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The Job Pack

The :term:`job pack` contains all the information needed for creating and running a Disco :term:`job`.

The first time any :term:`task` of a job executes on a Disco node, the job pack for the job is retrieved from the master, and :ref:`jobhome` is unzipped into a job-specific directory.

.. seealso:: The Python :class:`disco.job.JobPack` class.

File format:

+---------------- 4
| magic / version |
|---------------- 8 -------------- 12 ------------- 16 ------------- 20
| jobdict offset  | jobenvs offset | jobhome offset | jobdata offset |
|------------------------------------------------------------------ 128
|                           ... reserved ...                         |
|                               jobdict                              |
|                               jobenvs                              |
|                               jobhome                              |
|                               jobdata                              |

The current supported jobpack version is 0x0002. Limited support is provided for jobpacks of version 0x0001.

The Job Dict

The :term:`job dict` is a :term:`JSON` dictionary.

.. attribute:: jobdict.pipeline

   A list of tuples (tuples are lists in JSON), with each tuple
   specifying a stage in the pipeline in the following form::

       stage_name, grouping

   Stage names in a pipeline have to be unique.  ``grouping``
   should be one of :term:`split`, :term:`group_label`,
   :term:`group_all`, :term:`group_node` and

   .. seealso:: :ref:`pipeline`

.. attribute:: jobdict.input

   A list of inputs, with each input specified in a tuple of the
   following form::

       label, size_hint, url_location_1, url_location_2, ...

   The ``label`` and ``size_hint`` are specified as integers,
   while each ``url_location`` is a string.  The ``size_hint`` is
   a hint indicating the size of this input, and is only used to
   optimize scheduling.

.. attribute:: jobdict.worker

   The path to the :term:`worker` binary, relative to the
   :term:`job home`.  The master will execute this binary after it
   unpacks it from :ref:`jobhome`.

.. attribute:: jobdict.prefix

   String giving the prefix the master should use for assigning a
   unique job name.

   .. note:: Only characters in ``[a-zA-Z0-9_]`` are allowed in the prefix.

.. attribute:: jobdict.owner

   String name of the owner of the :term:`job`.

.. attribute:: jobdict.save_results

   Boolean that when set to true tells Disco to save the job
   results to DDFS.  The output of the job is then the DDFS tag
   name containing the job results.

   .. versionadded:: 0.5


The following applies to jobdict attributes in jobpack version 0x0001. Support for this version might be removed in a future release.

.. attribute:: jobdict.input

   A list of urls or a list of lists of urls.  Each url is a

   .. note::
          An inner list of urls gives replica urls for the same
          data.  This lets you specify redundant versions of an
          input file.  If a list of redundant inputs is specified,
          the scheduler chooses the input that is located on the
          node with the lowest load at the time of scheduling.
          Redundant inputs are tried one by one until the task
          succeeds.  Redundant inputs require that :attr:`map?` is

   .. note::
          In the pipeline model, the ``label`` associated with
          each of these inputs are all 0, and all inputs are
          assumed to have a ``size_hint`` of 0.

   .. deprecated:: 0.5

.. attribute::

   Boolean telling whether or not this job should have a
   :term:`map` phase.

   .. deprecated:: 0.5

.. attribute:: jobdict.reduce?

   Boolean telling whether or not this job should have a
   :term:`reduce` phase.

   .. deprecated:: 0.5

.. attribute:: jobdict.nr_reduces

   Non-negative integer that used to tell the master how many
   reduces to run.  Now, if the value is not 1, then the number of
   reduces actually run by the pipeline depends on the labels
   output by the tasks in the map stage.

   .. deprecated:: 0.5

.. attribute:: jobdict.scheduler

  * *max_cores* - use at most this many cores (applies
    to both map and reduce).  Default is ``2**31``.

  * *force_local* - always run task on the node where
    input data is located; never use HTTP to access
    data remotely.

  * *force_remote* - never run task on the node where
    input data is located; always use HTTP to access
    data remotely.

   .. versionadded:: 0.5.2

Job Environment Variables

A :term:`JSON` dictionary of environment variables (with string keys and string values). The master will set these in the environment before running the :attr:`jobdict.worker`.

The Job Home

The :term:`job home` directory serialized into :term:`ZIP` format. The master will unzip this before running the :attr:`jobdict.worker`. The :term:`worker` is run with this directory as its working directory.

In addition to the :term:`worker` executable, the :term:`job home` can be populated with files that are needed at runtime by the worker. These could either be shared libraries, helper scripts, or parameter data.


The .disco subdirectory of the job home is reserved by Disco.

The job home is shared by all tasks of the same job on the same node. That is, if the job requires two map task and two reduce task executions on a particular node, then the job home will be unpacked only once on that node, but the worker executable will be executed four times in the job home directory, and it is also possible for some of these executions to be concurrent. Thus, the worker should take care to use unique filenames as needed.

Additional Job Data

Arbitrary data included in the :term:`job pack`, used by the :term:`worker`. A running worker can access the job pack at the path specified by jobfile in the response to the :ref:`TASK` message.

Creating and submitting a Job Pack

The jobpack can be constructed and submitted using the disco job command.