Ocamlnet 4 Reference Manual : Netchannels_tut

# Netchannels_tut

# Netchannels Tutorial

`Netchannels` is one of the basic modules of this library, because it provides some very basic abstractions needed for many other functions of the library. The key abstractions `Netchannels` defines are the types `in_obj_channel` and `out_obj_channel`. Both are class types providing sequential access to byte streams, one for input, one for output. They are comparable to the types `in_channel` and `out_channel` of the standard library that allow access to files. However, there is one fundamental difference: `in_channel` and `out_channel` are restricted to resources that are available through file descriptors, whereas `in_obj_channel` and `out_obj_channel` are just class types, and by providing implementations for them any kind of resources can be accessed.

## Motivation

In some respect, `Netchannels` fixes a deficiency of the standard library. Look at the module `Printf` which defines six variants of the `printf` function:

    val fprintf : out_channel -> ('a, out_channel, unit) format -> 'a
    val printf : ('a, out_channel, unit) format -> 'a
    val eprintf : ('a, out_channel, unit) format -> 'a
    val sprintf : ('a, unit, string) format -> 'a
    val bprintf : Buffer.t -> ('a, Buffer.t, unit) format -> 'a
    val kprintf : (string -> string) -> ('a, unit, string) format -> 'a

It is possible to write into six different kinds of print targets. The basic problem of this style is that the provider of a service function like `printf` must define it for every commonly used print target. The other solution is that the provider defines only one version of the service function, but that the caller of the function arranges the polymorphism. A `Netchannels`-aware `Printf` would have only one variant of `printf`:

    val printf : out_obj_channel -> ('a, out_obj_channel, unit) format -> 'a

The caller would create the right `out_obj_channel` object for the real print target:

    let file_ch = new output_file (file : out_channel) in
    printf file_ch ...

(printing into files), or:

    let buffer_ch = new output_buffer (buf : Buffer.t) in
    printf buffer_ch ...

(printing into buffers). Of course, this is only a hypothetical example. The point is that this library defines many parsers and printers, and that it is really a simplification for both the library and the user of the library to have this object encapsulation of I/O resources.

## Programming with `in_obj_channel`

For example, let us program a function reading a data source line by line, and returning the sum of all lines which must be integer numbers. The argument `ch` is an open `Netchannels.in_obj_channel`, and the return value is the sum:

    let sum_up (ch : in_obj_channel) =
      let sum = ref 0 in
      try
        while true do
          let line = ch # input_line() in
          sum := !sum + int_of_string line
        done;
        assert false
      with
        End_of_file ->
          !sum

The interesting point is that the data source can be anything: a channel, a string, or any other class that implements the class type `in_obj_channel`.

This expression opens the file `"data"` and returns the sum of this file:

    let ch = new input_channel (open_in "data") in
    sum_up ch

The class `Netchannels.input_channel` is an implementation of the type `in_obj_channel` where every method of the class simply calls the corresponding function of the module `Pervasives`. (By the way, it would be a good idea to close the channel afterwards: `ch#close_in()`. We will discuss that below.)

This expression sums up the contents of a constant string:

    let s = "1\n2\n3\n4" in
    let ch = new input_string s in
    sum_up ch

The class `Netchannels.input_string` is an implementation of the type `in_obj_channel` that reads from a string that is treated like a channel.

The effect of using the `Netchannels` module is that the same implementation `sum_up` can be used to read from multiple data sources, as it is sufficient to call the function with different implementations of `in_obj_channel`.

## The details of `in_obj_channel`

The properties of any class that implements `in_obj_channel` can be summarized as follows:

- After the object has been created (`new`), the netchannel is open. The netchannel remains open until it is explicitly closed (method `close_in : unit -> unit`). When you call a method of a closed netchannel, the exception `Closed_channel` is raised (even if you try to close the channel again).
- The methods

        really_input : string -> int -> int -> unit
        input_char : unit -> char
        input_byte : unit -> int
        input_line : unit -> string

  work like their counterparts of the standard library. In particular, the end of file condition is signaled by rasising `End_of_file`.
- The method

        input : string -> int -> int -> int

  works like its counterpart of the standard library, except that the end of the file is also signaled by `End_of_file`, and not by the return value 0.
- The method `pos_in : int` returns the current byte position of the channel in a way that is logically consistent with the input methods: After reading `n` bytes, the method must return a position that is increased by `n`. Usually the position is zero after the object has been created, but this is not specified. Positions are available even for file descriptors that are not seekable.
- There is intentionally no `seek_in` method. Seekable channels are currently out of scope, as netstring focuses on non-seekable channels.

## Programming with `out_obj_channel`

The following function outputs the numbers of an `int list` sequentially on the passed netchannel:

    let print_int_list (ch : out_obj_channel) l =
      List.iter
        (fun n ->
           ch # output_string (string_of_int n);
           ch # output_char '\n';
        )
        l;
      ch # flush()

The following statements write the output into a file:

    let ch = new output_channel (open_out "data") in
    print_int_list ch [1;2;3]

And these statements write the output into a buffer:

    let b = Buffer.create 16 in
    let ch = new output_buffer b in
    print_int_list ch [1;2;3]

Again, the caller of the function `print_int_list` determines the type of the output destination, and you do not need several functions for several types of destination.

## The details of `out_obj_channel`

The properties of any class that implements `out_obj_channel` can be summarized as follows:

- After the object has been created (`new`), the netchannel is open. The netchannel remains open until it is explicitly closed (method `close_out : unit -> unit`). When you call a method of a closed netchannel, the exception `Closed_channel` is raised (even if you try to close the channel again).
- The methods

        output : string -> int -> int -> int
        really_output : string -> int -> int -> unit
        output_char : char -> unit
        output_byte : int -> unit
        output_string : string -> unit

  work like their counterparts of the standard library. There is usually an output buffer, but this is not specified. By calling `flush : unit -> unit`, the contents of the output buffer are forced to be written to the destination.
- The method

        output_buffer : Buffer.t -> unit

  works like `Buffer.output_channel`, i.e. the contents of the buffer are printed to the channel.
- The method

        output_channel : ?len:int -> in_obj_channel -> unit

  reads data from the argument `in_obj_channel` and prints them to the output channel. By default, the input channel is read until the EOF position. If the `len` argument is passed, at most this number of bytes are copied from the input channel to the output channel. The input channel remains open in all cases.
- The method `pos_out : int` returns byte positions that are logically consistent: After writing `n` bytes, the method must return a position that is increased by `n`. Usually the position is zero after the object has been created, but this is not specified. Positions are available even for file descriptors that are not seekable.
- There is intentionally no `seek_out` method. Seekable channels are currently out of scope, as netstring focuses on non-seekable channels.

## How to close channels

As channels may use file descriptors for their implementation, it is very important that all open channels are closed after they have been used; otherwise the operating system will certainly get out of file descriptors. The simple way,

    let ch = new <channel_class> args ... in
    ... do something ...
    ch # close_in() or close_out()

is dangerous because an exception may be raised between channel creation and the `close_*` invocation. An elegant solution is to use `with_in_obj_channel` and `with_out_obj_channel`, as in:

    with_in_obj_channel             (* or with_out_obj_channel *)
      (new <channel_class> ...)
      (fun ch ->
         ... do something ...
      )

This programming idiom ensures that the channel is always closed after usage, even in the case of exceptions.

Complete examples:

    let sum = with_in_obj_channel
                (new input_channel (open_in "data"))
                sum_up ;;

    with_out_obj_channel
      (new output_channel (open_out "data"))
      (fun ch -> print_int_list ch ["1";"2";"3"]) ;;

## Examples: HTML Parsing and Printing

In the Netstring library there are lots of parsers and printers that accept netchannels as data sources and destinations, respectively. One of them is the `Nethtml` module providing an HTML parser and printer. A few code snippets how to call them, just to get used to netchannels:

    let html_document =
      with_in_obj_channel
        (new input_channel (open_in "myfile.html"))
        Nethtml.parse ;;
    with_out_obj_channel
      (new output_channel (open_out "otherfile.html"))
      (fun ch -> Nethtml.write ch html_document) ;;

## Transactional Output Channels

Sometimes you do not want that generated output is directly sent to the underlying file descriptor, but rather buffered until you know that everything worked fine. Imagine you program a network service, and you want to return the result only when the computations are successful, and an error message otherwise. One way to achieve this effect is to manually program a buffer:

    let network_service ch =
      try
        let b = Buffer.create 16 in
        let ch' = new output_buffer b in
        ... computations, write results into ch' ...
        ch' # close_out;
        ch # output_buffer b
      with
        error ->
          ... write error message to ch ...

There is a better way to do this, as there are transactional output channels. This type of netchannels provide a buffer for all written data like the above example, and only if data is explicitly committed it is copied to the real destination. Alternatively, you can also rollback the channel, i.e. delete the internal buffer. The signature of the type `trans_out_obj_channel` is:

    class type trans_out_obj_channel = object
      inherit out_obj_channel
      method commit_work : unit -> unit
      method rollback_work : unit -> unit
    end

They have the same methods as `out_obj_channel` plus `commit_work` and `rollback_work`. There are two implementations, one of them keeping the buffer in memory, and the other using a temporary file:

    let ch' = new buffered_trans_channel ch

And:

    let ch' = new tempfile_trans_channel ch

In the latter case, there are optional arguments specifiying where the temporary file is created.

Now the network service would look like:

    let network_service transaction_provider ch =
      try
        let ch' = transaction_provider ch in
        ... computations, write results into ch' ...
        ch' # commit_work();
        ch' # close_out()     (* implies ch # close_out() *)
      with
        error ->
          ch' # rollback_work();
          ... write error message to ch' ...
          ch' # commit_work();
          ch' # close_out()   (* implies ch # close_out() *)

You can program this function without specifying which of the two implementations is used. Just call this function as

    network_service (new buffered_trans_channel) ch

or

    network_service (new tempfile_trans_channel) ch

to determine the type of transaction buffer.

Some details:

- The method `commit_work` copies all uncommitted data to the underlying channel, and flushes all buffers.
- When `rollback_work` is called the uncommitted data are deleted.
- The method `flush` does not have any effect.
- The reported position adds the committed and the uncommitted amounts of data. This means that `rollback_work` resets the position to the value of the last `commit_work` call.
- When the transactional channel is closed, the underlying channel is closed, too. By default, the uncommitted data is deleted, but the current implementations can optionally commit data in this case.

## Pipes and Filters

The class `pipe` is an `in_obj_channel` and an `out_obj_channel` at the same time (i.e. the class has the type `io_obj_channel`). A pipe has two endpoints, one for reading and one for writing (similar in concept to the pipes provided by the operating system, but note that our pipes have nothing to do with the OS pipes). Of course, you cannot read and write at the same time, so there must be an internal buffer storing the data that have been written but not yet read. How can such a construction be useful? Imagine you have two routines that run alternately, and one is capable of writing into netchannels, and the other can read from a netchannel. Pipes are the missing communication link in this situation, because the writer routine can output into the pipe, and the reader routine can read from the buffer of the pipe. In the following example, the writer outputs numbers from 1 to 100, and the reader sums them up:

    let pipe = new pipe() ;;
    let k = ref 1 ;;
    let writer() =
      if !k <= 100 then (
        pipe # output_string (string_of_int !k);
        incr k;
        if !k > 100 then pipe # close_out() else pipe # output_char '\n';
      ) ;;
    let sum = ref 0 ;;
    let reader() =
      let line = pipe # input_line() in
      sum := !sum + int_of_string line ;;
    try
      while true do
        writer();
        reader()
      done
    with
      End_of_file ->
        () ;;

The `writer` function prints the numbers into the pipe, and the `reader` function reads them in. By closing only the output end Of the pipe the `writer` signals the end of the stream, and the `input_line` method raises the exception `End_of_file`.

Of course, this example is very simple. What does happen when more is printed into the pipe than read? The internal buffer grows. What does happen when more is tried to read from the pipe than available? The input methods signal this by raising the special exception `Buffer_underrun`. Unfortunately, handling this exception can be very complicated, as the reader must be able to deal with partial reads.

This could be solved by using the `Netstream` module. A netstream is another extension of `in_obj_channel` that allows one to look ahead, i.e. you can look at the bytes that will be read next, and use this information to decide whether enough data are available or not. Netstreams are explained in another chapter of this manual.

Pipes have another feature that makes them useful even for "normal" programming. You can specify a conversion function that is called when data is to be transferred from the writing end to the reading end of the pipe. The module `Netencoding.Base64` defines such a pipe that converts data: The class `encoding_pipe` automatically encodes all bytes written into it by the Base64 scheme:

    let pipe = new Netencoding.Base64.encoding_pipe() ;;
    pipe # output_string "Hello World";
    pipe # close_out() ;;
    let s = pipe # input_line() ;;

`s` has now the value `"SGVsbG8gV29ybGQ="`, the encoded form of the input. This kind of pipe has the same interface as the basic pipe class, and the same problems to use it. Fortunately, the Netstring library has another facility simplifying the usage of pipes, namely filters.

There are two kinds of filters: The class `Netchannels.output_filter` redirects data written to an `out_obj_channel` through a pipe, and the class `Netchannels.input_filter` arranges that data read from an `in_obj_channel` flows through a pipe. An example makes that clearer. Imagine you have a function `write_results` that writes the results of a computation into an `out_obj_channel`. Normally, this channel is simply a file:

    with_out_obj_channel
      (new output_channel (open_out "results"))
      write_results

Now you want that the file is Base64-encoded. This can be arranged by calling `write_results` differently:

    let pipe = new Netencoding.Base64.encoding_pipe() in
    with_out_obj_channel
      (new output_channel (open_out "results"))
      (fun ch ->
        let ch' = new output_filter pipe ch in
        write_results ch';
        ch' # close_out()
      )

Now any invocation of an output method for `ch'` actually prints into the filter, which redirects the data through the `pipe`, thus encoding them, and finally passing the encoded data to the underlying channel `ch`. Note that you must close `ch'` to ensure that all data are filtered, it is not sufficient to flush output.

It is important to understand why filters must be closed to work properly. The problem is that the Base64 encoding converts triples of three bytes into quadruples of four bytes. Because not every string to convert is a multiple of three, there are special rules how to handle the exceeding one or two bytes at the end. The pipe must know the end of the input data in order to apply these rules correctly. If you only flush the filter, the exceeding bytes would simply remain in the internal buffer, because it is possible that more bytes follow. By closing the filter, you indicate that the definite end is reached, and the special rules for trailing data must be performed. \- Many conversions have similar problems, and because of this it is a good advice to always close output filters after usage.

There is not only the class `output_filter` but also `input_filter`. This class can be used to perform conversions while reading from a file. Note that you often do not need to close input filters, because input channels can signal the end by raising `End_of_file`, so the mentioned problems usually do not occur.

There are a number of predefined conversion pipes:

- `Netencoding.Base64.encoding_pipe`: Performs Base64 encoding
- `Netencoding.Base64.decoding_pipe`: Performs Base64 decoding
- `Netencoding.QuotedPrintable.encoding_pipe`: Performs QuotedPrintable encoding
- `Netencoding.QuotedPrintable.decoding_pipe`: Performs QuotedPrintable decoding
- `Netconversion.conversion_pipe`: Converts the character encoding form charset A to charset B

## Defining Classes for Object Channels

As subtyping and inheritance are orthogonal in O'Caml, you can simply create your own netchannels by defining classes that match the `in_obj_channel` or `out_obj_channel` types. E.g.

    class my_in_channel : in_obj_channel =
    object (self)
      method input s pos len = ...
      method close_in() = ...
      method pos_in = ...
      method really_input s pos len = ...
      method input_char() = ...
      method input_line() = ...
      method input_byte() = ...
    end

Of course, this is non-trivial, especially for the `in_obj_channel` case. Fortunately, the Netchannels module includes a "construction kit" that allows one to define a channel class from only a few methods. A closer look at `in_obj_channel` and `out_obj_channel` shows that some methods can be derived from more fundamental methods. The following class types include only the fundamental methods:

    class type raw_in_channel = object
      method input : string -> int -> int -> int
      method close_in : unit -> unit
      method pos_in : int
    end

    class type raw_out_channel = object
      method output : string -> int -> int -> int
      method close_out : unit -> unit
      method pos_out : int
      method flush : unit -> unit
    end

In order to define a new class, it is sufficient to define this raw version of the class, and to lift it to the full functionality. For example, to define `my_in_channel`:

    class my_raw_in_channel : raw_in_channel =
    object (self)
      method input s pos len = ...
      method close_in() = ...
      method pos_in = ...
    end
    class my_in_channel =
      in_obj_channel_delegation (lift_in (`Raw(new my_raw_in_channel)))

The function `Netchannels.lift_in` can lift several forms of incomplete channel objects to the full class type `in_obj_channel`. There is also the corresponding function `Netchannels.lift_out`. Note that lifting adds by default another internal buffer to the channel that must be explicitly turned off when it is not wanted. The rationale for this buffer is that it avoids some cases with extremely poor performance which might be surprising for many users.

The class `in_obj_channel_delegation` is just an auxiliary construction to turn the `in_obj_channel` object returned by `lift_in` again into a class.

## Some FAQ

- Netchannels add further layers on top of the built-in channels or file descriptors. Does this make them slow?

  Of course, Netchannels are slower than the underlying built-in I/O facilities. There is at least one, but often even more than one method call until the data is transferred to or from the final I/O target. This costs time, and it is a good idea to reduce the number of method calls for maximum speed. Especially the character- or byte-based method calls should be avoided, it is better to collect data and pass them in larger chunks. This reduces the number of method calls that are needed to transfer a block of data.

  However, some classes implement buffers themselves, and data are only transferred when the buffers are full (or empty). The overhead for the extra method calls is small for these classes. The classes that implement their own buffers are the transactional channels, the pipes, and all the classes with "buffer" in their name.

  Netchannels are often stacked, i.e. one netchannel object transfers data to an underlying object, and this object passes the data to further objects. Often buffers are involved, and data are copied between buffers several times. Of course, these copies can reduce the speed, too.

- Why do Netchannels not support seeking?

  Netchannels were invented to support the implementation of network protocols. Network endpoints are not seekable.

- What about `printf` and `scanf`?

  In principle, methods for `printf` and `scanf` could be added to `out_obj_channel` and `in_obj_channel`, respectively, as recent versions of O'Caml added the necessary language means (polymorphic methods, `kprintf`, `kscanf`). However, polymorphic methods work only well when the type of the channel object is always annotated (e.g. as `(ch : out_obj_channel) # printf ...`), so this is not that much better than `ch # output_string (sprintf ...)`.

- Can I pass an `in_obj_channel` to an ocamllex-generated lexer?

  Yes, just call `Netchannels.lexbuf_of_in_obj_channel` to turn the `in_obj_channel` into a `lexbuf`.

- Do Netchannels support non-blocking I/O?

  Yes and no. Yes, because you can open a descriptor in non-blocking mode, and create a netchannel from it. When the program would block, the `input` and `output` methods return 0 to indicate this. However, the non-raw methods cannot cope with these situations.

- Do Netchannels support multiplexed I/O?

  No, there is no equivalent to `Unix.select` on the level of netchannels.

- Can I use Netchannels in multi-threaded programs?

  Yes. However, shared netchannels are not locked, and strange things can happen when netchannels are used by several threads at the same time.

- Can I use pipes to communicate between threads?

  This could be made work, but it is currently not the case. A multithreading-aware wrapper around pipes could do the job.

- Pipes call external programs to do their job, don't they?

  No, they do not call external programs, nor do they need any parallel execution threads. Pipes are just a tricky way of organizing buffers.

- How do I define my own conversion pipe?

  Look at the sources `netencoding.ml`, it includes several examples of conversion pipes.