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WPI's Exception Handling Patterns and CWL

Broadly speaking, the CWL standards have little support for exception handling.

Back to the list of WPI pattern categories

WPI Exception Types

"Work Item Failure"

CommandLineTools can define what exit codes represent successful execution, or temporary or permanenet task execution failures. While there isn't a specific CWL construct to verify the correct execution of a previous step, one can evaluate any outputs (or even the workflow level inputs provided by the user) and halt execution if they do not satisfy some criteria.

Starting with CWL v1.2 there can be alternative and/or supplementary execution paths to cope with a step that ran without error but did not produce some desired output or meet a particular threshold. However, as of CWL v1.2, there is no ability in CWL to loop over a remediation step until a particular type of result is obtained.

"Deadline Expiry"

CWL has no concept of a task deadline.

Starting with CWL v1.1 it is possible to annotate CWL CommandLineTools with a ToolTimeLimit that specified the maximum computation time that may pass once execution of a CommandLineTool has begun.. However the result of missing the time limit is a permanent failure and CWL does have any automatic recovery mechanism for permanent failures.

"Resource Unavailability"

While the lack of needed resources can happen to a CWL workflow execution, there is no CWL construct to automatically scale or otherwise attempt to recover from such a situation. The providers of CWL compatible workflow execution are allowed to implement their own solutions, otherwise manual recovery is required.

"External Trigger"

There is no concept of triggers in CWL.

"Constraint Violation"

Beyond validation of the workflow description and the type compatibility between processes, the only mechanism to check for constraint violation is pickValue. pickValue appeared in version 1.2 of the CWL standards to primarily cope with the results of conditional execution using the when modifier on Workflow steps.

"Exception Handling at Work Item Level" patterns

There are no CWL constructs corresponding to any of the WPI exception handling at the work item level patterns. However, many of these techniques are used automatically by many CWL executors.