Architecture
The Sync Framework runtime provides synchronization functionality, without being tied to any data store or data transport protocols. By providing data source specific synchronization providers, any data source can be supported. For example, using proper synchronization providers, files can be synchronized across computers, project updates synchronized across project participants, or media synchronized across devices. Sync Framework ships with three providers: Microsoft Sync Services for ADO.NET, Sync Services for File Systems, and Sync Services for SSE. Sync Services can be used to synchronize devices by supplying providers for the device. Similarly, PIM software such as Microsoft Office Outlook and media libraries such as Windows Media Player can also be supported by providing suitable providers.
The providers are used to enumerate the items in a data store, each identified by an Item ID. In addition, they also have to maintain synchronization metadata and the state of the data store, so that changes can be enumerated quickly. The metadata is maintained for every instance of the data store (replica) that the provider is attached to. The metadata maintained includes the replica ID, tick count (representing progression in time), conflict log, tombstone log, and the set of the changes the data store has seen (knowledge). A replica ID and tick count pair makes up a version and encodes the state of the data store until that time. Sync Framework defines a set of operation for the Knowledge object for a replica: Contains which determines if the store contains a specified change, Union to merge two knowledge sets, Project to project out the knowledge for a subset of the items, and Exclude to create a new knowledge set without the changes for a subset of the items. The metadata is managed by the metadata storage service which uses an in-process SQL Server Compact database to store the metadata on a per-provider basis.
The Sync Services API operates by creating a synchronization session, represented by a Session object. A synchronization session synchronizes data across two synchronization providers - one for the source data store and the other for the destination. Instances of both the providers are passed to the Session object. During a synchronization session, the destination provider sends the knowledge set of the store. The source provider compares the knowledge of the destination with the change set in the source to enumerate the changes and then transfer it to the destination. The destination provider makes sure the changes are not conflicting and merges the changes and updates the knowledge.
- Snapshot sync (download-only sync): The data in the data source (or a subset of it) is synchronized with clients.
- Upload-only sync: Data in the client is merged to the source replica.
- Bidirectional sync: Both the data sources can be modified independently and changes are synchronized with each other. An n-level sync is achieved by performing multiple bidirectional synchronizations.
Read more about this topic: Microsoft Sync Framework
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