Webcron

webcron is the term for a time-based job scheduler hosted on a web server. The name derives its roots from the phrase web server and the Unix daemon cron. A webcron solution enables users to schedule jobs to run within the web server environment on a web host that does not offer a shell account or other means of scheduling jobs.

Many web hosts offer shell accounts or some sort of built-in job scheduler such as cron that makes it easy for users to schedule jobs. Such hosts run jobs as command-line applications that may optionally communicate with the web server. A webcron solution, however, runs entirely within the purview of the web server environment of a web host. This allows a webcron solution to operate on hosts that do not offer a job scheduler such as cron or a shell account. A webcron solution will also run equally well on hosts that do offer users such capabilities but is designed as a substitution or replacement.

A webcron solution is made up of two pieces. The first piece is a script that will execute the tasks that resides somewhere accessible via a URL. The second piece is to use a scheduling provider that contacts the URL of the script at regular intervals.

Before setting up a schedule with a scheduling provider, a user must set up a script that runs on the web server. Most web hosts have restrictions on the length of time a single instance of a script may execute. Many web hosts also have limitations on CPU and RAM resource usage. Users of webcron solutions on shared hosting providers must be careful to not repeatedly exceed their web host's limitations so as to not get kicked off. A script that runs a long time must take into account that it may be terminated at any point by a web server process. Users may implement a state machine, which allows the script to operate across multiple invocations and run within the limitations imposed by a web host.

There are many third-party webcron scheduling providers on the web. These services accept a URL and a frequency schedule to retrieve, or ping, the specified URL. Most providers have restrictions built into their system to avoid overloading their servers and to encourage users to sign up for premium accounts.

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Wikipedia