Adding Functionality to Apache NiFi
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Exceptions within the Processor

During the execution of the onTrigger method of a Processor, many things can potentially go awry. Common failure conditions include:

  • Incoming data is not in the expected format.

  • Network connections to external services fail.

  • Reading or writing data to a disk fails.

  • There is a bug in the Processor or a dependent library.

Any of these conditions can result in an Exception being thrown from the Processor. From the framework perspective, there are two types of Exceptions that can escape a Processor: ProcessException and all others.

If a ProcessException is thrown from the Processor, the framework will assume that this is a failure that is a known outcome. Moreover, it is a condition where attempting to process the data again later may be successful. As a result, the framework will roll back the session that was being processed and penalize the FlowFiles that were being processed.

If any other Exception escapes the Processor, though, the framework will assume that it is a failure that was not taken into account by the developer. In this case, the framework will also roll back the session and penalize the FlowFiles. However, in this case, we can get into some very problematic cases. For example, the Processor may be in a bad state and may continually run, depleting system resources, without providing any useful work. This is fairly common, for instance, when a NullPointerException is thrown continually. In order to avoid this case, if an Exception other than ProcessException is able to escape the Processor's onTrigger method, the framework will also "Administratively Yield" the Processor. This means that the Processor will not be triggered to run again for some amount of time. The amount of time is configured in the file but is 10 seconds by default.