# Module Http_client

    module Http_client: sig .. end

HTTP 1.1 client

Note for beginners: There is a simplified interface called `Http_client.Convenience`.
Implements the following advanced features:

- chunked message transport
- persistent connections
- connections in pipelining mode ("full duplex" connections)
- modular authentication methods, currently Basic and Digest
- event-driven implementation; allows concurrent service for several network connections
- HTTP proxy support, also with Basic and Digest authentication
- SOCKS proxy support (1)
- HTTPS support via extension module `Https_client` (in library `equeue-ssl`). HTTPS proxies are also supported (CONNECT method) (1)
- Automatic and configurable retry of failed idempotent requests
- Redirections can be followed automatically
- Compressed message bodies can be automatically decoded (gzip only, method `set_accept_encoding`) (1)

Left out:

- multipart messages, including multipart/byterange
- content digests specified by RFC 2068 and 2069 (2)
- content negotiation (2)
- conditional and partial GET (2)
- client-side caching (2)
- HTTP/0.9 compatibility

(1) Since Ocamlnet-3.3

(2) These features can be implemented on top of this module if really needed, but there is no special support for them.

Related modules/software:

- `Http_fs` allows you to access HTTP servers in the style of filesystems
- WebDAV: If you are looking for WebDAV there is an extension of this module: Webdav, which is separately available.

Thread safety

The module can be compiled such that it is thread-safe. In particular, one has to link the http_client_mt.cm`xo` object, and thread-safety is restricted to the following kinds of usage:

- The golden rule is that threads must not share pipeline objects. If every thread uses its own pipeline, every thread will have its own set of state variables. It is not detected if two threads errornously share a pipeline, neither by an error message nor by implicit serialization. Strange things may happen.
- The same applies to the other objects, e.g. http_call objects
- The `Convenience` module even serializes; see below.

# Types and Exceptions

    exception Bad_message of string

The server sent a message which cannot be interpreted. The string indicates the reason.

    exception No_reply

There was no response to the request because some other request failed earlier and it was not allowed to send the request again.

    exception Too_many_redirections

While following redirections the limit has been reached

    exception Name_resolution_error of string

Could not resolve this name - same as `Uq_engines.Host_not_found`

    exception URL_syntax_error of string

This URL cannot be parsed after a redirection has been followed.

    exception Timeout of string

A timeout. The string explains which connection is affected. New since Ocamlnet-3.3.

    exception Proxy_error of int

An error status from a proxy. This is only used when extra proxy messages are used to configure the proxy (e.g. the CONNECT message).

    exception Response_too_large

The length of the response exceeds the configured maximum

    exception Http_protocol of exn

The request could not be processed because the exception condition was raised. The inner exception is one of the above defined.

    exception Http_error of (int * string)

Deprecated. The server sent an error message. The left component of the pair is the error code, the right component is the error text. This exception is only used by `get_resp_body`, and by the `Http_client.Convenience` module.

    type status = [ `Client_error
           | `Http_protocol_error of exn
           | `Redirection
           | `Server_error
           | `Successful
           | `Unserved ]

Condensed status information of a HTTP call:

- ``Unserved`: The call has not yet been finished
- ``HTTP_protocol_error e`: An error on HTTP level occurred. Corresponds to the exception `Http_protocol`.
- ``Successful`: The call is successful, and the response code is between 200 and 299.
- ``Redirection`: The call is successful, and the response code is between 300 and 399.
- ``Client_error`: The call failed with a response code between 400 and 499.
- ``Server_error`: The call failed for any other reason.

`type `'a` how_to_reconnect =`

`|` | `Send_again` | `(*` | Send the request automatically again | `*)`
`|` | `Request_fails` | `(*` | Drop the request | `*)`
`|` | `Inquire of `('a -> bool)`` | `(*` | If the function return `true` send again, otherwise drop the request. | `*)`
`|` | `Send_again_if_idem` | `(*` | Default behaviour: `Send_again` for idempotent methods (GET, HEAD), `Request_fails` for the rest | `*)`

How to deal with automatic reconnections, especially when the connection crashes.
`type `'a` how_to_redirect =`

`|` | `Redirect` | `(*` | Perform the redirection | `*)`
`|` | `Do_not_redirect` | `(*` | No redirection | `*)`
`|` | `Redirect_inquire of `('a -> bool)`` | `(*` | If the function return `true` redirect, otherwise do not redirect. It is legal to set the `Location` header as part of the action performed by the function. (Should be an absolute http URL.) | `*)`
`|` | `Redirect_if_idem` | `(*` | Default behaviour: `Redirect` for idempotent methods (GET, HEAD), `Do_not_redirect` for the rest | `*)`

    type private_api

The private part of the `http_call` class type

    type response_body_storage = [ `Body of unit -> Netmime.mime_body
           | `Device of unit -> Uq_io.out_device
           | `File of unit -> string
           | `Memory ]

How to create the response body:

- ``Memory`: The response body is in-memory
- ``File f`: The response body is stored into the file whose name is returned by `f()`
- ``Body f`: The response body is stored into the object returned by `f()`
- ``Device f`: The response is directly forwarded to the device obtained by `f()` (new since Ocamlnet-3.3)

When the function `f` is called in the latter cases the response header has already been received, and can be retrieved with the `response_header` method of the call object. Also, `response_status_text`, `response_status_code`, and `response_status` return meaningful values.
`type ``synchronization =`

`|` | `Sync` | `(*` | The next request begins after the response of the last request has been received. | `*)`

`|`

`Pipeline of `int``

`(*`

The client is allowed to send several requests without waiting for responses. The number is the maximum number of unreplied requests that are allowed. A typical value: 5. If you increase this value, the risk becomes higher that requests must be repeatedly sent to the server in the case the connection crashes. Increasing is recommended if you send a bigger number of GET or HEAD requests to the server. Decreasing is recommended if you send large POST or PUT requests to the server.

Values > 8 are interpreted as 8.

`*)`

This type determines whether to keep requests and responses synchronized or not.

The first request/response round is always done in Sync mode, because the protocol version of the other side is not known at that moment. `Pipeline` requires HTTP/1.1.

In previous versions of netclient there was a third option, `Sync_with_handshake_before_request_body`. This option is no longer necessary because the HTTP specification has been updated in the meantime, and there is a better mechanism now (the `Expect` header is set).

    type resolver = Unixqueue.unix_event_system ->
           string -> (Unix.inet_addr option -> unit) -> unit

A name resolver is a function `r` called as `r esys name reply`. As `name` the name to resolve is passed. The resolver must finally call `reply` with either the resolved address or with `None`, indicating an error in the latter case. The event system `esys` can be used to carry out the resolution process in an asynchronous way, but this is optional.

Only 1:1 resolution is supported, 1:n resolution not.

    type channel_binding_id = int

A channel binding identifies a requirement for the transport channel, especially whether plain HTTP is sufficient, or HTTPS needs to be used, and if so, whether there are further requirements for the SSL context. There are the predefined IDs:

- `Http_client.http_cb_id` for HTTP connections
- `Http_client.https_cb_id` for HTTPS connections without user certificates
- `Http_client.proxy_only_cb_id` for the restriction that this protocol can only be used via a web proxy

`type ``http_options = {`

`` | `synchronization : `synchronization`;` | `(*` | Default: `Pipeline 5`. | `*)`
`` | `maximum_connection_failures : `int`;` | `(*` | This option limits the number of connection attempts. Default: 2 | `*)`

``

`maximum_message_errors : `int`;`

`(*`

This option limits the number of protocol errors tolerated per request. If a request leads to a protocol error, the connection is shut down, the server is connected again, and the request is tried again (if the kind of the message allows retransmission). If a request repeatedly fails, this option limits the number of retransmissions. Default: 2

`*)`

`` | `inhibit_persistency : `bool`;` | `(*` | This option turns persistent connections off. Default: `false` It is normally not necessary to change this option. | `*)`
`` | `connection_timeout : `float`;` | `(*` | If there is no network transmission for this period of time, the connection is shut down, and tried again. Default: 300.0 (seconds) It may be necessary to increase this value if HTTP is used for batch applications that contact extremely slow services. | `*)`
`` | `number_of_parallel_connections : `int`;` | `(*` | The client keeps up to this number of parallel connections to a single content server or proxy. Default: 2 You may increase this value if you are mainly connected with an HTTP/1.0 proxy. | `*)`
`` | `maximum_redirections : `int`;` | `(*` | The maximum number of redirections per message | `*)`
`` | `handshake_timeout : `float`;` | `(*` | The timeout when waiting for "100 Continue". Default: 1.0 | `*)`
`` | `resolver : `resolver`;` | `(*` | The function for name resolution | `*)`
`` | `configure_socket : `Unix.file_descr -> unit`;` | `(*` | A function to configure socket options | `*)`
`` | `schemes : `(string * Neturl.url_syntax * int option * channel_binding_id) list`;` | `(*` | The list of supported URL schemes. The tuples mean `(scheme, syntax, default_port, cb)`. By default, the schemes "http", "https", and "ipp" are supported. | `*)`
`` | `verbose_status : `bool`;`
`` | `verbose_request_header : `bool`;`
`` | `verbose_response_header : `bool`;`
`` | `verbose_request_contents : `bool`;`
`` | `verbose_response_contents : `bool`;`
`` | `verbose_connection : `bool`;`

``

`verbose_events : `bool`;`

`(*`

Enable various debugging message types.

- `verbose_status`: reports about status of received documents
- `verbose_request_header`: prints the header sent to the server
- `verbose_request_contents`: prints the document sent to the server
- `verbose_response_header`: prints the header of the answer from the server
- `verbose_response_contents`: prints the document received from the server
- `verbose_connection`: reports many connection events; authentication, too.
- `verbose_events`: everything about the interaction with `Unixqueue`

By default, `verbose_status` and `verbose_connection` are enabled. Note that you also have to set `Debug.enable` to `true` to see any log message at all!

`*)`

}
Options for the whole pipeline. It is recommended to change options the following way:

        let opts = pipeline # get_options in
        let new_opts = { opts with <field> = <value>; ... } in
        pipeline # set_options new_opts

New fields can be added anytime to this record, and this style of changing options is transparent to field additions.

    type header_kind = [ `Base | `Effective ]

The ``Base` header is set by the user of `http_call` and is never changed during processing the call. The ``Effective` header is a copy of the base header at the time the request is sent. The effective header contains additions like `Content-length` and authentication info.

    class type http_call = object .. end

The container for HTTP calls

    class type transport_channel_type = object .. end

# HTTP methods

    class virtual generic_call : object .. end

This class is an implementation of `http_call`.
The following classes are implementations for the various HTTP methods. These classes do not initialize the call object.

    class get_call : http_call

    class trace_call : http_call

    class options_call : http_call

    class head_call : http_call

    class post_call : http_call

    class put_call : http_call

    class delete_call : http_call

The following classes initialize the request message of the call (header and body). These classes are also backward compatible to the classes found in earlier versions of netclient.

    class get : string -> http_call

Argument: URI

    class trace : string -> int -> http_call

Arguments: URI, maximum number of hops

    class options : string -> http_call

Argument: URI or "*"

    class head : string -> http_call

Argument: URI

    class post : string -> (string * string) list -> http_call

Arguments: URI, parameter list to be transferred as application/x-www-form-urlencoded body

    class post_raw : string -> string -> http_call

Arguments: URI, body

    class put : string -> string -> http_call

Arguments: URI, body

    class delete : string -> http_call

Argument: URI

# Authentication

    class type key = object .. end

A `key` is a user/password combination for a certain realm

    val key : user:string ->
           password:string -> realm:string -> domain:string list -> key

Create a key object

    class type key_handler = object .. end

    class key_ring : ?uplink:#key_handler -> unit -> object .. end

The `key_ring` is a cache for keys.

    class type auth_session = object .. end

An `auth_session` represents an authenticated session

    class type auth_handler = object .. end

An authentication handler has the capability of adding the necessary headers to messages.

    class basic_auth_handler : ?enable_auth_in_advance:bool -> #key_handler -> auth_handler

Basic authentication.

    class digest_auth_handler : ?enable_auth_in_advance:bool -> #key_handler -> auth_handler

Digest authentication.

    class unified_auth_handler : #key_handler -> auth_handler

Support both digest and basic authentication, with preference to digest.

    class basic_auth_method : object .. end

Deprecated.

    class digest_auth_method : basic_auth_method

Deprecated.

# Transport

A connection cache is an object that keeps connections open that are currently unused. A connection cache can be shared by several pipelines.

    type connection_cache = Http_client_conncache.connection_cache

    val close_connection_cache : connection_cache -> unit

Closes all descriptors known to the cache

    val create_restrictive_cache : unit -> connection_cache

A restrictive cache closes connections as soon as there are no pending requests.

    val create_aggressive_cache : unit -> connection_cache

This type of cache tries to keep connections as long open as possible. The consequence is that users are responsible for closing the descriptors (by calling `close_connection_cache`) when the cache is no longer in use.

    val http_cb_id : channel_binding_id

Identifies a channel binding to pure HTTP (without SSL), with or without web proxies

    val https_cb_id : channel_binding_id

Identifies a channel binding to anonymous HTTPS (i.e. no client certificates), with or without web proxies.

    val proxy_only_cb_id : channel_binding_id

Identifies a channel binding to web proxy connections. Use this to e.g. send an FTP URL to a web proxy via HTTP

    val new_cb_id : unit -> channel_binding_id

Allocates and returns a new ID

    val http_transport_channel_type : transport_channel_type

Transport via HTTP

# Pipelines

    class pipeline : object .. end

A pipeline is a queue of HTTP calls to perform
Example using the pipeline:

     let call = new get "http://server/path" in
     let pipeline = new pipeline in
     pipeline # add call;
     pipeline # run();    (* Now the HTTP client is working... *)
     match call # status with
     | `Successful -> ...
     | ...

# Convenience module for simple applications

Do `open Http_client.Convenience` for simple applications.

    module Convenience: sig .. end

# Debugging

    module Debug: sig .. end