Why a scheduling tool?¶
When running and maintaining a complex system like TYPO3, there are a number of tasks that need to be automated or executed in the small hours, when no one is around to press the button. Indeed quite a few extensions come with some command-line scripts meant to be run regularly using cron jobs. When each of these scripts need their separate entry in the server's crontab, maintenance complexity increases, as well as the cost of migration. Furthermore there's no simple way to keep an overview of and manage these tasks inside TYPO3.
The Scheduler aims to address this issue.
Scripts can be developed as Scheduler tasks by extending a base class provided by the Scheduler. They can then be registered with the Scheduler. At that point it becomes possible to set up a schedule for them by using the Scheduler's BE module.
The BE module provides an overview of all scheduled tasks and some indication of their status, e.g. are they currently running or are they late, did something wrong happen during last execution, etc. It is also possible to manually start the execution of tasks from the BE module.
The Scheduler provides a command-line tool to be run by TYPO3's command-line dispatcher. Only this script needs to be registered in the server's cron tab for all other recurring tasks to be executed. Indeed every time the Scheduler is launched by the cron daemon, it will look for all tasks that are due (or overdue) and execute them.
When a task is executed by the Scheduler it is marked as being executed in the corresponding database record (in the field called "serialized_executions"). When the task has finished running, the execution is removed from the database record. This mechanism makes it possible to know that a given task is currently running and also helps prevent multiple executions. Indeed it may be that a task requires more time to run than the frequency it is set up for. In such a case a new run may be started which is not always desirable. It is possible to deny such parallel (or multiple) executions.
Whenever a task starts or ends a message is written to TYPO'3 system log (viewable in SYSTEM > Log). A message is also written when a parallel execution has been blocked. This makes it possible to follow what happens, since the main purpose of the Scheduler is to run things when nobody is watching.
A task that fails may report on the reasons for failure using exceptions. Such a message will be logged in the Scheduler's database table and will be displayed in the BE module.
There's no default output to the command-line as scheduled tasks are designed to run in the background.
A few terms need to be defined more precisely:
Task : this word is used quite generally throughout this document, sometimes to cover different meanings. A task is really a piece of code that does a precise task and can be registered with the Scheduler in order to execute that piece of code at a precise time, recurrently or not.
Task class : this is the type of task. The "test" task is one particular task class. Its function is to send an email. The "sleep" task is another task class.
Registered task : an instance of a task class that has been registered with the Scheduler. A given task class may be registered several times, for example if it needs to be executed with different parameters.