[raft] Add a session proto message for idempotency support (#6680)

proto/raft.proto

@@ -294,6 +294,25 @@ enum FinalizeOperation {
   COMMIT = 2;
 }
 
+// A client session that is used to support idempotency on the raft state
+// machine.
+message Session {
+  // An unique identifier of the client session.
+  // Example: 018fcb2f-cfab-718b-974e-64907df56881
+  bytes id = 1;
+  // Each BatchCmdRequest has its unique <id, index>. The index is
+  // monotonically increasing.
+  // Example: 1717100891000000000
+  uint64 index = 2;
+  // A timestamp when the session will be cleaned up; and when the state machine
+  // can safely clean up the entry.
+  int64 expiry_usec = 3;
+
+  // Optional. The response associated with the command associated with the
+  // index.
+  optional bytes rsp_data = 4;
+}
+
 message BatchCmdRequest {
   // Header will be validated by the state machine if set.
   Header header = 1;
@@ -311,6 +330,9 @@ message BatchCmdRequest {
 
   // Any post-commit hooks that should be run after a batch is *committed*.
   repeated PostCommitHook post_commit_hooks = 5;
+
+  // Session is used to guarantee idempotency for writes on the state machine.
+  optional Session session = 6;
 }
 
 message BatchCmdResponse {
@@ -467,6 +489,9 @@ message Header {
   //  Reads will use LINEARIZABLE by default but STALE can be set if the client
   //  can tolerate stale data.
   ConsistencyMode consistency_mode = 4;
+
+  // Session is used to guarantee idempotency for writes on the state machine.
+  optional Session session = 5;
 }
 
 message RangeSet {