An Android service that synchronizes data between an Android device and a server
A SyncAdapter is an Android Service that synchronizes data between an Android device and a backend server. SyncAdapters can be part of an Android program, or can be a standalone entity. SyncAdapters are associated with an Account, which defines a collection of data that belongs to a specific application.
The purpose of a SyncAdapter is to ensure that data is kept consistent on the Android device and a server. When the data is changed on the server, the SyncAdapter must identify that a change has been made, and update the data on the Android device to reflect the change.
The most common type of data that is synchronized using SyncAdapters are your Contacts list. Whenever you add a contact on your phone, the contact is also sent to the Google server and stored within. Similarly, if you add a contact from a different Google service, such as GMail, the SyncAdapter will identify that a contact has been added, and replicate the data onto your Android device.
Any type of data can be synced with a backend server, its only dependent on the type of data that your application is dealing with. Other common data types to be synced in this way are Calendars and To-Do/Task lists.
The reason why SyncAdapters exist is to create a local copy of the data, so that it can be accessed quickly without the need for a data connection. For example, if your contacts are kept synced, then you can access your contacts quickly on your Android device regardless of whether you have data access or not - it's all locally-stored data independent from network requirements.
SyncAdapters will usually send a 'check' message to the server on a periodic basis, say every 30 minutes, to ask whether any changes have been made during this time. This is only a small data exchange, and doesn't consume much network data. Further 'synchronization' steps only occur if the data has changed and needs to be updated on the Android device accordingly.
