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workflows.md

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Workflows

Serverledge accepts workflows defined by users through a subset of the JSON-based Amazon States Language, currently in use by AWS Step Functions.

Tasks

Serverledge workflows currently comprise 4 types of tasks:

  • Simple: a task that wraps a function. This is the only task that executes user-defined functions.
  • Choice: a task with N alternative branches, each associated with a condition; execution control and input data are transferred to the first branch whose condition is evaluated as true
  • FanOut: a task with N outputs that copies (or scatters) the input to all the outputs (with subsequent nodes activated in parallel); experimental
  • FanIn: a task with N inputs that waits for the termination of all the parent nodes, and then merges the results in one output. The node fails after a specified timeout.

Other special types of tasks are always present and pre-built when using the APIs:

  • Start: the task from which the workflow starts executing (not associated with any function)
  • End: the final task of the workflow (not associated with any function)
  • Success: a task that -- as soon it is activated -- terminates workflow execution reporting success
  • Fail: a task that -- as soon it is activated -- terminates workflow execution reporting a failure

Writing Functions

Simple tasks execute a regular Serverledge function. Any function previously registered in Serverledge can be associated with a Simple task. The only additional requirement is that functions used in workflows must be associated with a Signature upon creation (which is optional in general).

Signature

A signature specifies the name and the type of the inputs accepted by the function, as well as the type of the returned outputs. For instance, a Fibonacci function might have a single integer input, and produce a single integer output.

The signature can be specified when creating a function through the CLI.

Inputs can be specified using the --input (or -i for short) option, while outputs are specified using the --output (or -o for short) option.

Both inputs and outputs are specified using the syntax name:type, where type is one of the following strings: Int, Float, Text, Bool.

Example:

bin/serverledge-cli create --function inc \
     --memory 128 \
     --src examples/inc.py \
     --runtime python310 \
     --handler "inc.handler"\
     --input "n:Int" --output "m:Int"

Example

The file examples/workflow-simple.json contains an example ASL definition of a workflow. To register it:

bin/serverledge-cli create-workflow -s examples/workflow-simple.json -f myWorkflow

To execute it with input n=2:

bin/serverledge-cli invoke-workflow -f myWorkflow -p "input:2"