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
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.
- 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
- 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
- Stack limit to use for the workers. The default is inherited
If you need to control resource usage you may consider the
spawn option 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:
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.
- - Use
- 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
- 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
- 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
- Port number that identifies the server.
- One of
accept for a new connection or
keep_alive if 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.
- See also
- - http_add_worker/2 may be used to create (temporary) extra
- 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.
- 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:
- 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.