This repo shows a simple example of how you can dynamically construct a matrix strategy with GitHub Actions.

The example uses workflow_dispatch to allow the workflow to be executed manually with some inputs, just to easily demonstrate the mechanism:

When the default inputs are selected, the workflow will dynamically construct a strategy matrix in JSON during the execution of the First Job. The JSON will be taken as an output from the first job and used as the value for the strategy: key of the Second Job. It uses fromJSON to take the JSON string and parse it into something GitHub Actions can understand.

The result is the following being dynamically constructed:

strategy:
  matrix:
    stage: ["dev", "tst"]
  fail-fast: true
  max-parallel: 1

This results in an ordered execution of the required stages, due to the max-parallel being defined as 1.

Setting fail-fast to true (default) causes GitHib Actions to cancel all matrix in-progress or queued jobs. Which would mean, in this example, that if dev fails then tst will not run.

If Run in order is unchecked then the Second Job is not constrained to max-parallel: 1 and normal parallel job execution takes place:

This also allows for the stage values to be dynamically constructed. So, for example, we could uncheck Run Dev:

would construct the matrix as follows:

strategy:
  matrix:
    stage: ["tst"]
  fail-fast: false
  max-parallel: 2

Obviously meaning that only the tst environment will be processed in this workflow execution.

This is only a simple example but you can imagine you could construct quite a complex matrix strategy if so desired.