RFC: across step

029-across-step/proposal.md

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+# `across` step
+
+This proposal introduces a mechanism by which a build plan can be executed
+multiple times with different `((vars))`, in service of build matxies, pipeline
+matrixes, and - building on [`((var))` sources][var-sources-rfc] - dynamic
+variations of both.
+
+
+## Motivation
+
+* Support dynamic multi-branch workflows:
+  [concourse/concourse#1172][multi-branch-issue]
+* Support build matrixes and pipeline matrixes.
+* Replace a common, and very flawed, usage pattern of `version: every`, which
+  has proven to be very complicated to support.
+
+
+## Proposal
+
+The `across` step modifier is given a list containing vars and associated
+values to set when executing the step. The step will be executed across all
+combinations of var values.
+
+### Static var values
+
+The set of values can be static, as below:
+
+```yaml
+task: unit
+vars: {go_version: ((.:go_version))}
+across:
+- var: go_version
+  values: [1.12, 1.13]
+```
+
+This will run the `unit` task twice: once with `go_version` set to `1.12`, and
+again with `1.13`.
+
+### Dynamic var values from a var source
+
+Rather than static values, var values may be pulled from a var source
+dynamically:
+
+```yaml
+var_sources:
+- name: booklit-prs
+  type: github-prs
+  config:
+    repository: vito/booklit
+    access_token: # ...
+
+plan:
+- set_pipeline: pr
+  instance_vars: {pr_number: ((.:pr.number))}
+  across:
+  - var: pr
+    source: booklit-prs
+```
+
+The above example will run the `set_pipeline` step across the set of all GitHub
+PRs, returned through a `list` operation on the `booklit-prs` var source.
+
+### Running across a matrix of var values
+
+Multiple vars may be listed to form a matrix:
+
+```yaml
+set_pipeline: pr-go
+instance_vars:
+  pr_number: ((.:pr.number))
+  go_version: ((.:go_version))
+across:
+- var: pr
+  source: booklit-prs
+- var: go_version
+  values: [1.12, 1.13]
+```
+
+This will run 2 * (# of PRs) `set_pipeline` steps, setting two pipelines per
+PR: one for Go 1.12, and one for Go 1.13.
+
+### Controlling parallelism with `max_in_flight`
+
+By default, the steps are executed serially to prevent surprising load
+increases from a dynamic var source suddenly returning a ton of values.
+
+To run steps in parallel, a `max_in_flight` must be specified as either `all`
+or a number - its default is `1`. Note: this value is specified on each `var`,
+rather than the entire step.
+
+With `max_in_flight: all`, no limit on parallelism will be enforced. This would
+be typical for when a small, static set of values is specified, and it would be
+annoying to keep the number in sync with the set:
+
+```yaml
+task: unit
+vars: {go-version: ((.:go-version))}
+across:
+- var: go-version
+  values: [1.12, 1.13]
+  max_in_flight: all
+```
+
+With `max_in_flight: 3`, a maximum of 3 var values would be executed in
+parallel. This would be typically set for values coming from a var source,
+which may change at any time, or especially large static values.
+
+```yaml
+set_pipeline: pr
+instance_vars: {pr_number: ((.:pr.number))}
+across:
+- var: pr
+  source: booklit-prs
+  max_in_flight: 3
+```
+
+When multiple `max_in_flight` values are configured, they are multiplicative,
+building on the concurrency of previously listed vars:
+
+```yaml
+set_pipeline: pr
+instance_vars:
+  pr_number: ((.:pr.number))
+  go_version: ((.:go_version))
+across:
+- var: pr
+  source: booklit-prs
+  max_in_flight: 3
+- var: go_version
+  values: [1.12, 1.13]
+  max_in_flight: all
+```
+
+This will run 6 `set_pipeline` steps at a time, focusing on 3 PRs and setting
+Go 1.12 and Go 1.13 pipelines for each in parallel.
+
+Note that setting a `max_in_flight` on a single `var` while leaving the rest as
+their default (`1`) effectively sets an overall max-in-flight.
+
+### Triggering on changes
+
+With `trigger: true` configured on a var, the build plan will run on any change
+to the set of vars - i.e. when a var value is added, removed, or changes.
+
+```yaml
+var_sources:
+- name: booklit-prs
+  type: github-prs
+  config:
+    repository: vito/booklit
+    access_token: # ...
+
+plan:
+- set_pipeline: pr
+  instance_vars: {pr_number: ((.:pr.number))}
+  across:
+  - var: pr
+    source: booklit-prs
+    trigger: true
+```
+
+Note that this can be applied to either static `values:` or dynamic vars from
+`source:` - both cases just boil down to a comparison against the previous
+build's set of values.
+
+### Modifier syntax precedence
+
+The `across` step is a *modifier*, meaning it is attached to another step.
+Other examples of modifiers are `timeout`, `attempts`, `ensure`, and the `on_*`
+family of hooks.
+
+In terms of precedence, `across` would bind more tightly than `ensure` and
+`on_*` hooks, but less tightly than `across` and `timeout`.
+
+`ensure` and `on_*` bind to the `across` step so that they may be run after the
+full matrix completes.
+
+`attempts` binds to the inner step because it doesn't seem to make a whole lot
+of sense to retry the *entire matrix* because one step failed. The individual
+steps should be retried instead.
+
+`timeout` binds to the inner step because otherwise a `max_in_flight` could
+cause the timeout to be exceeded before some steps even get a chance to run.
+
+```yaml
+task: unit
+timeout: 1h # interrupt the task after 1 hour
+attempts: 3 # attempt the task 3 times
+across:
+- var: go_version
+  values: [1.12, 1.13]
+on_failure: # do something after all steps complete and at least one failed
+```
+
+To apply `ensure` and `on_*` hooks to the nested step, rather than the `across`
+step modifier, the `do:` step may be utilized:
+
+```yaml
+do:
+- task: unit
+  on_failure: # runs after each individual step completes and fails
+across:
+- var: go_version
+  values: [1.12, 1.13]
+on_failure: # runs after all steps complete and at least one failed
+```
+
+This can be rewritten in a slightly more readable syntax by placing the `do:`
+below the `across:`:
+
+```yaml
+across:
+- var: go_version
+  values: [1.12, 1.13]
+do:
+- task: unit
+  on_failure: # runs after each individual step completes and fails
+on_failure: # runs after all steps complete and at least one failed
+```
+
+### Failing fast
+
+When a step within the matrix fails, the `across` step will continue to run the
+remaining steps in the matrix. However, the `across` step itself will still
+fail after all steps have completed.
+
+With `fail_fast: true` applied to the `across` step, execution will halt on the
+first failure, and any currently running steps (via `max_in_flight`) will be
+interrupted:
+
+```yaml
+task: unit
+across:
+- var: go_version
+  values: [1.12, 1.13]
+fail_fast: true
+```
+
+Note: this is the first time a step *modifier* has had additional sibling
+fields. In the event of a field conflict, the `do:` step may be utilized as a
+work-around.
+
+### Var scoping and shadowing
+
+The inner step will be run with a local var scope that inherits from the outer
+scope and is initialized with the across step's var values.
+
+Given that it runs with its own scope, it follows that the vars in the local
+scope, along with any vars newly set within the scope, are not accessible
+outside of the `across` step.
+
+When a var set by the `across` step shadows outer vars, a warning will be
+printed.
+
+Example:
+
+```yaml
+plan:
+- get_var: foo
+  trigger: true
+- run: print
+  type: debug
+  params: {value: ((.:foo))}
+- across:
+  - var: foo
+    values: [one, two]
+  do:
+  - run: print
+    type: debug
+    params: {value: ((.:foo))}
+- run: print
+  type: debug
+  params: {value: ((.:foo))}
+```
+
+Assuming the `get_var` step produces a value of `zero`, this build plan should
+result in the following output:
+
+```
+zero
+WARNING: across step shadows local var 'foo'
+one
+two
+zero
+```
+
+
+## Open Questions
+
+* n/a
+
+
+## New Implications
+
+* Using `across` with var sources implies the addition of a `list` action for
+  listing the vars from a var source. We could build on this to show the list
+  of available vars in the UI, which would really help with troubleshooting
+  credential manager access and knowing what vars are available.
+
+  Obviously we wouldn't want to show credential values, so `list` should only
+  include safe things like credential paths.
+
+* Combining the [`set_pipeline` step][set-pipeline-rfc], [`((var))`
+  sources][var-sources-rfc], [instanced pipelines][instanced-pipelines-rfc],
+  [pipeline archiving][pipeline-archiving-rfc], and the `across` step can be
+  used for end-to-end automation of pipelines for branches and pull requests:
+
+  ```yaml
+  set_pipeline: pr
+  instance_vars: {pr_number: ((.:pr.number))}
+  across:
+  - var: pr
+    source: prs
+    trigger: true
+  ```
+
+[set-pipeline-rfc]: https://github.com/concourse/rfcs/pull/31
+[instanced-pipelines-rfc]: https://github.com/concourse/rfcs/pull/34
+[pipeline-archiving-rfc]: https://github.com/concourse/rfcs/pull/33
+[var-sources-rfc]: https://github.com/concourse/rfcs/pull/39
+[multi-branch-issue]: https://github.com/concourse/concourse/issues/1172