Under the hood, the whole system is based on the model of “Task providers”. Task providers are systems from which the WMA can aggregate tasks:
At the moment, three tasks providers are supported: Microsoft SharePoint, Microsoft Exchange and Microsoft Project Server. This means, that the users will see the tasks from those three sources, in their “My Tasks” area. Adding own task providers in not supported at the moment. Different providers are actually very different underlying systems, and there is no easy, “unique” way to do the aggregation and synchronization. Microsoft was planning to include an official API and methodology for adding own providers, but it never made its way in production bits, and it seems that it got a bit off the agenda. Pity, since there is great demand for it – that is actually always the first question being asked, after users learn of existence of this service application.
WMA initiates the task sync with providers through so called “provider refreshes”. In the WMA lingo, a refresh is an initiation of the task aggregation (sync) process. For SharePoint and Microsoft Project, provider refreshes are triggered on demand. That means, there is no background process refreshing the tasks for the users in some regular time intervals. Refresh is triggered at that very moment when a user accesses her MyTasks section. A small spinning circle indicates that a refresh is in progress. After refresh is completed, tasks are “reloaded”, and user gets to see her tasks. There is an artificial limit of one refresh every five minutes, not to jeopardize the overall SharePoint server performace. This limit can be changed with PowerShell commands for SharePoint 2013 (there is no GUI for Work Management Application management). For SharePoint Online in Office 365, this limit of one refresh per five minuts cannot be changed.
Provider refresh for Exchange tasks is somewhat different, and it is based on a background timer job (“Work Management Synchronize with Exchange”) which actually does the sync. The job is initially set to run every minute.
Tasks are internally cached in a hidden list in user’s My Site. That means that all the tasks which provider refresh discovers, and eventually updates, are locally stored in the WmaAggregatorList_User in the MySite.