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# mod_esi behaviour (inets v9.4.2)

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Erlang Server Interface

This module defines the Erlang Server Interface (ESI) API. It is a more efficient way of writing Erlang scripts for your `Inets` web server than writing them as common CGI scripts.

# Summary

## Types

env()

Environment data associated with a request.

session_id()

Identifies the requesting client.

## Callbacks: ESI Callback Functions

'Function'(SessionID, Env, Input)

Called by `mod_esi` in response to requests.

## Functions

deliver(SessionID, Data)

Sends data from an ESI script back to the client.

# Types

# env()

(not exported)

    -type env() ::
              {server_software, string()} |
              {server_name, string()} |
              {gateway_interface, string()} |
              {server_protocol, string()} |
              {server_port, integer()} |
              {request_method, string()} |
              {remote_adress, inet:ip_address()} |
              {peer_cert, undefined | no_peercert | public_key:der_encoded()} |
              {script_name, string()} |
              {http_LowerCaseHTTPHeaderName, string()}.

Environment data associated with a request.

## Possible values

- `{server_software, string()}` - Indicates the inets version.

- `{server_name, string()}` - The local hostname.

- `{gateway_interface, string()}` - Legacy string used in CGI, just ignore.

- `{server_protocol, string()}` - HTTP version, currently "HTTP/1.1"

- `{server_port, integer()}` - Servers port number.

- `{request_method, "GET" | "PUT" | "DELETE" | "POST" | "PATCH"}` - HTTP request method.

- `{remote_adress, inet:ip_address()}` - The clients ip address.

- `{peer_cert, undefined | no_peercert | DER:binary()}` - For TLS connections where client certificates are used this will be an ASN.1 DER-encoded X509-certificate as an Erlang binary. If client certificates are not used the value will be `no_peercert`, and if TLS is not used (HTTP or connection is lost due to network failure) the value will be `undefined`.

- `{script_name, string()}` - Request URI

- `{http_LowerCaseHTTPHeaderName, string()}` - example: `{http_content_type, "text/html"}`

# session_id()

(since OTP 28.0)

    -opaque session_id()

Identifies the requesting client.

# Callbacks: ESI Callback Functions

# 'Function'(SessionID, Env, Input)

(optional)

    -callback 'Function'(SessionID, Env, Input) -> {continue, State} | _
                            when
                                SessionID :: session_id(),
                                Env :: [env()],
                                Input :: string() | ChunkedData,
                                ChunkedData ::
                                    {first, Data :: binary()} |
                                    {continue, Data :: binary(), State :: term()} |
                                    {last, Data :: binary(), State :: term()},
                                State :: term().

Called by `mod_esi` in response to requests.

`Module` must be found in the code path and export `Function` with an arity of three. An `erl_script_alias` must also be set up in the configuration file for the web server, see the ESI properties documentation.

The `Module` and `Function` that are called depend on the URL. See the ESI introductory documentation for more details.

`mod_esi:deliver/2` shall be used to generate the response to the client, and `SessionID` shall be passed as the first argument.

## Chunking

This function may be called several times to chunk the response data. Notice that the first chunk of data sent to the client must at least contain all HTTP header fields that the response will generate. If the first chunk does not contain the end of HTTP header, that is, `"\r\n\r\n"`, the server assumes that no HTTP header fields will be generated. This behaviour depends on the `httpd` configuration, see below.

## Parameters

- `SessionID`: request identifier.

  Pass this to `mod_esi:deliver/2` when generating a response.

- `Env`: environment data of the request, see `env/0`.

- `Input`: query data of a GET request or the body of a PUT or POST request.

  The default behavior (legacy reasons) for delivering the body, is that the whole body is gathered and converted to a string. But if the httpd config parameter `max_client_body_chunk` is set, the body will be delivered as binary chunks instead. The maximum size of the chunks is either `max_client_body_chunk` or decided by the client if it uses HTTP chunked encoding to send the body.

  When using the chunking mechanism, this callback must return `{continue, State::term()}` for all calls where `Input` is `{first, Data::binary()}` or `{continue, Data::binary(), State::term()}`. When `Input` is `{last, Data::binary(), State::term()}` the return value will be ignored.

  The input `State` is the last returned `State`, in it the callback can include any data that it needs to keep track of when handling the chunks.

#### Note

Note that if the body is small all data may be delivered in only one chunk and then the callback will be called with `{last, Data::binary(), undefined}` without getting called with `{first, Data::binary()}`.

## Setting a response status

To set the response status code, the special `status` response header can be sent. For instance, to acknowledge creation of a resource and send an empty JSON response body, one could pass the following:

    "status: 201 Created\r\ncontent-type: application/json\r\n\r\n{}"

# Functions

# deliver(SessionID, Data)

    -spec deliver(SessionID, Data) -> ok | {error, Reason}
                     when SessionID :: session_id(), Data :: iolist(), Reason :: bad_sessionID.

Sends data from an ESI script back to the client.

This function is only intended to be used from functions called by the ESI interface to deliver parts of the content to the user.

#### Note

If any HTTP header fields are added by the script, they must be in the first call to `deliver/2`, and the data in the call must be a string. Calls after the headers are complete can contain binary data to reduce copying overhead. Do not assume anything about the data type of `SessionID`. `SessionID` must be the value given as input to the ESI callback function that you implemented.