Batch Processing

pScheduler can be used to run a series of tasks, called a batch, from the command line and return the results for further processing by other programs.

Each task is referred to as a job.

Jobs can be done singly or in multiple iterations with the latter being possible serially or in parallel.

Invocation

The batch processor is started using a pscheduler command:

pscheduler batch [ OPTIONS ] [ INPUT-FILE ]

The OPTIONS control the batch processor’s behavior and can be listed using the --help option.

The INPUT-FILE is a path to the input. If not provided or is -, input will be taken from the standard input.

The final result (see Output) is sent to the standard output.

Error and diagnostic output will be sent to standard error.

Input

Input to the batch processor is JSON in the form of a single object containing three pairs, global, jobs and, optionally, a schema.

The schema describes which version of the batch specification is in use. If none is provided, it defaults to 1. Specific instances where other values are required are described below.

The global Pair

The global pair is an optional JSON object containing data and transforms provided or applied to all jobs. It contains the following pairs, all optional:

data (Any JSON) - Data made available to all jq transforms as a variable named $global.

transform-pre (pScheduler jq Transform) - A transform applied to the task object in each job before anything else is done.

transform-post (pScheduler jq Transform) - A transform applied to the task object in each job after transform-pre and the job’s task-transform have been applied.

The jobs Pair

The jobs pair is an array of objects, each containing a single job:

"jobs": [
    { ... Job 1 .. },
    { ... Job 2 .. },
    { ... Job n .. }
]

A job is described in a JSON object containing the following pairs:

label (String) - A label for the job, used for reference in debugging output.

enabled (Boolean) - Determines whether or not the job is run. Defaults to true.

iterations (Number or JQ Transform) - The number of times to run the specified task. If a JQ transform is provided, the script should calculate and return the number of iterations as an integer and the schema should be set to at least 3. Input to the transform is null; any external data required should be accessed via $global.

parallel (Boolean) - Whether or not the job’s iterations should be run in parallel. This defaults to false and implies sync-start (see below) unless sync-start is explicitly set false.

setup-time (Boolean) - The amount of time expected for pScheduler to set up a single run. The default of PT15S should be more than sufficient in most cases. This is ignored if not doing a synchronized start (see sync-start, below).

backoff (String) - ISO8601 duration indicating how long each iteration run in parallel waits before being submitted to pScheduler. The first will have no backoff, the second will have the indicated backoff, the third will have twice that, etc. This value is ignored if parallel is false.

sync-start (Boolean) - If running in parallel, set the start time of all iterations to be the same. The time is based on the number of times the task is run, backoff and setup-time. This value is ignored if parallel is false. Note that tasks subject to restrictions on being run at the same time will not necessarily start in sync (or at all if no slip is allowed as part of the task’s schedule section.

task (Object or Array of Objects) - A pScheduler task specification as would be produced using the task command’s --export switch. Note that if the specification contains a schedule, those parameters will be ignored. If the value is an array, the contents of the array will be treated as a set of tasks and the number of iterations will be set to its length. This overrides the contents of iterations. Also note that a minimal batch containing a single task can be exported using the --export and --export-format batch switches.

task-transform - A jq transform that operates on the task’s value for each iteration to make iteration-specific changes. The $iteration variable is provided to indicate which iteration (starting with 0) is being transformed. The script should operate on the input in place.

For example, this job will run five sequential rtt tests to www.perfsonar.net with 5, 10, 15, 20 and 25 pings sent. The task-transform adds a count to the test specification that is calculated based on the iteration:

{
    "label": "rtt",
    "iterations": 5,
    "parallel": false,
    "task": {
        "test": {
            "type": "rtt",
            "spec": {
                "schema": 1,
                "dest": "www.perfsonar.net"
            }
        }
    },
    "task-transform": {
        "script": [
            ".test.spec.count = ($iteration + 1) * 5"
        ]
    }
}

continue-if - A jq transform that determines, based on the results of a job, whether the batch processor should continue to the next job or abort the batch. The input given to the transform is the same as the value of the results pair in the output as described below. For example:

[
  {
    "task": { ... },
    "runs": [
      {
        "application/json": {
          "schema": 1,
          "duration": "PT2S",
          "succeeded": true
        },
        "text/plain": " ... ",
        "text/html": " ... "
      }
    ]
  }
]

The transform should return `true for the batch to continue with the next job or false to abort the batch without processing any subsequent jobs. Any other value is treated as an error and the batch will be aborted with no results.

A continue-if that decides whether to continue based on the success or failure of the first run in a job would look like this:

{
    "task": { ... },
    "continue-if": {
        "script": ".[0].runs[0].\"application/json\".succeeded"
    }
 }

Output

Once all jobs have been completed, the batch processor will output a copy of the input with the addition of a results pair in each job containing information about what tasks were run and the results they produced.

The results pair is an array of JSON objects, with one element per iteration. Each object contains the following pairs:

task (pScheduler Task Specification) - The task that was submitted to pScheduler and run.

runs (Array of pScheduler Results) - An array of the results produced by the task. In most cases, there will be a single element, but for tasks that return multiple results (e.g., latencybg), there will be more than one. Each result is a JSON object containing pairs named application/json, text/plain and text/html for each of the formats in which pScheduler can produce a result.

The batch pair includes diagnostic information about the batch.

Invocation from Python

The batch processor can be invoked from Python on any system where pScheduler’s Python library is installed. (On CentOS, this would be the python-pscheduler package.)

For example:

#!/usr/bin/env python3

import pscheduler.batchprocessor
import sys


batch = { ... }

def debug(message):
    """
    Callback function for the batch processor to emit a line of
    debug.
    """
    print(message, file=sys.stderr)

processor = pscheduler.batchprocessor.BatchProcessor(batch)

# Leave out the debug argument for no debugging.
# This can be invoked multiple times to run the same batch repeatedly.
result = processor(debug=debug)

Tips and Tricks

Running Different Tasks as Part of the Same Job

Different tests can be run in parallel by using the task-transform to alter the contents of the test pair for each iteration.

• Put an array of the tests to be run in the task’s reference pair. The length of the array should be the same as the specified iterations.

• Leave the task’s test section as an empty object ({}).

• Add a task-transform that replaces the test with an element from the array (e.g., .test = .reference.tests[$iteration]).

This example runs a three-minute-long streaming latency test with a throughput test to the same host during the second minute. The backoff value makes the througput test sleep for one minute before it is scheduled and started so there’s latency data produced beforehand and afterward.:

{
    "label": "different-in-parallel",
    "iterations": 2,
    "parallel": true,
    "backoff": "PT1M",
    "task": {
        "reference": {
            "tests": [
                {
                    "type": "latencybg",
                    "spec": {
                        "dest": "ps.example.net",
                        "duration": "PT3M"
                    }
                },
                {
                    "type": "throughput",
                    "spec": {
                        "dest": "ps.example.net",
                        "duration": "PT1M"
                    }
                }
            ]

        },
        "#": "This is intentionally empty:",
        "test": { }
    },
    "task-transform": {
        "script": [
            "# Replace the test section of the task with one of the",
            "# tests in the reference block based on the iteration.",
            ".test = .reference.tests[$iteration]"
        ]
    }
}