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One of key benefits of a message-based systems is the loose coupling between participants; the message sender and recipient make no (or few) assumptions about each other's identity. If a message recipient retrieves a message from a message channel, it generally does not know nor care which application put the message on the channel. The message is by definition self-contained and is not associated with a specific sender. This is one of the architectural strengths of message-based systems.

However, the same property can make debugging and analyzing dependencies very difficult. If we are not sure where a message goes, how can we assess the impact of a change in the message format? Likewise, if we don't know which application published a message it is difficult to correct a problem with the message.

How can we effectively analyze and debug the flow of messages in a loosely coupled system?

Therefore, attach a Message History to the message. The Message History is a list of all applications that the message passed through since its origination.

The Message History maintains a list of all components that the message passed through. Every component that processes the message (including the originator) adds one entry to the list. The Message History should be part of the message header because it contains system-specific control information. Keeping this information in the header separates it from the message body that contains application specific data.

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Related patterns: Aggregator, Control Bus, Message Router, Message Store, Process Manager, Publish-Subscribe Channel, Recipient List


Enterprise Integration Patterns Find the full description of this pattern in:
Enterprise Integration Patterns
Gregor Hohpe and Bobby Woolf
ISBN 0321200683
650 pages
Addison-Wesley
Creative Commons License Parts of this page are available under the Creative Commons Attribution license. You can reuse the pattern icon, the pattern name, the problem and solution statements (in bold), and the sketch under this license. Other portions of the text, such as text chapters or the full pattern text, are protected by copyright.

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Table of Contents
Revision History
Preface
Introduction
Solving Integration Problems using Patterns
Integration Styles
File Transfer
Shared Database
Remote Procedure Invocation
Messaging
Messaging Systems
Message Channel
Message
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
Request-Reply
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
Splitter
Aggregator
Resequencer
Composed Msg. Processor
Scatter-Gather
Routing Slip
Process Manager
Message Broker
Message Transformation
Envelope Wrapper
Content Enricher
Content Filter
Claim Check
Normalizer
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
Detour
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
Appendices
Bibliography