Simple Text Queue
#################

Why and what it is
==================

**teaqueue** is a command pair able to process sequentially items such as
files from a list of filenames in a text file by calling a given command for
each of them. You use **GNU** tools like `find` to populate your queue, or use
**vim** to reorder it.

It can be used to:

- transcode a batch of video files such as your preferred technical
  presentations to a format accepted by your mobile phone,

- retrieve a bunch of URLs or other remote operation,

- sequential operation on anything that can be encoded on a single line (json
  data, base64 data, etc).

It is really useful for operations taking a lot of time when you want to be
able to stop processing items in the queue and resume it. Thus, when stopped
you can optionally change the order of items, add items, or remove items from
the queue.

Why `tea` ???
-------------

Because I love tea.

Usage
=====

Usage is linked directly to manipulation of simple text files. You use text
files to make the queue, to add or remove items from it, to change priority of
items. *UNIX is the way*.

I give some examples in this section. Take into account `teaqueue-server`
takes `queue.txt` in the current directory by default as queue file modifying
it when interrupted to remove processed items from it. By default, processed
items goes to `done.txt`. This files serves to remove processed items from
`queue.txt` when the queue is interrupted and serves as a log file for
processed items.

Video batch transcoding
-----------------------

Firstly list filenames in the queue and serve the queue::

    find -type f -iname '*.webm' > queue.txt
    teaqueue-server

Then ask `teaqueue-client` to process each files with a transcoding command::

    teaqueue-client transcode_my_file.sh

This is all if you just want to process each file and do not interrupt it. If
you want to interrupt processing of the queue, modify it and resume it, first
press `Ctrl-C` in the `teaqueue-server` console and modify `queue.txt`
accordingly::

    find ../another_dir -type '*.webm' >> queue.txt

Or reorder some items::

    vim queue.txt

Then run again `teaqueue-server`, clients will continue to ask new items from
the queue::

    teaqueue-server transcode_my_file.sh

`transcode_my_file.sh` is a called a worker and is a simple script which calls
`ffmpeg` or another program to convert your files. You can find examples of
workers inside the `worker_examples` directory.

Write a worker
==============

In order to be useful you certainly have to write a worker script. Writing
a worker is simple, all you have to do is to store the standard input line and
work with it.

Echo worker
-----------

For example, a useless worker, but simple to understand the principle is just
a worker echo each item it receive:

.. code:: shell

    #!/bin/sh
    line="$(cat)"
    echo "Printing line '$line' and sleeping $1 seconds"
    sleep $1

You can use it with a `teaqueue-client`::

    teaqueue-client ./echo_sleep.sh 10

Remote files
------------

If you want to be able to use `teaqueue-client` on another host than the
`teaqueue-server` you can download the files (sharing it on the server side is
done through anything you want) from the host, then work on it and push back
the transformed files. Just write the worker in consequence:

.. code:: shell

    #!/bin/sh
    filename="$(cat)"
    echo "$filename" | nc -q0 serverin 1338 > file
    transcode.sh file > fileout
    (echo "$filename"; cat fileout) | nc -q0 serverout 1338

This example assume you share your files with simple netcat commands, but you
can adapt with anything you want: wget, scp, etc.

History
=======

Normally, if you have a worker writing to standard output every file
processed, you could write something like this::

    find -iname '*.ext' > queue.txt
    cat queue.txt | worker > done.txt
    # here you can interrupt the service at any time (Ctrl-C/SIGTERM)
    comm -23 queue.txt done.txt > remaining_queue.txt

But if this is something you want to do again and again, it might be
convenient to write a little wrapper to do it for us. This is how **teaqueue**
was born.

Limitations
===========

**teaqueue** is not perfect and does not try to be. There is no security and
some rare race conditions.

Known race condition:

- If someone interrupts `teaqueue-server` just after the line has been sent
  but before it gets printed out, the line will not be filtered in the
  `queue.txt` file. Thus it will be send again to the next demanding client.

License
=======

**teaqueue** is shared to you under the GPL-3 license conditions. See
`LICENSE.txt`.