Step 7 - Add messaging
The approval blocks waiting for a decision. Messages give callers a way to interact with it while it waits.
Two Operations get added to the workflow sample problem. Which message type each one uses is decided by what the caller needs back, not by preference.
| Operation | Message type | Why this type |
|---|---|---|
remindApprover | Signal | Fire-and-forget. The caller does not need a response, only for the nudge to happen. |
submitDecision | Update | Changes state and returns a result the caller needs — confirmation the decision was recorded. |
That is the whole rule. If the caller can proceed without hearing anything back, a Signal is enough. If the caller needs to know what the message did, it needs an Update.
Add the handlers to the Workflow
On the Workflow, add a Signal handler that increments the reminder count and an Update handler that records the decision and unblocks the wait.
The Update is what ends the approval. It records APPROVED or DENIED, which satisfies the condition the Workflow is blocked on, and the Workflow then returns that decision as its result.
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Expose them as Nexus Operations
Both use TemporalOperationHandler, and they divide along the line described in The Nexus-aware Client: a Signal is sync messaging, and an Update is an async backing.
Signal
Send the Signal through the Client, then return a synchronous result. The Operation completes immediately, during the handler call.
A synchronous handler must finish inside the 10-second handler deadline, and the budget you actually get is smaller: the clock starts on the caller's side and the request still has to route through matching.
Sending one Signal is comfortably inside it. A handler that sends several messages, or does slow work before returning, is not. Overrunning gives the caller a context deadline exceeded error, which it then retries with exponential backoff until the schedule-to-close timeout expires.
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Update
Start the Update on the Client. This is an async backing: the Operation completes when the Update completes, and its result is delivered through the Nexus completion callback. If the Update happens to come back already complete — a retried request, or one that failed validation — the result returns synchronously instead.
An Update-backed Operation carries two requirements. It targets a Workflow that already exists, so a submitDecision for a purchase with no approval running fails. And because it is an async backing, there is at most one per Operation invocation, though a handler can still combine it with sync side effects.
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Do not poll for the decision
There is one design mistake worth naming, because it is the most common one in this shape: reaching for a message to fetch the final decision.
The decision is the result of requestApproval, and it reaches the caller without anyone asking for it.
When the handler started the approval, Nexus attached a completion callback to that Workflow. The moment the Workflow returns, the handler's Namespace delivers the callback to the caller's Nexus Machinery, which records a NexusOperationCompleted event in the caller Workflow's history. The caller Worker picks that up on its next Workflow Task, and the caller Workflow resumes with the decision. See the asynchronous Operation lifecycle for the full sequence.
A caller that instead asks the approval for its status in a loop is polling for something already on its way.
Messages are for changing a running approval or nudging it along, not for collecting its outcome. remindApprover asks the approver again. submitDecision supplies the decision and confirms it landed. Neither is a way to read the result.
Both also stop working the moment the approval completes. The Temporal Service accepts a Signal or an Update only while the Workflow is still running, and rejects one sent to a closed Workflow with NOT_FOUND: workflow execution already completed. That happens as soon as the decision lands, not when the Retention Period later expires and the Execution is deleted. A second submitDecision for an approval that has already been decided fails this way, and so does any attempt to use these Operations to look up a past decision.
If more than one system needs the outcome, see step 8 for how additional callers attach to a running approval and receive the same decision.
Next
Step 8 - Send messages - call the messaging Operations, and handle an approval that may not exist yet.
Back to the Microservice Development Walkthrough overview.
- Workflow message passing for Signals and Updates.
- Handling messages for handler constraints.
- Temporal Operation Handler for the sync messaging and async backing distinction.