# Slurm REST API

Slurm provides a REST API daemon named slurmrestd. This daemon is designed to allow clients to communicate with Slurm via a REST API (in addition to the command line interface (CLI) or C API).

Please see OpenAPI Plugin Release Notes for changes between release versions.

To view the currently generated API reference: Slurm REST API Reference

## Prerequisites

slurmrestd requires additional libraries for compilation:

- HTTP Parser (>= v2.6.0)
- LibYAML (optional)
- JSON-C
- JWT Authentication (optional)

## Stateless

Slurmrestd is stateless as it does not cache or save any state between requests. Each request is handled in a thread and then all of that state is discarded. Any request to slurmrestd is completely synchronous with the Slurm controller (slurmctld or slurmdbd) and is only considered complete once the HTTP response code has been sent to the client. Slurmrestd will hold a client connection open while processing a request. Slurm database commands are committed at the end of every request, on the success of all API calls in the request.

Sites are strongly encouraged to setup a caching proxy between slurmrestd and clients to avoid having clients repeatedly call queries, causing usage to be higher than needed (and causing lock contention) on the controller.

## Run modes

Slurmrestd currently supports two run modes: inet service mode and listening mode.

### Inet Service Mode

The Slurmrestd daemon acts as an Inet Service treating STDIN and STDOUT as the client. This mode allows clients to use inetd, xinetd, or systemd socket activated services and avoid the need to run a daemon on the host at all times. This mode creates an instance for each client and does not support reusing the same instance for different clients.

### Listening Mode

The Slurmrestd daemon acts as a full UNIX service and continuously listens for new TCP connections. Each connection and request are independently authenticated.

## Configuration

slurmrestd can be configured either by environment variables or command line arguments. Please see the doc/man/man1/slurmrestd.8 man page for details.

## OpenAPI Specification (OAS)

Slurmrestd is compliant with OpenAPI 3.0.2 . The OAS can be viewed at the following URLs:

- /openapi.json
- /openapi.yaml
- /openapi/v3

The OAS is designed to be the primary documentation for the request methods. There are several third party tools that automatically generate documentation against the OAS output listed by openapi.tools.

NOTE: Slurm attempts to strictly comply with the relevant OpenAPI standards. For reasons of compatibility, Slurm may be tested against publicly available OpenAPI client generators, but Slurm does not directly support any of them as they are outside the control of SchedMD and may change at anytime. The goal is to comply with the standards, supporting as many clients as possible, without favoring any one client. Sites are always welcome to write their own clients that are OpenAPI compliant. As a rule, SchedMD will debug the HTTP sent to and received by slurmrestd but will not directly debug any client source code.

Tested compatibility by OpenAPI plugins:

- openapi/v0.0.35:
  - v4.x of OpenAPI-generator
- openapi/v0.0.36:
  - v4.x of OpenAPI-generator
- openapi/v0.0.37:
  - v4.x of OpenAPI-generator
  - v5.x of OpenAPI-generator
- openapi/dbv0.0.36:
  - v4.x of OpenAPI-generator
- openapi/dbv0.0.37:
  - v4.x of OpenAPI-generator
  - v5.x of OpenAPI-generator

## Plugins

As of Slurm 20.11, the REST API uses plugins for authentication and generating content. As of Slurm-21.08, the OpenAPI plugins are available outside of slurmrestd daemon and other slurm commands may provide or accept the latest version of the OpenAPI formatted output. This functionality is provided on a per command basis. These plugins can be optionally listed or selected via command line arguments as described in the slurmrestd documentation.

### Plugin life cycle

Plugins provided upstream with Slurm in any release are considered supported by that release. In general, only the latest versioned plugins will receive minor bug fixes but all will receive security fixes. Due to the nature of the plugins, one plugin can be supported across multiple Slurm releases to ensure (limited) backward client compatibility. SchedMD is currently only explicitly deprecating plugins per the OpenAPI standard method of setting "Deprecated: True" in the method path. Any plugin that is not marked as deprecated will continue to exist in the next Slurm Major release pending any critical issues which will be announced separately. Any plugin marked for deprecation will be removed on the next Slurm major release. In general, the last three plugins have been retained in each Slurm major release with the oldest getting marked as deprecated.

Sites are always encouraged to use the latest stable plugin version available for code development. There is no guarantee of compatibility between different versions of the same plugin with clients. Clients should always make sure to validate their code when migrating to newer versions of plugins. Plugin versions will always be included in the path for every method provided by a given plugin to ensure no two plugins will provide the same path.

As the plugins utilize the Slurm API internally, plugins that existed in previous versions of Slurm should continue to be able to communicate with the two previous versions of Slurm, similar to other components of Slurm. Newer plugins may have required RPC changes which will exclude them from working with previous Slurm versions. For instance, the openapi/dbv0.0.36 plugin will not be able to communicate with any slurmdbd servers prior to the slurm-20.11 release.

As with all other plugins in Slurm, sites are more than welcome to write their own plugins and are suggested to submit them as code contributions via bugzilla, tagged as a contribution. The plugin API provided may change between releases which could cause site specific plugins to break.

## Security

The Slurm REST API is written to provide the necessary functionality for clients to control Slurm using REST commands. It is not designed to be directly internet facing. Only unencrypted and uncompressed HTTP communications are supported. Slurmrestd also has no protection against man in the middle or replay attacks. Slurmrestd should only be placed in a trusted network that will communicate with a trusted client.

Any site wishing to expose Slurm REST API to the internet or outside of the cluster should at the very least use a proxy to wrap all communications with TLS v1.3 (or later). You should also add monitoring to reject any client who repeatedly attempts invalid logins at either the network perimeter firewall or at the TLS proxy. Any client filtering that can be done via a proxy is suggested to avoid common internet crawlers from talking to slurmrestd and wasting system resource or even causing higher latency for valid clients. Sites are recommended to use shorter lived JWT tokens for clients and renew often, possibly via non-Slurm JWT generator to avoid having to enforce JWT lifespan limits. It is also suggested that sites use an authenticating proxy to handle all client authentication against the sites preferred Single Sign On (SSO) provider instead of Slurm scontrol generated tokens. This will prevent any unauthenticated client from connecting to slurmrestd.

The Slurm REST API is an HTTP server and all general possible precautions for security of any web server should be applied. As these precautions are site specific, it is highly recommended that you work with your site's security group to ensure all policies are enforced at the proxy before connecting to slurmrestd.

Slurm tries not to give potential attackers any hints when there are authentication failures. This results in the client getting this rather terse message: `Authentication failure`. When this happens, take a look at the logs for the relevant Slurm daemon (i.e. slurmdbd, slurmctld or slurmd) for information about the actual issue.

## JSON Web Token (JWT) Authentication

slurmrestd supports using JWT to authenticate users. JWT can be used to authenticate user over REST protocol.

- User Name Header: X-SLURM-USER-NAME
- JWT Header: X-SLURM-USER-TOKEN

SlurmUser or root can provide alternative user names to act as a proxy for the given user. While using JWT authentication, slurmrestd should be run as a unique, unprivileged user and group. Slurmrestd should be provided an invalid SLURM_JWT environment variable at startup to activate JWT authentication. This will allow users to provide their own JWT tokens while authenticating to the proxy and ensuring against any possible accidental authorizations.

When using JWT, it is important that AuthAltTypes=auth/jwt be configured in your slurm.conf for slurmrestd.

## Local Authentication

slurmrestd supports using UNIX domain sockets to have the kernel authenticate local users. By default, slurmrestd will not start as root or SlurmUser or if the user's primary group belongs to root or SlurmUser. Slurmrestd must be located in the Munge security domain in order to function and communicate with Slurm in local authentication mode.

## Authenticating Proxy

There is a wide array of authentication systems that a site could choose from, if using JWT authentication doesn't meet your requirements. An authenticating proxy is setup with a JWT token assigned to the SlurmUser that can then be used to proxy for any user on the cluster. This ability is only allowed for SlurmUser and the root users, all other tokens will only work with their locally assigned users.

If using a third-party authenticating proxy, it is expected that it will provide the correct HTTP headers (X-SLURM-USER-NAME and X-SLURM-USER-TOKEN) to slurmrestd along with the user's request.

Slurm places no requirements on the authenticating proxy beyond its being HTTP 1.1 compliant and that it provides the correct HTTP headers to allow client authentication. Slurm will explicitly trust the HTTP headers provided and has no way to verify them (beyond the proxy's trusted token X-SLURM-USER-TOKEN). Any authenticating proxy will need to follow your site's security policies and ensure that the proxied requests come from the correct user. These requirements are standard to any authenticated proxy and are not Slurm specific.

A working trivial example can be found in an internal tool used for testing and training. It uses PHP and NGINX to provide the authentication logic.

Last modified 16 March 2023