Ocamlnet 3 Reference Manual : Netcgi_porting

# Netcgi_porting

# Porting `netcgi1` programs to `netcgi2`

The library `netcgi2` is a revised version of the old `cgi` library which is now also referred to as `netcgi1`. As any software, `netcgi1` aged, and suffered more and more from inadequate interfaces. Because of this it became necessary to improve the interfaces from grounds up. The result is `netcgi2`, a new major version that tries to continue the good parts of `netcgi1` while replacing its problematic edges.

When this text is written, `netcgi2` is still being developed, and subject of discussion.

## Why porting and when?

It is not possible to use `netcgi1` and `netcgi2` at the same time in the same application. This means that one cannot gradually upgrade from `netcgi1` to `netcgi2` by using more and more of the `netcgi2` features. Instead of this, it is necessary to switch from `netcgi1` to `netcgi2` at one point in the lifetime of the web application.

The main benefit is that you have access to the newest `netcgi2` features. There are already a number of connectors that are not present in `netcgi1` (newer AJP protocol version, SCGI). Furthermore, new features will only be added to `netcgi2`.

However, if your application is already at or near its end of lifetime, there is no need to port it to `netcgi2`. The `netcgi1` library will remain in Ocamlnet 2, and bugs will be fixed.

## Module Organization

The new organization is very simple:

- `Netcgi` defines all basic types. Previously, this was done in the two modules `Netcgi_env` and `Netcgi_types`
- For every connector c there is one module `Netcgi_`c implementing it. Especially the classic CGI connector is now in `Netcgi_cgi`. Previously, the CGI connector was defined in `Netcgi`, and there used to be several modules per connector.
- `Netcgi_common` defines service functions to define new connectors.

There is also a module `Netcgi1_compat` trying to ease porting. See below for a discussion.

## Interface Changes

Most of the types remain the same, or almost the same. A few changes have been done:

- Immutability of arguments: A `Netcgi.cgi_argument` is no longer writable. Furthermore, the list of arguments in a `Netcgi.cgi_activation` can no longer be modified. There are some new service functions to modify lists of arguments in case one needs such a list.
- Enhanced cookie API: Cookie formats newer than the old Netscape format are supported. Old and new cookie types can be transformed into each other. See the module `Netcgi.Cookie`.
- Exception Handling: The `Netcgi_common.HTTP` exception can be used to exit from a processor at any time. There is the notion of an exception handler for web-related exceptions.
- Simplified Environments: The CGI environments `Netcgi.cgi_environment` have been simplified. It is only distinguished between two states: Output headers have been/ have not been sent. Other processing states are hidden by the implementation.
- Improved finalization: All CGI arguments are finalized at the end of the request ensuring that temporary files are deleted. It is also possible to register further finalizers using the `Netcgi.cgi_activation.at_exit` method.

The connectors, however, are now created in very different ways. This is mainly driven by uniformity: There should be a method of creating web connectors that works for every kind of connector. Because of this, the code instantiating connectors in application must always be changed so it matches the new, uniform conventions. Fortunately, this code is usually not very extensive.

## Porting strategies

### Strategy: Use new API

In the long term this is the best strategy. In principle, one has to distinguish between

- program parts that access `netcgi` values, and
- program parts that connect the `netcgi` application with the web server.

Porting the first parts is fairly simple, because the types of the `netcgi` values do not change much. For example, the function `web_page` for `netcgi1`

    (* This is [netcgi1] code! *)
    let web_page (cgi : Netcgi_types.cgi_activation) =
      let webarg = cgi # argument_value "webarg" in
      cgi # set_header();
      cgi # output # output_string ("The argument is: " ^ webarg)

would read in the version for `netcgi2` as follows:

    (* This is [netcgi2] code! *)
    let web_page (cgi : Netcgi.cgi_activation) =
      let webarg = cgi # argument_value "webarg" in
      cgi # set_header();
      cgi # output # output_string ("The argument is: " ^ webarg)

The only change is that the type `cgi_activation` is now defined in the module `Netcgi` and no longer in `Netcgi_types`. It is expected that this simple way of porting applies to almost all parts of `netcgi` applications.

By the way, the type `cgi_activation` can now be abbreviated as `cgi`, as this is the type name that needs to be written down most frequently.

#### The new CGI connector

In `netcgi1`, the CGI connector is selected by instantiating the class `Netcgi.std_activation`, as in:

    (* This is [netcgi1] code! *)
    let cgi = new Netcgi.std_activation() in
    process cgi

It is assumed that `process` is a function taking a `cgi_activation` as argument, and processing the request.

The corresponding `netcgi2` call is:

    (* This is [netcgi2] code! *)
    Netcgi_cgi.run process

As you see, `Netcgi_cgi.run` is now responsible for calling `process`.

#### The new FastCGI connector

In `netcgi1` there are several ways of using FastCGI. The most common is to call `Netcgi_fcgi.serv` as in:

    (* This is [netcgi1] code! *)
    Netcgi_fcgi.serv process optype

It is assumed that `process` is a function taking a `cgi_activation` as argument, and processing the request. `optype` is a valid operating type.

The corresponding `netcgi2` call is:

    (* This is [netcgi2] code! *)
    let process' cgi = process (cgi :> Netcgi.cgi_activation) in
    Netcgi_fcgi.run ~output_type:optype process'

Note that the argument of `process'` is a slightly extended version of `cgi_activation`, so you usually need the coercion to cut off the additional part of the object interface.

#### The new AJP connector

The new connector supports now the AJP version 1.3 - this is the default version used by Jakarta and mod_jk. In `netcgi1`, only version 1.2 of the AJP protocol was supported. The new protocol version is no big improvement, however. It uses a slightly more compact representation of the data. The biggest plus is better support of SSL.

In `netcgi1` there was some special machinery around the AJP connector to create worker processes. This code has been completely removed in favor of `netplex`, the new general-purpose server framework. Because of that, porting AJP applications is probably a bit of work, and we cannot give a receipt here how to do that.

### Strategy: Use `Netcgi1_compat`

If you want to use the new connectors but currently do not have time to check all your code for changes, there is a special helper module called `Netcgi1_compat` that provides a `netcgi1`-compatible API on top of either `netcgi1` or `netcgi2`.

Because `Netcgi1_compat` is available in both `netcgi1` and `netcgi2` you can write code that can be compiled for both versions without needing to change anything. Note, however, that this module is not 100% identical in both versions - the `netcgi2` version includes some additional functions that converts values from their `netcgi1` representation to their `netcgi2` representation and vice versa. Unfortunately, this makes the two versions of this module binary-incompatible, so you have to recompile your code for either `netcgi1` or `netcgi2`.

The `Netcgi1_compat` module simply contains the relevant parts of the `netcgi1` API as submodules. That means you can access

- the `netcgi1` version of the module `Netcgi_types` as `Netcgi1_compat.Netcgi_types`
- the `netcgi1` version of the module `Netcgi_env` as `Netcgi1_compat.Netcgi_env`
- the `netcgi1` version of the module `Netcgi` as `Netcgi1_compat.Netcgi`

Other modules of `netcgi1` are not covered by the compatibility API. In `Netcgi`, the `custom_activation` class has been left out.

You can usually port code to using this API by either

- prefixing these module names in source code with `Netcgi1.`, e.g. `new Netcgi.std_activation()` would be turned into `new Netcgi1_compat.Netcgi.std_activation()`
- opening the module `Netcgi1` at the beginning of each .ml and .mli file by an `open Netcgi1_compat`.

Except `Netcgi.std_activation`, the `netcgi1` way of creating a connector for classic CGI, there are no connectors in the compatibility API. If you need one, you must take it directly from either `netcgi1` or `netcgi2`. For example, to connect using FastCGI:

    (* This is code for both [netcgi1] and [netcgi2]! *)
    let process (cgi : Netcgi1_compat.Netcgi.cgi_activation) =
      ...

    (* This is [netcgi2] code! *)
    let process_netcgi2 cgi2 =
      let cgi1 = Netcgi1_compat.Netcgi_types.to_compat_activation cgi2 in
      process cgi1
    Netcgi_fcgi.run ~output_type process'