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* Overview
Lager (as in the beer) is a logging framework for Erlang. Its purpose is
to provide a more traditional way to perform logging in an erlang application
that plays nicely with traditional UNIX logging tools like logrotate and
* Features
- Finer grained log levels (debug, info, notice, warning, error, critical,
alert, emergency)
- Logger calls are transformed using a parse transform to allow capturing
Module/Function/Line/Pid information
- When no handler is consuming a log level (eg. debug) no event is even sent
to the log handler
- Supports multiple backends, including console and file. More are planned.
* Usage
To use lager in your application, you need to define it as a rebar dep or have
some other way of including it in erlang's path. You can then add the
following option to the erlang compiler flags
{parse_transform, lager_transform}
Alternately, you can add it to the module you wish to compile with logging
-compile([{parse_transform, lager_transform}]).
Once you have built your code with lager, you can then generate log messages
by doing the following:
lager:error("Some message")
lager:warning("Some message with a term: ~p", [Term])
The general form is lager:Severity() where Severity is one of the log levels
mentioned above.
* Configuration
To configure lager's backends, you use an application variable (probably in
your app.config):
{lager, [
{handlers, [
{lager_console_backend, info},
{lager_file_backend, [
{"error.log", error, 10485760, "$D0", 5},
{"console.log", info, 10485760, "$D0", 5}
The available configuration options for each backend are listed in their
module's documentation.
* Error logger integration
Lager is also supplied with a error_logger handler module that translates
traditional erlang error messages into a friendlier format and sends them into
lager itself to be treated like a regular lager log call. To disable this, set
the lager application variable `error_logger_redirect' to `false'.
The error_logger handler will also log more complete error messages (protected
with use of trunc_io) to a "crash log" which can be referred to for further
information. The location of the crash log can be specified by the crash_log
application variable. If undefined it is not written at all.
Messages in the crash log are subject to a maximum message size which can be
specified via the crash_log_msg_size application variable.
* Runtime loglevel changes
You can change the log level of any lager backend at runtime by doing the
lager:set_loglevel(lager_console_backend, debug).
Or, for the backend with multiple handles (files, mainly):
lager:set_loglevel(lager_file_backend, "console.log", debug).
Lager keeps track of the minium log level being used by any backend and
supresses generation of messages lower than that level. This means that debug
log messages, when no backend is consuming debug messages, are effectively
free. A simple benchmark of doing 1 million debug log messages while the
minimum threshold was above that takes less than half a second.
* Internal log rotation
Lager can rotate its own logs or have it done via an external process. To
use internal rotation, use the last 3 values in the file backend's
configuration tuple. For example
{"error.log", error, 10485760, "$D0", 5}
This tells lager to log error and above messages to "error.log" and to
rotate the file at midnight or when it reaches 10mb, whichever comes first
and to keep 5 rotated logs, in addition to the current one. Setting the
count to 0 does not disable rotation, it instead rotates the file and keeps
no previous versions around. To disable rotation set the size to 0 and the
date to "".
The "$D0" syntax is taken from the syntax newsyslog uses in newsyslog.conf.
The relevant extract follows:
Day, week and month time format: The lead-in character
for day, week and month specification is a `$'-sign.
The particular format of day, week and month
specification is: [Dhh], [Ww[Dhh]] and [Mdd[Dhh]],
respectively. Optional time fields default to
midnight. The ranges for day and hour specifications
hh hours, range 0 ... 23
w day of week, range 0 ... 6, 0 = Sunday
dd day of month, range 1 ... 31, or the
letter L or l to specify the last day of
the month.
Some examples:
$D0 rotate every night at midnight
$D23 rotate every day at 23:00 hr
$W0D23 rotate every week on Sunday at 23:00 hr
$W5D16 rotate every week on Friday at 16:00 hr
$M1D0 rotate on the first day of every month at
midnight (i.e., the start of the day)
$M5D6 rotate on every 5th day of the month at
6:00 hr
To configure the crash log rotation, the following application variables are
- crash_log_size
- crash_log_date
- crash_log_count
See the .app.src file for further details.
* Syslog Support
Lager syslog output is provided as a seperate application;
[[][lager_syslog]]. It is packaged as a
separate application so Lager itself doesn't have an indirect dependancy on a
port driver. Please see the lager_syslog README for configuration information.