32/Flux Job Execution Protocol Version 1

This specification describes Version 1 of the Flux Job Execution Protocol implemented by the job manager and job execution system.




Jim Garlick <garlick@llnl.gov>




The key words “MUST”, “MUST NOT”, “REQUIRED”, “SHALL”, “SHALL NOT”, “SHOULD”, “SHOULD NOT”, “RECOMMENDED”, “MAY”, and “OPTIONAL” in this document are to be interpreted as described in RFC 2119.


The job execution service launches Flux job shells on execution targets at the behest of the job manager. The job shells in turn launch one or more user processes that comprise the job. The job execution service thus acts as an intermediary between the job manager and a set of job shells.

RFC 16 describes the division of labor among Flux job services, and how they share information via the KVS Job Schema. The Flux Execution Protocol version 1 defines the minimal interactions between the job manager and the execution service needed to synchronize the concurrent progress of multiple jobs through their running phase.

As a reminder, the job execution service runs as the unprivileged Flux instance owner. In a multi-user Flux instance, it launches Flux job shells via the IMP as described in RFC 15, with the IMP managing the user transition after authenticating the signed job request from the KVS. The security aspects of launching a job as another user are not reflected in the job execution protocol.

Design Criteria

The job execution protocol must adhere to these criteria:

  • Maintain a clean separation of concerns between job manager, scheduler, execution service, and Flux job shell.

  • Avoid presenting obstacles to the scaling of job size, the number of jobs running concurrently, or job throughput.

  • Communicate job problems to the job manager, such that the job manager can use this information to raise job exceptions.

  • Support partial release of allocated resources to the scheduler, in case one or more execution targets cannot be expeditiously finalized.

  • Communicate high level job results to the job manager upon job completion.

  • Support execution service reload.

  • Support execution service override by the Flux simulator.


As with the scheduler RPCs described in RFC 27, <service>.start RPCs use the job ID to match requests and responses, and set the RFC 6 matchtag message field to zero. It follows that:

  • The job ID MUST appear in the <service>.start request and response message payloads.

  • There SHALL NOT be more than one <service>.start request in flight for each job, since otherwise a request could not be uniquely matched to a response using the job ID.

  • The errnum field in <service>.start response messages MUST be set to zero, even if the response indicates an error. Otherwise, the message payload could not include the job ID since RFC 6 defines the payload of an error response to be optional error text.

  • The job manager SHALL treat a conventional Flux error response to <service>.start with a nonzero errnum field as an execution service fatality, and SHALL not send further requests to the execution service until it receives a new job-manager.exec-hello request.

The other RPCs behave conventionally.


The Flux execution service SHALL register a service name with the job manager on initialization. This service MAY be job-exec or another name. The execution service calls the job-manager.exec-hello RPC whose request payload SHALL be a JSON object containing with one REQUIRED key:


(string) execution service name


  "service": "job-exec"

If an execution service is already loaded, the job manager SHALL allow the new one to override it.

The response payload SHALL be empty on success. The job manager SHALL issue a failure response if any jobs have an outstanding start request to an existing execution service. The execution service SHALL treat a failure response to exec-hello as fatal.

Start Request

Once the execution service is registered, the job manager SHALL send <service>.start requests for any jobs that have been allocated resources. Each start request begins a streaming RPC that remains active while the job is running. The request payload SHALL be a JSON object containing the following REQUIRED keys:


(integer) the job ID


(integer) the submitting userid


(object) jobspec object (RFC 14)


(boolean) Set to True if broker has been restarted and job should still be running.


  "id": 1552593348,
  "userid": 5588,
  "jobspec": {},
  "reattach": false,

The response payload SHALL be a JSON object containing the following REQUIRED keys:


(integer) the job ID, used by the job manager to match the response back to the request


(string) the type of response (see below)


(object) type-dependent data (see below)

There are four response types:


Indicates that the job shells have started. data is an empty object. Example:

  "id": 1552593348,
  "type": "start",
  "data": {},

Release R fragment to job-manager. data contains two keys: ranks (string), an idset representing subset of execution targets whose resources may be released; and final (boolean) a flag indicating whether all the job’s execution targets have now been released. Example:

  "id": 1552593348,
  "type": "release",
  "data": {
    "ranks": "0-2",
    "final": true,

Raise an exception on the job as described in RFC 21. data contains two required keys: severity (integer), the exception severity; and type (string), the exception type. A third key, note (string), is a human readable description of the exception which the job manager SHALL include in the exception context if present. Example:

  "id": 1552593348,
  "type": "exception",
  "data": {
    "severity": 0,
    "type": "timeout",
    "note": "resource allocation expired",

Job is complete. data contains one required key: status (integer), the numerically greatest wait status returned by the set of job shells. Example:

  "id": 1552593348,
  "type": "finish",
  "data": {
    "status": 143,

An exception response MAY be sent at any point. start and/or finish responses MAY be omitted depending on when a fatal exception occurs. The execution service MUST always send a release response with final set to True. The final release response SHALL be the last response sent by the execution service for a given job ID and is interpreted as “end of stream” by the job manager.