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Workflow Entry, Routing, and Termination

This document defines standard semantics for:

  • workflow initiation (entry selection),
  • step routing (Petri-net arcs),
  • branch termination,
  • playbook completion (quiescence),
  • optional finalization/aggregation,

without requiring reserved step names such as step: start or step: end.

The playbook root sections remain:

  • metadata, keychain (optional), executor (optional), workload, workflow, workbook (optional)

The workflow section remains an array of steps.

1. Model and terminology​

1.1 Steps as Petri-net transitions​

A workflow is modeled as a Petri net / state machine where:

  • a token represents a unit of control ready to run a step,
  • a step is a transition that consumes a token when it starts,
  • step.next.arcs[] defines outgoing arcs that produce new tokens for downstream steps.

1.2 Server vs worker responsibilities​

  • The server (control plane) is responsible for:

    • selecting the entry step,
    • evaluating step admission (step.spec.policy.admit.rules),
    • evaluating routing (step.next.spec.mode + step.next.arcs[].when),
    • scheduling step-runs to workers,
    • detecting completion (quiescence),
    • persisting the event log and maintaining projections.

  • The worker (data plane) is responsible for:

    • executing the step pipeline (step.tool tasks in order),
    • applying task policy control flow (retry/jump/break/fail/continue) via task.spec.policy.rules,
    • emitting task and step outcome events.

2. Entry selection (initial marking)​

2.1 Default entry rule (MUST)​

If no explicit entry is configured, the entry step MUST be the first step in the workflow array:

  • entry_step := workflow[0].step

This ensures deterministic initiation while keeping the workflow as a simple ordered list.

2.2 Optional override (MAY)​

The runtime MAY support an executor policy to override entry selection:

executor:
  spec:
    entry_step: "<step_name>"

If executor.spec.entry_step is present:

  • the server MUST select that step as entry.
  • the referenced step name MUST exist in the workflow array.

2.3 Initial token​

At execution start, the server MUST create exactly one initial token targeting entry_step unless an implementation explicitly supports multi-token starts.

3. Step enablement​

3.1 Admission policy (server-side)​

current DSL has no step.when field. Enablement is expressed via step admission policy:

  • step.spec.policy.admit.rules[]

Admission rule shape:

spec:
  policy:
    admit:
      rules:
        - when: "{{ <bool expr> }}"
          then: { allow: true|false }
        - else:
            then: { allow: true|false }

Admission rule:

  • a token targeting a step is admitted when the server evaluates allow: true.
  • if admit is omitted, admission defaults to allow.

3.2 Disabled steps​

If admission evaluates to deny:

  • the token MUST NOT be scheduled to a worker.
  • the runtime MUST define one of:
    • token remains pending until it becomes enabled (default), or
    • token is discarded (optional policy).

Standard default:

  • token remains pending (to support gates/approvals via future ctx patches).

4. Routing via step.next arcs​

4.1 Router semantics​

A step may define a next router object with outgoing arcs:

next:
  spec:
    mode: exclusive|inclusive
  arcs:
    - step: next_step_name
      when: "{{ <bool expr> }}"   # optional (default true)
      args: { ... }               # optional

After a step reaches a terminal outcome (step.done or step.failed, or loop.done when a loop is present), the server MUST evaluate step.next.arcs[] to determine which new tokens to emit.

4.2 Arc guard evaluation​

For each arc:

  • if next.arcs[].when is omitted, it MUST be treated as true.
  • if present, it is evaluated server-side using available state:
    • event (boundary event), workload, ctx, and incoming args
  • the evaluation result determines whether the arc matches.

4.3 Router mode​

next.spec.mode controls fan-out:

Defaults:

  • if omitted, exclusive MUST be assumed.

Behavior:

  • exclusive: first matching arc (YAML order) wins; emit exactly one downstream token.
  • inclusive: all matching arcs fire; emit one downstream token per match (fan-out).

4.4 Arc payloads (args)​

If an arc includes args, they MUST be bound into the downstream token as immutable args for that step-run.

args should remain small and reference-first. Large payloads MUST be externalized and passed as references.

5. Default routing and branch termination​

5.1 No arcs​

If a step has no next section:

  • it represents a leaf transition.
  • after it completes, no downstream token is emitted by routing.

5.2 No matching arcs​

If a step has next.arcs[] but none match:

  • the runtime MUST treat this as branch termination by default:
    • no downstream tokens are emitted.

Optional policy (MAY):

  • treat “no match” as an error and fail execution. If supported, it MUST be explicitly enabled by policy (e.g., executor.spec.no_next_is_error: true).

Standard default:

  • “no match” terminates the branch.

6. Workflow termination (quiescence)​

6.1 Quiescence definition (MUST)​

An execution is complete when all are true:

  1. No runnable tokens exist
  • there are no enabled tokens that can be scheduled.
  1. No in-flight step-runs exist
  • there are no leased/running step runs on workers (including retries in progress).
  1. No pending fan-in trackers exist (if fan-out mode is enabled)
  • there are no incomplete join/fan-in groups preventing downstream routing.

This is the Petri-net notion of reaching a marking with no enabled transitions and no active firings.

6.2 Completion status​

The server MUST compute final status based on terminal step outcomes and policies, e.g.:

  • success when all completed branches ended normally,
  • failed if any branch ended in a terminal failure and policy requires fail-fast or fail-on-any-error,
  • partial if policy allows partial completion.

(Exact final status policy is implementation-defined, but must be explicit.)

7. Optional finalization / aggregation (without step: end)​

7.1 final_step policy (MAY)​

To support “run once at the end” behavior without embedding reserved end steps, the runtime MAY support:

executor:
  spec:
    final_step: "<step_name>"

Semantics:

  • After quiescence is reached, if final_step is configured and not yet executed:
    • the server MUST schedule a single token to run final_step.
  • After final_step reaches terminal outcome, the execution completes.

7.2 Final step inputs​

The runtime SHOULD provide a finalization summary to final_step via args, including:

  • execution id,
  • counts of steps/branches,
  • references to result sets (not full payloads),
  • failure summaries if any.

7.3 Final step failures​

If final_step fails, final execution status MUST follow policy, e.g.:

  • fail the execution (default), or
  • mark partial and retain references for inspection.

8. Relationship to noop steps​

kind: noop is a valid tool kind to support:

  • pure routing steps,
  • emitting context patches (set_ctx, set_iter) via task policy rules,
  • decision points inside pipelines.

However:

  • workflow initiation and termination MUST NOT require noop steps or reserved step names.
  • entry and termination are defined by the rules above.

9. Summary (Standard defaults)​

  • Entry step is workflow[0] unless overridden by executor.spec.entry_step.
  • Steps are enabled by admission (step.spec.policy.admit; default allow).
  • Routing is defined by step.next:
    • default exclusive, ordered first-match wins.
  • If no arcs match, the branch terminates by default.
  • Execution completes by quiescence (no runnable tokens, no in-flight runs, no pending fan-in).
  • Optional executor.spec.final_step allows a single end-of-run aggregation step without requiring step: end.