thread_httpd.pl -- Threaded HTTP server

Most code doesn't need to use this directly; instead use library(http/http_server), which combines this library with the typical HTTP libraries that most servers need.

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.
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.
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.

make_socket(+Address, :OptionsIn, -OptionsOut) is det[private]

Create the HTTP server socket and worker pool queue. OptionsOut is quaranteed to hold the option queue(QueueId).

Arguments:

OptionsIn

- is qualified to allow passing the module-sensitive ssl option argument.

make_addr_atom(+Scheme, +Address, -Atom) is det[private]

Create an atom that identifies the server's queue and thread resources.

create_server(:Goal, +Address, +Options) is det[private]

Create the main server thread that runs accept_server/2 to listen to new 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.

accept_server(:Goal, +Initiator, +Options)[private]

The goal of a small server-thread accepting new requests and posting them to the queue of workers.

close_server_socket(+Options)[private]

Close the server socket.

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.

create_workers(+Options)[private]

Create the pool of HTTP worker-threads. Each worker has the alias http_worker_N.

resize_pool(+Queue, +Workers) is det[private]

Create or destroy workers. If workers are destroyed, the call waits until the desired number of waiters is reached.

http_worker(+Options)[private]

Run HTTP worker main loop. Workers simply wait until they are passed an accepted socket to process a client.

If the message quit(Sender) is read from the queue, the worker stops.

open_client(+Message, +Queue, -Goal, -In, -Out, +Options, -ClientOptions) is semidet[private]

Opens the connection to the client in a worker from the message sent to the queue by accept_server/2.

open_client(+Message, +Goal, -In, -Out, -ClientOptions, +Options) is det[private]

check_keep_alive_connection(+In, +TimeOut, +Peer, +In, +Out) is semidet[private]

Wait for the client for at most TimeOut seconds. Succeed if the client starts a new request within this time. Otherwise close the connection and fail.

done_worker[private]

Called when worker is terminated due to http_workers/2 or a (debugging) exception. In the latter case, recreate_worker/2 creates a new worker.

recreate_worker(+Status, +Queue) is semidet[private]

Deal with the possibility that threads are, during development, killed with abort/0. We recreate the worker to avoid that eventually we run out of workers. If we are aborted due to a halt/0 call, thread_create/3 will raise a permission error.

The first clause deals with the possibility that we cannot write to user_error. This is possible when Prolog is started as a service using some service managers. Would be nice if we could write an error, but where?

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_process(Message, Queue, +Options)[private]

Handle a single client message on the given stream.

http_close_connection(+Request)

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

close_connection(+Peer, +In, +Out)[private]

Closes the connection from the server to the client. Errors are currently silently ignored.

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 http:create_pool/1 to create it. If this predicate succeeds the operation is retried.

create_pool(+Pool)[private]

Lazy creation of worker-pools for the HTTP server. This predicate calls the hook http:create_pool/1. If the hook fails it creates a default pool of size 10. This should suffice most typical usecases. Note that we get a permission error if the pool is already created. We can ignore this.

thread_repeat_wait(:Goal) is multi[private]

Acts as repeat, thread_idle(Goal), choosing whether to use a long or short idle time based on the average firing rate.

new_rate_mma(+N, +Resolution, -State) is det[private]

update_rate_mma(!State, -MMA) is det[private]

Implement Modified Moving Average computing the average time between requests as an exponential moving averate with alpha=1/N.

Arguments:

Resolution

- is the time resolution in 1/Resolution seconds. All storage is done in integers to avoid the need for stack freezing in nb_setarg/3.