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The way to recover lost updates during a disconnection, or rather, to update the client with the latest situation of every Items is through the snapshot. The concept of a snapshot may vary depending on the type of subscription and is present both for MERGE, DISTINCT, and COMMAND mode. Please refer to section 3.4.1 for full details.
A JavaScript client can call this method setRequestedSnapshot to request the snapshot from the server and so it will receive all snapshot updates and then the real time updates.