📖 Verb Reference

This page documents all 31 saga step types ("verbs") supported by the engine.

Quick-reference table

Verb	License group	One-liner
action	common	Dispatch a step to an external worker and pause until it replies.
decision	common	Evaluate a stored rule table and branch on its output.
switch	common	Evaluate a CEL expression and branch on the string result.
error	common	Immediately fail the saga with a code and message.
noop	common	Placeholder — do nothing and advance.
end	(terminal)	End the run as succeeded.
set_var	common	Write a literal or CEL-computed value into a variable.
transform	common	Evaluate a CEL expression and store the result.
merge	common	Merge a CEL-evaluated map into an existing variable.
filter	common	Keep list elements that satisfy a CEL predicate.
map	common	Transform each element of a list with a CEL expression.
assert	common	Fail the saga if a CEL expression is not truthy.
log	common	Emit a structured log line at a chosen level.
metric_emit	observability	Append a named metric event to the run's audit stream.
http_request	common / external_io_advanced¹	Issue a synchronous outbound HTTP request.
webhook_emit	external_io_advanced	POST a JSON payload to an external URL.
wait_duration	waits	Pause for a Go duration (e.g. "5s", "1h30m").
wait_until	waits	Pause until an RFC3339 absolute timestamp.
wait_for_signal	events_and_signals	Pause until a named external signal arrives (with optional timeout).
wait_for_event	events_and_signals	Pause until a matching event topic + header subset arrives.
emit_signal	events_and_signals	Send a signal to another (or the same) run.
emit_event	events_and_signals	Publish an event via the configured EventEmitter.
while	loops_and_recovery	Loop while a CEL condition holds; exit via branching.
try_catch	loops_and_recovery	Push an error-handler frame; jump to catch on any step error.
cancel	loops_and_recovery	Cancel a run (self or a target).
parallel	parallel_control	Fan out N branches and join when a strategy is satisfied.
foreach	parallel_control	Fan out one branch per element of a CEL-evaluated list.
sub_saga	compositions	Start a child workflow and pause the parent until it finishes.
spawn_saga	compositions	Fire-and-forget: start a child workflow and continue immediately.
manual_approval	human_interaction	Create a user task; pause until an assignee submits it.
collect_input	human_interaction	Like manual_approval but form_schema is required.

¹ http_request is common when method is GET with no secret_ref; all other configurations require the external_io_advanced group.

---

💡 License groups gate verbs at publish time and at runtime. In development the StubAllowAll licensing resolver (used by saga.InMemory() and saga.New with no Licensing set) permits every group. In production, override per-request with the X-Feature-Override header or the FeatureOverrides field on the run start request.

---

Core / Control

action

Dispatches the step to a named worker process and pauses the saga until the worker replies.

Input	Required	Notes
step.Action	✅	"service.name" — must contain a dot. Identifies the worker queue.
step.Inputs	optional	Forwarded verbatim to the worker as inputs.

Output: The worker's result map is merged directly into Variables. There is no out_var — whatever keys the worker returns become top-level variables.

⚠️ In embedded mode (saga.InMemory()) the action verb publishes to an in-process publisher. You still need a worker goroutine (or service mode) to actually handle the dispatch; a plain in-memory saga with no registered worker handler will leave the run paused indefinitely.

Example: examples/workflows/action.json

---

decision

Evaluates a stored decision-table rule and returns its output map. The engine reads result["branch"] to pick a route from step.Branches.

Input	Required	Notes
rule_id	✅	Stable ID of a published RuleDefinition.
inputs_map	optional	map[string]string — maps rule input keys to variable names. Omit to pass Variables directly.

Output: The rule's full output map (including branch) is merged into Variables.

Example: examples/workflows/decision.json

---

switch

Evaluates a CEL expression to a string and routes via step.Branches. Simpler than decision when no rule table is needed.

Input	Required	Notes
expr	✅	CEL expression over Variables; must produce a string.

Output: {"branch": "<result>"} — the engine picks step.Branches[<result>].Next. An unknown branch value is a runtime error.

Example: examples/workflows/switch.json

---

error

Immediately fails the saga as a non-retryable error.

Input	Required	Notes
code	✅	Error code string surfaced in the run's error record.
message	optional	Human-readable description.

Output: None — the run terminates as failed.

Example: examples/workflows/error.json

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noop

Does nothing. Advances to step.Next. Useful as a placeholder during development or as a join point for multiple branches.

Inputs: none. Output: empty map.

Example: examples/workflows/noop.json

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end

Marks the run as succeeded. Every workflow needs at least one end step. It is dispatched inline by the engine (not via a queue) so no license group applies.

Inputs: none. Output: none — the saga terminates.

---

Data

set_var

Writes a value to a variable. Use it to seed variables before CEL verbs read them.

Input	Required	Notes
out_var	✅	Destination variable name. Dotted keys (a.b.c) write into nested maps.
value	one of value/expr	Literal value — passed through unchanged.
expr	one of value/expr	CEL expression over Variables; result is written. expr wins when both are present.

Output: {out_var: <value>}.

Example: examples/workflows/set_var.json

---

transform

Evaluates a CEL expression and writes the result to a named variable. Equivalent to set_var with expr.

Input	Required	Notes
expr	✅	CEL expression over Variables.
out_var	✅	Destination variable name.

Output: {out_var: <result>}.

Example: examples/workflows/transform.json

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merge

Evaluates a CEL expression that must produce a map, then deep-merges it into an existing variable.

Input	Required	Notes
from	✅	CEL expression → map.
into	✅	Name of the destination variable. Dotted paths are supported.

Output: The merged variable value under its existing key.

Example: examples/workflows/merge.json

---

filter

Keeps list elements where a CEL predicate is truthy.

Input	Required	Notes
list	✅	CEL expression → list.
expr	✅	CEL predicate; the current element is bound as _.
out_var	✅	Variable to write the filtered list to.

Output: {out_var: [filtered list]}.

Example: examples/workflows/filter.json

---

map

Transforms every element of a list with a CEL expression.

Input	Required	Notes
list	✅	CEL expression → list.
expr	✅	CEL transform; element bound as _.
out_var	✅	Variable to write the mapped list to.

Output: {out_var: [mapped list]}.

Example: examples/workflows/map.json

---

assert

Fails the saga if a CEL expression is not truthy. Use it for invariant checks mid-workflow.

Input	Required	Notes
expr	✅	CEL boolean expression.
code	optional	Error code emitted on failure (default "assertion_failed").

Output: Empty map on success; non-retryable error on failure.

Example: examples/workflows/assert.json

---

Observability

log

Emits a structured log line via the engine's logger.

Input	Required	Notes
message	✅	Log message string.
level	optional	"info" (default) | "warn" | "error".

Output: Empty map.

Example: examples/workflows/log.json

---

metric_emit

Appends a named metric event to the run's audit stream. (Prometheus side-channel wiring is planned for a future release.)

Input	Required	Notes
name	✅	Metric name string.
value	✅	Numeric value.
labels	optional	map[string]string of label key/value pairs.

Output: Empty map.

Example: examples/workflows/metric_emit.json

---

I/O

http_request

Issues a synchronous outbound HTTP request and merges the response into Variables.

Input	Required	Notes
url	✅	Target URL.
method	optional	HTTP method (default "GET").
headers	optional	map[string]any → request headers.
body	optional	Any value; JSON-marshalled into the request body.
timeout_s	optional	Request timeout in seconds (default 30).
secret_ref	optional	Secret key resolved via the Secrets resolver → set as Authorization header.
out_var	optional	Output prefix (default "http_result").

Output keys (with out_var = "http_result"):

• http_result — parsed JSON body (or raw string if non-JSON).
• http_result_status — int64 HTTP status code.
• http_result_headers — map[string]string of response headers.

⚠️ License group is common only for GET with no secret_ref. Any other method or authenticated request requires the external_io_advanced group.

Example: examples/workflows/http_request.json

---

webhook_emit

POSTs a JSON payload to an external URL, with optional HMAC-SHA256 request signing.

Input	Required	Notes
url	✅	Target URL.
body	✅	Any value; JSON-marshalled.
secret_ref	optional	Secret key → X-Webhook-Sig: sha256=<hex> header.
timeout_s	optional	Timeout in seconds (default 15).
headers	optional	Additional request headers.
async	optional	bool; default false. When true, fires the request in a goroutine and returns immediately (failures are logged only).
out_var	optional	Output prefix (default "webhook_result").

Output (sync mode): {out_var}_status — int64 HTTP status code.
Output (async mode): {out_var}_async: true.

Example: examples/workflows/webhook_emit.json

---

Timing

wait_duration

Pauses the saga for a duration expressed as a Go duration string.

Input	Required	Notes
duration	✅	Go duration string e.g. "5s", "1h30m", "72h".

The engine's timer dispatcher wakes the run when the deadline passes.

Example: examples/workflows/wait_duration.json

---

wait_until

Pauses the saga until an absolute point in time.

Input	Required	Notes
timestamp	✅	RFC3339 datetime string, e.g. "2026-01-01T09:00:00Z".

Example: examples/workflows/wait_until.json

---

Events & Signals

wait_for_signal

Pauses the saga until a named external signal arrives via POST /api/v1/sagas/{run_id}/signal/{name}.

Input	Required	Notes
name	✅	Signal name to await.
timeout_s	optional	Max seconds to wait. Omit to wait indefinitely.

Timeout routing: when the deadline fires while the signal has not yet arrived, the engine checks step.Branches["timeout"].Next first. If that branch exists, the run routes there instead of step.Next — handy for escalation paths.

💡 Wire a timeout branch to an escalation step to handle missed approvals or SLA breaches without any extra polling.

Example: examples/workflows/wait_for_signal.json

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wait_for_event

Pauses the saga until an event with a matching topic (and optional header subset) arrives via the event bus.

Input	Required	Notes
topic	✅	Event topic / RabbitMQ routing key to await.
headers	optional	map[string]any — incoming event headers must contain all of these key/value pairs (string equality).
timeout_s	optional	Number of seconds to wait. On timeout the run routes to the step's timeout branch if defined, else to next. Omitted = wait indefinitely.

💡 Add a "timeout" entry to the step's branches to escalate when no matching event arrives before timeout_s (same pattern as wait_for_signal).

Example: examples/workflows/wait_for_event.json

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emit_signal

Sends a signal to another run (the send-side complement of wait_for_signal). If the target is currently paused awaiting that signal, it is consumed and the target advances immediately.

Input	Required	Notes
run_id	✅	UUID of the target run.
name	✅	Signal name.
payload	optional	map[string]any carried with the signal.

Output: Empty map.

Example: examples/workflows/emit_signal.json

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emit_event

Publishes an event via the configured EventEmitter (in-process when embedded; RabbitMQ in service mode).

Input	Required	Notes
topic	✅	Event topic / routing key.
headers	optional	map[string]any → map[string]string.
payload	optional	map[string]any event payload.

Output: Empty map.

💡 In embedded mode the in-process emitter both wakes runs awaiting the topic and runs the trigger dispatcher, so matching record_transition triggers start new runs — parity with service mode (no broker needed).

Example: examples/workflows/emit_event.json

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Loops & Recovery

while

Loops while a CEL condition evaluates to true.

Input	Required	Notes
condition	✅	CEL boolean expression over Variables.
max_iterations	optional	Default 100; hard cap 10000. Prevents runaway loops.

Output: {"branch": "continue" | "exit"}.

Wiring pattern:

• step.Branches.continue → next — first step of the loop body.
• step.Branches.exit → next — first step after the loop.
• The body's last step sets next back to the while step.

An iteration counter is maintained at Variables._while.<step_id>.iter.

Example: examples/workflows/while.json

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try_catch

Pushes an error-handler frame. If any step inside the protected region errors, the saga jumps to the catch step instead of failing. The error context is written to Variables._error.

Input	Required	Notes
try	✅	[]string of step IDs in the protected region. Used by ValidateDefinition to reject disallowed nesting (e.g. parallel inside try).
catch	✅	Step ID to jump to on error.

Wiring: set step.Next to the first step inside the try body. The body's last step sets next to whatever comes after the protected region.

Example: examples/workflows/try_catch.json

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cancel

Cancels a run.

Input	Required	Notes
run_id	optional	UUID of the target run. Omit (or set to the current run's ID) to self-cancel — the current run ends as cancelled.
reason	optional	Human-readable reason string.

Self-cancel: returns ErrSagaCancelled; the run ends immediately as cancelled.
Target-cancel: cancels the target run and the current run continues to step.Next.

⚠️ When you cancel a run that is a child of a parallel join, the join may be left waiting if the join strategy expects all children. The cancelled child is counted as terminal, so with join_strategy: "all" the parent will eventually time out or remain paused unless all other children also complete.

Example: examples/workflows/cancel.json

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Parallelism

parallel

Fans out N branches as child runs and pauses the parent until the join strategy is satisfied.

Input	Required	Notes
branches	✅	[]any of branch objects or a CEL string → list. Each branch: long-form {"start": "step_id", "steps": [...]} or short-form {"type": "...", "inputs": {...}}.
join_strategy	optional	"all" (default) — wait for every branch. "quorum" — wake after quorum_n successes.
quorum_n	required when quorum	Positive integer ≤ branch count. Can also be a CEL string evaluated at runtime.

Output: Each branch's variables are aggregated into Variables._parallel.<step_id>.branches when the parent wakes.

⚠️ Remaining branches continue to run after a quorum wake — they are not cancelled.

Example: examples/workflows/parallel.json

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foreach

Fans out one child run per element of a CEL-evaluated list (parallel mode only in v1).

Input	Required	Notes
list	✅	CEL expression → list.
body	✅	[]any of step objects forming the loop body.
start	✅	ID of the first step inside body.
parallel	optional	bool; default true. Sequential mode is not yet supported — use while with an index counter for sequential loops.

Each child run receives Variables._foreach_item (the element) and Variables._foreach_index (zero-based index). An empty list advances without spawning.

Example: examples/workflows/foreach.json

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Composition (Call Tree)

sub_saga

Starts a named workflow as a child saga and pauses the parent until the child reaches a terminal state.

Input	Required	Notes
workflow_id	✅	Stable ID of the child WorkflowDefinition.
inputs	optional	map[string]any passed as the child's initial inputs.
entrypoint	optional	Named entry point on the child definition (see Entrypoints). Defaults to Start.

Output: Empty map on parent resume (child variables are not automatically merged; wire a set_var/transform after if needed).

Example: examples/workflows/sub_saga.json

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spawn_saga

Starts a named workflow as a fire-and-forget child. The parent continues immediately to step.Next without waiting.

Input	Required	Notes
workflow_id	✅	Stable ID of the child workflow.
inputs	optional	map[string]any passed as the child's initial inputs.
entrypoint	optional	Named entry point on the child. Defaults to Start.

Output: Empty map; parent is not paused.

Example: examples/workflows/spawn_saga.json

---

Human Interaction

manual_approval

Creates a user task and pauses the saga until the assignee submits it via POST /api/v1/sagas/{run_id}/user_task/{task_id}/submit.

Input	Required	Notes
assignee	✅	User ID or role expected to submit.
due_in	optional	Go duration string — sets due_at = now + due_in.
form_schema	optional	map[string]any rendered in the admin UI.

Output: None on pause; the submitted form data is available after the run resumes.

Example: examples/workflows/manual_approval.json

---

collect_input

Like manual_approval but form_schema is required. Use this when the workflow needs structured data from the user (remediation notes, parameters, etc.) rather than a simple approve/reject.

Input	Required	Notes
assignee	✅	User ID or role.
form_schema	✅	map[string]any schema — must be non-empty.
due_in	optional	Go duration deadline.

Output: None on pause.

Example: examples/workflows/collect_input.json