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Software Architecture Guide

Message Filter

Explanation

Section titled “Explanation”

A component sometimes receives messages it has no interest in. The Message Filter addresses this problem. It is a small processing step placed in front of that component.

How it works

The filter reads the content of each incoming message and compares it with a set of criteria. If the message matches, the filter passes it on. If not, the filter discards it.

The filter has only one output channel. That makes it a special, very simple kind of router. A router sends messages onward to a destination. See Message Routing for the wider family. A channel is the path messages travel along. See Message Channel.

In common frameworks

  • In Apache Camel, the filter uses a predicate. A predicate is a test that answers true or false. Only messages with a true result continue. Messages with a false result are not routed further.
  • In Spring Integration, the filter decides whether to pass or drop a message. It can look at a header value or at the message content.

What happens to rejected messages

Spring Integration offers choices beyond a silent drop:

  • Send rejected messages to a dedicated discard channel.
  • Raise an error when a message is rejected.

Example

Section titled “Example”

An order system publishes every order event to one channel. A shipping component only cares about orders marked “ready to ship”. A filter in front of shipping checks each event. Ready orders pass through. All other events are discarded, or sent to a discard channel for review. The shipping component never sees the other events.

Trade-offs

Section titled “Trade-offs”

Discarding a message is a decision that is easy to forget. A message that fails the condition is gone unless the design routes it to a discard channel or raises an error. Silent dropping keeps the flow simple. A discard channel or an error makes rejections visible, at the cost of extra channels or error handling to manage.

A Message Filter passes matching messages to its single output channel and discards the rest.

Diagram source 
flowchart LR
  A[Input channel] --> F{Message Filter: criteria match?}
  F -- yes --> B[Output channel]
  F -- no --> D[Discarded]

Checks

Section titled “Checks”
  • Each filter in the design states the content criteria that decide whether a message passes.
  • The design states what happens to messages that do not match: dropped, sent to a discard channel, or raised as an error.
  • Each filter has a single output channel.
Section titled “Related”

Sources

Section titled “Sources”