# Module Xdr_mstring

    module Xdr_mstring: sig .. end

Managed Strings

Managed strings are used in XDR context for constant strings that are stored either as string or as memory (bigarray of char).

A managed string `ms` is declared in the XDR file as in

          typedef _managed string ms<>;

In the encoded XDR stream there is no difference between strings and managed strings, i.e. the wire representation is identical. Only the Ocaml type differs to which the managed string is mapped. This type is `Xdr_mstring.mstring` (below).

In the RPC context there is often the problem that the I/O backend would profit from a different string representation than the user of the RPC layer. To bridge this gap, managed strings have been invented. Generally, the user can determine how to represent strings (usually either as an Ocaml string, or as memory), and the I/O backend can request to transform to a different representation when this leads to an improvement (i.e. copy operations can be saved).

Only large managed strings result in a speedup of the program (at least several K).

## How to practically use managed strings

There are two cases: The encoding case, and the decoding case. In the encoding case the `mstring` object is created by the user and passed to the RPC library. This happens when a client prepares an argument for calling a remote procedure, or when the server sends a response back to the caller. In the decoding case the client analyzes the response from an RPC call, or the server looks at the arguments of an RPC invocation. The difference here is that in the encoding case user code can directly create `mstring` objects by calling functions of this module, whereas in the decoding case the RPC library creates the `mstring` objects.

For simplicity, let us only look at this problem from the perspective of an RPC client.

Encoding. Image a client wants to call an RPC, and one of the arguments is a managed string. This means we finally need an `mstring` object that can be put into the argument list of the call.

This library supports two string representation specially: The normal Ocaml `string` type, and `Netsys_mem.memory` which is actually just a bigarray of char's. There are two factories `fac`,

- `Xdr_mstring.string_based_mstrings`, and
- `Xdr_mstring.memory_based_mstrings`,

and both can be used to create the `mstring` to pass to the RPC layer. It should be noted that this layer can process the `memory` representation a bit better. So, if the original `data` value is a string, the factory for `string` should be used, and if it is a char bigarray, the factory for `memory` should be used. Now, the `mstring` object is created by

- `let mstring = fac # create_from_string data pos len copy_flag`, or by
- `let mstring = fac # create_from_memory data pos len copy_flag`.

Of course, if `fac` is the factory for strings, the `create_from_string` method works better, and if `fac` is for `memory`, the `create_from_memory` method works better. `pos` and `len` can select a substring of `data`. If `copy_flag` is `false`, the `mstring` object does not copy the data if possible, but just keeps a reference to `data` until it is accessed; otherwise if `copy_flag` is `true`, a copy is made immediately. Of couse, delaying the copy is better, but this requires that `data` is not modified until the RPC call is completed.

Decoding. Now, the call is done, and the client looks at the result. There is also an `mstring` object in the result. As noted above, this `mstring` object was already created by the RPC library (and currently this library prefers string-based objects if not told otherwise). The user code can now access this `mstring` object with the access methods of the `mstring` class (see below). As these methods are quite limited, it makes normally only sense to output the `mstring` contents to a file descriptor.

The user can request a different factory for managed strings. The function `Rpc_client.set_mstring_factories` can be used for this purpose. (Similar ways exist for managed clients, and for RPC servers.)

Potential. Before introducing managed strings, a clean analysis was done how many copy operations can be avoided by using this technique. Example: The first N bytes of a file are taken as argument of an RPC call. Instead of reading these bytes into a normal Ocaml string, an optimal implementation uses now a `memory` buffer for this purpose. This gives:

- Old implementation with strings and ocamlnet-2: Data is copied six times from reading it from the file until writing it to the socket.
- New implementation with memory-based mstrings: Data is copied only twice! The first copy reads it from the file into the input buffer (a `memory` value), and the second copy writes the data into the socket.

Part of the optimization is that `Unix.read` and `Unix.write` do a completely avoidable copy of the data which is prevented by switching to `Netsys_mem.mem_read` and `Netsys_mem.mem_write`, respectively. The latter two functions exploit an optimization that is only possible when the data is `memory`-typed.

The possible optimizations for the decoding side of the problem are slightly less impressive, but still worth doing it.

## Interface

    class type mstring = object .. end

The object holding the string value

    class type mstring_factory = object .. end

The object creating new `mstring` objects

    val string_based_mstrings : mstring_factory

Uses strings to represent mstrings

    val string_to_mstring : ?pos:int -> ?len:int -> string -> mstring

Represent a string as mstring (no copy)

    val memory_based_mstrings : mstring_factory

Uses memory to represent mstrings. The memory bigarrays are allocated with `Bigarray.Array1.create`

    val memory_to_mstring : ?pos:int -> ?len:int -> Netsys_mem.memory -> mstring

Represent memory as mstring (no copy)

    val paligned_memory_based_mstrings : mstring_factory

Uses memory to represent mstrings. The memory bigarrays are allocated with `Netsys_mem.alloc_memory_pages` if available, and `Bigarray.Array1.create` if not.

    val memory_pool_based_mstrings : Netsys_mem.memory_pool -> mstring_factory

Uses memory to represent mstrings. The memory bigarrays are obtained from the pool. The length of these mstrings is limited by the blocksize of the pool.

    val length_mstrings : mstring list -> int

returns the sum of the lengths of the mstrings

    val concat_mstrings : mstring list -> string

concatenates the mstrings and return them as single string. The returned string may be shared with one of the mstrings passed in.

    val prefix_mstrings : mstring list -> int -> string

`prefix_mstrings l n`: returns the first `n` chars of the concatenated mstrings `l` as single string

    val blit_mstrings_to_memory : mstring list -> Netsys_mem.memory -> unit

blits the mstrings one after the other to the memory, so that they appear there concatenated

    val shared_sub_mstring : mstring -> int -> int -> mstring

`shared_sub_mstring ms pos len`: returns an mstring that includes a substring of `ms`, starting at `pos`, and with `len` bytes. The returned mstring shares the buffer with the original mstring `ms`

    val shared_sub_mstrings : mstring list -> int -> int -> mstring list

Same for a list of mstrings

    val copy_mstring : mstring -> mstring

Create a copy

    val copy_mstrings : mstring list -> mstring list

Create a copy

    type named_mstring_factories = (string, mstring_factory) Hashtbl.t