Add support for run constraints

[alexanderdean]

This is the idea that Factotum should only execute if a "run constraint" specified on the CLI resolves to true.

This is a powerful feature (taken from Snowplow's internal executor), which lets you have the same Factotum invocation in the cron on multiple boxes, but only one box will execute the job.

Example cronfile:

BOX1=box1.acme.internal
BOX2=box2.acme.internal

/opt/factotum-0.4.0/factotum first-dag.factotum --constraint "host,${BOX1}"
/opt/factotum-0.4.0/factotum secnd-dag.factotum --constraint "host,${BOX2}"

To start with, we only allow one constraint, and the only allowed constraint is host, and the implied check is ==.

[ninjabear]

Hey!

This functionality can be reproduced like this:

{
    "schema": "iglu:com.snowplowanalytics.factotum/factfile/jsonschema/1-0-0",
    "data": {
        "name": "constraint demo",
        "tasks": [
            {
                "name": "box check",
                "executor": "shell",
                "command": "if [ \"{{box_name}}\" = \"1\" ]; then exit 1; else exit 0; fi",
                "arguments": [],
                "dependsOn": [],
                "onResult": {
                    "terminateJobWithSuccess": [ 0 ],
                    "continueJob": [ 1 ]
                }
            },
            {
                "name": "execute",
                "executor": "shell",
                "command": "echo",
                "arguments": [ "beta" ],
                "dependsOn": [ "box check" ],
                "onResult": {
                    "terminateJobWithSuccess": [ 3 ],
                    "continueJob": [ 0 ]
                }
            }
        ]
    }
}

and running like this:

factotum run ./noop_constraint.factfile --env '{"box_name": "1"}'

This may not be specific enough for this use-case, but I thought I'd mention it anyway

[alexanderdean]

Hey @ninjabear - that's really neat. An emergent property of Factotum!

I think we need to keep this as a feature however, because otherwise we have to mutate all of our factfiles to add this check...