finagle
A client and server for the mux protocol described in com.twitter.finagle.mux.
Package mux implements a generic RPC multiplexer with a rich protocol.
Package mux implements a generic RPC multiplexer with a rich protocol. Mux is itself encoding independent, so it is meant to use as the transport for other RPC systems (eg. thrift). In OSI terminology, it is a pure session layer.
In the below description, all numeric values are unsigned and in
big-endian byte order. The schema size:4 body:10 defines the
field size to be 4 bytes, followed by 10 bytes of the field body. The
schema key~4 defines the field key to be defined by 4 bytes
interpreted as the size of the field, followed by that many bytes
comprising the field itself--it is shorthand for keysize:4 key:keysize.
Groups are denoted by parenthesis; * denotes repetition of the
previous schema 0 or more times, while {n} indicates repetition
exactly n times. Unspecified sizes consume the rest of the frame:
they may be specified only as the last field in the message.
All strings in Mux are Utf-8 encoded, and are never null-terminated.
Messages in mux are framed with a 4-byte big-endian size header, followed by 1 byte describing the message type and a 3-byte tag; or, diagrammatically: size:4 type:1 tag:3. The remainder of the frame (size-4 bytes) contains the body. Its format depends on the message type, documented below.
Tag 0 designates a "marker" T message that expects no reply. Some messages may be split into an ordered sequence of fragments. Tag MSB=0 denotes the last message in such a sequence, making the tag namespace 23 bits. The tag is otherwise arbitrary, and is chosen by the sender of the T message.
Currently, only Tdispatch and Rdispatch messages may be split into an ordered sequence of fragments. TdispatchError message ends a Tdispatch sequence and an Rerr ends an Rdispatch sequence.
Message types, interpreted as a two's complement, 1-byte integer are numbered as follows: positive numbers are T-messages; their negative complement is the corresponding R message. T-messages greater than 63 (correspondingly R-messages smaller than -63) are session messages. The message number -128 is reserved for Rerr. All other messages are application messages. Middle boxes may forward application messages indiscriminately. Because of an early implementation bug, two aliases exist: 127 is Rerr, and -62 is Tdiscarded.
The protocol is full duplex: both the server and client may send T messages initiating an exchange.
Messages are designated as "T messages" or "R messages", T and R being stand-ins for transmit and receive. A T message initiates an exchange and is assigned a free tag by the sender. A reply is either an R message of the same type (Rx replies to Tx for some x), or an Rerr, indicating a session layer error. R messages are matched to their T messages by tag, and the reply concludes the exchange and frees the tag for future use. Implementations should reuse small tag numbers.
size:4 Tinit:1 tag:3 version:2 (key~4 value~4)* reinitializes a session. Clients typically send this at the beginning of the session. When doing so, the sender may issue no more T messages until the corresponding size:4 Rinit:1 tag:3 version:2 (key~4 value~4)* has been received. After the Rinit was received, all connection state has been reset (outstanding tags are invalidated) and the stream is resumed according to the newly negotiated parameters. Prior to the first Tinit, the session operates at version 1. Rinit's version field is the accepted version of the session (which may be lower than the one requested by Tinit).
size:4 Treq:1 tag:3 n:1 (key:1 value~1){n} body: initiates the request described by its body. The request body is delivered to the application. The request header contains a number of key-value pairs that describe request metadata.
Keys for Treq messages are as follows:
2. traceflag: a bitmask describing trace flags. Currently, the only defined flag is bit 0 which enables "debug mode", asking the server to force trace sampling.
size:4 Tdispatch:1 tag:3 nctx:2 (ckey~2 cval~2){nc} dst~2 nd:2 (from~2 to~2){nd} body: implements destination dispatch. Tdispatch messages carry a set of keyed request contexts, followed by a logical destination encoded as a UTF-8 string. A delegation table follows describing rewrite rules that apply to this request.
size:4 Rreq:1 tag:3 status:1 body: replies to a request. Status codes are as follows: 0=OK; the body contains the reply. 1=ERROR; the body contains a string describing the error. 2=NACK; a negative acknowledgment, the body contains a string describing the reason.
size:4 Rdispatch:1 tag:3 status:1 nctx:2 (key~2 value~2){nctx} body: replies to a Tdispatch request. Status codes are as in Rreq. Replies can include request contexts. MuxFailure flags are currently sent via Rdispatch contexts under the "MuxFailure" key. See the MuxFailure flags section below.
size:4 Rerr:1 tag:3 why: indicates that the corresponding T message produced an error. Rerr is specifically for server errors: the server failed to interpret or act on the message. The body carries a string describing the error.
size:4 Tdrain:1 tag:3 is a request sent by the server telling the client to stop sending new requests. A client acknowledges this with an Rdrain message.
size:4 Tping:1 tag:3 is sent by either party to check the liveness of its peer; these should be responded to immediately with a Rping message.
size:4 Tdiscarded:1 tag:3 discard_tag:3 why: is a marker message and therefore has a tag value of 0. discard_tag indicates the tag of the Tdispatch to be discarded by the client. This can be used as a hint for early termination. Why is a string describing why the request was discarded. Note that it does *not* free the server from the obligation of replying to the original Treq.
size:4 Tlease:1 tag:3 unit:1 howmuch:8 is a marker message indicating that a lease has been issued for howmuch units. As a marker message, its tag value must be 0. Unit '0' is reserved for duration in milliseconds. Whenever a lease has not been issued, a client can assume it holds an indefinite lease. Adhering to the lease is optional, but the server may reject requests or provide degraded service should the lease expire. This is used by servers to implement features like garbage collection avoidance.
Failure flags are read and written as an 8 byte integer. Unrecognized flags will be ignored silently, but should all be considered reserved for future use.
Flag Value Meaning Restartable 1 << 0 Request is safe to re-issue Rejected 1 << 1 Request was rejected/Nacked by the server NonRetryable 1 << 2 Request should not be retried