Conditional Invocation

FaaSr supports the conditional invocation of actions in the workflow. This is useful in workflows where actions may or may not execute depending on the return value (true or false) of a predecessor.

Expressing conditional invocation in the workflow

Consider the example below, where the action DownloadData always invokes TestData unconditionally (black edges denote unconditional execution). Now suppose TestData invokes a user function (in R or Python) my_test() that returns either True or False.

Then, depending on the return value of my_test(), either ComputeT is invoked (return value True, green edge), or ComputeF is invoked (return value False, red edge) - but not both.

Finally, the last action in the graph ComputeNext is invoked unconditionally. In effect, this implements the behavior of an if-then-else conditional execution in the workflow.

Constraints

There are constraints that need to be observed when expressing conditional execution:

• Cycles are not allowed (the workflow graph must still be a DAG)
• A node in a graph cannot have incident edges of different types (conditional and unconditional).

The example below shows an invalid use of conditionals where ComputeNext has incident edges of different types: