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108 changes: 70 additions & 38 deletions modules/ROOT/pages/scatter-gather-concept.adoc
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Expand Up @@ -4,11 +4,12 @@ include::_attributes.adoc[]
endif::[]
:page-aliases: scatter-gather-xml-reference.adoc

The Scatter-Gather component is a routing event processor that processes a Mule event through different parallel processing routes that contain different event processors. Each route receives a reference to the Mule event and executes a sequence of one or more event processors. Each of these routes uses a separate thread to execute the event processors, and the resulting Mule event can be either the same Mule event without modifications or a new Mule event with its own payload, attributes, and variables. The Scatter-Gather component then combines the Mule events returned by each processing route into a new Mule event that is passed to the next event processor only after every route completes successfully.
The Scatter-Gather component (`<scatter-gather />`) is a routing event processor that processes a Mule event through different parallel processing routes that contain different event processors, which are connector operations or other Mule components. Each route receives a reference to the Mule event and executes a sequence of one or more event processors. Each of these routes uses a separate thread to execute the event processors, and the resulting Mule event can be either the same Mule event without modifications or a new Mule event with its own payload, attributes, and variables. The Scatter-Gather component then combines the Mule events returned by each processing route into a new Mule event that is passed to the next event processor only after every route completes successfully.

The Scatter-Gather component executes each route in parallel, not sequentially. Parallel execution of routes can greatly increase the efficiency of your Mule application and may provide more information than sequential processing.
The Scatter-Gather component executes each route in parallel, not sequentially. Parallel execution of routes can greatly increase the efficiency of your Mule application and can provide more information than sequential processing.

The Scatter-Gather component works with repeatable streams. It does not process nonrepeatable streams, which can be read only once before they are lost. By default, all streams are repeatable in Mule unless a component's streaming strategy is configured to be nonrepeatable.
//TODO: clarify seemingly conflicting info: "It does not process nonrepeatable streams" vs. can be "configured to be nonrepeatable"
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The Scatter-Gather component works with repeatable streams by default. It does not process nonrepeatable streams, which can be read only once before they are lost. By default, all streams are repeatable in Mule unless a component's streaming strategy is configured to be nonrepeatable.

The following diagram details the behavior of the Scatter-Gather component:

Expand All @@ -23,24 +24,24 @@ WARNING: Configure your Scatter-Gather component to have at least two routes; ot

== Variable Propagation

Every route starts with the same initial variable values. Modifications to a variable within a specific route do not affect other routes. So, if a variable is added or modified in one route, then, after aggregation, the value is defined by that route. If a variable is added or modified by more than one route, the value is added to a list of all the values defined for that variable within all the routes, for example:
Every route starts with the same initial variable values. Modifications to a variable within a specific route do not affect other routes. So, if a variable is added or modified in one route, then, after aggregation, the value is defined by that route. If a variable is added or modified by more than one route, the value is added to an array of all the values defined for that variable within all the routes, for example:

[source,xml,linenums]
----
<set-variable variableName="var1" value="var1"/>
<set-variable variableName="var2" value="var2"/>
<scatter-gather doc:name="Scatter-Gather" doc:id="abc665e0-6119-4ecb-9f8b-52dbcbb1d488" >
<route >
<set-variable variableName="var2" value="newValue"/>
<route >
<set-variable variableName="var2" value="newValue"/>
<set-variable variableName="var3" value="appleVal"/>
</route>
<route >
<set-variable variableName="var3" value="bananaVal"/>
</route>
<route >
<set-variable variableName="var3" value="otherVal"/>
</route>
<route >
<set-variable variableName="var3" value="bananaVal"/>
</route>
<route >
<set-variable variableName="var3" value="otherVal"/>
<set-variable variableName="var4" value="val4"/>
</route>
</route>
</scatter-gather>
----

Expand All @@ -50,17 +51,19 @@ After aggregation, the variables are:

== Error Handling Inside Scatter-Gather Routes

You can use a Try scope in each route of a Scatter-Gather component to handle any errors that might be generated by a route’s event processors. After every route executes, if any has failed with an error, then the Scatter-Gather component throws an error of type `MULE:COMPOSITE_ROUTING`, and event processing does not proceed past the Scatter-Gather component in the flow. Instead, the flow branches to your error-handling event processors.
You can use a Try scope in each route of a Scatter-Gather component to handle any errors generated by a route’s event processors. After every route executes, if any failed with an error, the Scatter-Gather component throws an error of type `MULE:COMPOSITE_ROUTING`, and event processing does not proceed past the Scatter-Gather component in the flow. Instead, the flow branches to your error-handling event processors.

Because the `MULE:COMPOSITE_ROUTING` error object gathers not only errors from routes that failed but also Mule events from successfully completed routes, your application can use the error-handling event processors to process Mule events from the routes that completed
Because the `MULE:COMPOSITE_ROUTING` error object gathers not only errors from routes that failed but also Mule events from successfully completed routes, your application can use the error-handling event processors to process Mule events from the routes that completed.


To illustrate how this works, consider the following two cases:

* The routes in a Scatter-Gather component each contain a Try scope. +
One of the routes generates an error that is successfully handled by that route’s Try scope through an `on-error-continue` error handler, so the route is completed successfully. The Scatter-Gather component consolidates the Mule events from all routes into a new Mule event and passes the consolidated event to the next event processor.
* The routes in a Scatter-Gather component each contain a Try scope.
+
One of the routes generates an error that is successfully handled by that route’s Try scope through an `on-error-continue` error handler, so the route completes successfully. The Scatter-Gather component consolidates the Mule events from all routes into a new Mule event and passes the consolidated event to the next event processor.

* One of the routes in a Scatter-Gather component does not contain a Try scope or contains a Try scope with an error handler that cannot handle the error type, or the error handler is an `on-error-propagate` type. +
* One of the routes in a Scatter-Gather component does not contain a Try scope or contains a Try scope with an error handler that cannot handle the error type, or the error handler is an `on-error-propagate` type:
+
An error occurs in this route, causing the route to fail, which in turn causes the Scatter-Gather component to throw a `MULE:COMPOSITE_ROUTING` error. The flow branches to your error-handling event processors, which are able to process the Mule events from the completed routes.

Example of handling these errors:
Expand All @@ -69,14 +72,14 @@ Example of handling these errors:
<flow name="errorHandler">
<scatter-gather>
<route>
<raise-error type="APP:MYERROR"/>
<raise-error type="APP:MYERROR" description="My Error Example"/>
</route>
<route>
<set-payload value="apple"/>
</route>
</scatter-gather>
<error-handler>
<on-error-continue type="MULE:COMPOSITE_ROUTING">
<on-error-continue type="MULE:COMPOSITE_ROUTING">
<!-- This will have the error thrown by the first route -->
<logger level="WARN" message="#[error.errorMessage.payload.failures['0']]"/>
<!-- This will be a null value -->
Expand All @@ -91,52 +94,80 @@ Example of handling these errors:
</flow>
----

The Logger output looks like this (edited for readability):

[source, logs]
----
WARN ...LoggerMessageProcessor:
org.mule.runtime.core.internal.message.ErrorBuilder$DeserializableErrorImplementation
{
description=My Error
detailedDescription=My Error
errorType=APP:MYERROR
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@dukesphere dukesphere Oct 21, 2022 •
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@IvanAndresFritzler will check this

The issue is actually the empty On Error Propagate data that returns after calling the Raise Error component in the component.


ERROR 2022-10-31 15:03:52,151 [[MuleRuntime].uber.05: [w-scatter-gather-example].
  w-scatter-gather-exampleFlow.CPU_LITE @194953c6] 
  [processor: w-scatter-gather-exampleFlow/processors/2/route/3/processors/0; 
   event: e7a8db20-5967-11ed-9b15-147ddaaf4f97]
  org.mule.runtime.core.internal.exception.OnErrorPropagateHandler: 
  Exception(s) were found for route(s): 

Route 3: 
********************************************************************************
Message               : My Error Example
Element               : (None)
Element DSL           : (None)
Error type            : (None)
FlowStack             : (None)

cause=org.mule.runtime.api.exception.DefaultMuleException
errorMessage=-
suppressedErrors=[]
childErrors=[]
}
WARN ...LoggerMessageProcessor: null
WARN ...LoggerMessageProcessor: null
WARN ...LoggerMessageProcessor:
org.mule.runtime.core.internal.message.DefaultMessageBuilder$MessageImplementation
{
payload=apple
mediaType=*/*
attributes=<not set>
attributesMediaType=*/*
}
----

== Handle Timeout Errors in a Scatter-Gather

If you configure a timeout for a Scatter-Gather component and a route does not complete processing before the timeout expires, the route throws a `MULE:TIMEOUT` error. This error is then handled the same way as any other error generated from a route: after each route completes (either by processing success or by throwing a `MULE:TIMEOUT` error), the successful results and errors are collected together in the Scatter-Gather component `MULE:COMPOSITE_ROUTING` error, which is then processed in your configured error handler.
If you configure a timeout for a Scatter-Gather component and a route does not complete processing before the timeout expires, the route throws a `MULE:TIMEOUT` error. This error is then handled the same way as any other error generated from a route: after each route completes (either by processing successfully or by throwing a `MULE:TIMEOUT` error), the successful results and errors are collected into the Scatter-Gather component `MULE:COMPOSITE_ROUTING` error, which is then processed in your configured error handler.

== Example Project
In Anypoint Studio, you can download and open the example project _Scatter-Gather Flow Control_ from Anypoint Exchange to learn more about how to use the Scatter-Gather component. This example shows the usage of the scatter-gather control flow to aggregate data in parallel and return the result in JSON.
In Anypoint Studio, you can download and open the example project https://www.mulesoft.com/exchange/org.mule.examples/scatter-gather-flow-control/[Scatter-Gather Flow Control^] from Anypoint Exchange to learn more about how to use the Scatter-Gather component. This example shows the use of the scatter-gather control flow to aggregate data in parallel and return the result in JSON.

The example uses prepared data as input for two resources that should be aggregated. The data represents information about two contacts and has the following structure:
The example uses prepared data as input for two resources to be aggregated. The data represents information about two contacts and has the following structure:

|===
|Resource|`firstname`|`surname`|`phone`|`email`
|contacts-1.csv
|John
|Doe
|096548763
|[email protected]
|+[email protected]+

|contacts-2.csv
|Jane
|Doe
|091558780
|[email protected]
|+[email protected]+
|===

DataWeave is used to aggregate the data. The information about the contacts is aggregated to a JSON structure that represents data from both resources.
The example uses DataWeave to aggregate the data. The contact information is aggregated to a JSON structure that represents data from both resources.

To download and open this example project while you are in Anypoint Studio, click the Exchange icon in the upper-left corner. Then, in the window that opens, log into Anypoint Exchange and search on the name of the project.
To download and open this example project through Anypoint Studio, click the Exchange icon in the upper-left corner. Then, in the window that opens, log into Anypoint Exchange and search on the name of the project.

== Scatter-Gather XML Reference

[%header%autowidth.spread,cols="a,a"]
|===
|Element |Description
| `scatter-gather` |Sends a request message to multiple targets concurrently. It collects the responses from all routes, and aggregates them into a single message.
| `scatter-gather` |Sends a request message to multiple targets concurrently. This scope collects the responses from all routes and aggregates them into a single message.
2+| *Attributes*
|`timeout` |Sets the timeout for responses from sent messages, in milliseconds. A value of 0 or lower than 0 means no timeout.
|`timeout` |Sets the timeout for responses from sent messages, in milliseconds. A value of 0 or less than 0 means no timeout.

| `maxConcurrency` |Determines the maximum number of concurrent routes to process.

| `maxConcurrency` |Determines the maximum amount of concurrent routes to process. +
By default all routes run in parallel.
* By default all routes run in parallel.

By setting this value to 1, scatter-gather processes the routes sequentially.
* By setting this value to 1, `scatter-gather` processes the routes sequentially.

| `target` | The name of the target variable.
| `target` | The name of the xref:target-variables.adoc[target variable].

| `targetValue` | Value of the data to store in the target variable. +
If not set, the default value is `#[payload]`. +
| `targetValue` | Value of the data to store in the target variable.
If not set, the default value is `#[payload]`.
This field accepts any value that a variable accepts:

* Any supported data type.
Expand All @@ -158,7 +189,8 @@ This field accepts any value that a variable accepts:

== See Also

* xref:about-components.adoc[Core Components]
* xref:about-components.adoc[]
* xref:transaction-management.adoc#tx_scopes_routers[How Transactions Affect Scopes and Routers]
* xref:error-handling.adoc[Error Handling]
* xref:try-scope-concept.adoc[Try Scope]
* xref:error-handling.adoc[]
* xref:try-scope-concept.adoc[]
* xref:about-mule-event.adoc[]