Merge pull request #15 from liftbridge-io/msg_type

Update protocol

Author: tylertreat

README.md

@@ -21,12 +21,16 @@ with the following header and be encoded as a `Message` (defined in
 
 ### Liftbridge Envelope Header
 
+All Liftbridge messages and RPCs sent over NATS are prefixed with an envelope
+header. This includes client-facing messages, such as publishes and acks, as
+well as internal RPCs like replication.
+
 ```
 0               8               16              24              32
 ├───────────────┴───────────────┴───────────────┴───────────────┤
 │                          Magic Number                         │
 ├───────────────┬───────────────┬───────────────┬───────────────┤
-│    Version    │   HeaderLen   │     Flags     │    Reserved   │
+│    Version    │   HeaderLen   │     Flags     │    MsgType    │
 ├───────────────┴───────────────┴───────────────┴───────────────┤
 │                       CRC-32C (optional)                      │
 └───────────────────────────────────────────────────────────────┘
@@ -45,6 +49,8 @@ bumped if the envelope format changes or if the message encoding changes in a
 non-backwards-compatible way. Adding fields to the messages should not require
 a version bump.
 
+Currently, the only supported protocol version is v0, i.e. `0x00`.
+
 #### HeaderLen [1 byte]
 
 The header length is the offset of the payload. This is included primarily for
@@ -55,9 +61,31 @@ safety.
 The flag bits are defined as follows:
 
 | Bit | Description     |
-| --- | --------------- |
+| :-- | :-------------- |
 | 0   | CRC-32C enabled |
 
+#### MsgType [1 byte]
+
+This is the Liftbridge-specific message type the envelope contains:
+
+| MsgType | Name                      | Description                                            | Internal |
+| :------ | :------------------------ | :----------------------------------------------------- | :------- |
+| 0       | Publish                   | Client-published message                               | no       |
+| 1       | Ack                       | Server-published ack                                   | no       |
+| 2       | ReplicationRequest        | Request to replicate partition data                    | yes      |
+| 3       | ReplicationResponse       | Response to ReplicationRequest                         | yes      |
+| 4       | RaftJoinRequest           | Request to join Raft cluster                           | yes      |
+| 5       | RaftJoinResponse          | Response to RaftJoinRequest                            | yes      |
+| 6       | LeaderEpochOffsetRequest  | Request for partition leader's latest offset for epoch | yes      |
+| 7       | LeaderEpochOffsetResponse | Response to LeaderEpochOffsetRequest                   | yes      |
+| 8       | PropagatedRequest         | Request forwarded to metadata leader                   | yes      |
+| 9       | PropagatedResponse        | Response to PropagatedRequest                          | yes      |
+| 10      | ServerInfoRequest         | Request for cluster information                        | yes      |
+| 11      | ServerInfoResponse        | Response to ServerInfoRequest                          | yes      |
+| 12      | PartitionStatusRequest    | Request to get partition status                        | yes      |
+| 13      | PartitionStatusResponse   | Response to PartitionStatusRequest                     | yes      |
+| 14      | PartitionNotification     | Signal new data is available for partition             | yes      |
+
 #### CRC-32C [4 bytes, optional]
 
 The CRC-32C (Castagnoli) is the checksum of the payload (i.e. from HeaderLen to