EIPinCPI - Resequencer

[bhalchandraswcg]

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This week, we'll study a variation of Aggregator Pattern known as Resequencer.

When do I use this pattern?

An aggregator is used when multiple related messages need to be combined into a single message. When these multiple messages need to be combined in a sequence a Resequencer is used. For example, order items may be split to be processed individually, however, the receiver wants them together and in sequence. In this case, a sequencer can be used to combine order items in a sequence.

Resequencer in CPI

In CPI, Aggregator component is used to implement a sequencer. For the demonstration, I'll reuse the example from Aggregator Pattern blog. For resequencing, the element 'Sequence' has been added to each message.

These are the three order items:

<OrderItem>

    <Id>1</Id>

    <OrderId>001</OrderId>

    <NextOrderItemId>2</NextOrderItemId>

    <Sequence>1</Sequence>

</OrderItem>

<OrderItem>

    <Id>2</Id>

    <OrderId>001</OrderId>

    <NextOrderItemId>3</NextOrderItemId>

    <Sequence>2</Sequence>

</OrderItem>

<OrderItem>

    <Id>3</Id>

    <OrderId>001</OrderId>

    <Sequence>3</Sequence>

</OrderItem>

Integration Flow

This integration flow exposes Reseqencer as an HTTPS service using HTTPS Adapter. Step 'Resequencer' is an Aggregator component that is responsible for sequencing the order items. The Log step uses a Groovy Script to log the output.

The Resequencer's configuration is as follows:

Tab	Parameter	Value
Correlation	Correlation Expression	/OrderItem/OrderId
Aggregation Strategy	Incoming Format	XML (Same Format)
Aggregation Strategy	Aggregation Algorithm	Combine in Sequence
Aggregation Strategy	Message Sequence Expression	/OrderItem/Sequence
Aggregation Strategy	Last Message Condition	not(boolean(/OrderItem/NextOrderItemId))
Aggregation Strategy	Completion Timeout (in min)	60
Aggregation Strategy	Data Store Name	Orders

Here, the Aggregation Algorithm 'Combine in Sequence' tells Aggregator to behave like a Resequencer. Whereas, Message Sequence Expression tells the element to be used to order the messages.

Output

Here, the message with Sequence 2 was sent first, the message with Sequence 1 was sent second, and finally, the message with Sequence 3 was sent. However, in the output, the messages are ordered as per the Sequence element.

<multimap:Messages xmlns:multimap="http://sap.com/xi/XI/SplitAndMerge">

    <multimap:Message1>

        <OrderItem>

            <Id>1</Id>

            <OrderId>001</OrderId>

            <NextOrderItemId>2</NextOrderItemId>

            <Sequence>1</Sequence>

        </OrderItem>

        <OrderItem>

            <Id>2</Id>

            <OrderId>001</OrderId>

            <NextOrderItemId>3</NextOrderItemId>

            <Sequence>2</Sequence>

        </OrderItem>

        <OrderItem>

            <Id>3</Id>

            <OrderId>001</OrderId>

            <Sequence>3</Sequence>

        </OrderItem>

    </multimap:Message1>

</multimap:Messages>

Conclusion

The Resequencer is a special form of Aggregator. It is used with multiple related messages need to be combined in a single message in sequence.

References/Further Readings

• Resequencer Pattern in Enterprise Integration Patterns


• EIPinCPI
  
  
  
  • Aggregator
  
  
  
  


• CPI Components
  
  
  
  • Aggregator
  
  
  • HTTPS Adapter
  
  
  • Groovy Script
  
  
  
  

Hope this helps,
Bala

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