RabbitMQ in clustered mode and (b) your Users generally follow a lot of other Users and (c) they connect and disconnect frequently.  This is because the effect of (c) is to frequuently update the routing table, possibly with a lot of binding/routing data (b), and then replicating it across the cluster (a).

>>> Let us know if you want more info about other ways to do this.


Case3: User A is connected to both servers and UserA consumer is
subscribed to UserA queue. If UserB sends any message to its exchange
which is routed to Queue of UserA. At this moment of time I want all
messages stored in queue to be delievered plus any other incoming
message should also be routed.

>>> In any case, to achieve this, just start consuming messages from Queue A.

>>> I assume by 'delivered', you mean 'delivered to UserA's consumer'.  Where is that running?




Case4: User A is connected to both servers and UserA consumer is
subscribed to UserA queue. Now User disconnects from Server 2. If
UserB sends any message to its exchange which is routed to Queue of
UserA. Again I want to store all messages.

>>> To achieve this, just stop consuming messages from Queue A.  They will be stored in Queue A.



Case5: If user A is disconnected from both servers. Initial state is
achieved again.

>>> When User A disconnects from Server 1, then Server 1 can tell RabbitMQ to delete Queue A (along with its contents)

Cheers,

alexis






On Fri, Jul 3, 2009 at 5:05 PM, GAGAN ARORA<gaganarora.itm at gmail.com> wrote:
> Hi Alexis
> Attaching a doc having architecture diagram and cases desired. Hope this
> would make things more clear to you.
> Thanks
> Gagan Arora
>
> On Fri, Jul 3, 2009 at 8:23 PM, Alexis Richardson
> <alexis.richardson at gmail.com> wrote:
>>
>> Gagan,
>>
>> I'm a bit confused about what you are trying to do.  I think what you
>> describe is possible but I am not sure what role is being played by
>> your two servers.  Is RabbitMQ running on both of them, one of them,
>> or neither?  You may find it easier to just use one RabbitMQ server.
>>
>> Can you send a drawing of what your desired configuration is?
>>
>> alexis
>>
>>
>>
>>
>> On Fri, Jul 3, 2009 at 3:08 PM, GAGAN ARORA<gaganarora.itm at gmail.com>
>> wrote:
>> > Hi
>> > I am developing a system in which a user will connect to two servers. On
>> > connecting to first server he will become online and will send his
>> > presence
>> > update but he will receive presence updates only once he is connected to
>> > other server as his consumer subscribes to his queue after connecting to
>> > second server.
>> > I am using one exchange and one queue per user. So if one user comes
>> > online
>> > then he will send his presence update to his exchange and users bound to
>> > that will receive updates.
>> > I need to configure a queue which would hold messages for user who has
>> > just
>> > came online until he is connected to other server. After connecting to
>> > second server all messages will be consumed. All messages sent to queue
>> > will
>> > be dropped once user goes offline.
>> > Is it possible?
>> > Thanks
>> > Gagan Arora
>> > _______________________________________________
>> > rabbitmq-discuss mailing list
>> > rabbitmq-discuss at lists.rabbitmq.com
>> > http://lists.rabbitmq.com/cgi-bin/mailman/listinfo/rabbitmq-discuss
>> >
>> >
>
>