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streaming JSON RPC over TCP

node-fast: streaming JSON RPC over TCP

Fast is a simple RPC protocol used in Joyent's Triton and Manta systems, particularly in the Moray key-value store. This README contains usage notes. For developers, see

This module includes:

  • client library interface
  • server library interface
  • fastcall, a command-line tool for making Fast RPC requests
  • fastserve, a command-line Fast server for demo and testing
  • fastbench, a command-line tool for very basic client load generation

This rewrite was done to fix a number of issues with service discovery and connection management in the previous node-fast client. Service discovery and connection management in this client are managed by the caller using something like cueball.

The library interfaces here follow Joyent's Best Practices for Error Handling. In particular, all operational errors are emitted asynchronously. It is a programmer error to call any of the public APIs with missing or incorrectly-typed arguments. All arguments are required unless otherwise specified.


Start the Fast server:

$ fastserve 2030 > server.log &

Use fastcall to invoke the date RPC method inside the client:

$ fastcall 2030 date '[]'

Or try the yes method, an RPC version of yes(1):

$ fastcall 2030 yes '[ { "value": { "hello": "world" }, "count": 3 } ]'

For an example client and server, see the fastcall and fastserve programs.


The Fast protocol is intended for internal systems. It does not support authentication. Neither the client nor server implementations are hardened against byzantine behavior (e.g., denial-of-service attacks), though they are expected to correctly handle all network-related failure modes and byzantine input.

Previous implementations of the Fast protocol supported cancellation, but it was dangerous to use with servers that did not support it, and there was no way to tell if the server did support it. As a result, this implementation does not support cancellation of in-flight requests. (There's an abandon() function in the client API, but it only causes the request stream to fail. The underlying RPC continues executing and incoming messages are ignored.)

Early versions of the original node-fast module used a buggy CRC implementation. Since changing the CRC algorithm would require a flag day among deployed components, this module continues to use the buggy CRC implementation.


Kang server

The Fast client and server provide functions suitable for use with kang, a small library for exposing debugging information over an HTTP API.

The server-side kang functions report:

  • server-wide statistics about connections created, requests started, requests completed, and requests failed;
  • per-connection state information (including time accepted and errors seen) and statistics about requests started, completed, and failed; and
  • per-request state information (including time started)

These enable basic monitoring of server activity and health. The connection and request state information allows you to see which clients are connected, how long they've been connected, and how long individual requests have been running, which is often helpful in identifying leaked or hung requests.

The client-side kang functions report per-client and per-request state and statistics.

The client and server only provide functions; you have to hook this up to a kang server. The built-in demo server ("fastserve") and benchmarking tool ("fastbench") demonstrate how to do that.

DTrace probes

The Fast client and server provide DTrace probes and scripts in the "bin" directory that use these probes to show client and server activity. These scripts, the probes, and their arguments may change over time.

Fast client probes:

Probe name Event Arg0 Arg1 Arg2 Arg3
rpc-start Client begins issuing an RPC call (int) Client identifier (int) Message identifier (string) RPC method name (json) object with "rpcargs" and optional "timeout".
rpc-data Client receives 'data' message for outstanding RPC (int) Client identifier (int) Message identifier (json) Received data -
rpc-done Client finishes processing RPC (int) Client identifier (int) Message identifier (json) May contain "error" describing any error that occurred. -

Fast client scripts in "bin" directory:

  • fastclatency: trace latency of all client RPC requests. Prints power-of-two histogram of request latency when the script exits.
  • fastcsnoop: dump out client RPC activity (all "start", "data", and "done" events).

Note that the client identifier is only unique within a process, and the message identifier is only unique for a given client. Both are only unique over a given period of time (i.e., client ids and message ids may be reused). See the sample scripts for how to use these correctly.

Fast server probes:

Probe name Event Arg0 Arg1 Arg2 Arg3
conn-create Client connection created (int) server identifier (int) client identifier (string) client label -
conn-destroy Client connection destroyed (int) server identifier (int) client identifier - -
rpc-start Server starts processing an RPC call (int) server identifier (int) client identifier (int) request identifier (string) RPC method name
rpc-done Server finishes processing an RPC call (int) server identifier (int) client identifier (int) request identifier -

Fast server script in "bin" directory:

  • fastssnoop: dump out server RPC activity (client connection create/destroy and rpc start/done).

Similar to the client, the server identifier is only unique within the process, and the request identifier is only unique within the client. See the sample scripts for how to use these correctly.

The client and server also use bunyan for logging. On systems with DTrace support, you can use runtime log snooping (bunyan -p) to observe what the client and server would be logging at the finest-grained log levels, even if you haven't enabled those.

Client API

Consumers of the client API are responsible for maintaining persistent connections to the server. The cueball module is recommended for this purpose. Clients pass connected sockets to the FastClient constructor:

FastClient class

A FastClient attaches to a Node net.Socket object. The FastClient makes RPC calls by sending Fast messages over the socket and reading responses in the form of Fast messages from the socket.

Constructor arguments:

Name Type Meaning
log object bunyan-style logger
transport net.Socket underlying TCP connection to the server
nRecentRequests positive int number of recent requests to track for debugging purposes

While consumers are responsible for handling socket errors, the FastClient will watch the transport for error and end events in order to proactively fail any outstanding requests.

Public methods:

  • rpc(args): initiate an RPC request
  • rpcBufferAndCallback(args): initiate an RPC request and buffer incoming data
  • request.abandon(): abandon an RPC request
  • detach(): detach client from underlying socket

This class emits error when there's a problem with the underlying socket (other than an error emitted by the socket itself) that prevents any requests from completing. This would usually be a protocol error of some sort.

rpc(args): initiate an RPC request to the remote server

Named arguments:

Name Type Meaning
rpcmethod string name of the RPC method to invoke on the server
rpcargs array JSON-serializable array of RPC call arguments
timeout integer (optional) milliseconds after which to abandon the request if it has not already completed. The default is that there is no timeout.
log object (optional) bunyan logger for this request. If not specified, a child logger of the client-level logger will be used.
ignoreNullValues boolean (optional) if true, null data values will be accepted from the server and dropped. These are otherwise considered a protocol error.

The return value is an object-mode stream that consumers use to interact with the request. Objects sent by the server to the client are made available via this stream. The stream emits end when the server successfully completes the request. The stream emits error when the server reports an error, or if there's a socket error or a protocol error. Consumers need not proactively abandon requests that fail due to a socket error.

Keep in mind that with any distributed system, failure of an RPC request due to a socket error, protocol error, network failure, or timeout does not mean that the RPC did not complete successfully or even that it is not still running. The server may have successfully completed the request, or failed it for a different reason, or may still be running it when a network error occurs. Consumers must keep this in mind in designing RPC protocols and responses to failure.

As with other Node streams, the request stream will emit exactly one end or error event, after which no other events will be emitted.

rpcBufferAndCallback(args, callback): initiate an RPC request and buffer response

This is a convenience function for making RPC calls when the server is expected to return a bounded number of objects and the client doesn't intend to process the results in a streaming way. This function buffers up to maxObjectsToBuffer objects and invokes callback when the request is complete, as:

callback(err, data, ndata)


  • err is the error emitted by the request
  • data is an array of buffered data objects, which will have at most maxObjectsToBuffer elements
  • ndata is a non-negative integer describing the total number of data objects received. This may be larger than data.length only if some objects were dropped because maxObjectsToBuffer objects had already been buffered.

Note that data and ndata will always be present and valid, even if err is non-null, since some number of data objects may have been received before the request error.

Named arguments:

Name Type Meaning
rpcmethod string see arguments to rpc()
rpcargs array see arguments to rpc()
timeout integer see arguments to rpc()
log object see arguments to rpc()
maxObjectsToBuffer non-negative integer maximum number of received data objects that may be buffered. Subsequently received objects will be dropped. Callers can tell whether this happened by looking at the ndata argument to the callback.

request.abandon(): abandon an RPC request

Callers may invoke abandon() on the return value from rpc(args) to abandon the request. If the request is not yet complete, then it will emit an error indicating the abandonment, and no more data will be emitted.

This does not actually notify the server. The server will still process the request.

Additionally, the client will continue to maintain state about this request until whenever the request would have otherwise terminated (i.e., until the request completes normally, the client is detached from the transport, or there's a transport error). This can in principle result in holding onto a small amount of memory for some time. Since consumers are expected to identify and respond to transport errors (e.g., using TCP KeepAlive or the like), this should only be possible if the server itself has hung responding to the request, and the resources will be freed when the server is restarted or disappears (as detected by the underlying transport) or if the consumer drops its references to this Fast client.

detach(): detach client from underlying socket

This method causes the client to stop sending data on the socket and stop reading from the socket. Any outstanding RPC requests are failed as though the socket had emitted an error.

This can be used during a graceful shutdown (e.g., of a command-line tool) to tear down a Fast client and its associated socket. This could also be used if the caller wanted to stop using this particular client immediately (e.g., because the remote server is no longer registered in the caller's service discovery mechanism and should not be used any more).

Server API

FastServer class

Each FastServer instance wraps a net.Socket server socket. The server keeps track of clients connected to it.

Named arguments for the constructor:

Name Type Meaning
log object bunyan-style logger
server net.Socket underlying server socket

Public methods:

  • registerRpcMethod(args): register an RPC method handler
  • close(): shut down the server

registerRpcMethod(args): register an RPC method handler

Registers a JavaScript function to invoke for incoming RPC requests. Named arguments include:

Name Type Meaning
rpcmethod string name of the method, as clients will specify it when making RPC calls
rpchandler function JavaScript function to invoke for each incoming request

The RPC handler function will be invoked as rpchandler(rpc), where rpc is an RPC context object. This is a function-oriented interface for accessing information about the RPC, including the request identifier, method name, and arguments. It provides the following read-only methods:

  • rpc.connectionId(): returns a unique identifier for this connection
  • rpc.requestId(): returns a unique identifier for this request
  • rpc.methodName(): returns the client-specified name of this request
  • rpc.argv(): returns the array of arguments provided by the client for the request
  • report failure of the RPC request with the specified error

The rpc object is also an object-mode stream that the handler can use to emit values and report request completion. Values are sent to the client by writing them to the stream. Flow control is supported, provided the handler follows conventions for that (i.e., using pipe() or checking the return value of write()). When the request has completed, the handler should end the stream with its end() method.

The handler should report failure by invoking, where err is an error describing the failure. The handler should not emit data or end the request gracefully after reporting an error.

close(): shut down the server

This method shuts down the server by disconnecting outstanding requests from their underlying connections and then destroying those underlying client sockets. The consumer should close the underlying server socket first in order to ensure no new connections will be created. Any newly-created connections will be dealt with, but without closing the server socket, there is no guarantee that this process will converge.

Since the interface for RPC handlers does not currently provide a way to inform those handlers that the request has been cancelled because of a case like this, handlers for outstanding requests continue as normal, and any data emitted is ignored. As a result, though, these handlers may continue running even after this function has been called and client sockets are destroyed.

Protocol overview

The Fast protocol is intended for use with TCP. Typically, a Fast server listens for TCP connections on a well-known port, and Fast clients connect to the server to make RPC requests. Clients can make multiple connections to the server, but each connection represents a logically separate client. Communication between client and server consist of discrete messages sent over the TCP connection. Each message contains:

Field Type Purpose
msgid 32-bit integer identifies messages related to a given request
status 8-bit integer indicates what kind of message this is
data raw JSON data depends on message status

Messages have headers that include additional information, like payload length and checksum. The physical format is described in detail in lib/fast_protocol.js.

There are three allowed values for status:

Status value Status name Description
0x1 DATA From clients, indicates an RPC request. From servers, indicates one of many values emitted by an RPC call.
0x2 END Indicates the successful completion of an RPC call. Only sent by servers.
0x3 ERROR Indicates the failed completion of an RPC call. Only sent by servers.

For all messages, the data field contains properties:

Field Type Purpose
m object describes the RPC method being invoked string name of the RPC method being invoked
m.uts number (optional) timestamp of message creation, in microseconds since the Unix epoch
d object or array varies by message status

In summary, there are four kinds of messages.

Client initiates an RPC request. The client allocates a new message identifier and sends a DATA message with set to the name of the RPC method it wants to invoke. Arguments are specified by the array data.d. Clients may issue concurrent requests over a single TCP connection, provided they do not re-use a message identifier for separate requests.

Server sends data from an RPC call. RPC calls may emit an arbitrary number of values back to the client. To emit these values, the server sends DATA messages with data.d set to an array of non-null values to be emitted. All DATA messages for the same RPC request have the same message identifier that the client included in its original DATA message that initiated the RPC call.

Server completes an RPC call successfully. When an RPC call completes successfully, the server sends an END event having the same message identifier as the one in the client's original DATA message that initiated the RPC call. This message can contain data as well, in which case it should be processed the same way as for a DATA message.

Server reports a failed RPC call. Any time before an END message is generated for an RPC call, the server may send an ERROR message having the same message identifier as the one in the client's original DATA message that initiated the RPC call.

By convention, the m fields ( and m.uts) are populated for all server messages, even though is redundant.

The RPC request begins when the client sends the initial DATA message. The RPC request is finished when the server sends either an ERROR or END message for that request. In summary, the client only ever sends one message for each request. The server may send any number of DATA messages and exactly one END or ERROR message.