# PrEp Plugin API

## Overview

This document describes the Slurm PrEp — short for "Pr"olog and "Ep"ilog — plugins API. It is intended as a resource to programmers wishing to write their own Slurm prep plugins.

The purpose of the prep plugin APIs to provide a native C interface to the same hooks traditionally used by the Prolog, Epilog, PrologSlurmctld, and EpilogSlurmctld scripts. Those interfaces are now implemented through the prep/script plugin, and that plugin serves as a good example of how to approach development of additional plugins.

Slurm PrEp plugins must conform to the Slurm Plugin API with the following specifications:

const char plugin_name[]="full text name"

A free-formatted ASCII text string that identifies the plugin.

const char plugin_type[]="major/minor"

The major type must be "prep". The minor type can be any suitable name for the type of prep plugin.

const uint32_t plugin_version

If specified, identifies the version of Slurm used to build this plugin and any attempt to load the plugin from a different version of Slurm will result in an error. If not specified, then the plugin may be loaded by Slurm commands and daemons from any version, however this may result in difficult to diagnose failures due to changes in the arguments to plugin functions or changes in other Slurm functions used by the plugin.

Slurm can be configured to use multiple prep plugins if desired through the PrEpPlugins configuration option. Additional plugins should be comma-separated. Note that, by default, the prep/script plugin is loaded if that option is not set, but will not be loaded if an explict setting has been made. Thus, if you do set that option, and intend to still use the Prolog, Epilog, PrologSlurmctld, and/or EpilogSlurmctld options you will need to ensure both your additional plugin and prep/script are set.

Special care must be used when developing against the prep_p_prolog_slurmctld() or prep_p_epilog_slurmctld() interfaces. These functions are called while the slurmctld holds a number of internal locks, and need to return quickly otherwise slurmctld responsiveness and system throughput will be impacted. For simple logging, this is not required, and the "async" option can be left to false. But, especially for anything communicating with an external API or spawning additional processes, it is highly recommended to first make a local copy of any job record details required, and then spawn a separate processing thread — which, by default, will not have inherited any slurmctld locks — to continue processing. You must set the async return value to true, and call the corresponding prolog_slurmctld_callback() or epilog_slurmctld_callback() function before the thread exits. These callbacks are provided as function pointers when the slurmctld starts through prep_p_register_callbacks() call, and these function pointers should be cached locally in your plugin.

## API Functions

All of the following functions are required. Functions which are not implemented must be stubbed.

int init(void)

Description:
Called when the plugin is loaded, before any other functions are called. Put global initialization here.

Returns:
SLURM_SUCCESS on success, or
SLURM_ERROR on failure.

void fini(void)

Description:
Called when the plugin is removed. Clear any allocated storage here.

Returns: None.

Note: These init and fini functions are not the same as those described in the dlopen (3) system library. The C run-time system co-opts those symbols for its own initialization. The system _init() is called before the Slurm init(), and the Slurm fini() is called before the system's _fini().

void prep_p_register_callbacks(prep_callbacks_t *callbacks)

Description:
This function is called by the slurmctld to pass function pointer addresses used with asynchronous operation with the prep_p_prolog_slurmctld() and prep_p_epilog_slurmctld() interfaces. These pointers must be saved if asynchronous operation is used, otherwise this function can be an empty stub.

Arguments:
callbacks (input) contains function pointers for use with asynchronous operation within the slurmctld

Returns: None.

int prep_p_prolog(job_env_t *job_env, slurm_cred_t *cred)

Description:
Called within the slurmd as root before the first step of a job starts on the compute node.

Arguments:
job_env (input) details from the step launch request
cred (input) launch credential with additional verifiable launch details signed by the slurmctld

Returns:
SLURM_SUCCESS on success, or
SLURM_ERROR on failure, will cause job failure.

int prep_p_epilog(job_env_t *job_env, slurm_cred_t *cred)

Description:
Called within the slurmd as root after all job steps have completed.

Arguments:
job_env (input) details from the step launch request
cred (input) launch credential with additional verifiable launch details signed by the slurmctld

Returns:
SLURM_SUCCESS on success, or
SLURM_ERROR on failure, will cause job failure.

int prep_p_prolog_slurmctld(job_record_t *job_ptr, bool *async)

Description:
Called within the slurmctld before a job launches.

Arguments:
job_ptr (input) raw job record
async (output) set to true if this interface has spawned a separate processing thread that must complete before the job starts execution

Returns:
SLURM_SUCCESS on success, or
SLURM_ERROR on failure, will cause job failure.

int prep_p_epilog_slurmctld(job_record_t *job_ptr, bool *async)

Description:
Called within the slurmctld before a job launches.

Arguments:
job_ptr (input) raw job record
async (output) set to true if this interface has spawned a separate processing thread that must complete before the job is marked complete

Returns:
SLURM_SUCCESS on success, or
SLURM_ERROR on failure, will cause job failure.

Last modified 22 April 2020