A running Node.js process—or even a job that started successfully—does not prove a scheduled gaming experiment completed its work. To detect a missed job, give every expected cohort window a stable identity, record completion only after its durable operation commits, and have an independent watchdog compare that evidence with an explicit deadline. If evidence is missing or uncertain, freeze that cohort’s progression and apply rollback only under a defined, auditable policy.
What a cron health check must prove
A conventional process heartbeat answers whether a process is responsive. A job-start event answers whether execution began. Neither proves that the scheduled business operation finished or that its changes persisted. The health check needs evidence tied to the expected work: one completion record for each experiment, cohort, and scheduled window, written only after the durable operation succeeds.
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Use a stable run key such as experiment_id/cohort/window. The window identifies the expected occurrence, so retries refer to the same logical run rather than creating new completion claims. Include the experiment version so any later rollback can be bound to the version it reverses.
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Keep the schedule, execution, completion, and monitoring observations distinct. A useful record includes:
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- Run key: the stable experiment/cohort/window identity.
- Scheduled time: when that window was expected to run.
- Start time: when an attempt began; diagnostic information, not proof of health.
- Durable completion time: when the required write or other durable operation committed.
- Observed time: when the watchdog learned the completion result.
- Attempt metadata: attempt identifiers and diagnostic details, stored separately from the logical completion state.
- Experiment version: the version of the cohort operation the record covers.
Judge whether work was late using scheduled time and durable completion time. Retain observed time to show when the monitoring system learned the outcome; a delayed observation is not the same as a late operation.
Make recording completion idempotent on the run key. That alone does not make the cohort mutation idempotent: guard the underlying state change with its own transaction or deduplication boundary. If an attempt times out after a write may have succeeded, treat the result as unknown until the durable store is checked. Do not turn uncertainty into a second mutation or a premature success claim.
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How the independent watchdog detects a missed window
The watchdog should read the expected schedule and completion records independently of the job being monitored. If the job never ran, it cannot be responsible for reporting its own absence. For each expected window, compare its durable completion record with a deadline; record the watchdog observation and the decision it makes.
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- Look up completion evidence. Match on the stable run key. A start record or process heartbeat does not satisfy this check.
- Apply the deadline. Set it using measured queue delay and execution duration for the workload, plus an explicit allowance for clock skew. Revisit it when load or cohort size changes.
- Persist the outcome. Record whether the window is on time, late, missing, or uncertain, along with the observation time and any resulting cohort decision.
A five-minute check cadence and a seven-minute deadline can illustrate how cadence and grace period differ, but they are not universal thresholds. The deadline should reflect the actual workload and capacity.
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Choose missed-occurrence and overlap behavior explicitly
Scheduler behavior when an occurrence is missed or a run is still active varies by framework and version. NestJS durable workflows document cron expressions with five fields, or six when seconds lead; every intervals; and RFC 5545 rrule schedules. UTC is the documented default timezone. In that framework, the documented missed-occurrence default is skip, with once and all alternatives; the documented overlap default is also skip, with allow, cancel-previous, and buffer-one alternatives. These are NestJS workflow semantics, not guarantees of every Node.js cron library. See the NestJS durable workflows documentation and verify the behavior of the scheduler and version you deploy.
The node-schedule package documents retrieving a scheduled fire date, which can help compare intended timing with actual invocation timing and preserve delay diagnostics. Check the installed package’s documentation and version rather than assuming all schedulers expose the same information: node-schedule documentation.
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Decide whether to skip, catch up, allow, or prevent overlapping runs based on the work’s semantics and capacity. For example, if a five-minute schedule routinely requires eight minutes at expected capacity, choose deliberately among preventing overlap, partitioning the work, or revising the cadence and completion objective. Adding retries does not add processing capacity.
Use missing evidence to control cohort progression
Advance a cohort only when its expected window has a durable, on-time completion and the evidence path is available. A successful window in another cohort, or a healthy worker process, says nothing about this cohort’s missing run.
- Missing or late evidence at the deadline: freeze progression for the affected cohort while the outcome is resolved.
- Unreadable completion store or uncertain execution outcome: treat the state as unknown and freeze rather than declaring the cohort healthy.
- Rollback required by policy: make the action idempotent and bind it to the experiment version being reversed.
- Completion arrives after a decision: retain it as late evidence. Do not erase or silently rewrite the earlier decision.
Persist decision history with the cohort scope, run key, relevant experiment version, observation time, and action. That history lets operators distinguish a deliberate freeze or rollback from a missing log entry, and supports review without letting one cohort’s status decide another’s outcome.
Validate the failure paths with fixed UTC times
Acceptance checks should exercise the evidence and decision boundaries, not merely confirm that a cron callback fires. Use fixed UTC timestamps so deadline comparisons are repeatable.
- An expected window completes durably before its deadline.
- One cohort has no completion record when its deadline passes, while another cohort completes.
- A late completion appears after a rollback decision; the earlier decision remains in history.
- Duplicate attempts use the same stable key without creating duplicate completion claims or repeating the cohort mutation.
- The completion store cannot be read; the watchdog reports uncertainty and does not mark the cohort healthy.
- A run exceeds the schedule cadence, exercising the configured overlap behavior.
Where an external heartbeat monitor fits
An external scheduled-job monitor can provide alerts when expected pings do not arrive. Evaluate whether it supports schedule and grace-period configuration, records pings only after successful work, offers useful alert channels and event history, and meets your self-hosting and data-ownership needs. Also check whether it can represent per-cohort run keys. Basic missed-run alerting does not, by itself, establish durable cohort evidence or make automated rollback safe.
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