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http_log.pl -- HTTP Logging module
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Simple module for logging HTTP requests to a file. Logging is enabled by loading this file and ensure the setting http:logfile is not the empty atom. The default file for writing the log is httpd.log. See library(settings) for details.

The level of logging can be modified using the multifile predicate http_log:nolog/1 to hide HTTP request fields from the logfile and http_log:password_field/1 to hide passwords from HTTP search specifications (e.g. /topsecret?password=secret).

Source http_log_stream(-Stream) is semidet
True when Stream is a stream to the opened HTTP log file. Opens the log file in append mode if the file is not yet open. The log file is determined from the setting http:logfile. If this setting is set to the empty atom (''), this predicate fails.

If a file error is encountered, this is reported using print_message/2, after which this predicate silently fails.

Before opening the log file, the message http_log_open(Term) is broadcasted. This message allows for creating the directory, renaming, deleting or truncating an existing log file.

Source http_log_close(+Reason) is det
If there is a currently open HTTP logfile, close it after adding a term server(Reason, Time). to the logfile. This call is intended for cooperation with the Unix logrotate facility using the following schema:
  • Move logfile (the HTTP server keeps writing to the moved file)
  • Inform the server using an HTTP request that calls http_log_close/1
  • Compress the moved logfile
- Suggested by Jacco van Ossenbruggen
Source http_log(+Format, +Args) is det
Write message from Format and Args to log-stream. See format/2 for details. Succeed without side effects if logging is not enabled.
Source password_field(+Field) is semidet[multifile]
Multifile predicate that can be defined to hide passwords from the logfile.
Source nolog(+HTTPField)[multifile]
Multifile predicate that can be defined to hide request parameters from the request logfile.
Source nolog_post_content_type(+Type) is semidet[multifile]
Multifile hook called with the Content-type header. If the hook succeeds, the POST data is not logged. For example, to stop logging anything but application/json messages:
:- multifile http_log:nolog_post_content_type/1.

http_log:nolog_post_content_type(Type) :-
   Type \= (application/json).
Type- is a term MainType/SubType
Source post_data_encoded(?Bytes:string, ?Encoded:string) is det
Encode the POST body for inclusion into the HTTP log file. The POST data is (in/de)flated using zopen/3 and base64 encoded using base64//1. The encoding makes long text messages shorter and keeps readable logfiles if binary data is posted.
Source http_logrotate(+Options) is det
Rotate the available log files. Note that there are two ways to deal with the rotation of log files:
  1. Use the OS log rotation facility. In that case the OS must (1) move the logfile and (2) have something calling http_log_close/1 to close the (moved) file and make this server create a new one on the next log message. If library(http/http_unix_daemon) is used, closing is achieved by sending SIGHUP or SIGUSR1 to the process.
  2. Call this predicate at scheduled intervals. This can be achieved by calling http_schedule_logrotate/2 in the context of library(http/http_unix_daemon) which schedules the maintenance actions.


Do not rotate if the log file is smaller than Bytes. The default is 1Mbytes.
Number of rotated log files to keep (default 10)
Compress the log files to the given format.
If true, rotate the log files in the background.
Source http_schedule_logrotate(When, Options)
Schedule log rotation based on maintenance broadcasts. When is one of:
Run each day at Hour:Min. Min is rounded to a multitude of 5.
weekly(Day, Hour:Min)
Run at the given Day and Time each week. Day is either a number 1..7 (1 is Monday) or a weekday name or abbreviation.
monthly(DayOfTheMonth, Hour:Min)
Run each month at the given Day (1..31). Note that not all months have all days.

This must be used with a timer that broadcasts a maintenance(_,_) message (see broadcast/1). Such a timer is part of library(http/http_unix_daemon).