What is Timer Job In sharepont 2010

Timer Jobs are recurring background processes that are managed by SharePoint. If you navigate to the Central Administration site, click on the Monitoring link from the main page, and then choose the Review job definitions link under the Timer Jobs section, then you’ll see a list of scheduled timer job instances. Notice that I did not say a list of timer jobs, but rather a list of scheduled timer job instances. Unfortunately, the term ‘Timer Job’ in SharePoint is a bit too general. A timer job really consists of three parts: Timer Job Definitions, Timer Job Instances, and Timer Job Schedules.

1. A timer job definition is a .NET class that inherits from the SPJobDefinition class and defines the execution logic of the Timer Job. Since it is just a .NET class, the Timer Job Definition has all of the things that you would expect a class to have: properties, methods, constructors, etc.
2. A timer job instance, as you may have guessed, is an object instance of the .NET Timer Job Definition class. You can have multiple instances of a timer job definition, which allows you to define a Timer Job Definition once, but vary the way it operates by specifying different property values for each Timer Job Instance. Whether you need one or many instances of a Timer Job Definition depends entirely on what you are trying to accomplish.
3. A timer job schedule is the last part of the puzzle. SharePoint exposes a series of pre-defined scheduling classes that derive from the SPSchedule class. A timer job instance must be associated with one of these schedules in order to run, even if you only want to run the timer job instance once.

Timer Jobs in the Central Administration UI

As seems standard with a number of constructs in SharePoint, Microsoft has made it a bit confusing for developers by using the term “Job Definition” in the SharePoint user interface that means something a bit different than what you would expect if you work them in code.
In Central Administration, you can review all of the “Job Definitions” by clicking on the Monitoring link from the main screen (1); then under the Timer Jobs heading clicking the Review Job Definitions (2) link. Since there is a SPJobDefinition class, you may be led to believe that this screen shows all of the timer job definitions that are available, but that is not the case. SharePoint does not have a mechanism to register a job definition by itself, so it does not have a list of the available job definitions. SharePoint only maintains a list of timer job instances that have been scheduled (i.e. SPJobDefinition instances with an associated SPSchedule). So the Review Job Definitions page is really a list of timer job instances that have been scheduled.

Adding to the confusion, there is also a link from the Review Job Definitions page to Scheduled Jobs. Since a timer job instance needs to be scheduled before it can be used, you would think this page contains a list of timer job instances that have been scheduled. Technically-speaking, it does, but it’s basically the same list that you find on the Review Job Definitions page with a different view. The main difference is that this page displays, and is sorted by, the Next Start Time (which informs you when the job will run next) instead of by Title. Timer jobs with schedules that have been disabled will also not appear on this list, so it may have a fewer number of items listed than the Review Job Definitions page. Clicking on the Title from either page takes you to the same timer job instance configuration page that allows you to modify the schedule of the timer job instance.