Jobs let you execute processing that would not necessarily fit in one single transaction, by splitting it into smaller tasks, executed in sequence.
A job represents the entire processing you want to achieve, whilst its tasks are each one a smaller piece of that larger processing.
A typical example is when you want to modify all documents in a given collection, or even in the entire database. For instance due to a change in the data model.
If the collection contains a large number of documents, maybe a significant portion of the entire database, then it is not possible to modify all of them in one single transaction. If you compute first a list of groups of say, 1,000 document URIs, then you can process the first group of 1,000 documents, then the next one, then each of them in sequence.
A job is the definition containing what to do, and how to do it. You can start a job, or "instantiate" it, creating an activity. The activity represents just one run of a job. One job can be run several times. Each piece of processing executed in an activity is a task.
In order to better understand jobs and tasks, it is useful to understand why we need them at all, when to use them, and how they are implemented.
If the query takes too much time or makes too many changes, it might be too big too fit in a single transaction. MarkLogic simply "kills" (and rollback) a query that takes too long or when the changes are too large to keep track of.
Also, if a query makes changes to a whole lot of documents, we do not want necessarily for the entire transaction to be rolledback in case of an error on one single document.
For some maintenance tasks, it is useful then to use a divide and conquer approach: first divide the entire set of changes to apply in smaller chunks, then apply the query to each of these chunks in a separate transaction.
A typical example is a change in the data model that requires to update all documents in the database (or a significant part of them). If the database is large enough, the query is likely to make too many changes to fit in one single transaction. In that case, you can rather define chunks of say, 1,000 document URIs, then execute the query that modifies a set of documents once for each chunk.
TODO: Still to be written...
MarkLogic provides a special app server on every cluster (technically, on each group on the cluster). The task server is basically a queue of the queries that have been "spawned" (scheduled to be executed asynchronously, in their own transaction). The task server is a powerful tool, but it comes with its own complexity, especially the complexity inherent to dealing with asynchronous processing.
The ML Console provides a layer on top of the task server. It let you provide 2 queries (one to identify the chunks of data to work on, and the second to actually work on each of them). Once a job is defined, you can then excute it. A job can be executed several times (e.g. a maintenance job to be executed on a regular basis).
It takes care of saving the chunks, starting a new task for each of them, makes sure the tasks are executed in sequence, and tracks the status of each task, and records the result or error for each task.
The ML Console also provides a handy web UI to define, execute, and monitor jobs.