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 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.
- Instead of binding to a TCP port, bind to a Unix Domain Socket at Path.
- Affects the message printed while the server is started. Interpreted as a URI relative to the server root.
- If provided, use this socket instead of the creating one and binding it to an address. The socket must be bound to an address.
- Determine the number of worker threads. Default is 5. This is fine for small scale usage. Public servers typically need a higher number.
- 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.
- Time to keep `Keep alive' connections alive. Default is 2 seconds.
- Stack limit to use for the workers. The default is inherited
mainthread. If you need to control resource usage you may consider the
spawnoption of http_handler/3 and library(thread_pool).
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
- 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.
- http_server_property(?Port, ?Property) is nondet
- True if Property is a property of the HTTP server running at
Port. Defined properties are:
- Goal used to start the server. This is often http_dispatch/1.
- Scheme is one of
- 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:
- 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 the server socket.
- http_stop_server(+Port, +Options)
- Stop the indicated HTTP server gracefully. First stops all workers, then stops the server.
- 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
- Port number that identifies the server.
- One of
acceptfor a new connection or
keep_aliveif a worker tries to reschedule itself.
- Identify the other end of the connection
- Number of messages waiting in the 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.
- 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.
- 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.
- 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.
- 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.
- 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:
- 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.
- 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
shortidle 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.