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Multiple Clients sharing data

When multiple client applications store data locally and share these with other application instances, data synchronization between different clients takes place. In order to support this case, a few fundamental considerations are necessary.

A Client is any application instance which stores data locally and which want to inform other instances about local changes as well as receiving updates from other instances. Each client instance that participates in the data exchange must have a unique clientId.

The clientVersion is a 64-bit integer that is used on the service to compare which Document is newer. If, for example, a client sent a Document with clientVersion=1 and the corresponding Document on the service has already clientVersion=2, the update request is discarded. A good practice is to set clientVersion = milliseconds-since-epoch, on the client when the record is updated locally.

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Synchronization Workflow

  1. Send local changes to the Service

  2. Ask Service for changes from other clients since the last request.

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Upload local changes

  • The client collects all local records which have localVersion > lastUploadClientVersion, all documents which have been changed locally since the last upload.

  • The client sends it local changes together with its clientId to the service: POST Docs,to update (upsert) all records on the Service. Don't forget to provide a clientVersion for each record!

  • The Service creates or updates all Documents on the service which do not exist or have a lower (or the same; in case clientVersion=0) clientVersion. The Service will also record the id of the client which sent the updates.

    • The Service sets the server date/time of the change in LastUpdateUtc .

  • The client stores/updates lastUploadClientVersion = max( localVersion), to remember the latest local change that was sent to the service.

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Download server-side changes

  • The client queries the Service for all changes since, which were not provided by the current client: POST /Docs/Query?origin=other&sinceUtc=${sinceUtc}

    • The Services responds all Documents with LastUpdateUtc ≥ sinceUtc where clientId which provided the Document is different from the querying clientId, to avoid infinitive loops (sending the same record forth and back).

The querying client cannot know about clientVersions which have been provided by other clients, and that is why clientVersion is not suitable to query changes. To query changes the server-site LastUpdateUtc is the information to rely on.

  • The client records newestKnownServiceRecordUtc = max( LastUpdateUtc) to be able to start the next request with an appropriate date.

  • The client updates its local database.

  • The client does not process the incoming clientId, because it is not of interest which client updated the record.

  • The client sets localRecord.clientVersion = serviceRecord.clientVersion.