Enterprise Integration Patterns
Messaging Patterns
Channel PurgerChannel PurgerMessaging Patterns » System Management

When Bobby and I worked on the simple JMS Request/Reply Example, we ran into a simple, but interesting problem. The example consists of a Requestor which sends a message to a Replier and waits for the response. The example uses two Point-to-Point Channels, RequestQueue and ReplyQueue (see picture).

We had plastered println statements all over the code so we can see what is going on. We started the Replier first, then the Requestor. Then a very odd thing happened. The Requestor console window claimed to have gotten a response before the Replier ever acknowledged receiving a request. A delay in the console output? Lacking any great ideas, we decided to shut the Replier down and re-ran the Requestor. Odd enough, we still received a response to our request! Magic? No, just a side effect of persistent messaging. A superfluous message was present on the ReplyQueue. When we started the Requestor it placed a new message on the RequestQueue and then immediately retrieved the extraneous reply message that was sitting on the ReplyQueue. We never noticed that this was not the reply to the request the Requestor just made! Once the Replier received the request message, it placed a new reply message on the ReplyQueue so that the 'magic' repeated during the next test. It can be amazing (or amazingly frustrating) how persistent, asynchronous messaging can play tricks on you in even the most simple scenarios!

How can you keep 'left-over' messages on a channel from disturbing tests or running systems?

Use a Channel Purger to remove unwanted messages from a channel.

A simple Channel Purger simply remove all the messages from a channel. This may be sufficient for test scenarios where we want to reset the system into a consistent state. If we are debugging a production system we may need to remove individual message or a set of messages based on specific criteria, such as the message ID or the values of specific message fields.


Example: Channel Purger in Amazon SQSNEW

Many hosted messaging systems have a built-in function to purge channels. For example, with Amazon's Simple Queuing Service (SQS), you can purge a queue simply from the web user interface:

Purging a queue in Amazon SQS

Related patterns: Correlation Identifier, Invalid Message Channel, Message Channel, Message Store, Point-to-Point Channel, JMS Request/Reply Example

Table of Contents
Solving Integration Problems using Patterns
Integration Styles
File Transfer
Shared Database
Remote Procedure Invocation
Messaging Systems
Message Channel
Pipes and Filters
Message Router
Message Translator
Message Endpoint
Messaging Channels
Point-to-Point Channel
Publish-Subscribe Channel
Datatype Channel
Invalid Message Channel
Dead Letter Channel
Guaranteed Delivery
Channel Adapter
Messaging Bridge
Message Bus
Message Construction
Command Message
Document Message
Event Message
Return Address
Correlation Identifier
Message Sequence
Message Expiration
Format Indicator
Interlude: Simple Messaging
JMS Request/Reply Example
.NET Request/Reply Example
JMS Publish/Subscribe Example
Message Routing
Content-Based Router
Message Filter
Dynamic Router
Recipient List
Composed Msg. Processor
Routing Slip
Process Manager
Message Broker
Message Transformation
Envelope Wrapper
Content Enricher
Content Filter
Claim Check
Canonical Data Model
Interlude: Composed Messaging
Synchronous (Web Services)
Asynchronous (MSMQ)
Asynchronous (TIBCO)
Messaging Endpoints
Messaging Gateway
Messaging Mapper
Transactional Client
Polling Consumer
Event-Driven Consumer
Competing Consumers
Message Dispatcher
Selective Consumer
Durable Subscriber
Idempotent Receiver
Service Activator
System Management
Control Bus
Wire Tap
Message History
Message Store
Smart Proxy
Test Message
Channel Purger
Interlude: Systems Management Example
Instrumenting Loan Broker
Integration Patterns in Practice
Case Study: Bond Trading System
Concluding Remarks
Emerging Standards
Revision History

Enterprise Integration Patterns

Find the full description of this pattern in:
Enterprise Integration Patterns
Gregor Hohpe and Bobby Woolf
ISBN 0321200683
650 pages

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