Pattern: Scatter-Gather

How do you maintain the overall message flow when a message needs to be sent to multiple recipients, each of which may send a reply?

Use a Scatter-Gather that broadcasts a message to multiple recipients and re-aggregates the

responses back into a single message.

The Scatter-Gather routes a request message to the a number of recipients. It then uses an Aggregator to collect the responses and distill them into a single response message.

There are two variants of the Scatter-Gather that use different mechanisms to send the request messages to the intended recipients:

• Distribution via a Recipient List allows the Scatter-Gather to control the list of recipients but requires the Scatter-Gather to be aware of each recipient’s message channel.
• Auction-style Scatter-Gathers use a Publish-Subscribe Channel to broadcast the request to any interested participant. This option allows the Scatter-Gather to use a single channel but at the same time relinquishes control.

Example: Combining Patterns

We can now use the Scatter-Gather to implement the widget and gadget order processing example. We can combine the Scatter-Gather with the Composed Message Processor to process each incoming

order, sequence it into individual items, then pass each item up for a bid, then aggregate the bids

for each item into a combined bid response, and lastly aggregate all bid responses into a complete

quote. This is a very real example how multiple integration patterns can be combined into a

complete solution. The composition of individual patterns into larger patterns allows us to

discuss the solution at a higher level of abstraction. It also allows us to modify details of the

implementation without affecting other components.