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# RPC protocol
## Data encoding
All integral data is encoded in little endian format.
## Protocol negotiation
The negotiation works by exchanging negotiation frame immediately after connection establishment. The negotiation frame format is:
uint8_t magic[8] = SSTARRPC
uint32_t len
uint8_t data[len]
The negotiation frame data is itself composed of multiple records, one for each feature number present. Feature numbers begin at zero and will be defined by later versions of this document.
struct negotiation_frame_feature_record {
uint32_t feature_number;
uint32_t len;
uint8_t data[len];
}
A `negotiation_frame_feature_record` signals that an optional feature is present in the client, and can contain additional feature-specific data. The feature number will be omitted in a server response if an optional feature is declined by the server.
Actual negotiation looks like this:
Client Server
--------------------------------------------------------------------------------------------------
send negotiation frame
recv frame
check magic (disconnect if magic is not SSTARRPC)
send negotiation frame back
recv frame
check magic (disconnect if magic is not SSTARRPC)
### Supported features
#### Compression
feature_number: 0
data : opaque data that is passed to a compressor factory
provided by an application. Compressor factory is
responsible for negotiation of compression algorithm.
If compression is negotiated request and response frames are encapsulated in a compressed frame.
#### Timeout propagation
feature_number: 1
data : none
If timeout propagation is negotiated request frame has additional 8 bytes that hold timeout value
for a request in milliseconds. Zero value means that timeout value was not specified.
If timeout is specified and server cannot handle the request in specified time frame it my choose
to not send the reply back (sending it back will not be an error either).
#### Connection ID
feature_number: 2
uint64_t conenction_id : RPC connection ID
Server assigns unique connection ID for each connection and sends it to a client using
this feature.
#### Stream parent
feature_number: 3
uint64_t connection_id : RPC connection ID representing a parent of the stream
If this feature is present it means that the connection is not regular RPC connection
but stream connection. If parent connection is closed or aborted all streams belonging
to it will be closed as well.
Stream connection is a connection that allows bidirectional flow of bytes which may carry one or
more messages in each direction. Stream connection should be explicitly closed by both client and
server. Closing is done by sending special EOS frame (described below).
#### Isolation
feature number: 4
uint32_t isolation_cookie_len
uint8_t isolation_cookie[len]
The `isolation_cookie` field is used by the server to select a
`seastar::scheduling_group` (or equivalent in another implementation) that
will run this connection. In the future it will also be used for rpc buffer
isolation, to avoid rpc traffic in one isolation group from starving another.
The server does not directly assign meaning to values of `isolation_cookie`;
instead, the interpretation is left to user code.
##### Compressed frame format
uint32_t len
uint8_t compressed_data[len]
after compressed_data is uncompressed it becomes regular request, response or streaming frame
## Request frame format
uint64_t timeout_in_ms - only present if timeout propagation is negotiated
uint64_t verb_type
int64_t msg_id
uint32_t len
uint8_t data[len]
msg_id has to be positive and may never be reused.
data is transparent for the protocol and serialized/deserialized by a user
## Response frame format
int64_t msg_id
uint32_t len
uint8_t data[len]
if msg_id < 0 enclosed response contains an exception that came as a response to msg id abs(msg_id)
data is transparent for the protocol and serialized/deserialized by a user
## Stream frame format
uint32_t len
uint8_t data[len]
len == 0xffffffff signals end of stream
data is transparent for the protocol and serialized/deserialized by a user
## Exception encoding
uint32_t type
uint32_t len
uint8_t data[len]
### Known exception types
USER = 0
UNKNOWN_VERB = 1
#### USER exception encoding
uint32_t len
char[len]
This exception is sent as a reply if rpc handler throws an exception.
It is delivered to a caller as std::runtime_error(char[len])
#### UNKNOWN_VERB exception encoding
uint64_t verb_id
This exception is sent as a response to a request with unknown verb_id, the verb id is passed back as part of the exception payload.
## More formal protocol description
request_stream = negotiation_frame, { request | compressed_request }
request = verb_type, msg_id, len, { byte }*len
compressed_request = len, { bytes }*len
response_stream = negotiation_frame, { response | compressed_response }
response = reply | exception
compressed_response = len, { byte }*len
streaming_stream = negotiation_frame, { streaming_frame | compressed_streaming_frame }
streaming_frame = len, { byte }*len
compressed_streaming_frame = len, { byte }*len
reply = msg_id, len, { byte }*len
exception = exception_header, serialized_exception
exception_header = -msg_id, len
serialized_exception = (user|unknown_verb)
user = len, {byte}*len
unknown_verb = verb_type
verb_type = uint64_t
msg_id = int64_t
len = uint32_t
byte = uint8_t
negotiation_frame = 'SSTARRPC' len32(negotiation_frame_data) negotiation_frame_data
negotiation_frame_data = negotiation_frame_feature_record*
negotiation_frame_feature_record = feature_number len {byte}*len
feature_number = uint32_t
Note that replies can come in order different from requests, and some requests may not have a reply at all.
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