.. _quick_start_guide:

Users' Quick Start Guide
========================

`HTCondor <https://htcondor.org>`_ is a system for dynamically sharing
computational resources between competing computational tasks.  As an
HTCondor user, you will describe your computational tasks as a series
of independent, asynchronous "jobs."  You access computational resources
managed by HTCondor by submitting (or "placing") job descriptions at an
HTCondor "access point" (AP), also known as a "submit node."  HTCondor
locates an appropriate machine for each job,
packages up the job and ships it off to that machine for execution.
Machines providing resources to HTCondor are therefore known as execution
points (EP).

This guide covers submitting and observing the successful completion
of a first, example job.  It then suggests extensions that you can apply to
your own jobs.

This guide presumes that

* HTCondor is running
* You have access to a machine within the pool that may submit jobs,
  termed an Access Point (AP).
* You are logged in to and working on the AP.  (If you
  just finished :ref:`getting HTCondor <getting_htcondor>`, the one machine
  you just installed is this AP.)
* Your program executable, your submit description file, and any needed
  input files are all on the file system of the AP.
* Your job (the program executable) is able to run without any
  interactive input.  Standard input (from the keyboard), standard output
  (seen on the display), and standard error (seen on the display) may still
  be used, but their contents will be redirected from/to files.

What is a Job?
--------------

"Job" is a very specific term in HTCondor. A job is the atomic unit of work.
A job may use multiple cores on one machine, but one job may not (in general)
run across more than one machine.  To effectively use HTCondor, you will
need to divide your total work (often called a workflow) into a number
of jobs.  These atomic units of work run asynchronously with respect to each other, but 
may be connected by input and output files.  Each job is described by a
Job ClassAd, which is usually created by the system from a submit description file.
HTCondor is a High Throughput system, which means it has been designed to 
effectively manage hundreds of thousands of jobs.  Attributes of jobs that
must be defined include the executable or script to run, the amount of memory, CPU
and other machine resources it needs, and descriptions of the file inputs it need.
The set of files used by a job is called the "sandbox".  There is an input sandbox,
the input files that exist before a job starts; the output sandbox, the set of files
created by the job; and a scratch sandbox, the set of files made as the job runs.


A First HTCondor Job
--------------------

For HTCondor to run a job, it must be given details such as the names
and location of the executable and all needed input files. These details
are specified in a submit description file.

.. rubric:: The executable

Before presenting the details of the submit description file, consider this
first HTCondor job.  It is a sleep job that waits for 6 seconds and then
exits.  While most aspects of HTCondor are identical on Linux (or Mac) and
Windows machines, awareness of the AP's operating system will lead
to a better understanding of jobs and job submission.

This first executable program is a shell script (Linux or Mac) or batch file
(Windows).  The file that represents this differs based on operating
system; the Linux (or Mac) version is shown first, and
the Windows version is shown second.  To try this example,
log in to the AP, and use an editor to type in or copy and paste
the file contents.  Name the resulting file ``sleep.sh`` if the AP 
is Linux (or Mac) operating system, and name the resulting file ``sleep.bat``
if the AP is running Windows.  Note that you will need to
know whether the operating system on your AP is a Linux (or Mac)
operating system or Windows.

.. code-block:: shell
    :caption: Linux (or Mac) executable, a shell script

    #!/bin/bash
    # file name: sleep.sh

    TIMETOWAIT="6"
    echo "sleeping for $TIMETOWAIT seconds"
    /bin/sleep $TIMETOWAIT

.. code-block:: bat
    :caption: Windows executable, a batch file

    :: file name: sleep.bat
    @echo off

    set TIMETOWAIT=6
    echo sleeping for %TIMETOWAIT% seconds
    choice /D Y /T %TIMETOWAIT% > NUL

For a Linux (or Mac) AP only, change the ``sleep.sh`` file to be
executable by running the following command:

.. code-block:: shell

    chmod u+x sleep.sh

.. rubric:: The contents of the submit description file

The submit description file describes the job.  To submit this sample
job, again use an editor to create the file ``sleep.sub``.  The submit
description file contents for this job differs on Linux (or Mac) and Windows
machines only in the name of the script or batch file:

.. code-block:: condor-submit
    :caption: Linux (and Mac) submit description file

    # sleep.sub -- simple sleep job

    executable              = sleep.sh

    log                     = sleep.log
    output                  = sleep.out
    error                   = sleep.err 

    should_transfer_files   = Yes
    when_to_transfer_output = ON_EXIT

    request_cpus            = 1
    request_memory          = 512M
    request_disk            = 1G

    queue

.. code-block:: condor-submit
    :caption: Windows submit description file

    # sleep.sub -- simple sleep job

    executable              = sleep.bat

    log                     = sleep.log
    output                  = sleep.out
    error                   = sleep.err

    should_transfer_files   = Yes
    when_to_transfer_output = ON_EXIT

    request_cpus            = 1
    request_memory          = 512M
    request_disk            = 1G

    queue

The first line of this submit description file is a comment.  Comments
begin with the # character.  Comments do not span lines.

Each line of the submit description file has the form

.. code-block:: condor-submit

    command_name = value

The command name is case insensitive and precedes an equals sign.  Values
to right of the equals sign are likely to be case sensitive, especially
in the case that they specify paths and file names.

Next in this file is a specification of the ``executable`` to run.  It
specifies the program that becomes the HTCondor job.  For this example, it
is the file name of the Linux (or Mac) script or Windows batch file. A  full
path and executable name, or a path and executable relative to the current
working directory may be specified.

The ``log`` command causes a job event log file named ``sleep.log`` to be
created on the AP once the job is submitted.  A log is not
necessary, but it can be incredibly useful in figuring out what happened or
is happening with a job.

HTCondor must be told how many resources your job needs on an Execution
Point in order to run.  This allows HTCondor to run as many jobs as
possible on each EP without overloading them.  Jobs must declare the
number of CPUs, the amount of memory and disk they need.  Special jobs
may need to request other resources, such as GPUs or licenses.  Ask your
administrator if your jobs requires such things.  The amount of cpus
is unit less, but memory and disk requires can have a "M" for megabyte,
"G" for Gigabyte suffix for legibility.  Without the suffix, memory
units are megabytes and disk kilobytes.

.. code-block:: condor-submit

    request_cpus            = 1
    request_memory          = 512M
    request_disk            = 1G


If this script/batch file were to be invoked from the command line, and
outside of HTCondor, its single line of output

.. code-block:: text

    sleeping for 6 seconds

would be sent to standard output (the display).  When submitted as an HTCondor
job, standard output of the job is on that EP, and thus unavailable.  HTCondor
captures standard output in a file due to the :subcom:`output` command in the submit
description file.  This example names the redirected standard output file
``sleep.out``, and this file is returned to the AP when the job completes.  The
same structure is specified for standard error, as specified with the :subcom:`error`
command.

The commands

.. code-block:: condor-submit

    should_transfer_files   = Yes
    when_to_transfer_output = ON_EXIT

direct HTCondor to explicitly send the needed files, including the executable,
to the machine where the job executes.  These commands will likely not be
necessary for jobs in which the AP and the EP (the Execution Point, or worker
node) access a shared file system.  However, including these commands
will allow this first sample job to work under a large variety of pool
configurations.

The :subcom:`queue` command tells HTCondor to run one instance of this job.

.. rubric:: Submitting the job

With this submit description file, all that remains is to hand off the job to
HTCondor.  Note that the :subcom:`queue` command should be the last command in the
file.  Commands after the :subcom:`queue` are  ignored.  Otherwise, the order of
commands with the file does not matter. Assuming the current working directory
contains the ``sleep.sub`` submit description file and the executable
(``sleep.sh`` or ``sleep.bat``), the command line

.. code-block:: shell

    condor_submit sleep.sub

submits the job to the AP. If the submission is successful, the terminal will
display a response that identifies the job, of the form

.. code-block:: console

    Submitting job(s).
    1 job(s) submitted to cluster 6.

.. rubric:: Monitoring the job

Once the job has been submitted, command line tools may help you follow along
with the progress of the job.  The ``condor_q`` command prints a listing of
all your jobs currently in the queue.  For example, a short time after Kris
submits the sleep job from a Linux (or Mac) AP on a pool that has
no other queued jobs, the output may appear as

.. code-block:: console

    $ condor_q
    -- Submitter: example.wisc.edu : <192.0.2.44:56550> : example.wisc.edu
     ID      OWNER            SUBMITTED     RUN_TIME ST PRI SIZE CMD
        6.0   kris            2/13 10:49   0+00:00:03 R  0   97.7 sleep.sh

    1 jobs; 0 completed, 0 removed, 0 idle, 1 running, 0 held, 0 suspended

The first column of output from ``condor_q`` identifies the job; the
identifier is composed of two integers separated by a period.  The first
integer is known as a cluster number, and it will be the same for each of
the potentially many jobs submitted by a single invocation of
``condor_submit``.  The second integer in the identifier is known as a
process ID, and it distinguishes between distinct job instances that have
the same cluster number.  These values start at 0.

Of interest in this output, the job is running, and it has used 3 seconds
of time so far.

At job completion, the log file contains

.. code-block:: text

    000 (006.000.000) 02/13 10:49:04 Job submitted from host: <192.0.2.44:46062>
    ...
    001 (006.000.000) 02/13 10:49:24 Job executing on host: <192.0.2.5:43051?PrivNet=cs.wisc.edu>
    ...
    006 (006.000.000) 02/13 10:49:30 Image size of job updated: 100000
            0  -  MemoryUsage of job (MB)
            0  -  ResidentSetSize of job (KB)
    ...
    005 (006.000.000) 02/13 10:49:31 Job terminated.
            (1) Normal termination (return value 0)
                    Usr 0 00:00:00, Sys 0 00:00:00  -  Run Remote Usage
                    Usr 0 00:00:00, Sys 0 00:00:00  -  Run Local Usage
                    Usr 0 00:00:00, Sys 0 00:00:00  -  Total Remote Usage
                    Usr 0 00:00:00, Sys 0 00:00:00  -  Total Local Usage
            23  -  Run Bytes Sent By Job
            113  -  Run Bytes Received By Job
            23  -  Total Bytes Sent By Job
            113  -  Total Bytes Received By Job
            Partitionable Resources :    Usage  Request Allocated
               Cpus                 :                 1         1
               Disk (KB)            :   100000   100000   2033496
               Memory (MB)          :        0       98      2001
    ...

Each event in the job event log file is separated by a line containing three
periods.  For each event, the first 3-digit value is an event number.

.. rubric:: Removing a job

Successfully submitted jobs will occasionally need to be removed from the
queue.  The ``condor_rm`` command with the job identifier as a command line
argument removes jobs.  Kris' job may be removed from the queue with

.. code-block:: shell

    condor_rm 6.0

Specification of the cluster number only as with the command

.. code-block:: shell

    condor_rm 6

will cause all jobs within that cluster to be removed.

The science Job Example
-----------------------

A second example job illustrates aspects of file specification for the
job.  Assume that the program executable is called ``science.exe``.  This
program does not use standard input or output; instead, the command line
to invoke this program specifies two input files and one output file.  For
this example, the command line to invoke ``science.exe`` (not as an HTCondor
job) will be

.. code-block:: shell

    science.exe infile-A.txt infile-B.txt outfile.txt

While the name of the executable is specified in the submit description file
with the :subcom:`executable` command, the remainder of the command line will be
specified with the :subcom:`arguments` command.

Here is the submit description file for this job:

.. code-block:: condor-submit

    # science1.sub -- run one instance of science.exe
    executable              = science.exe
    arguments               = "infile-A.txt infile-B.txt outfile.txt"

    transfer_input_files    = infile-A.txt,infile-B.txt
    should_transfer_files   = IF_NEEDED
    when_to_transfer_output = ON_EXIT

    request_cpus            = 1
    request_memory          = 512M
    request_disk            = 1G

    max_retries             = 2
    log                     = science1.log
    queue

The input files ``infile-A.txt`` and ``infile-B.txt`` will need to be
available on the Execution Point within the pool where the job
runs.  HTCondor cannot interpret command line arguments, so it cannot know
that these command line arguments for this job specify input and output
files.  The submit command :subcom:`transfer_input_files` instructs HTCondor to
transfer these input files from the machine where the job is submitted to the
machine chosen to execute the job.  The default operation of HTCondor is to
transfer all files created by the job on the EP back to the
AP.  Therefore, there is no specification of the ``outfile.txt``
output file.

This example submit description file modifies the commands that direct
the transfer of files from AP to EP and back again.

.. code-block:: condor-submit

    should_transfer_files   = IF_NEEDED
    when_to_transfer_output = ON_EXIT

These values are the HTCondor defaults, so are not needed in this example.
They are included to direct attention to the capabilities of HTCondor.  The
:subcom:`should_transfer_files` command specifies whether HTCondor should assume the
existence of a file system shared by the AP and the EP.  Where there is a
shared file system, a correctly configured pool of machines will not need to
transfer the files from one machine to the other, as both can access the shared
file system.  Where there is not a shared file system, HTCondor must transfer
the files from one machine to the other.  The specification ``IF_NEEDED`` asks
HTCondor to use a shared file system when one is detected, but to transfer the
files when no shared file system is detected.  When files are to be
transferred, HTCondor automatically sends the executable as well as a file
representing standard input; this file would be specified by the :subcom:`input`
submit command, and it is not relevant to this example.  Other files are
specified in a comma separated list with :subcom:`transfer_input_files`, as they are
in this example.

When the job completes, all files created by the executable as it ran are
transferred back to the AP.

HTCondor assumes that if the job exits of its own accord, with an exit code
of zero, that indicates success, and any non-zero exit code is a failure.
By default, when the job exits, it will leave the queue.  If you would
like a job that exits with a non-zero exit code to be restarted some
number of times until it does, set :subcom:`max_retries` in the submit file like
so:

.. code-block:: condor-submit

   max_retries = 2

Expanding the science Job and the Organization of Files
-------------------------------------------------------

A further example promotes understanding of how HTCondor makes the
submission of lots of jobs easy.  Assume that the ``science.exe`` job
is to be run 40 times.  If the input and output files were exactly the
same for each run, then only the last line of the given submit description
file changes: from

.. code-block:: condor-submit

    queue

to

.. code-block:: condor-submit

    queue 40

It is likely that this does not produce the desired outcome, as the output
file created, ``outfile.txt``, has the same name for each queued instance
of the job, and thus this file of results for each run conflicts.  Chances
are that the input files also must be distinct for each of the 40 separate
instances of the job.  HTCondor offers the use of a macro that can uniquely
name each run's input and output file names.  The ``$(Process)`` macro causes
substitution by the process ID from the job identifier.  The submit
description file for this proposed solution uniquely names the files:

.. code-block:: condor-submit

    # science2.sub -- run 40 instances of science.exe
    executable              = science.exe
    arguments               = "infile-$(Process)A.txt infile-$(Process)B.txt outfile$(Process).txt"

    transfer_input_files    = infile-$(Process)A.txt,infile-$(Process)B.txt
    should_transfer_files   = IF_NEEDED
    when_to_transfer_output = ON_EXIT

    request_cpus            = 1
    request_memory          = 512M
    request_disk            = 1G

    max_retries             = 2
    log                     = science2.log
    queue 40

The 40 instances of this job will have process ID values that run from 0 to
39.  The two input files for process ID 0 are ``infile-0A.txt`` and
``infile-0B.txt``, the ones for process ID 1 will be ``infile-1A.txt`` and
``infile-1B.txt``, and so on, all the way to process ID 39, which will be
files ``infile-39A.txt`` and ``infile-39B.txt``.  Using this macro for
the output file naming of each of the 40 jobs creates ``outfile0.txt`` for
process ID 0; ``outfile1.txt`` for process ID 1; and so on, to
``outfile39.txt`` for process ID 39.

This example does not scale well as the number of jobs increases,
because the number of files in the same directory becomes unwieldy.  Assume
now that there will be 100 instances of the ``science.exe`` job, and each
instance has distinct input files, and produces a distinct output file.  A
recommended organization introduces a unique directory for each job
instance.  The following submit description file facilitates this organization
by specifying the directory with the :subcom:`initialdir` command.  The directories
for this example are named ``run0``, ``run1``, etc. all the way to ``run99``
for the 100 instances of the following example submit file:

.. code-block:: condor-submit

    # science3.sub -- run 100 instances of science.exe, with
    #  unique directories named by the $(Process) macro

    executable              = science.exe
    arguments               = "infile-A.txt infile-B.txt outfile.txt"

    should_transfer_files   = IF_NEEDED
    when_to_transfer_output = ON_EXIT

    initialdir              = run$(Process)
    transfer_input_files    = infile-A.txt,infile-B.txt

    request_cpus            = 1
    request_memory          = 512M
    request_disk            = 1G

    max_retries             = 2
    log                     = science3.log
    queue 100

The input and output files for each job instance can again be the initial
simple names that do not incorporate the ``$(Process)`` macro. These files are
distinct for each run due to their placement within a uniquely named
directory.  This organization also works well for executables that do not
facilitate command line naming of input or output files.

Here is a listing of the files and directories on the AP within
this suggested directory structure.  The files created due to submitting and
running the jobs are shown preceded by an asterisk (*).  Only a subset of the
100 directories are shown.  Directories are identified using the Linux (and
Mac) convention of appending the directory name with a slash character (/).

.. code-block:: text

    science.exe
    science3.sub
    run0/
        infile-A.txt
        infile-B.txt
        * outfile.txt
        * science3.log
    run1/
        infile-A.txt
        infile-B.txt
        * outfile.txt
        * science3.log
    run2/
        infile-A.txt
        infile-B.txt
        * outfile.txt
        * science3.log

Submitting a GPU Job
--------------------

This example demonstrates how to submit a job that requests GPU resources.
HTCondor can automatically match jobs that need GPUs with execution points
that have GPU devices available.

The submit description file shows how to request a GPU using the
:subcom:`request_gpus` command:

.. code-block:: condor-submit

    # gpu_simple.sub -- simple GPU job example
    # This job requests one GPU and demonstrates how GPU assignments
    # are exposed to the job via the CUDA_VISIBLE_DEVICES environment variable.

    universe = vanilla

    # Script executed on the execute node
    executable = gpu_check.sh

    # Request one CPU core and one GPU device
    request_cpus = 1
    request_gpus = 1

    # Memory requirement for the job
    request_memory = 2GB

    # Standard HTCondor log, output, and error files
    output = gpu.out
    error  = gpu.err
    log    = gpu.log

    queue

When HTCondor assigns a GPU to your job, it sets the ``CUDA_VISIBLE_DEVICES``
environment variable to indicate which GPU device(s) have been allocated.
Most GPU-accelerated applications automatically use this environment variable
to determine which GPU to use.

Here is an example executable script that checks for GPU assignment:

.. code-block:: bash

    #!/bin/bash
    # gpu_check.sh
    # This script runs on the execute node and prints GPU information

    echo "=== HTCondor GPU Job Check ==="
    echo "Hostname: $(hostname)"
    echo "Date: $(date)"

    echo
    echo "CUDA_VISIBLE_DEVICES = $CUDA_VISIBLE_DEVICES"
    echo

    # If NVIDIA utilities are available, show GPU info
    if command -v nvidia-smi >/dev/null 2>&1; then
        echo "nvidia-smi output:"
        nvidia-smi
    else
        echo "nvidia-smi not found (this is OK on non-NVIDIA systems)"
    fi

    echo
    echo "Job completed successfully."

Remember to make the script executable on Linux or Mac systems:

.. code-block:: console

    $ chmod u+x gpu_check.sh

This example can be used to verify GPU job submission works correctly,
even on systems without physical GPUs. The job will match with execution
points that have GPUs available. For actual GPU computation, replace
``gpu_check.sh`` with your GPU-accelerated application.

Creating Workflows with DAGMan
------------------------------

It is common to have a sequence of jobs that need to run in a particular order.
The most common reason is the output one job becomes the input of another.
While you could place the first job, and manually check up on the first job
until it finishes, and only then place the second job, HTCondor comes with a
workflow manager called DAGMan. Here is a brief introduction to DAGMan. For
more detailed information see :ref:`DAGMan documentation`.

.. warning::

    This example assumes all files are within the same working directory for
    simplicity. It is important to think about how the workflow directory structure
    before executing a workflow of jobs.

To start, this example has two jobs. The first is a simulator job that produces
input for the second analysis job. These simple jobs are described in the HTCondor
Job Description Language as follows:

.. tabs::

    .. code-tab:: condor-submit Simulation Job

        # simulator.sub
        executable            = simulator.exe
        arguments             = "-seed 12345 -outfile science.sim"

        output                = simulation.out
        error                 = simulation.err
        log                   = simulation.log

        transfer_output_files = science.sim

        request_cpus          = 2
        request_memory        = 2G
        request_disk          = 1G

        queue

    .. code-tab:: condor-submit Analysis Job

        # analyzer.sub
        executable            = analyzer.exe
        arguments             = "-infile science.sim"

        output                = analysis.out
        error                 = analysis.err
        log                   = analysis.log

        transfer_input_files  = science.sim
        transfer_output_files = results.txt

        request_cpus          = 1
        request_memory        = 1G
        request_disk          = 5G

        queue

From here the two jobs and the dependency between them can be
described in a DAG file utilizing the DAG Description Language
(DDL) as follows:

.. code-block:: condor-dagman

    # my-workflow.dag

    # Describe two nodes within the DAG named simulator and analyzer
    JOB simulator simulator.sub
    JOB analyzer analyzer.sub

    # Create ordered dependency between simulator and analyzer nodes
    PARENT simulator CHILD analyzer

This DAG file creates a workflow consisting of two nodes named
``simulator`` and ``analyzer``. Each node has the similarly named
submit description files (``*.sub``) to be executed. The DAG also
informs that the ``analyzer`` node is dependent on the ``simulator``
node such that the former only executes once the latter executes
successfully.

From here the workflow can be executed by running the following command:

.. code-block:: console

    $ condor_submit_dag my-workflow.dag

.. note::

    This example is a simple introduction DAGMan workflows and only touches
    the surface. There is much more in regards to executing/managing DAGMan
    workflows and various capabilities of DAGMan.

Where to Go from Here
---------------------

.. What we really want here is a link to a nice page in the user manual
.. that briefly describes HTCondor's major features and/or what you'd use
.. them for, as kind of a road map to the user manual.

* Consider watching our
  `video tutorial <https://www.youtube.com/watch?v=p2X6s_7e51k&list=PLO7gMRGDPNumCuo3pCdRk23GDLNKFVjHn>`_
  for new users.
* `Additional tutorials <https://www.youtube.com/playlist?list=PLO7gMRGDPNumCuo3pCdRk23GDLNKFVjHn>`_
  about other aspects of using HTCondor are available
  in our `YouTube channel <https://www.youtube.com/channel/UCd1UBXmZIgB4p85t2tu-gLw>`_.
* Slides from `past HTCondor Weeks <https://htcondor.org/past_condor_weeks.html>`_ -- our annual conference -- include the tutorials given there.
* The :doc:`../users-manual/index` is a good reference.
* If you like what you've seen but want to run more jobs simultaneously, the
  :doc:`administrator's quick start guide <../getting-htcondor/admin-quick-start>`
  will help you make more of your machines available to run jobs.
