thread_httpd.pl -- Threaded HTTP server

This library defines the HTTP server frontend of choice for SWI-Prolog. It is based on the multi-threading capabilities of SWI-Prolog and thus exploits multiple cores to serve requests concurrently. The server scales well and can cooperate with library(thread_pool) to control the number of concurrent requests of a given type. For example, it can be configured to handle 200 file download requests concurrently, 2 requests that potentially uses a lot of memory and 8 requests that use a lot of CPU resources.

On Unix systems, this library can be combined with library(http/http_unix_daemon) to realise a proper Unix service process that creates a web server at port 80, runs under a specific account, optionally detaches from the controlling terminal, etc.

Combined with library(http/http_ssl_plugin) from the SSL package, this library can be used to create an HTTPS server. See <plbase>/doc/packages/examples/ssl/https for an example server using a self-signed SSL certificate.

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.
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.
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 det

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.

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_enough_workers(+Queue, +Why, +Peer) is det

Check that we have enough workers in our queue. If not, call the hook http:schedule_workers/1 to extend the worker pool. This predicate can be used by accept_hook/2.

http:schedule_workers(+Data:dict) is semidet[multifile]

Hook called if a new connection or a keep-alive connection cannot be scheduled immediately to a worker. Dict contains the following keys:

port:Port: Port number that identifies the server.
reason:Reason: One of accept for a new connection or keep_alive if a worker tries to reschedule itself.
peer:Peer: Identify the other end of the connection
waiting:Size: Number of messages waiting in the queue.
queue:Queue: Message queue used to dispatch accepted messages.

Note that, when called with reason:accept, we are called in the time critical main accept loop. An implementation of this hook shall typically send the event to thread dedicated to dynamic worker-pool management.

See also: - http_add_worker/2 may be used to create (temporary) extra workers.

thread_httpd:message_level(+Exception, -Level)[multifile]

Determine the message stream used for exceptions that may occur during server_loop/5. Being multifile, clauses can be added by the application to refine error handling. See also message_hook/3 for further programming error handling.

http_requeue(+Header)

Re-queue a connection to the worker pool. This deals with processing additional requests on keep-alive connections.

http_close_connection(+Request)

Close connection associated to Request. See also http_requeue/1.

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.