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Content Synchronization

Content Synchronization. March 2nd 2005. Synchronization. Needed whenever multiple copies of an object may be updated independently After synchronization objects should be in same state Conflict resolution Domain specific solutions Can be often automated. Different types of synchronization.

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Content Synchronization

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  1. Content Synchronization March 2nd 2005 T-110.456

  2. Synchronization • Needed whenever multiple copies of an object may be updated independently • After synchronization objects should be in same state • Conflict resolution • Domain specific solutions • Can be often automated T-110.456

  3. Different types of synchronization • One way or two way • Slow • Transmit all information • Slow • Fast • Transmit only changes • Devices need to keep track of all changes T-110.456

  4. Issues with mobile devices • The usual suspects: • Electric power, computing power, slow connections, limited data storage • Divide the synchronization workload asymmetrically • Client / server, server doing most of the work • Client still needs to track changes • Standard solution: SyncML • Defines data representation and synchronization protocol T-110.456

  5. SyncML - Overview • Can be used with different bearers • Radio, Bluetooth, Infrared, WLAN... • Messages are in XML • WBXML used to save bandwidth • Synchronization protocol • Initialization • Negotiate capabilities between client & server • Negotiate sync type • Authentication • Data transfer T-110.456

  6. Example synchronization T-110.456

  7. SyncML - synchronization types • Seven different synchronization types • Slow / fast two-way • Slow / fast one-way from client to server • Slow / fast one-way from server to client • Server initiated sync • Normally, client always initiates sync • Server can request initiation from client • The two-way syncs are required by the standard, others are optional T-110.456

  8. SyncML - message structure • Information is transferred in packages • Packages can consist of multiple messages • Message size can be limited by the bearer • Large objects may be sent in multiple messages • Package contains related information • e.g. client modifications, initialization T-110.456

  9. SyncML - conflict resolution • SyncML does not define how conflicts are resolved • Out of scope as resolving is domain-specific • Mechanism for reporting conflicts and actions taken • Status codes for different types of conflicts • delete, update • Status codes for outcomes • Server win, client win, merge, copy • Conflicts are resolved in the server T-110.456

  10. SyncML - mappings • Server identifiers are typically long while mobile terminals will benefit from shorter identifiers • SyncML uses two sets of identifiers: • GUID: Globally Unique IDentifier • LUID: Locally Unique IDentifier • Client always uses LUIDs, and server maintains a mapping between LUIDs and GUIDs. T-110.456

  11. Example ID mapping T-110.456

  12. SyncML - device capabilities • There is a DTD defining the device capability description • Exchanging capability information is useful • Server can avoid sending information that the client can not use • Removes possible sources of ambiguity • For example, if client sends a contact entry without a picture, was the picture deleted deliberately or discarded because client does not support pictures T-110.456

  13. SyncML - miscellaneous • Sync anchors next and last • Provides a way to see if an attempt failed • Some error cases require slow sync • If database contains large items, such as photographs, this can be wasteful as also unchanged data is transmitted • In some cases message digests could be used to save bandwidth T-110.456

  14. Using digests to save bandwidth • General idea: • Client sends digests of all items to server • Server compares received digests to stored information and deduces the changes made by client • Server sends changed items to client and requests changed items from client • Server reconstructs the change log regarding changed items in order to avoid sending unchanged information • Not supported in the standard • Utility depends on many factors • Frequency errors requiring slow sync • Amount of large items in databases T-110.456

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