Scheduler — Amazon EventBridge Scheduler

AWS docs: https://docs.aws.amazon.com/scheduler/latest/APIReference/Welcome.html

EventBridge Scheduler is served as a REST-JSON API at AWS’s own paths, so an unmodified SDK or aws scheduler … call reaches it. This implementation focuses on schedule groups, schedules, tagging, and clock-driven target dispatch to every target type EventBridge rules reach.

---

Behavior Notes

• Request paths — AWS’s own bindings, taken from the pinned Smithy model:

  | Operation | Binding | | --- | --- | | CreateSchedule | POST /schedules/{Name} — GroupName in the body | | GetSchedule | GET /schedules/{Name}?groupName= | | UpdateSchedule | PUT /schedules/{Name} — GroupName in the body | | DeleteSchedule | DELETE /schedules/{Name}?groupName= | | ListSchedules | GET /schedules?ScheduleGroup= | | CreateScheduleGroup | POST /schedule-groups/{Name} | | GetScheduleGroup | GET /schedule-groups/{Name} | | DeleteScheduleGroup | DELETE /schedule-groups/{Name} | | ListScheduleGroups | GET /schedule-groups | | TagResource / UntagResource / ListTagsForResource | POST / DELETE / GET /tags/{ResourceArn} |

  A schedule is addressed by name alone; its group is never a path segment. /tags/{ResourceArn} is shared with API Gateway, EKS and Pipes, and is dispatched on the scheduler segment of the resource ARN.

  Releases up to and including 0.0.1-alpha.33 served these operations under an emulator-invented /_scheduler/ prefix instead, so every SDK and CLI call answered 501. That prefix has been removed rather than kept as an alias.

• Default schedule group:

  • default is auto-seeded and cannot be deleted.
  • DeleteScheduleGroup deletes the schedules inside the group, as AWS does.

• Supported schedule expressions:

  • rate(...)
  • at(...)
  • cron(...) (AWS-style 6-field form). Each field takes *, ?, a value, a comma-separated list, a range (9-17) or a step (*/5, 0/15, 9-17/4); a step over a range walks that range. The L, W and # day specifiers and the three-letter month and day names are not supported, and an expression using one is refused by CreateSchedule.
  • A cron expression is evaluated by advancing field by field, so a sparse schedule — yearly, say — costs the same per tick as a frequent one.

• Validation on CreateSchedule and UpdateSchedule:

  • The schedule and group names must match the model’s constraint — 1–64 characters of [0-9a-zA-Z-_.].
  • The ScheduleExpression must be one the engine can evaluate. An expression it cannot parse is refused up front rather than accepted and reported as an engine error on every tick, which would leave a schedule that reads correctly in GetSchedule and never fires.
  • FlexibleTimeWindow.Mode is required and must be OFF or FLEXIBLE; State must be ENABLED or DISABLED.

• UpdateSchedule replaces, as AWS’s does. The request carries the whole schedule, so any optional member the caller omits — Description, ScheduleExpressionTimezone, State, StartDate, EndDate, or anything inside Target — ends up unset, and State returns to its ENABLED default. Read the schedule, change what you mean to change, and send the result back. What survives is the schedule’s identity, again as on AWS: its name, group, ARN and CreationDate.

  Releases up to and including 0.0.1-alpha.33 merged instead, keeping an omitted member at its stored value.

• Pagination and filtering:

  • ListSchedules and ListScheduleGroups honour MaxResults (1–100, a full page when omitted) and NextToken.
  • ListSchedules filters on NamePrefix and State; ListScheduleGroups filters on NamePrefix.
  • A NextToken that cannot be decoded is answered with a ValidationException rather than silently restarting at the first page, which an SDK paginator would read as a legitimate page and loop on.
  • Both operations return the full stored object rather than AWS’s ScheduleSummary/ScheduleGroupSummary shape. That is a superset, so an SDK deserialises it unchanged.

• Background scheduler engine:

  • Polls on a 1-second clock ticker.
  • Uses the injected clock, so integration tests can advance time quickly.
  • A tick hands each due schedule to a pool of delivery workers rather than delivering it on the tick itself, so a target that is slow, unreachable or working through its RetryPolicy delays only its own schedule. A schedule is never in flight twice, so its firings stay in order; a tick that finds a schedule still mid-delivery leaves it due and skips it.

• Target dispatch:

  • Delivery goes through the same internal dispatcher EventBridge rules and Pipes use, so a target ARN behaves identically on a schedule and on a rule. A firing is replayed against the emulator’s own API, which means a missing function, queue, topic, stream or state machine produces that service’s own AWS error rather than a silent no-op.
  • Supported target types: Lambda (async invoke), SQS, SNS, Step Functions, Kinesis, Firehose, ECS (RunTask) and EventBridge event buses (PutEvents).
  • Target parameters honoured: SqsParameters.MessageGroupId, KinesisParameters.PartitionKey, EventBridgeParameters (Source and DetailType), and EcsParameters (TaskDefinitionArn, TaskCount, LaunchType, PlatformVersion, Group, NetworkConfiguration). The remainder of AWS’s EcsParameters shape — tags, placement constraints and strategy, capacity provider strategy — is accepted and ignored.
  • Target.Input is delivered verbatim. A target with no Input receives a generated {"source":"aws.scheduler","time":…,"id":…} envelope.
  • A target type Overcast cannot fire is rejected at CreateSchedule and UpdateSchedule with a ValidationException, rather than being accepted and dropped at fire time. This is stricter than AWS, which delivers to ~270 services through templated and universal (arn:aws:scheduler:::aws-sdk:…) targets; the refusal fails locally and loudly instead of leaving a schedule that looks correct and never fires. An ECS target without EcsParameters.TaskDefinitionArn, and an event-bus target without EventBridgeParameters, are refused for the same reason.

• Retries and dead-lettering:

  • RetryPolicy.MaximumRetryAttempts is honoured, capped at 6 total attempts. Retries run back to back on the delivery worker that owns the firing, with no backoff, so AWS’s default of 185 attempts is not replayed: a target with no RetryPolicy is attempted once. Other schedules are unaffected while a firing retries. EventBridge rule targets behave the same way.
  • RetryPolicy.MaximumEventAgeInSeconds is honoured — once the payload is older than the budget, no further attempt is made.
  • DeadLetterConfig.Arn is honoured for SQS queues, which is the only dead-letter target AWS supports. The payload is sent to the queue after the final failed attempt.
  • A firing that cannot be delivered and has no dead-letter queue is logged at ERROR with the sink’s own message — it is never dropped silently.

• Not implemented:

  • FlexibleTimeWindow is stored and returned, but a schedule always fires at its exact due tick rather than being jittered across the window.
  • ScheduleExpressionTimezone is stored and returned, but cron(...) and at(...) are evaluated against the emulator’s own clock rather than the named zone.
  • KmsKeyArn is stored and returned, but nothing is encrypted with it — the emulator holds schedule data in plaintext, so no Decrypt call is ever made against the key.

Summary

Category	✅ Supported
Schedule Groups	4
Schedules	5
Tags	3

---

Endpoints

Schedule Groups

Operation	Status	Notes	AWS Docs
CreateScheduleGroup	✅ Supported	Creates a named group	docs
GetScheduleGroup	✅ Supported	Returns group metadata	docs
ListScheduleGroups	✅ Supported	Lists groups in region	docs
DeleteScheduleGroup	✅ Supported	Deletes group (except default)	docs

Schedules

Operation	Status	Notes	AWS Docs
CreateSchedule	✅ Supported	POST /schedules/{Name}; GroupName in the body, defaulting to default; rejects a target type Overcast cannot fire	docs
GetSchedule	✅ Supported	GET /schedules/{Name}; ?groupName selects the group	docs
UpdateSchedule	✅ Supported	PUT /schedules/{Name}; GroupName in the body; replaces the whole schedule, so an omitted member is unset; rejects a target type Overcast cannot fire	docs
DeleteSchedule	✅ Supported	DELETE /schedules/{Name}; ?groupName selects the group	docs
ListSchedules	✅ Supported	GET /schedules; optional ?ScheduleGroup filter	docs

Tags

Operation	Status	Notes	AWS Docs
TagResource	✅ Supported	Merges tags on ARN, at the shared /tags/{ResourceArn} path; refuses the keys and values AWS refuses	docs
UntagResource	✅ Supported	Removes ?TagKeys from ARN	docs
ListTagsForResource	✅ Supported	Returns the modeled TagList, ordered by key	docs