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).

 log_error(+Error)[private]

There was an error writing the log file. The message is printed using print_message/2 and execution continues according to the setting http:on_log_error, which is one of:

retry

Close the log file. The system will try to reopen it on the next log event, recovering from the error. Note that the most common case for this is probably running out of disc space.

exit

exit(Code)

Stop the server using halt(Code). The exit variant is equivalent to exit(1).

The best choice depends on your priorities. Using retry gives priority to keep the server running. Using exit guarantees proper log files and thus the ability to examine these for security reasons. An attacker may try to flood the disc, causing a successful DoS attack if exit is used and the ability to interact without being logged if retry is used.

 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. Opening is retried every minute when a new message arrives.

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.

 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

author

- Suggested by Jacco van Ossenbruggen

 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.

 log_started(+Request, +Id, +Stream) is det[private]

Write log message that Request was started to Stream.

Arguments:

Filled

- with sequence identifier for the request

 log_request(+Request, -Log)[private]

Remove passwords from the request to avoid sending them to the logfiles.

 password_field(+Field) is semidet[multifile]

Multifile predicate that can be defined to hide passwords from the logfile.

 nolog(+HTTPField)[multifile]

Multifile predicate that can be defined to hide request parameters from the request logfile.

 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).

Arguments:

Type	- is a term MainType/SubType

 add_post_data(+Request0, -Request) is det[private]

Add a request field post_data(Data) if the setting http:log_post_data is an integer > 0, the content length < this setting and nolog_post_content_type/1 does not succeed on the provided content type.

 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.

 log_completed(+Code, +Status, +Bytes, +Id, +CPU, +Stream) is det[private]

Write log message to Stream from a call_cleanup/3 call.

Arguments:

Status	- 2nd argument of call_cleanup/3
Id	- Term identifying the completed request
CPU0	- CPU time at time of entrance
Stream	- Stream to write to (normally from http_log_stream/1).

 log_check_deleted(+Stream) is semidet[private]

If the link-count of the stream has dropped to zero, the file has been deleted/moved. In this case the log file is closed and log_check_deleted/6 will open a new one. This provides some support for cleaning up the logfile without shutting down the server.

See also

- logrotate(1) to manage logfiles on Unix systems.

 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.

Options:

min_size(+Bytes)

Do not rotate if the log file is smaller than Bytes. The default is 1Mbytes.

keep_logs(+Count)

Number of rotated log files to keep (default 10)

compress_logs(+Format)

Compress the log files to the given format.

background(+Boolean)

If true, rotate the log files in the background.

 compress_file(+File, +Format)[private]

Compress a file according to Format. Currently only supports gzip.

 http_schedule_logrotate(When, Options)

Schedule log rotation based on maintenance broadcasts. When is one of:

daily(Hour:Min)

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).

 http_consider_logrotate[private]

Perform a log rotation if the schedule is met