Tip Queries¶

Ceramic streams are identified by a URI called StreamIds. Nodes that want to sync a stream need to query the network for the tip of that stream using its StreamId.

Protocol¶

A node resolving a Ceramic URI sends a query message to the network and then listens for responses with the candidates for the current tip of the stream. Any node that is interested in the same stream on the network and has stored its tips will respond with a response message. All messages are sent on the libp2p pubsub topic for the network the node is configured for.

Query Message¶

type QueryMessage = {
  typ: MsgType.QUERY // 1
  id: string
  stream: StreamID
}

Where:

• typ - the message is a query message, enum 1
• stream - the streamId that is being queried or resolved
• id - a multihash base64url.encode(sha265(dagCBOR({typ:1, stream: streamId}))), can generally be treated as a random string that is used to pair queries to responses

Response Message¶

type ResponseMessage = {
  typ: MsgType.RESPONSE // 2
  id: string
  tips: Map<StreamId, CID> 
}

Where:

• typ - the message is a response message, enum 2
• id - id of the query that this message is a response to
• tips - map of StreamID to CID of stream tip

Examples¶

TypeScript Definitions¶

enum MsgType { // Ceramic Pub/Sub message type.
  UPDATE = 0,
  QUERY = 1,
  RESPONSE = 2,
  KEEPALIVE = 3,
}

type QueryMessage = {
  typ: MsgType.QUERY
  id: string
  stream: StreamID
}

type ResponseMessage = {
  typ: MsgType.RESPONSE
  id: string
  tips: Map<string, CID>
}