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A logging framework for Erlang/OTP
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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 syslog.

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  • 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.
  • Rewrites common OTP error messages into more readable messages
  • Support for pretty printing records encountered at compile time
  • Tolerant in the face of large or many log messages, won't out of memory the node
  • Supports internal time and date based rotation, as well as external rotation tools
  • Syslog style log level comparison flags
  • Colored terminal output (requires R16+)


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 enabled:

-compile([{parse_transform, lager_transform}]).

Before logging any messages, you'll need to start the lager application. The lager module's start function takes care of loading and starting any dependencies lager requires.


You can also start lager on startup with a switch to erl:

erl -pa path/to/lager/ebin -s lager

Once you have built your code with lager and started the lager application, 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.


To configure lager's backends, you use an application variable (probably in your app.config):

{lager, [
  {handlers, [
    {lager_console_backend, info},
    {lager_file_backend, [{file, "error.log"}, {level, error}]},
    {lager_file_backend, [{file, "console.log"}, {level, info}]}

The available configuration options for each backend are listed in their module's documentation.

Custom Formatting

All loggers have a default formatting that can be overriden. A formatter is any module that exports format(#lager_log_message{},Config#any()). It is specified as part of the configuration for the backend:

{lager, [
  {handlers, [
    {lager_console_backend, [info, {lager_default_formatter, [time," [",severity,"] ", message, "\n"]}]},
    {lager_file_backend, [{file, "error.log"}, {level, error}, {formatter, lager_default_formatter},
      {formatter_config, [date, " ", time," [",severity,"] ",pid, " ", message, "\n"]}]},
    {lager_file_backend, [{file, "console.log"}, {level, info}]}

Included is lager_default_formatter. This provides a generic, default formatting for log messages using a "semi-iolist" as configuration. Any iolist allowed elements in the configuration are printed verbatim. Atoms in the configuration are treated as metadata properties and extracted from the log message. The metadata properties date,time, message, and severity will always exist. The properties pid, file, line, module, function, and node will always exist if the parser transform is used.

["Foo"] -> "Foo", regardless of message content.
[message] -> The content of the logged message, alone.
[{pid,"Unknown Pid"}] -> "<?.?.?>" if pid is in the metadata, "Unknown Pid" if not.
[{pid, ["My pid is ", pid], "Unknown Pid"}] -> if pid is in the metadata print "My pid is <?.?.?>", otherwise print "Unknown Pid"

Optionally, a tuple of {atom(),semi-iolist()} can be used. The atom will look up the property, but if not found it will use the semi-iolist() instead. These fallbacks can be nested or refer to other properties.

[{pid,"Unknown Pid"}] -> "<?.?.?>" if pid is in the metadata, "Unknown Pid" if not.
[{server,[$(,{pid,"Unknown Server"},$)]}}] -> user provided server metadata, otherwise "(<?.?.?>)", otherwise "(Unknown Server)"

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' tofalse'.

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 set to 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.

Overload Protection

Prior to lager 2.0, the gen_event at the core of lager operated purely in synchronous mode. Asynchronous mode is faster, but has no protection against message queue overload. In lager 2.0, the gen_event takes a hybrid approach. it polls its own mailbox size and toggles the messaging between synchronous and asynchronous depending on mailbox size.

{async_threshold, 20},
{async_threshold_window, 5}

This will use async messaging until the mailbox exceeds 20 messages, at which point synchronous messaging will be used, and switch back to asynchronous, when size reduces to 20 - 5 = 15.

If you wish to disable this behaviour, simply set it to 'undefined'. It defaults to a low number to prevent the mailbox growing rapidly beyond the limit and causing problems. In general, lager should process messages as fast as they come in, so getting 20 behind should be relatively exceptional anyway.

If you want to limit the number of messages per second allowed from error_logger, which is a good idea if you want to weather a flood of messages when lots of related processes crash, you can set a limit:

{error_logger_hwm, 50}

It is probably best to keep this number small.

Runtime loglevel changes

You can change the log level of any lager backend at runtime by doing the following:

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.

Syslog style loglevel comparison flags

In addition to the regular log level names, you can also do finer grained masking of what you want to log:

info - info and higher (>= is implicit)
=debug - only the debug level
!=info - everything but the info level
<=notice - notice and below
<warning - anything less than warning

These can be used anywhere a loglevel is supplied, although they need to be either a quoted atom or a string.

Internal log rotation

Lager can rotate its own logs or have it done via an external process. To use internal rotation, use the 'size', 'date' and 'count' values in the file backend's config:

[{name, "error.log"}, {level, error}, {size, 10485760}, {date, "$D0"}, {count, 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 used:

  • 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 separate 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.

Older Backends

Lager 2.0 changed the backend API, there are various 3rd party backends for lager available, but they may not have been updated to the new API. As they are updated, links to them can be re-added here.

Record Pretty Printing

Lager's parse transform will keep track of any record definitions it encounters and store them in the module's attributes. You can then, at runtime, print any record a module compiled with the lager parse transform knows about by using the lager:pr/2 function, which takes the record and the module that knows about the record:

lager:info("My state is ~p", [lager:pr(State, ?MODULE)])

Often, ?MODULE is sufficent, but you can obviously substitute that for a literal module name. lager:pr also works from the shell.

Colored terminal output

If you have erlang R16 or higher, you can tell lager's console backend to be colored. Simply add

{colored, true}

To lager's application environment config. If you don't like the default colors, they are also configurable, see the app.src file for more details.


Lager supports basic support for redirecting log messages based on log message attributes. Lager automatically captures the pid, module, function and line at the log message callsite. However, you can add any additional attributes you wish:

lager:warning([{request, RequestID},{vhost, Vhost}], "Permission denied to ~s", [User])

Then, in addition to the default trace attributes, you'll be able to trace based on request or vhost:

lager:trace_file("logs/", [{vhost, ""}], error)

To persist metadata for the life of a process, you can use lager:md/1 to store metadata in the process dictionary:

lager:md([{zone, forbidden}])

Note that lager:md will only accept a list of key/value pairs keyed by atoms.

You can also omit the final argument, and the loglevel will default to 'debug'.

Tracing to the console is similar:

lager:trace_console([{request, 117}])

In the above example, the loglevel is omitted, but it can be specified as the second argument if desired.

You can also specify multiple expressions in a filter, or use the '*' atom as a wildcard to match any message that has that attribute, regardless of its value.

Tracing to an existing logfile is also supported, if you wanted to log warnings from a particular module to the default error.log:

lager:trace_file("log/error.log", [{module, mymodule}], warning)

To view the active log backends and traces, you can use the lager:status() function. To clear all active traces, you can use lager:clear_all_traces().

To delete a specific trace, store a handle for the trace when you create it, that you later pass to lager:stop_trace/1:

{ok, Trace} = lager:trace_file("log/error.log", [{module, mymodule}]),

Tracing to a pid is somewhat of a special case, since a pid is not a data-type that serializes well. To trace by pid, use the pid as a string:

lager:trace_console([{pid, "<0.410.0>"}])

As of lager 2.0, you can also use a 3 tuple while tracing, where the second element is a comparison operator. The currently supported comparison operators are:

  • '<' - less than
  • '=' - equal to
  • '>' - greater than
lager:trace_console([{request, '>' 117}, {request, '<' 120}])

Using '=' is equivalent to the 2-tuple form.

Setting the truncation limit at compile-time

Lager defaults to truncating messages at 4096 bytes, you can alter this by using the {lager_truncation_size, X} option. In rebar, you can add it to erl_opts:

{erl_opts, [{parse_transform, lager_transform}, {lager_truncation_size, 1024}]}.

You can also pass it to erlc, if you prefer:

erlc -pa lager/ebin +'{parse_transform, lager_transform}' +'{lager_truncation_size, 1024}' file.erl
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