Adding an executor

This tutorial will write an executor for PBSPro using the PSI/J batch scheduler executor interface.

It should be useful when writing an executor for any HPC style scheduler that looks like SLURM or PBSPro.

What Is an Executor and Why Might You Want to Add One?

PSI/J provides a common interface for obtaining allocations on compute resources. Usually, those compute resources will already have a batch scheduler in place (for example, SLURM).

A PSI/J executor is the code that tells the core of PSI/J how to interact with a batch scheduler so that it can provide a common interface to applications.

A PSI/J executor needs to implement the abstract methods defined on the psij.job_executor.JobExecutor base class. The documentation for that class has reference material for each of the methods that won’t be repeated here.

For batch scheduler systems, the BatchSchedulerExecutor subclass provides further useful structure to help implement JobExecutor. This tutorial will focus on using BatchSchedulerExecutor as a base, rather than implementing JobExecutor directly.

The batch scheduler executor is based on a model where interactions with a local resource manager happen via command line invocations. For example, with PBS qsub and qstat commands are used to submit a request and to see status.

To use BatchSchedulerExecutor for a new local resource manager that uses this command line interface, subclass BatchSchedulerExecutor and add in code that understands how to form the command lines necessary to submit a request for an allocation and to get allocation status. This tutorial will do that for PBSPro.

Adding an Executor

First set up a directory structure:

mkdir project/
cd project/
mkdir psijpbs
mkdir psij-descriptors

We’re going to create three source files in this directory structure:

  • psijpbs/ - This will contain the bulk of the code.

  • psijpbs/pbspro.mustace - This will contain a template for a PBS Pro job submission file.

  • psij-descriptors/ - This file tells the PSI/J core what this package implements.


  • You have the psij-python package installed and are able to run whatever basic verification you think is necessary.

  • You are able to submit to PBS Pro on a local system.

First, we’ll build a skeleton that won’t work, and see that it doesn’t work in the test suite. Then we’ll build up to the full functionality.

A Not-implemented Stub

Add the project directory to the Python path directory:


Create a simple BatchSchedulerExecutor subclass that does nothing new in psijpbs/

from psij.executors.batch.batch_scheduler_executor import BatchSchedulerExecutor

class PBSProJobExecutor(BatchSchedulerExecutor):

and create a descriptor file to tell PSI/J about this, psij-descriptors/

from packaging.version import Version

from psij._descriptor import _Descriptor

__PSI_J_EXECUTORS__ = [_Descriptor(name='pbspro', version=Version('0.0.1'),

Now, run the test suite. It should fail with an error reporting that the resource manager specific methods of BatchSchedulerExecutor have not been implemented:

PYTHONPATH=$PWD/src:../wheverever/project/ pytest 'tests/' --executors=pbspro
TypeError: Can't instantiate abstract class PBSProJobExecutor with abstract methods generate_submit_script, get_cancel_command, get_status_command, get_submit_command, job_id_from_submit_output, parse_status_output, process_cancel_command_output

That error message tells us what we need to implement. There are three broad pieces of functionality:

  • Submitting a job:

  • Requesting job status:

  • Cancelling a job:


Let’s implement all of these with stubs that return a NotImplementedError that we will then flesh out:

class PBSProJobExecutor(BatchSchedulerExecutor):

 def generate_submit_script(*args, **kwargs):
      raise NotImplementedError

  def get_submit_command(*args, **kwargs):
      raise NotImplementedError

  def job_id_from_submit_output(*args, **kwargs):
      raise NotImplementedError

  def get_cancel_command(*args, **kwargs):
      raise NotImplementedError

  def process_cancel_command_output(*args, **kwargs):
      raise NotImplementedError

  def get_status_command(*args, **kwargs):
      raise NotImplementedError

  def parse_status_output(*args, **kwargs):
      raise NotImplementedError

Now running the same pytest command will give a different error further along into attempting to submit a job:

>       assert config
E       AssertionError

This default BatchSchedulerExecutor code needs a configuration object and none was supplied.

A configuration object can contain configuration specific to this particular executor. For now we are not going to specify a custom configuration object and instead will reuse the BatchSchedulerExecutorConfig supplied by the PSI/J core.

Define a new __init__ method that will define a default configuration:

from psij.executors.batch.batch_scheduler_executor import BatchSchedulerExecutorConfig


  def __init__(self, *, url, config):
      if config is None:
          config = BatchSchedulerExecutorConfig()
      super().__init__(url=url, config=config)

The URL parameter can be ignored entirely for batch scheduler systems.

Running pytest again, we get as far as seeing PSI/J is trying to do submit-related stuff:

   def generate_submit_script(*args, **kwargs):
>       raise NotImplementedError
E       NotImplementedError

../tutorial-play/psijpbs/ NotImplementedError

Implementing Job Submission

To implement submission, we need to implement three methods:

You can read the docstrings for each of these methods for more information, but briefly the submission process is:

  1. generate_submit_script generates a submit script specific to the batch scheduler.

  2. get_submit_command returns the command line necessary to submit that script to the batch scheduler.

The output of that command should be interpreted by job_id_from_submit_output to extract a batch scheduler specific job ID, which can be used later when cancelling a job or getting job status.

In line with other PSI/J executors, we’re going to delegate script generation to a template based helper. So add a line to initialize a TemplatedScriptGenerator in the executor initializer, pointing at a (as yet non-existent) template file, and replace generate_submit_script with a delegated call to TemplatedScriptGenerator:

from pathlib import Path
from psij import Job
from typing import Dict, TextIO
from psij.executors.batch.script_generator import TemplatedScriptGenerator

class PBSProJobExecutor(BatchSchedulerExecutor):


    def __init__(self, url: Optional[str] = None, config: Optional[PBSProExecutorConfig] = None):
        if not config:
            config = BatchSchedulerExecutorConfig()
        super().__init__(url=url, config=config)
        self.generator = TemplatedScriptGenerator(config, Path(__file__).parent / 'pbspro.mustache')

    def generate_submit_script(self, job: Job, context: Dict[str, object],
                               submit_file: TextIO) -> None:
        self.generator.generate_submit_script(job, context, submit_file)

Now the test will fail with an error like this, because that template doesn’t exist:

E       FileNotFoundError: [Errno 2] No such file or directory: '/home/you/project/psijpbs/pbspro.mustache'

Here is a very simple template for PBS, lacking a lot of features. A full template should use all of the fields in the supplied psij.Job object, as well as any executor-specific parameters supplied in the config parameter to __init__:


#PBS -N="{{.}}"


#PBS -l walltime={{.}}
#PBS --{{key}}="{{value}}"


#PBS -e /dev/null
#PBS -o /dev/null

cd "{{.}}"

exec &>> "{{psij.script_dir}}/$PBS_JOBID.out"

{{#psij.launch_command}}{{.}} {{/psij.launch_command}}

{{!we redirect to a file tied to the native ID so that we can reach the file with attach().}}
echo "$?" > "{{psij.script_dir}}/$"

Next, the test will fail because get_submit_command is missing. This method is going to give a command line to run to submit the tempate-generated submit file. In PBS, that submission happens by running a command like this:

> qsub c.submit

Here’s an implementation of get_submit_command that will make such a command:

from typing import List

def get_submit_command(self, job: Job, submit_file_path: Path) -> List[str]:
    return ['qsub', str(submit_file_path.absolute())]

The implementation so far is enough to get jobs to run in PBS, but not enough for PSI/J to make sense of what it has submitted.

The final step in submission is implementing job_id_from_submit_output. This interprets the output of the submit command to find the batch schedulers’s job ID for the newly created job.

In the PBS Pro case, as shown in the example above, that is pretty straightforward. The entire output is the job ID:

def job_id_from_submit_output(self, out: str) -> str:
    return out.strip()

That’s enough to get jobs submitted using PSI/J, but not enough to run the test suite. Instead, the test suite will appear to hang, because the PSI/J core code gets a bit upset by status monitoring methods raising a NotImplementedError.

Implementing Status

PSI/J needs to ask the batch scheduler for the status of jobs that it has submitted. This can be done with BatchSchedulerExecutor by overriding these two methods, which we stubbed out as not-implemented earlier on:

Here’s an implementation for get_status_command:

from typing import Collection

def get_status_command(self, native_ids: Collection[str]) -> List[str]:
    ids = ','.join(native_ids)
    return ['qstat',  '-f', '-F', 'json', '-x'] + list(native_ids)

This constructs a command line which looks something like this:

qstat -f -F json -x

The parameters change the default behavior of qstat to something more useful for parsing: -f asks for full output, with -x including information for completed jobs (which is normally suppressed) and -F json asking for the output to be formatted as JSON (rather than a default text tabular view).

This JSON output, which is passed to parse_status_output looks something like this (with a lot of detail removed):

           "comment":"Job run at Mon Jan 24 at 08:39 on (edtb-01[0]:ncpus=1) and finished",

Here is an implementation for parse_status_output, as well as a helper dictionary _STATE_MAP:

import json
from psij import JobState, JobStatus
from psij.executors.batch.batch_scheduler_executor import check_status_exit_code

    'Q': JobState.QUEUED,
    'R': JobState.ACTIVE,
    'F': JobState.COMPLETED

class PBSProJobExecutor: ...

    def parse_status_output(self, exit_code: int, out: str) -> Dict[str, JobStatus]:
        check_status_exit_code(_QSTAT_COMMAND, exit_code, out)
        r = {}

        report = json.loads(out)
        jobs = report['Jobs']
        for native_id in jobs:
            native_state = jobs[native_id]["job_state"]
            state = _STATE_MAP(native_state)

            msg = jobs[native_id]["comment"]
            r[native_id] = JobStatus(state, message=msg)

        return r

parse_status_output is given both the stdout and the exit code of qstat and must either transcribe that into a dictionary of psij.JobStatus objects describing the state of each job, or raise an exception.

This implementation uses a helper, psij.executors.batch.batch_scheduler_executor.check_status_exit_code(), which will raise an exception if qstat exited with a non-zero exit code. Then, it assumes that the qstat output is JSON and deserialises, and for each job in the JSON, it uses two fields to create a psij.JobStatus object: a human readable message is taken from the PBS comment field, and a machine readable status is converted from a single letter PBS status (such as F for finished, or Q for queued) into a PSI/J psij.JobState via the _STATE_MAP dictionary.

With these status methods in place, the pytest command from before should execute to completion.

We still haven’t implemented the cancel methods, though. That will be revealed by running a broader range of tests:

PYTHONPATH=$PWD/src:$PYTHONPATH pytest 'tests' --executors=pbspro

which should give this error (amongst others—this commandline formation is ugly and I’d like it to work more along the lines of make test):

FAILED tests/[pbspro] - NotImplementedError

Implementing Cancel

The two methods to implement for cancellation follow the same pattern as for submission and status:

It looks like you don’t actually need to implement process_cancel_command_output beyond the stub we already have, to make the abstract class mechanism happy. Maybe that’s something that should change in psi/j?

Here’s an implementation of get_cancel_command:

def get_cancel_command(self, native_id: str) -> List[str]:
    return ['qdel', native_id]

That’s enough to tell PBS Pro how to cancel a job, but it isn’t enough for PSI/J to know that a job was actually cancelled: the JobState from parse_status_output will still return a state of COMPLETED, when we actually want CANCELED. That’s because the existing job marks a job as COMPLETED whenever it reaches PBS Pro state F—no matter how the job finished.

So here’s an updated parse_status_output which checks the Exit_status field in the qstat JSON to see if it exited with status code 265—that means that the job was killed with signal 9. and if so, marks the job as CANCELED instead of COMPLETED:

def parse_status_output(self, exit_code: int, out: str) -> Dict[str, JobStatus]:
    check_status_exit_code('qstat', exit_code, out)
    r = {}

    report = json.loads(out)
    jobs = report['Jobs']

    for native_id in jobs:
        job_report = jobs[native_id]
        native_state = job_report["job_state"]
        state = _STATE_MAP[native_state]

        if state == JobState.COMPLETED:
            if 'Exit_status' in job_report and job_report['Exit_status'] == 265:
                state = JobState.CANCELED

        msg = job_report["comment"]
        r[native_id] = JobStatus(state, message=msg)

    return r

This isn’t necessarily the right thing to do: some PBS installs will use 128+9 = 137 to represent this instead of 256 + 9 = 265, according to the PBS documentation.

What’s Missing?

The biggest thing that was omitted was in the mustache template. A psij.Job object contains lots of options which could be transcribed into the template (otherwise they will be ignored). Have a look at the docstrings for Job and at other templates in the PSI/J source code for examples.

The _STATE_MAP given here is also not exhaustive: if PBS Pro qstat returns a different state for a job than what is in it, this will break. So make sure you deal with all the states of your batch scheduler, not just a few that seem obvious.

How to Distribute Your Executor

If you want to share your executor with others:

  1. You can make a Python package and distribute that as an add-on without needing to interact with the PSI/J project.

  2. You can make a pull request against the PSI/J repo.