sysloggerl: A syslog logger for Erlang

sysloggerl is a pure Erlang/OTP application for logging messages to syslog daemons, using UDP sockets. It provides a customizable error_logger event handler and can handle several loggers with different logging options. For now, it only implements the RFC 3164 protocol.

sysloggerl is distributed under the terms of the 2-clause BSD license; see COPYING.

Table of contents

• Installation
• Rebar
• Autotools
• How it works!
• Play with loggers
• Log messages
• Syslog error_logger handler
• Configuration
• Advanced feature: The syslog wrappers
• API

Installation

Rebar

If you use rebar, you can run the following command to build the application:

rebar compile

The testsuite is implemented using the Common Test framework. To run it with rebar:

rebar ct

Autotools

If you use the Autotools and make(1), run the following commands to build and install the application:

# Generate Autotools files.
autoreconf -vif

# Build the application.
./configure
make

# Run the testsuite
make check USE_COVER=yes

# Install it.
sudo make install

The default installation path is your Erlang's distribution libraries directory (see code:lib_dir()).

How it works!

First of all, before using sysloggerl, you must start it:

1> application:start(sysloggerl).

Then, you can use syslog module to log messages, to set new loggers or to retrieve info about existing ones.

To ease its use, the sysloggerl application exports useful types and two records (see sysloggerl.hrl file):

-record(priority, {facility  :: syslog:facility(),
                   log_level :: syslog:loglevel()}).

-record(logger,   {name       :: any(),
                   ident      :: string(),
                   udp_socket :: inet:socket(),
                   priority   :: syslog:priority(),
                   options    :: proplists:proplist()}).

Play with loggers

The sysloggerl application can handle several loggers, with different logging options. So it is possible to send messages with different identification strings, different priorities, to different syslog servers.

At any time, you can add a new logger or update an existing one:

2> rr("./include/sysloggerl.hrl").
3> Prio = syslog:priority(user, info).
4> Opts = [{host, "192.168.200.15"}, {port, 514}].
5> {ok, Logger} = syslog:set(my_logger, "test", Prio, Opts).
6> NewPrio = syslog:set_loglevel(debug, Prio).
7> {ok, _} = syslog:set(Logger#logger{priority=NewPrio}).

All existing loggers (or a specific one using its name) can be retrieved:

8> syslog:loggers().
[#logger{name = my_logger,ident = "test",
         udp_socket = #Port<0.802>,
         priority = #priority{facility = user,log_level = debug},
         options = [{host,"192.168.200.15"},{port,514}]},
 #logger{name = error_logger_syslog,ident = "error_logger",
         udp_socket = #Port<0.756>,
         priority = #priority{facility = user,log_level = info},
         options = []},
 #logger{name = default,ident = "sysloggerl",
         udp_socket = #Port<0.748>,
         priority = #priority{facility = user,log_level = notice},
         options = [log_pid]}]

9> syslog:logger(my_logger).
#logger{name = my_logger,ident = "test",
        udp_socket = #Port<0.802>,
        priority = #priority{facility = user,log_level = debug},
        options = [{host,"192.168.200.15"},{port,514}]}

By default, sysloggerl is started with a default logger and, if enabled, a logger dedicated to messages reported by the error logger. their names are, respectively, default and error_logger_syslog.

To remove a logger, you should simply call syslog:unset/1.

Log messages

The easiest way to log messages is to use the default logger, i.e by calling the logging API without any logger name:

10> syslog:log("Hello World!").
11> syslog:log(debug, "Hello World!").
12> syslog:warning_msg("I am on ~p", [node()]).

To use a specific logger, you should use the logging API with the corresponding logger name:

13> syslog:log(my_logger, info, "Hello World!", []).
14> syslog:log(my_logger, debug, "Hello World!", []).
15> syslog:warning_msg(my_logger, "I am on ~p", [node()]).

Note: any Erlang term can be used as logger name.

Syslog error_logger handler

By default, an event handler is added to log events reported by the error logger, named error_logger_syslog:

16> gen_event:which_handlers(error_logger).
[error_logger_syslog,error_logger].

It can be disabled by setting enable_error_logger flag to false in the application environment.

Standard events sent to the error logger (using error_logger:*_msg/1,2 functions) are logged on one line, with the level as prefix:

Mar 17 13:56:46 myrkr error_logger: INFO: I'm <0.33.0>
Mar 17 13:56:46 myrkr error_logger: WARNING: be careful
Mar 17 13:56:46 myrkr error_logger: ERROR: This is an error

On their side, Standard report events (sent using error_logger:*_report/1 functions) are logged with a format similar to the standard event handler:

Mar 17 14:47:16 myrkr error_logger: [1/4] = INFO REPORT ====
Mar 17 14:47:16 myrkr error_logger: [2/4]     tag1: data1
Mar 17 14:47:16 myrkr error_logger: [3/4]     a_term
Mar 17 14:47:16 myrkr error_logger: [4/4]     tag2: data
Mar 17 14:47:26 myrkr error_logger: [1/2] = ERROR REPORT ====
Mar 17 14:47:26 myrkr error_logger: [2/2] Serious error in my module

Note that User defined reports are also logged, following the same format.

By default, the error_logger_syslog handler will also log supervisor reports, crash reports and progress reports. These logs can be removed by switching on, respectively, no_supervisor_report, no_crash_report or no_progress_report parameters in the application environment.

Configuration

You can configure the default behaviour of the sysloggerl by setting following parameters in the application environment:

• {default_syslog_host, string()}

  Specifies the host of the Syslog daemon to which messages will be sent by default. Default value: "localhost".

• {default_syslog_port, pos_integer()}

  Specifies the port of the Syslog daemon to which messages will be sent by default. Default value: 514.

• {default_ident, string()}

  Specifies the identification string which will be prepended by default to logged messages. Default value: "sysloggerl".

• {default_facility, syslog:facility()}

  Specifies the default Syslog facility used to log messages. Default value: user.

• {default_loglevel, syslog:loglevel()}

  Specifies the default minimum level to log messages. Default value: notice.

By default, sysloggerl logs events reported by the error logger using a dedicated logger (Standard and SASL's events are logged). There are additional parameters to configure this logger:

• {enable_error_logger, boolean()}

  Enables/disables the logger dedicated to messages reported by the error logger. Default value: true.

• {error_logger_ident, string()}

  Overwrites the default identification string for messages received from the error logger. Default value: "error_logger".

• {error_logger_facility, syslog:facility()}

  Specifies the Syslog facility used to log error logger events. Default value: user.

• {error_logger_loglevel, syslog:loglevel()}

  Sets the minimum level to log error logger events. Default value: info.

• {error_logger_depth, -1 | non_neg_integer()}

  Controls the depth of the structures written when a report event is received. When the specified depth is reached, everything below this level is replaced by "...". Default value: -1.

• {error_logger_line_length, non_neg_integer()}

  Specifies the maximum line length of the structures written when a report event is received. Default value: 80.

• {error_logger_tty, boolean()}

  Enables/disables printout of standard events to the tty. See error_logger:tty/1. It also enables/disables supervisor reports, crash reports and progress reports. Default value: false.

• {no_crash_report, boolean()}

  Enables/disables filtering out crash reports. Default value: false.

• {no_supervisor_report, boolean()}

  Enables/disables filtering out supervisor reports. Default value: false.

• {no_progress_report, boolean()}

  Enables/disables filtering out progress reports. Default value: false.

sysloggerl_app module can be used to dynamically changed the value of these parameters. To take effet, the sysloggerl application must be reloaded:

17> sysloggerl_app:check_and_set_param(default_ident, "my_default_logger").
18> sysloggerl_app:reload().
19> syslog:logger(default).
#logger{name = default,ident = "my_default_logger",
        udp_socket = #Port<0.748>,
        priority = #priority{facility = user,log_level = notice},
        options = [log_pid]}]

Advanced feature: The syslog wrappers

For performance reasons, sysloggerl application tries to filter out unwanted log messages by comparing their log level with the minimum level of the used logger. So, for a logger with a minimum log level set to notice, all debug and info messages will be dropped by the application. This avoids unnecessary UDP traffic.
Of course this is not a mandatory. You could always set the minimum level of a logger to debug and let the syslog daemon do its job with respect of its configuration.
Internally, loggers are stored in an ETS table. So a lookup is required for each message, regardless if it will be sent or not. This is not really huge but on an heavily used application, such calls could be not negligible.

Next, a typical and encouraged usage for applications that log their messages through sysloggerl is to define a dedicated logger. But it would seem annoying to use the syslog API by repeating the logger name for each log messages.

To solve these problems, you can wrap your logger in a logger module, using the syslog_wrapper API:

20> Prio = syslog:priority(user, info).
21> Opts = [{host, "192.168.200.15"}, {port, 514}].
22> syslog_wrapper:create(my_app_logger, "my_app", Prio, Opts).
ok
23> my_app_logger:get_name().
{syslog_wrapper, my_app_logger}
24> my_app_logger:debug_msg("Just a test!").
ok

When you call syslog_wrapper:create/4, a module is compiled on the fly.
Such created logger modules can be updated by re-calling syslog_wrapper:create/4. So, like with sysloggerl loggers, it is possible to dynamically adapt its log level to your needs. And at any time you can remove it by calling syslog_wrapper:destroy/1 using the module name as argument:

25> syslog_wrapper:destroy(my_app_logger).
26> code:is_loaded(my_app_logger).
false

Important: To do its job, syslog_wrapper needs to have access to its own source file. It starts to search it in the src subdirectory directly under the top directory of the sysloggerl application (by calling code:lib_dir(sysloggerl, src). Then, if not found, it retrieves the source attribute returned by syslog_wrapper:module_info(compile).

API

Modules
sysloggerl_app
syslog
syslog_wrapper