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title: Migrating the Scatter-Gather Router
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---
---
product: Mule Runtime
version: 4.4
---

# Migrating the Scatter-Gather Router

> For the full documentation index, see: https://docs.mulesoft.com/llms.txt

The most important change in Scatter-Gather router between Mule 3 and Mule 4 is in the
`AggregationStrategy` used to consolidate the results of the different routes. Instead of providing
a Java class with the aggregation logic, you can perform the aggregation with a DataWeave
transformation.

The payload Scatter-Gather returns is a collection of messages, where each the result of each route.
Those messages are accessible with DataWeave.

## Example

This is a Mule 3 Scatter-Gather with a `custom-aggregation-strategy` element referencing the Java
class with the aggregation strategy:

<scatter-gather doc:name="Scatter-Gather">
    <custom-aggregation-strategy class="com.example.poc.CustomSGAggregationStrategy"/>
    <processor-chain>
        ...
    </processor-chain>
    <processor-chain>
        ...
    </processor-chain>
</scatter-gather>
<logger message="#\[payload\]" level="INFO" doc:name="Logger"/>

And the aggregation strategy implementation is:

package com.example.poc;

import ...

public class CustomSGAggregationStrategy implements AggregationStrategy { @Override public MuleEvent
aggregate(AggregationContext context) throws MuleException { StringBuilder responseBuilder = new
StringBuilder(); MuleEvent result = null; ArrayList < MarketRate > marketRates = new ArrayList < >
(); for (MuleEvent event: context.collectEventsWithoutExceptions()) { String response = (String)
event.getMessage().getPayload(); String\[\] spiltResponse = StringUtils.split(response, ",");
MarketRate marketRate = new MarketRate(); marketRate.setEvent(event);
marketRate.setMarketName(spiltResponse\[0\]); marketRate.setMarketRate(new
Integer(spiltResponse\[1\])); marketRates.add(marketRate); System.out.println(marketRate); }
Collections.sort(marketRates); result = DefaultMuleEvent.copy(marketRates.get(0).getEvent());
result.getMessage().setPayload(marketRates.get(0).toString()); return result; } }

For Mule 4, the Java class is removed, and the flow elements become:

<scatter-gather doc:name="Scatter-Gather">
    <route>
        ...
    </route>
    <route>
        ...
    </route>
</scatter-gather>
<ee:transform>
    <ee:message >
    	<ee:set-payload ><!\[CDATA\[%dw 2.0
output text/plain
---
(payload map ((response, index) -> (response.payload splitBy(','))\[1\]) orderBy ((rate, index) -> rate))\[0\]
\]\]\]></ee:set-payload>
    </ee:message>
</ee:transform>
<logger message="#\[payload\]" level="INFO" doc:name="Logger"/>

Notice the `ee:transform` element that replicates the logic of the `CustomSGAggregationStrategy`, in
DataWeave rather than Java.

## See Also

[About Scatter-Gather](https://docs.mulesoft.com/mule-runtime/4.4/scatter-gather-concept)

[About the Transform Component](https://docs.mulesoft.com/mule-runtime/4.4/transform-component-about)

[DataWeave Scripts](https://docs.mulesoft.com/dataweave/latest/dataweave-language-introduction)