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About the Mule Error

Mule execution failures result in a Mule Error. Each Mule Error has the following components:

Component Description Expression


A description regarding the problem



A type, used to characterize the problem and allow routing within an error handler



The underlying Java Throwable that resulted in the failure



An optional Mule Message regarding the problem


Child Errors

An optional collection of inner errors, used by elements like Scatter-Gather to provide aggregated route errors.


Each component in a Mule application declares the type of errors it throws so it’s easy for you to identify potential errors when designing a Mule flow.

For example, when an HTTP request fails with a 401 status code, the Mule Error looks like this:

Description: HTTP GET on resource ‘http://localhost:36682/testPath’ failed: unauthorized (401)
Cause: a ResponseValidatorTypedException instance
Error Message:  { "message" : "Could not authorize the user." }

About Error Types

In the example above, the error type is HTTP:UNAUTHORIZED, not simply UNAUTHORIZED.
Error types consist of both a namespace and an identifier, allowing you to distinguish the types according to their domain. For example, there are HTTP:NOT_FOUND and FILE:NOT_FOUND error types.
While connectors define their own namespace, core runtime errors have the implicit MULE one.
So MULE:EXPRESSION and EXPRESSION are interpreted as one.

Another important characteristic of error types is that they may have a parent type. For example, HTTP:UNAUTHORIZED has MULE:CLIENT_SECURITY as the parent, which, in turn, has MULE:SECURITY as the parent. This establishes error types as specifications of more global ones: an HTTP unauthorized error is a type of client security error, which is a type of a more broad security issue.

These hierarchies allow routing to be more general. For example, a handler for MULE:SECURITY catches HTTP unauthorized errors as well as OAuth errors.
Below you can see what the core runtime’s hierarchy looks like:

Error Hierarchy

All errors are either general or CRITICAL, the latter being so severe that they cannot be handled and will only be logged. At the top of the general hierarchy is ANY, which allows for matching all types under it. It’s important to note the UNKNOWN type, which is used when no clear reason for the failure is found. This error can only be handled through the ANY type to allow for specification of unclear errors in the future, without changing the existing behavior of the app.

When it comes to connectors, each connector defines its error type hierarchy considering the core runtime hierarchy, though CONNECTIVITY and RETRY_EXHAUSTED types are always present since they are common to all connectors.

About Custom Error Types

Custom error types can be defined when mapping errors or raising them. These errors require a specific custom namespace to distinguish them from other existing types within an application. This means that in an application using HTTP and DB, those two namespaces cannot be used. In fact, it’s best to stay away from existing connector namespaces and instead define namespaces related to the particular Mule application name or context.

For example, a customer aggregation API could use the CUSTOMER namespace for it’s custom error types and an order processing API could use the ORDER namespace.

About Error Mappings

In each operation of your flow, you can map the possible error types to a custom error type of your choosing. You can use these custom error types to differentiate exactly where an error occurred in your flow. For example, if your flow has two HTTP Request operations that reach out to different REST services, a connectivity failure on either produces the same error but by mapping each to different custom error types so that you can differentiate the error handling of each operation failure, as well as more quickly identify the source of the error in the Mule application logs.

In the example below, you can see how mappings allow granular error handling:

Example: XML Configuration for Mappings

<flow name="retrieveMatchingOrders">
  <http:request config-ref="customersConfig" path="/customer">
    <error-mapping sourceType="CONNECTIVITY" targetType="APP:CUSTOMER_API"/>
  <http:request config-ref="ordersConfig" path="/order">
    <error-mapping sourceType="CONNECTIVITY" targetType="APP:ORDER_API"/>
    <on-error-continue type="APP:CUSTOMER_API">
      <logger message="#['Could not retrieve customer data.']"/>
    <on-error-continue type="APP:ORDER_API">
      <logger message="#['Could not retrieve customer order data.']"/>