Module Faraday

Serialization primitives built for speed an memory-efficiency.

Faraday is a library for writing fast and memory-efficient serializers. Its core type and related operation gives the user fine-grained control over copying and allocation behavior while serializing user-defined types, and presents the output in a form that makes it possible to use vectorized write operations, such as the writev system call, or any other platform or application-specific output APIs.

A Faraday serializer manages an internal buffer and a queue of output buffers. The output bufferes may be a sub range of the serializer's internal buffer or one that is user-provided. Buffered writes such as write_string, write_char, write_bigstring, etc., copy the source bytes into the serializer's internal buffer. Unbuffered writes such as schedule_string, schedule_bigstring, etc., on the other hand perform no copying. Instead, they enqueue the source bytes into the serializer's write queue directly.

type bigstring = (char, Bigarray.int8_unsigned_eltBigarray.c_layout) Bigarray.Array1.t
type t

The type of a serializer.


val create : int -> t

create len creates a serializer with a fixed-length internal buffer of length len. See the Buffered writes section for details about what happens when len is not large enough to support a write.

val of_bigstring : bigstring -> t

of_bigstring buf creates a serializer, using buf as its internal buffer. The serializer takes ownership of buf until the serializer has been closed and flushed of all output.

Buffered Writes

A serializer manages an internal buffer for coalescing small writes. The size of this buffer is determined when the serializer is created. If the buffer does not contain sufficient space to service a caller's buffered write, the serializer will allocate a new buffer of the sufficient size and use it for the current and subsequent writes. The old buffer will be garbage collected once all of its contents have been flushed.

val write_string : t -> ?⁠off:int -> ?⁠len:int -> string -> unit

write_string t ?off ?len str copies str into the serializer's internal buffer.

val write_bytes : t -> ?⁠off:int -> ?⁠len:int -> Stdlib.Bytes.t -> unit

write_bytes t ?off ?len bytes copies bytes into the serializer's internal buffer. It is safe to modify bytes after this call returns.

val write_bigstring : t -> ?⁠off:int -> ?⁠len:int -> bigstring -> unit

write_bigstring t ?off ?len bigstring copies bigstring into the serializer's internal buffer. It is safe to modify bigstring after this call returns.

val write_gen : t -> length:('a -> int) -> blit:('a -> src_off:int -> bigstring -> dst_off:int -> len:int -> unit) -> ?⁠off:int -> ?⁠len:int -> 'a -> unit

write_gen t ~length ~blit ?off ?len x copies x into the serializer's internal buffer using the provided length and blit operations. See Bigstring.blit for documentation of the arguments.

val write_char : t -> char -> unit

write_char t char copies char into the serializer's internal buffer.

val write_uint8 : t -> int -> unit

write_uint8 t n copies the lower 8 bits of n into the serializer's internal buffer.

module BE : sig ... end

Big endian serializers

module LE : sig ... end

Little endian serializers

Unbuffered Writes

Unbuffered writes do not involve copying bytes to the serializers internal buffer.

val schedule_bigstring : t -> ?⁠off:int -> ?⁠len:int -> bigstring -> unit

schedule_bigstring t ?off ?len bigstring schedules bigstring to be written the next time the serializer surfaces writes to the user. bigstring is not copied in this process, so bigstring should only be modified after t has been flushed.

Querying A Serializer's State

val free_bytes_in_buffer : t -> int

free_bytes_in_buffer t returns the free space, in bytes, of the serializer's write buffer. If a write_* call has a length that exceeds this value, the serializer will allocate a new buffer that will replace the serializer's internal buffer for that and subsequent calls.

val has_pending_output : t -> bool

has_pending_output t is true if t's output queue is non-empty. It may be the case that t's queued output is being serviced by some other thread of control, but has not yet completed.

val pending_bytes : t -> int

pending_bytes t is the size of the next write, in bytes, that t will surface to the caller as a `Writev.

Control Operations

val yield : t -> unit

yield t causes t to delay surfacing writes to the user, instead returning a `Yield. This gives the serializer an opportunity to collect additional writes before sending them to the underlying device, which will increase the write batch size.

As one example, code may want to call this function if it's about to release the OCaml lock and perform a blocking system call, but would like to batch output across that system call. To hint to the thread of control that is performing the writes on behalf of the serializer, the code might call yield t before releasing the lock.

val flush : t -> (unit -> unit) -> unit

flush t f registers f to be called when all prior writes have been successfully completed. If t has no pending writes, then f will be called immediately. If yield was recently called on t, then the effect of the yield will be ignored so that client code has an opportunity to write pending output, regardless of how it handles `Yield operations.

val close : t -> unit

close t closes t. All subsequent write calls will raise, and any pending or subsequent yield calls will be ignored. If the serializer has any pending writes, user code will have an opportunity to service them before it receives the Close operation. Flush callbacks will continue to be invoked while output is shifted out of t as needed.

val is_closed : t -> bool

is_closed t is true if close has been called on t and false otherwise. A closed t may still have pending output.

val shift : t -> int -> unit

shift t n removes the first n bytes in t's write queue. Any flush callbacks registered with t within this span of the write queue will be called.

val drain : t -> int

drain t removes all pending writes from t, returning the number of bytes that were enqueued to be written and freeing any scheduled buffers in the process.


Low-level operations for runing a serializer. For production use-cases, consider the Async and Lwt support that this library includes before attempting to use this these operations directly.

type 'a iovec = {
buffer : 'a;
off : int;
len : int;

A view into iovec.buffer starting at and with length iovec.len.

type operation = [
| `Writev of bigstring iovec list
| `Yield
| `Close

The type of operations that the serialier may wish to perform.

  • `Writev iovecs: Write the bytes in iovecss reporting the actual number of bytes written by calling shift. You must accurately report the number of bytes written. Failure to do so will result in the same bytes being surfaced in a `Writev operation multiple times.
  • `Yield: Yield to other threads of control, waiting for additional output before procedding. The method for achieving this is application-specific, but once complete, the caller can proceed with serialization by simply making another call to operation or serialize.
  • `Close: Serialization is complete. No further output will generated. The action to take as a result, if any, is application-specific.
val operation : t -> operation

operation t is the next operation that the caller must perform on behalf of the serializer t. Users should consider using serialize before this function. See the documentation for the operation type for details on how callers should handle these operations.

val serialize : t -> (bigstring iovec list -> [ `Ok of int | `Closed ]) -> [ `Yield | `Close ]

serialize t writev sufaces the next operation of t to the caller, handling a `Writev operation with writev function and performing an additional bookkeeping on the caller's behalf. In the event that writev indicates a partial write, serialize will call yield on the serializer rather than attempting successive writev calls.

Convenience Functions

These functions are included for testing, debugging, and general development. They are not the suggested way of driving a serializer in a production setting.

val serialize_to_string : t -> string

serialize_to_string t runs t, collecting the output into a string and returning it. serialzie_to_string t immediately closes t and ignores any calls to yield on t.

val serialize_to_bigstring : t -> bigstring

serialize_to_string t runs t, collecting the output into a bigstring and returning it. serialzie_to_bigstring t immediately closes t and ignores any calls to yield on t.