This module provides the logic that is needed to integrate a process
into the Unix service (daemon) architecture. It deals with the following
aspects, all of which may be used/ignored and configured using
- Select the
port(s) to be used by the server
- Run the startup of the process as root to perform privileged
tasks and the server itself as unpriviledged user, for example
to open ports below 1000.
- Fork and detach from the controlling terminal
- Handle console and debug output using a file and/or the syslog
- Manage a pid file
The typical use scenario is to write a file that loads the following
- The application code, including http handlers (see http_handler/3).
- This library
In the code below,
?- [load]. loads the remainder of the webserver
code. This is often a sequence of use_module/1 directives.
The program entry point is http_daemon/0, declared using
initialization/2. This may be overruled using a new declaration after
loading this library. The new entry point will typically call
http_daemon/1 to start the server in a preconfigured way.
:- initialization(run, main).
Now, the server may be started using the command below. See
http_daemon/0 for supported options.
% [sudo] swipl mainfile.pl [option ...]
Below are some examples. Our first example is completely silent, running
on port 80 as user
% swipl mainfile.pl --user=www --pidfile=/var/run/http.pid
Our second example logs HTTP interaction with the syslog daemon for
debugging purposes. Note that the argument to
--debug= is a Prolog
term and must often be escaped to avoid misinterpretation by the Unix
shell. The debug option can be repeated to log multiple debug topics.
% swipl mainfile.pl --user=www --pidfile=/var/run/http.pid \
Broadcasting The library uses broadcast/1 to allow hooking certain
- Run after fork, just before starting the HTTP server. Can be used
to load additional files or perform additional initialisation, such as
starting additional threads. Recall that it is not possible to start
threads before forking.
- Run after starting the HTTP server.
- See also
- - The file <swi-home>/doc/packages/examples/http/linux-init-script
provides a /etc/init.d script for controlling a server as a normal
- To be done
- - Cleanup issues wrt. loading and initialization of xpce.
- Start the HTTP server as a daemon process. This predicate
processes the commandline arguments below. Commandline arguments
that specify servers are processed in the order they appear
using the following schema:
- Arguments that act as default for all servers.
--https=Spec is followed by
arguments for that server until the next
--https=Spec or the end of the options.
- If no
--https=Spec appears, one
HTTP server is created from the specified parameters.
--workers=10 --http --https
--http=localhost:8080 --workers=1 --https=8443 --workers=25
Start HTTP server at Port. It requires root permission and the
--user=User to open ports below 1000. The default
port is 80. If
--https is used, the default port is 443.
Only listen to the given IP address. Typically used as
--ip=localhost to restrict access to connections from
localhost if the server itself is behind an (Apache)
proxy server running on the same host.
Enable debugging Topic. See debug/3.
Write debug messages to the syslog daemon using Ident
When started as root to open a port below 1000, this option
must be provided to switch to the target user for operating
the server. The following actions are performed as root, i.e.,
before switching to User:
- open the
- write the pidfile
- setup syslog interaction
- Read the certificate, key and password file (
May be used in addition to
--user. If omitted, the login
group of the target user is used.
Write the PID of the daemon process to File.
Send output of the process to File. By default, all
Prolog console output is discarded.
If given as
--fork=false, the process
runs in the foreground.
Create a plain HTTP server. If the argument is missing or
true, create at the specified or default address. Else
use the given port and interface. Thus,
a server at port 80,
--http=8080 creates one at port
--http=localhost:8080 creates one at port
8080 that is only accessible from
--http, but creates an HTTPS server.
--cipherlist to configure SSL for
The server certificate for HTTPS.
The server private key for HTTPS.
File holding the password for accessing the private key. This
is preferred over using
--password=PW as it allows using
file protection to avoid leaking the password. The file is
read before the server drops privileges when started with
The password for accessing the private key. See also `--pwfile`.
One or more cipher strings separated by colons. See the OpenSSL
documentation for more information. Starting with SWI-Prolog
7.5.11, the default value is always a set of ciphers that was
considered secure enough to prevent all critical attacks at the
time of the SWI-Prolog release.
--no-fork and presents
the Prolog toplevel after starting the server.
Use the debugger to trace http_daemon/1.
Action to perform on
kill -HUP <pid>. Default is
(running make/0). Alternative is
quit, stopping the server.
Other options are converted by argv_options/3 and passed to
http_server/1. For example, this allows for:
Set the number of workers for the multi-threaded server.
http_daemon/0 is defined as below. The start code for a specific
server can use this as a starting point, for example for specifying
argv_options(Argv, _RestArgv, Options),
- See also
- - http_daemon/1
- Start the HTTP server as a daemon process. This predicate processes
a Prolog option list. It is normally called from http_daemon/0,
which derives the option list from the command line arguments.
Error handling depends on whether or not
interactive(true) is in
effect. If so, the error is printed before entering the toplevel. In
non-interactive mode this predicate calls
- Helper that is started from http_daemon/1. See http_daemon/1 for
options that are processed.
- option_servers(+Options, -Sockets:list)[private]
- Find all sockets that must be created according to Options. Each
socket is a term
server(Scheme, Address, Opts), where Address is
either a plain port (integer) or Host:Port. The latter binds the
port to the interface belonging to Host. For example:
socket(http, localhost:8080, Opts) creates an HTTP socket that
binds to the localhost interface on port 80. Opts are the
options specific for the given server.
- http_certificate_hook(+CertFile, +KeyFile, -Password) is semidet[multifile]
- Hook called before starting the server if the --https option is
used. This hook may be used to create or refresh the
certificate. If the hook binds Password to a string, this string
will be used to decrypt the server private key as if the
--password=Password option was given.
- start_server(+Server) is det[private]
- Start the HTTP server. It performs the following steps:
This predicate can be hooked using http_server_hook/1. This
predicate is executed after
- Setting I/O (e.g., to talk to the syslog daemon)
- Dropping root privileges (--user)
- Setting up signal handling
- Disable some development stuff.
- Re-enable the development environment. Currently re-enables xpce if
this was loaded, but not initialised and causes the interactive
toplevel to be re-enabled.
- setup_syslog(+Options) is det[private]
- Setup syslog interaction.
- setup_output(+Options) is det[private]
- Setup output from the daemon process. The default is to send all
output to a null-stream (see open_null_stream/1). With the
output(File), all output is written to File.
- write_pid(+Options) is det[private]
- If the option
pidfile(File) is present, write the PID of the
daemon to this file.
- switch_user(+Options) is det[private]
- Switch to the target user and group. If the server is started as
root, this option must be present.
- can_switch_user(Options) is det[private]
- Verify the user options before forking, so we can print the
message in time.
- server_redirect(+To, +Request)[private]
- Redirect al requests for this server to the specified server. To
is one of:
- A port (integer)
Redirect to the server running on that port in the same
Results from just passing
--redirect. Redirects to
an HTTPS server in the same Prolog process.
- A URL
Redirect to the the given URL + the request uri. This can
be used if the server cannot find its public address. For
--http --redirect=https://myhost.org --https
- setup_debug(+Options) is det[private]
- Initialse debug/3 topics. The
--debug option may be used
- kill_x11(+Options) is det[private]
- Get rid of X11 access if interactive is false.
- Prepare the server for signal handling. By default SIGINT and
SIGTERM terminate the server. SIGHUP causes the server to run
- This predicate runs in the main thread, waiting for messages
send by signal handlers to control the server. In addition, it
maintenance(Interval, Deadline) messages every
Interval seconds. These messages may be trapped using listen/2
for performing scheduled maintenance such as rotating log files,
cleaning stale data, etc.
- http_server_hook(+Options) is semidet[multifile]
- Hook that is called to start the HTTP server. This hook must be
http_server(Handler, Options). The default is
provided by start_server/1.
- http:sni_options(-HostName, -SSLOptions) is multi[multifile]
- Hook to provide Server Name Indication (SNI) for TLS
servers. When starting an HTTPS server, all solutions of this
predicate are collected and a suitable sni_hook/1 is defined
for ssl_context/3 to use different contexts depending on the
host name of the client request. This hook is executed
before privileges are dropped.