原文地址：https://spotify.github.io/snakebite/hadoop_rpc.html

Snakebite currently implements the following protocol in?snakebite.channel.SocketRpcChannel?to communicate with the NameNode.

Connection

The Hadoop RPC protocol works as described below. On connection, headers are sent to setup a session. After that, multiple requests can be sent within the session.

FunctionTypeDefault

Header	bytes	“hrpc”
Version	uint8	7
Auth method	uint8	80 (Auth method?SIMPLE)
Serialization type	uint8	0 (protobuf)
IpcConnectionContextProto length	uint32	?
IpcConnectionContextProto	bytes	?

Sending messages

When sending a message, the following is sent to the sever:

FunctionType

Length of the next two parts	uint32
RpcPayloadHeaderProto length	varint
RpcPayloadHeaderProto	protobuf?serialized?message
HadoopRpcRequestProto length	varint
HadoopRpcRequestProto	protobuf?serialized?message

varint?is a?Protocol Buffer variable int.

Note

The Java protobuf implementation uses?writeToDelimited?to prepend the message with their lenght, but the python implementation doesn’t implement such a method (yet).

Next to an?rpcKind?(snakebites default is?RPC_PROTOCOL_BUFFER), an?rpcOp?(snakebites default isRPC_FINAL_PAYLOAD), the?RpcPayloadHeaderProto?message defines a?callId?that is added in the RPC response (described below).

The?HadoopRpcRequestProto?contains a?methodName?field that defines what server method is called and a has a property?request?that contains the serialized actual request message.

Receiving messages

After a message is sent, the response can be read in the following way:

FunctionType

Length of the RpcResponseHeaderProto	varint
RpcResponseHeaderProto	bytes
Length of the RPC response	uint32
Serialized RPC response	bytes

The?RpcResponseHeaderProto?contains the?callId?of the request and a status field. The status can beSUCCESS,?ERROR?or?FAILURE. In case?SUCCESS?the rest of response is a complete protobuf response.

In case of?ERROR, the response looks like follows:

FunctionType

Length of the RpcResponseHeaderProto	varint
RpcResponseHeaderProto	bytes
Length of the RPC response	uint32
Length of the Exeption class name	uint32
Exception class name	utf-8?string
Length of the stack trace	uint32
Stack trace	utf-8?string

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