Erlcron

Erlcron provides testable cron like functionality for Erlang systems, with the ability to arbitrarily set the time and place along with fastforwarding through tests. See erlcron.erl for more documentation.

The synax of a job description is quite different from crontab. It is (in this author's opinion) easier to read and is much more in keeping with the Erlang tradition. It is not quite as expressive as cron but this can be compensated for by adding multiple jobs.

No output is logged or mailed to anyone. If you want output to be logged or mailed, you must explicitly specify that as part of the job.

This does not poll the system on a minute-by-minute basis like cron does. Each job is assigned to a single (Erlang) process. The time until it is to run next is calculated, and the process sleeps for exactly that long.

Unlike cron's one-minute resolution, erlcron has a 2-second resolution (actually 1 second, but after running the job, the process waits an extra second to avoid accidentally running it more than once.)

It does not handle Daylight Savings Time (or other cases when the system clock is altered) gracefully, and it is recommended that it be stopped and restarted on such occasions.

Cron Job Description Examples:

{{once, {3, 30, pm}},
    {io, fwrite, ["Hello, world!~n"]}}

{{once, {12, 23, 32}},
    {io, fwrite, ["Hello, world!~n"]}}

{{once, 3600},
    {io, fwrite, ["Hello, world!~n"]}}

{{daily, {every, {23, sec}, {between, {3, pm}, {3, 30, pm}}}},
    {io, fwrite, ["Hello, world!~n"]}}

{{daily, {3, 30, pm}},
    fun() -> io:fwrite("It's three thirty~n") end}

{{daily, [{1, 10, am}, {1, 07, 30, am}]},
    {io, fwrite, ["Bing~n"]}}

{{weekly, thu, {2, am}},
    {io, fwrite, ["It's 2 Thursday morning~n"]}}

{{weekly, wed, {2, am}},
    {fun() -> io:fwrite("It's 2 Wednesday morning~n") end}

{{weekly, fri, {2, am}},
    {io, fwrite, ["It's 2 Friday morning~n"]}}

{{monthly, 1, {2, am}},
    {io, fwrite, ["First of the month!~n"]}}

{{monthly, 4, {2, am}},
    {io, fwrite, ["Fourth of the month!~n"]}}

Adding a cron to the system:

Job = {{weekly, thu, {2, am}},
    {io, fwrite, ["It's 2 Thursday morning~n"]}}.

erlcron:cron(Job).

A simple way to add a daily cron:

erlcron:at({3, 30, pm}, Fun).

A simple way to add a job that will be run once in the future. Where 'once' is the number of seconds until it runs.

erlcron:once(300, Fun).

Cancel a running job.

erlcron:cancel(JobRef).

Get the current date time of the system.

erlcron:datetime().

Set the current date time of the system. Any tests that need to be run in the interim will be run as the time rolls forward.

erlcron:set_datetime(DateTime).

Set the current date time of the system on all nodes in the cluster. Any tests that need to be run in the interim will be run as the time rolls forward.

erlcron:multi_set_datetime(DateTime).

Set the current date time of the system on a specific set of nodes in the cluster. Any tests that need to be run in the interim will be run as the time rolls forward.

erlcron:multi_set_datetime(Nodes, DateTime).

The application cron can be pre-configured throught environment variables in the config file that all applications can load in the Erlang VM start. The app.config file can be as follow:

[
    {erlcron, [
        {crontab, [
            {{once, {3, 30, pm}}, {io, fwrite, ["Hello, world!~n"]}},

            {{once, {12, 23, 32}}, {io, fwrite, ["Hello, world!~n"]}},

            {{daily, {every, {23, sec}, {between, {3, pm}, {3, 30, pm}}}},
             {io, fwrite, ["Hello, world!~n"]}}

        ]}
    ]}
].

So, when the app will be started, all configurations will be loaded.

Note that the limitation is that in the config file is impossible load an anonymous function (or lambda function) so, you only can use {M,F,A} format.