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From Ken Giusti <kgiu...@redhat.com>
Subject Re: Proposed QIP: Support for Message Grouping in the broker.
Date Wed, 20 Jul 2011 15:57:55 GMT

Alan - thanks for the input, replies inline....

-K

----- Original Message -----
> On 07/01/2011 07:04 AM, Ken Giusti wrote:
> > Folks,
> >
> > Here's a second draft of the proposal. I've tried to incorporate the
> > feedback provided during the last week. And I have tried to limit
> > the proposed feature set a bit more.
> >
> > Opinions welcome, thanks -
> >
> > -K
> >
> >
> > # Message Groups
> >
> > ## Status
> >
> > Draft
> >
> > ## Summary
> >
> > This document describes a new feature that would allow an
> > application to
> > classify a set of related messages as belonging to a group. This
> > document also
> > describes two policies that the broker could apply when delivering a
> > message
> > group to one or more consumers.
> >
> >
> > ## Problem
> >
> > It would be useful to give an application the ability to classify a
> > set of
> > messages as belonging to a single unit of work. Furthermore, if the
> > broker can
> > identify messages belonging to the same unit of work, it can enforce
> > policies
> > that control how that unit of work can be consumed.
> >
> > For example, it may be desirable to guarantee that a particular set
> > of
> > messages are consumed by the same client, even in the case where
> > there are
> > multiple clients consuming from the same queue.
> >
> > In a different scenario, it may be permissible for different clients
> > to consume
> > messages from the same group, as long as it can be guaranteed that
> > the
> > individual messages are not processed in parallel. In other words,
> > the broker
> > would ensure that messages are processed by consumers in the same
> > order in
> > which they were enqueued.
> >
> > For example, assume we have a shopping application that manages
> > items in a
> > virtual shopping cart. A user may add an item to their shopping
> > cart, then
> > change their mind and remove it. If the application sends an "add"
> > message to
> > the broker, immediately followed by a "remove" message, they will be
> > queued in
> > the proper order - "add", then "remove".
> >
> > However, if there are multiple consumers, it is possible that once a
> > consumer
> > acquires the "add" message, a different consumer may acquire the
> > "remove"
> > message. This allows both messages to be processed in parallel,
> > which could
> > result in the "remove" operation being performed before the "add"
> > operation.
> >
> >
> > ## Solution
> >
> > This QIP proposes the following:
> >
> > 1) provide the ability for a message producer to designate a set of
> > messages
> > as belonging to the same group.
> >
> > 2) allow the broker to identify messages that belong to the same
> > group.
> >
> > 3) define policies for the broker that control the delivery of
> > messages
> > belonging to the same group.
> >
> > For #1:
> >
> > The sending application would define a message header that would
> > contain the
> > message's group identifier. The group identifier stored in that
> > header field
> > would be a string value determined by the application. Messages from
> > the same
> > group would have the same group identifier value.
> 
> The broker will likely have to keep state associated with a group. How
> does the
> broker know when a group ends so it can reclaim those resources?
> 


In this case, it probably would be sufficient to maintain a counter in the group's state.
 I'm assuming that we'll need a group lookup on message arrival to find that state (or create
it on the first received message).   Increment the counter when a message in that group is
enqueued, decrement when the message is dequeued.  If decremented to zero, delete the group.
  Performance impact of such an approach is TBD, of course - probably need to pool the state
objects to limit memory trashing, etc....



> > For #2:
> >
> > The key for the header that contains the group identifier would be
> > provided to
> > the broker via configuration.
> 
> A fixed name like 'qpid.group' seems less likely to confuse. Perhaps
> have id
> default to 'qpid.group' but be cofigurable to another name in the
> (unlikely?)
> event that it is required.
> 

That's a good idea.  The current AMQP 1.0 draft defines a message property header called "group-id",
perhaps we should use "qpid.group-id" as the default instead of "qpid.group"?


> >  From the broker's perspective, the number of different group id
> >  values would be
> > unlimited. And the value of group identifiers would not be provided
> > ahead of
> > time by configuration: the broker must learn them at runtime.
> >
> > For #3:
> >
> > This QIP defines two message group policies. Additional policies may
> > be
> > defined in the future.
> >
> > Policy 1: Exclusive Consumer
> >
> > With this policy, the broker would guarantee that all messages in a
> > group
> > would be delivered to the same client.
> >
> > This policy would be configured on a per-queue basis. When the first
> > message
> > of a new message group becomes available for delivery, the broker
> > will
> > associate that group with the next available consumer. The broker
> > would then
> > guarantee that all messages from that group are delivered to that
> > consumer
> > only.
> >
> > The broker will maintain the group/client association for the
> > lifetime of the
> > client. Should the client die or cancel its subscription, any
> > unacknowledged
> > messages in the group will be assigned to a different client
> > (preserving
> > message order). Group/client associations are not maintained across
> > broker
> > restart. These associations must be replicated in a clustered
> > broker.
> >
> >
> > Policy #2: Sequenced Consumers
> >
> > With this policy, the broker would guarantee that the order in which
> > messages
> > in a group are processed by consumers is the same order in which the
> > messages
> > where enqueued. This guarantee would be upheld even if the messages
> > of the
> > group are processed by different consumers. No two messages from the
> > same
> > group would be processed in parallel by different consumers.
> >
> > Specifically, for any given group, the broker allows only the first
> > N messages
> > in the group to be available for delivery to a particular consumer.
> > The value
> > of N would be determined by the selected consumer's configured
> > prefetch
> > capacity. The broker blocks access to the remaining messages in that
> > group by
> > any other consumer. Once the selected consumer has acknowledged that
> > first set
> > of delivered messages, the broker allows the next messages in the
> > group to be
> > available for delivery. The next set of messages may be delivered to
> > a
> > different consumer.
> >
> > This policy would be configured on a per-queue basis. Configuration
> > would
> > include designating the key of the application header that specifies
> > the group
> > id.
> >
> > Note will that, with this policy, the consuming application has to:
> >
> > 1. ensure that it has completely processed the data in a received
> > message
> > before accepting that message, as described in Section 2.6.2.
> > Transfer of
> > Responsibility, of the AMQP-0.10 specification.
> >
> > 2. ensure that messages are not selectively acknowledged or released
> > - order
> > must be preserved in both cases.
> 
> What happens if the consuming application fails to respect these
> requirements?
> 

Hmmm... good question.  My initial thoughts are that this would be considered an application
error, since preserving order is important for this feature.  Perhaps the broker should respond
with an error exception?


> 
> > Note well that in the case of either of these proposed policies,
> > distinct
> > message groups would not block each other from delivery. For
> > example, assume a
> > queue contains messages from two different message groups - say
> > group "A" and
> > group "B" - and they are enqueued such that "A"'s messages are in
> > front of
> > "B". If the first message of group "A" is in the process of being
> > consumed by a
> > client, then the remaining "A" messages are blocked, but the
> > messages of the
> > "B" group are available for consumption - even though it is "behind"
> > group "A"
> > in the queue.
> >
> >
> > ## Rationale
> >
> >
> > The solution described above allows an application to designate a
> > set of
> > related messages, and provides policies for controlling the
> > consumption of the
> > message group.
> >
> >   * Goal: allow dynamic values for the group identifiers (no
> >   preconfiguration of identifiers necessary)
> >   * Goal: the number of message groups "in flight" should only be
> >   limited by the available resources (no need to configure a hard
> >   limit).
> >   * Goal: manageability: visibility into the message groups
> >   currently on a given queue
> >   * Goal: manageability: purge, move, reroute messages at the group
> >   level.
> >
> > ## Implementation Notes
> >
> >   * Queues: support configuration of the group identifier header
> >   key.
> >   * Messages: provide access to group identifier.
> >   * Queues: identify head message of next available message group.
> >   * Queues: block the trailing messages in a given message group
> >   from being consumed.
> >   * Consumers: track the message group of the currently acquired
> >   message(s).
> >   * Clustering: ensure state is replicated within the cluster as
> >   needed.
> >
> > ## Consequences
> >
> >   * __Development:__ No changes to the development process.
> >   * __Release:__ No changes to the release process.
> >   * __Documentation:__ User documentation will be needed to explain
> >   the feature, and the steps to configure and manage the new
> >   feature.
> >   * __Configuration:__ Yes: per-queue group identifier header key is
> >   configurable. Queue state with regard to in-flight message groups
> >   needs to be visible. Additional methods to purge/move/reroute a
> >   message group.
> >   * __Compatibility:__ Unlikely - new feature that would need to be
> >   enabled manually. Applications wishing to use the feature would
> >   need to implement a message grouping policy, and ensure the
> >   processing of received message data is compliant with the desired
> >   policy.
> >
> > ## References
> >
> >   * None.
> >
> > ## Contributor-in-Charge
> >
> > Kenneth Giusti,<kgiusti@redhat.com>
> >
> > ## Contributors
> >
> >   * Ted Ross,<tross@redhat.com>
> >
> > ## Version
> >
> > 0.2
> >
> >
> 
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> Project: http://qpid.apache.org
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