Wednesday, 30 April 2014

IO::Iron gets command line tools

Now that all the functions of's IronMQ, IronCache and IronWorker services are turned into Perl client libraries, it is time to think about not only the programmer but also the tester and application supporter's needs. They require an easy and quick access to the services: command line tools.

Perl has several possible frameworks for creating command line utilities: e.g. CLI::Framework, App::Cmd, Badger, CLI::Application and CLI::Dispatch. From these I picked App::Cmd mostly because of its decentralized nature.

Command Line Tool Design for Continuous Integration

One of the principles of Continuous Integration states that "Everyone commits to the baseline every day". To make the programmers' load lighter every change should be in as much isolation as possible. Centrally located code which refers to individual parts of the system should always be generated automatically so the programmer does not need to remember and bother to keep up-to-date any kind of central index / reference table / central documentation / user reference or any other kind of central keeping place for things. Automatic code generation not only ensures that the "keeping place" is always up-to-date but also avoids typing errors. In Continuous Integration repository it also prevents or at least limits the possibility of merge conflicts in same files.

Equally important is to follow the practise of "Generate User Documentation from Program Code". App::Cmd is a good example of this. Actually, App::Cmd uses Getopt::Long::Descriptive which is its own small system (or framework) for processing command line options and parameters. The options and parameters are defined in a meta language (in a Perl hash data structure) and the Getopt::Long::Descriptive package uses this data structure to present the same options to user when needed, e.g. when user mistypes a parameter name.

my ($opt, $usage) = describe_options(
'my-program %o <some-arg>',
[ 'server|s=s', "the server to connect to", { required => 1 } ],
[ 'port|p=i', "the port to connect to", { default => 79 } ],
[ 'verbose|v', "print extra stuff" ],
[ 'help', "print usage message and exit" ],
becomes on the text terminal:

  my-program [-psv] [long options...] <some-arg>
    -s --server     the server to connect to
    -p --port       the port to connect to
    -v --verbose    print extra stuff
    --help          print usage message and exit

Loose Coupling at Runtime on Application Level

However, App::Cmd goes even further. When several distinctively different command functions are combined into one application, they are completely separated into individual files. This is a fine example of loose coupling inside one application. The commands do not know of each others' existence and neither do the programmers need to know of it. In IronMQ's case, the executable ironmq contains individual commands like 'add', 'delete', 'show' and 'list' but their existence is not documented permanently (statically) anywhere. One programmer works on one command and completes his or her work regardless of whether the other programmers have finished with the other commands. When user executes ironmq, App::Cmd framework discovers at runtime which commands and parameters are available.

This fastens the application development by making interconnected parts not reliant of each other. It speeds Continuous Integration and time to deployment.

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