http_server.pl -- HTTP server library

This library combines the core server functionality provided by several libraries that are needed by almost any web server. It exports the commonly used predicates from library(http/thread_httpd), library(http/http_dispatch), library(http/http_wrapper), library(http/http_parameters), library(http/html_write), library(http/http_json), and library(http/http_dyn_workers).

http_server(+Options) is det: Create an HTTP server using http_dispatch/1 for handling requests. See http_server/2 and http_dispatch/1 for details.

Re-exported predicates

The following predicates are exported from this file while their implementation is defined in imported modules or non-module files loaded by this module.

html_meta +Heads is det

This directive can be used to declare that an HTML rendering rule takes HTML content as argument. It has two effects. It emits the appropriate meta_predicate/1 and instructs the built-in editor (PceEmacs) to provide proper colouring for the arguments. The arguments in Head are the same as for meta_predicate or can be constant html. For example:

:- html_meta
    page(html,html,?,?).

reply_html_page(:Head, :Body) is det

reply_html_page(+Style, :Head, :Body) is det

Provide the complete reply as required by http_wrapper.pl for a page constructed from Head and Body. The HTTP Content-type is provided by html_current_option/1.

See also: - reply_html_partial/1 to avoid adding a DOCTYPE, and required outer HTML elements such as <html>.

is_json_content_type(+ContentType) is semidet

True if ContentType is a header value (either parsed or as atom/string) that denotes a JSON value.

http_read_json_dict(+Request, -Dict) is det

http_read_json_dict(+Request, -Dict, +Options) is det

Similar to http_read_json/2,3, but by default uses the version 7 extended datatypes.

http_read_json_dict(+Request, -Dict) is det

http_read_json_dict(+Request, -Dict, +Options) is det

Similar to http_read_json/2,3, but by default uses the version 7 extended datatypes.

reply_json_dict(+JSONTerm) is det

reply_json_dict(+JSONTerm, +Options) is det

As reply_json/1 and reply_json/2, but assumes the new dict based data representation. Note that this is the default if the outer object is a dict. This predicate is needed to serialize a list of objects correctly and provides consistency with http_read_json_dict/2 and friends.

reply_json_dict(+JSONTerm) is det

reply_json_dict(+JSONTerm, +Options) is det

http_parameters(+Request, ?Parms) is det

http_parameters(+Request, ?Parms, :Options) is det

Get HTTP GET or POST form-data, applying type validation, default values, etc. Provided options are:

attribute_declarations(:Goal): Causes the declarations for an attributed named A to be fetched using call(Goal, A, Declarations).
form_data(-Data): Return the data read from the GET or POST request as a list Name = Value. All data, including name/value pairs used for Parms, is unified with Data.

The attribute_declarations hook allows sharing the declaration of attribute-properties between many http_parameters/3 calls. In this form, the requested attribute takes only one argument and the options are acquired by calling the hook. For example:

    ...,
    http_parameters(Request,
                    [ sex(Sex)
                    ],
                    [ attribute_declarations(http_param)
                    ]),
    ...

http_param(sex, [ oneof(male, female),
                  description('Sex of the person')
                ]).

bug: - If both request parameters (?name=value&...) and a POST are present the parameters are extracted from the request parameters. Still, as it is valid to have request parameters in a POST request this predicate should not process POST requests. We will keep the current behaviour as the it is not common for a request to have both request parameters and a POST data of the type application/x-www-form-urlencoded.
In the unlikely event this poses a problem the request may be specified as [method(get)|Request].

http_parameters(+Request, ?Parms) is det

http_parameters(+Request, ?Parms, :Options) is det

Get HTTP GET or POST form-data, applying type validation, default values, etc. Provided options are:

attribute_declarations(:Goal): Causes the declarations for an attributed named A to be fetched using call(Goal, A, Declarations).
form_data(-Data): Return the data read from the GET or POST request as a list Name = Value. All data, including name/value pairs used for Parms, is unified with Data.

    ...,
    http_parameters(Request,
                    [ sex(Sex)
                    ],
                    [ attribute_declarations(http_param)
                    ]),
    ...

http_param(sex, [ oneof(male, female),
                  description('Sex of the person')
                ]).

bug: - If both request parameters (?name=value&...) and a POST are present the parameters are extracted from the request parameters. Still, as it is valid to have request parameters in a POST request this predicate should not process POST requests. We will keep the current behaviour as the it is not common for a request to have both request parameters and a POST data of the type application/x-www-form-urlencoded.
In the unlikely event this poses a problem the request may be specified as [method(get)|Request].

http_current_request(-Request) is semidet

Returns the HTTP request currently being processed. Fails silently if there is no current request. This typically happens if a goal is run outside the HTTP server context.

http_peer(+Request, -PeerIP:atom) is semidet

True when PeerIP is the IP address of the connection peer. If the connection is established via a proxy or CDN we try to find the initiating peer. Currently supports:

Fastly-client-ip
X-real-ip
X-forwarded-for
Direct connections

bug: - The X-forwarded-for header is problematic. According to Wikipedia, the original client is the first, while according to AWS it is the last.

http_server(:Goal, :Options) is det

Create a server at Port that calls Goal for each parsed request. Options provide a list of options. Defined options are

port(?Address): Port to bind to. Address is either a port or a term Host:Port. The port may be a variable, causing the system to select a free port. See tcp_bind/2.
unix_socket(+Path): Instead of binding to a TCP port, bind to a Unix Domain Socket at Path.
entry_page(+URI): Affects the message printed while the server is started. Interpreted as a URI relative to the server root.
tcp_socket(+Socket): If provided, use this socket instead of the creating one and binding it to an address. The socket must be bound to an address. Note that this also allows binding an HTTP server to a Unix domain socket (AF_UNIX). See socket_create/2.
workers(+Count): Determine the number of worker threads. Default is 5. This is fine for small scale usage. Public servers typically need a higher number.
timeout(+Seconds): Maximum time of inactivity trying to read the request after a connection has been opened. Default is 60 seconds. See set_stream/1 using the timeout option.
keep_alive_timeout(+Seconds): Time to keep `Keep alive' connections alive. Default is 2 seconds.
stack_limit(+Bytes): Stack limit to use for the workers. The default is inherited from the main thread. If you need to control resource usage you may consider the spawn option of http_handler/3 and library(thread_pool).
silent(Bool): If true (default false), do not print an informational message that the server was started.

A typical initialization for an HTTP server that uses http_dispatch/1 to relay requests to predicates is:

:- use_module(library(http/thread_httpd)).
:- use_module(library(http/http_dispatch)).

start_server(Port) :-
    http_server(http_dispatch, [port(Port)]).

Note that multiple servers can coexist in the same Prolog process. A notable application of this is to have both an HTTP and HTTPS server, where the HTTP server redirects to the HTTPS server for handling sensitive requests.

http_current_server(:Goal, ?Port) is nondet

True if Goal is the goal of a server at Port.

deprecated: - Use http_server_property(Port, goal(Goal))

http_server_property(?Port, ?Property) is nondet

True if Property is a property of the HTTP server running at Port. Defined properties are:

goal(:Goal): Goal used to start the server. This is often http_dispatch/1.
scheme(-Scheme): Scheme is one of http or https.
start_time(?Time): Time-stamp when the server was created.

http_workers(?Port, -Workers) is nondet

http_workers(+Port, +Workers:int) is det

Query or set the number of workers for the server at this port. The number of workers is dynamically modified. Setting it to 1 (one) can be used to profile the worker using tprofile/1.

See also: - library(http/http_dyn_workers) implements dynamic management of the worker pool depending on usage.

http_add_worker(+Port, +Options) is det

Add a new worker to the HTTP server for port Port. Options overrule the default queue options. The following additional options are processed:

max_idle_time(+Seconds): The created worker will automatically terminate if there is no new work within Seconds.

http_current_worker(?Port, ?ThreadID) is nondet

True if ThreadID is the identifier of a Prolog thread serving Port. This predicate is motivated to allow for the use of arbitrary interaction with the worker thread for development and statistics.

http_stop_server(+Port, +Options)

Stop the indicated HTTP server gracefully. First stops all workers, then stops the server.

To be done: - Realise non-graceful stop

http_spawn(:Goal, +Options) is det

Continue this connection on a new thread. A handler may call http_spawn/2 to start a new thread that continues processing the current request using Goal. The original thread returns to the worker pool for processing new requests. Options are passed to thread_create/3, except for:

pool(+Pool): Interfaces to library(thread_pool), starting the thread on the given pool.

If a pool does not exist, this predicate calls the multifile hook create_pool/1 to create it. If this predicate succeeds the operation is retried.

Undocumented predicates

The following predicates are exported, but not or incorrectly documented.