Introduction to Mule 4: Error Handlers

Execution failures are represented with Mule errors that have the following components:

For example, when an HTTP request fails with a 401 status code, a Mule error provides the following information:

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, HTTP:NOT_FOUND and FILE:NOT_FOUND). While connectors define their own namespace, core runtime errors have an implicit one: MULE:EXPRESSION and EXPRESSION are interpreted as one.

Another important characteristic of error types is that they might 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 kind of client security error, which is a type of a more broad security issue.

These hierarchies mean routing can be more general, since, for example, a handler for MULE:SECURITY will catch HTTP unauthorized errors as well as OAuth errors. Below you can see what the core runtime hierarchy looks like:

All errors are either general or CRITICAL, the latter being so severe that they cannot be handled. At the top of the general hierarchy is ANY, which allows 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 specifying the unclear errors in the future, without changing the existing app’s behavior.

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

Mule 4 has redesigned error handling by introducing the error-handler component, which can contain any number of internal handlers and can route an error to the first one matching it. Such handlers are on-error-continue and on-error-propagate, which both support matching through an error type (or group of error types) or through an expression (for advanced use cases). These are quite similar to the Mule 3 choice (choice-exception-strategy), catch (catch-exception-strategy), and rollback (rollback-exception-strategy) exceptions strategies However, they are much simpler and more consistent.

If an error is raised in Mule 4, an error handler will be executed and the error will be routed to the first matching handler. At this point, the error is available for inspection, so the handlers can execute and act accordingly, relative to the component where they are used (a Flow or Try scope):

Consider the following application where an HTTP listener triggers a Flow reference to another flow that performs an HTTP request. If everything goes right when a message is received (1 below), the reference is triggered (2), and the request performed (3), which results in a successful response (4).

If the HTTP request fails with a not found error (see 3 above) because of the error handler setup of inner-flow, then the error will be propagated (4), and the flow reference will fail (2). However, because primary-flow is handling that with an on-error-continue, this will execute (5), and a successful response will be returned (6).

If the request fails with an unauthorized error instead (3), then inner-flow will handle it with an on-error-continue by retrieving static content from a file (4). Then the Flow reference will be successful as well (2), and a successful response will be returned (5).

But what if another error occurred in the HTTP request? Although there are only handlers for NOT_FOUND and UNAUTHORIZED errors in the flow, errors are still propagated by default. This means that if no handler matches the error that is raised, then it will be re-thrown. For example, if the request fails with a method not allowed error (3), then it will be propagated, causing the flow reference to fail (2), and that propagation will result in a failure response (4).

The scenario above can be avoided by making the last handler match ANY, instead of just HTTP:UNAUTHORIZED. Notice how, below, all the possible errors of an HTTP request are suggested:

You can also match errors using an expression. For example, since the Mule Error is available during error handling, we can use it to match all errors with the HTTP namespace:

For the most part, Mule 3 only allows error handling at the flow level, forcing you to extract logic to a flow in order to address errors. In Mule 4, we’ve introduced a Try scope that you can use within a flow to do error handling of just inner components. The scope also supports transactions, which replaces the old Transactional scope.

In the previous example, On Error Propagate component is configured to propagate database connection errors, causing the try to fail and the flow’s error handler to execute. On Error Continue component is configured to handle any other error. The Try scope’s execution is successful for that case, which means that the flow’s execution continues with the next processor, an HTTP Request operation.

Mule 4 now also allows for mapping default errors to custom ones. The Try scope is useful, but if you have several equal components and want to distinguish the errors of each one, using a Try on them can clutter your app. Instead, you can add error mappings to each component, meaning that all or certain kind of errors streaming from the component will be mapped to another error of your choosing. If, for example, you are aggregating results from 2 APIs using an HTTP request component for each, you might want to distinguish between the errors of API 1 and API 2, since by default, their errors will be the same.

By mapping errors from the first request to a custom API_1 error and errors in the second request to API_2, you can route those errors to different handlers. The next example maps HTTP:INTERNAL_SERVER_ERROR so that different handling policies can be applied if the APIs go down (propagating the error in the first API and handling it in the second API).