The Mechanics of Global LSN and Vector Clocks in Session Consistency Guarantees

Session consistency uses a 'Session Token' (effectively a vector clock or LSN) passed between client and server to ensure read-your-own-writes within a specific version boundary.

When a client writes to Cosmos DB under Session consistency, the server returns an x-ms-session-token. This token contains the LSN (Log Sequence Number) of that specific write. In subsequent reads, the client sends this token back. The server ensures that the replica processing the read has at least reached that LSN. If the replica is lagging, the server waits or routes to a more up-to-date replica. This provides a 'Monotonic Read' guarantee for that specific client without the high cost of Strong consistency.