Componentpedia / Listings / lager

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.

• 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 sent to the log handler
• Supports multiple backends, including console and file.
• Supports multiple sinks
• 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
• Optional feature to bypass log size truncation ("unsafe")
• Supports internal time and date based rotation, as well as external rotation tools
• Syslog style log level comparison flags
• Colored terminal output (requires R16+)
• Map support (requires 17+)
• Optional load shedding by setting a high water mark to kill (and reinstall) a sink after a configurable cool down timer

We welcome contributions from the community. We are always excited to get ideas for improving lager.

If you are looking for an idea to help out, please take a look at our open issues - a number of them are tagged with Help Wanted and Easy - some of them are tagged as both! We are happy to mentor people get started with any of these issues, and they don't need prior discussion.

That being said, before you send large changes please open an issue first to discuss the change you'd like to make along with an idea of your proposal to implement that change.

• Large changes without prior discussion are likely to be rejected.
• Changes without test cases are likely to be rejected.
• Please use the style of the existing codebase when submitting PRs.

We review PRs and issues at least once a month as described below.

The lager maintainers intend to support the past three OTP releases from current on the main 3.x branch of the project. As of August 2019 that includes 22, 21 20

Lager may or may not run on older OTP releases but it will only be guaranteed tested on the previous three OTP releases. If you need a version of lager which runs on older OTP releases, we recommend you use either the 3.4.0 release or the 2.x branch.

We have (at least) monthly issue and PR triage for lager in the #lager room on the freenode IRC network every third Thursday at 2 pm US/Pacific, 10 pm UTC. You are welcome to join us there to ask questions about lager or participate in the triage.

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.

lager:start().

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")

Or:

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, [
  {log_root, "/var/log/hello"},
  {handlers, [
    {lager_console_backend, [{level, info}]},
    {lager_file_backend, [{file, "error.log"}, {level, error}]},
    {lager_file_backend, [{file, "console.log"}, {level, info}]}
  ]}
]}.

log_root variable is optional, by default file paths are relative to CWD.

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

Lager has traditionally supported a single sink (implemented as a gen_event manager) named lager_event to which all backends were connected.

Lager now supports extra sinks; each sink can have different sync/async message thresholds and different backends.

To use multiple sinks (beyond the built-in sink of lager and lager_event), you need to:

1. Setup rebar.config
2. Configure the backends in app.config

Each sink has two names: one atom to be used like a module name for sending messages, and that atom with _lager_event appended for backend configuration.

This reflects the legacy behavior: lager:info (or critical, or debug, etc) is a way of sending a message to a sink named lager_event. Now developers can invoke audit:info or myCompanyName:debug so long as the corresponding audit_lager_event or myCompanyName_lager_event sinks are configured.

In rebar.config for the project that requires lager, include a list of sink names (without the _lager_event suffix) in erl_opts:

{lager_extra_sinks, [audit]}

To be useful, sinks must be configured at runtime with backends.

In app.config for the project that requires lager, for example, extend the lager configuration to include an extra_sinks tuple with backends (aka "handlers") and optionally async_threshold and async_threshold_window values (see Overload Protection below). If async values are not configured, no overload protection will be applied on that sink.

[{lager, [
          {log_root, "/tmp"},

          %% Default handlers for lager/lager_event
          {handlers, [
                      {lager_console_backend, [{level, info}]},
                      {lager_file_backend, [{file, "error.log"}, {level, error}]},
                      {lager_file_backend, [{file, "console.log"}, {level, info}]}
                     ]},

          %% Any other sinks
          {extra_sinks,
           [
            {audit_lager_event,
             [{handlers,
               [{lager_file_backend,
                 [{file, "sink1.log"},
                  {level, info}
                 ]
                }]
              },
              {async_threshold, 500},
              {async_threshold_window, 50}]
            }]
          }
         ]
 }
].

All loggers have a default formatting that can be overridden. 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, [{level, info}, {formatter, lager_default_formatter},
      {formatter_config, [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 structure similar to Erlang's iolist which we call "semi-iolist":

• Any traditional iolist elements in the configuration are printed verbatim.
• Atoms in the configuration are treated as placeholders for lager metadata and extracted from the log message.
  • The placeholders date, time, message, sev and severity will always exist.
  • sev is an abbreviated severity which is interpreted as a capitalized single letter encoding of the severity level (e.g. 'debug' -> $D)
  • The placeholders pid, file, line, module, function, and node will always exist if the parse transform is used.
  • The placeholder application may exist if the parse transform is used. It is dependent on finding the applications app.src file.
  • If the error logger integration is used, the placeholder pid will always exist and the placeholder name may exist.
  • Applications can define their own metadata placeholder.
  • A tuple of {atom(), semi-iolist()} allows for a fallback for the atom placeholder. If the value represented by the atom cannot be found, the semi-iolist will be interpreted instead.
  • A tuple of {atom(), semi-iolist(), semi-iolist()} represents a conditional operator: if a value for the atom placeholder can be found, the first semi-iolist will be output; otherwise, the second will be used.
  • A tuple of {pterm, atom()} will attempt to lookup the value of the specified atom from the persistent_term feature added in OTP 21.2. The default value is "". The default value will be used if the key cannot be found or if this formatting term is specified on an OTP release before OTP 21.
  • A tuple of {pterm, atom(), semi-iolist()} will attempt to lookup the value of the specified atom from the persistent_term feature added in OTP 21.2. The default value is the specified semi-iolist(). The default value will be used if the key cannot be found or the if this formatting term is specified on an OTP release before OTP 21.

Examples:

["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"
[{server,{pid, ["(", pid, ")"], ["(Unknown Server)"]}}] -> user provided server metadata, otherwise "(<?.?.?>)", otherwise "(Unknown Server)"
[{pterm, pterm_key, <<"undefined">>}] -> if a value for 'pterm_key' is found in OTP 21 (or later) persistent_term storage it is used, otherwise "undefined"

By default, lager formats timestamps as local time for whatever computer generated the log message.

To make lager use UTC timestamps, you can set the sasl application's utc_log configuration parameter to true in your application configuration file.

Example:

%% format log timestamps as UTC
[{sasl, [{utc_log, true}]}].

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. You can also disable reformatting for OTP and Cowboy messages by setting variable error_logger_format_raw to true.

If you installed your own handler(s) into error_logger, you can tell lager to leave it alone by using the error_logger_whitelist environment variable with a list of handlers to allow.

{error_logger_whitelist, [my_handler]}

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 false 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.

Messages from error_logger will be redirected to error_logger_lager_event sink if it is defined so it can be redirected to another log file.

For example:

[{lager, [
         {extra_sinks,
          [
           {error_logger_lager_event, 
            [{handlers, [
              {lager_file_backend, [{file, "error_logger.log"}, {level, info}]}]
              }]
           }]
           }]
}].

will send all error_logger messages to error_logger.log file.

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. As of 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 async_threshold 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.

When the high-water mark is exceeded, lager can be configured to flush all event notifications in the message queue. This can have unintended consequences for other handlers in the same event manager (in e.g. the error_logger), as events they rely on may be wrongly discarded. By default, this behavior is enabled, but can be controlled, for the error_logger via:

{error_logger_flush_queue, true | false}

or for a specific sink, using the option:

{flush_queue, true | false}

If flush_queue is true, a message queue length threshold can be set, at which messages will start being discarded. The default threshold is 0, meaning that if flush_queue is true, messages will be discarded if the high-water mark is exceeded, regardless of the length of the message queue. The option to control the threshold is, for error_logger:

{error_logger_flush_threshold, 1000}

and for sinks:

{flush_threshold, 1000}

In some high volume situations, it may be preferable to drop all pending log messages instead of letting them drain over time.

If you prefer, you may choose to use the sink killer to shed load. In this operational mode, if the gen_event mailbox exceeds a configurable high water mark, the sink will be killed and reinstalled after a configurable cool down time.

You can configure this behavior by using these configuration directives:

{killer_hwm, 1000},
{killer_reinstall_after, 5000}

This means if the sink's mailbox size exceeds 1000 messages, kill the entire sink and reload it after 5000 milliseconds. This behavior can also be installed into alternative sinks if desired.

By default, the manager killer is not installed into any sink. If the killer_reinstall_after cool down time is not specified it defaults to 5000.

The unsafe code pathway bypasses the normal lager formatting code and uses the same code as error_logger in OTP. This provides a marginal speedup to your logging code (we measured between 0.5-1.3% improvement during our benchmarking; others have reported better improvements.)

This is a dangerous feature. It will not protect you against large log messages - large messages can kill your application and even your Erlang VM dead due to memory exhaustion as large terms are copied over and over in a failure cascade. We strongly recommend that this code pathway only be used by log messages with a well bounded upper size of around 500 bytes.

If there's any possibility the log messages could exceed that limit, you should use the normal lager message formatting code which will provide the appropriate size limitations and protection against memory exhaustion.

If you want to format an unsafe log message, you may use the severity level (as usual) followed by _unsafe. Here's an example:

lager:info_unsafe("The quick brown ~s jumped over the lazy ~s", ["fox", "dog"]).

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 minimum log level being used by any backend and suppresses 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.

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.

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:

[{file, "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
are:

  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

On top of the day, week and month time format from newsyslog, hour specification is added from PR #420

Format of hour specification is : [Hmm]
The range for minute specification is:

  mm      minutes, range 0 ... 59

Some examples:

  $H00    rotate every hour at HH:00
  $D12H30 rotate every day at 12:30
  $W0D0H0 rotate every week on Sunday at 00:00

To configure the crash log rotation, the following application variables are used:

• crash_log_size
• crash_log_date
• crash_log_count
• crash_log_rotator

See the .app.src file for further details.

Custom log rotator could be configured with option for lager_file_backend

{rotator, lager_rotator_default}

The module should provide the following callbacks as lager_rotator_behaviour

%% @doc Create a log file
-callback(create_logfile(Name::list(), Buffer::{integer(), integer()} | any()) ->
    {ok, {FD::file:io_device(), Inode::integer(), Size::integer()}} | {error, any()}).

%% @doc Open a log file
-callback(open_logfile(Name::list(), Buffer::{integer(), integer()} | any()) ->
    {ok, {FD::file:io_device(), Inode::integer(), Size::integer()}} | {error, any()}).

%% @doc Ensure reference to current target, could be rotated
-callback(ensure_logfile(Name::list(), FD::file:io_device(), Inode::integer(),
                         Buffer::{integer(), integer()} | any()) ->
    {ok, {FD::file:io_device(), Inode::integer(), Size::integer()}} | {error, any()}).

%% @doc Rotate the log file
-callback(rotate_logfile(Name::list(), Count::integer()) ->
    ok).

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 dependency on a port driver. Please see the lager_syslog README for configuration information.

There are lots of them! Some connect log messages to AMQP, various logging analytic services (bunyan, loggly, etc), and more. Looking on hex or using "lager BACKEND" where "BACKEND" is your preferred log solution on your favorite search engine is a good starting point.

Up to OTP 20:

try
    foo()
catch
    Class:Reason ->
        lager:error(
            "~nStacktrace:~s",
            [lager:pr_stacktrace(erlang:get_stacktrace(), {Class, Reason})])
end.

On OTP 21+:

try
    foo()
catch
    Class:Reason:Stacktrace ->
        lager:error(
            "~nStacktrace:~s",
            [lager:pr_stacktrace(Stacktrace, {Class, Reason})])
end.

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 sufficient, but you can obviously substitute that for a literal module name. lager:pr also works from the shell.

If you have Erlang R16 or higher, you can tell lager's console backend to be colored. Simply add to lager's application environment config:

{colored, true}

If you don't like the default colors, they are also configurable; see the .app.src file for more details.

The output will be colored from the first occurrence of the atom color in the formatting configuration. For example:

{lager_console_backend, [{level, info}, {formatter, lager_default_formatter},
  {formatter_config, [time, color, " [",severity,"] ", message, "\e[0m\r\n"]}]]}

This will make the entire log message, except time, colored. The escape sequence before the line break is needed in order to reset the color after each log message.

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/example.com.error", [{vhost, "example.com"}], 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. You may also use the special value ! to mean, only select if this key is not present.

Tracing to an existing logfile is also supported (but see Multiple sink support below):

lager:trace_file("log/error.log", [{module, mymodule}, {function, myfunction}], 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}]),
...
lager:stop_trace(Trace)

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 3.3.1, 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
• =< - less than or equal
• = - equal to
• != - not equal to
• > - greater than
• >= - greater than or equal

lager:trace_console([{request, '>', 117}, {request, '<', 120}])

Using = is equivalent to the 2-tuple form.

As of lager 3.3.1 you may also use the special filter composition keys of all or any. For example the filter example above could be expressed as:

lager:trace_console([{all, [{request, '>', 117}, {request, '<', 120}]}])

any has the effect of "OR style" logical evaluation between filters; all means "AND style" logical evaluation between filters. These compositional filters expect a list of additional filter expressions as their values.

The null filter has a special meaning. A filter of {null, false} acts as a black hole; nothing is passed through. A filter of {null, true} means everything passes through. No other values for the null filter are valid and will be rejected.

A special log level, silence can be used together with a filter in order to suppress specific log output. This can be useful if a backend has been configured for a particular log level, but a particular set of log messages clutters the log. If these come from a dependency, they might be difficult to remove entirely, and it might not be desirable to do so in general. In such situations, a trace filter with log level silence can turn them off selectively, while letting other messages through as before.

If using multiple sinks, there are limitations on tracing that you should be aware of.

Traces are specific to a sink, which can be specified via trace filters:

lager:trace_file("log/security.log", [{sink, audit_event}, {function, myfunction}], warning)

If no sink is thus specified, the default lager sink will be used.

This has two ramifications:

• Traces cannot intercept messages sent to a different sink.
• Tracing to a file already opened via lager:trace_file will only be successful if the same sink is specified.

The former can be ameliorated by opening multiple traces; the latter can be fixed by rearchitecting lager's file backend, but this has not been tackled.

Lager supports starting traces from its configuration file. The keyword to define them is traces, followed by a proplist of tuples that define a backend handler and zero or more filters in a required list, followed by an optional message severity level.

An example looks like this:

{lager, [
  {handlers, [...]},
  {traces, [
    %% handler,                         filter,                message level (defaults to debug if not given)
    {lager_console_backend,             [{module, foo}],       info },
    {{lager_file_backend, "trace.log"}, [{request, '>', 120}], error},
    {{lager_file_backend, "event.log"}, [{module, bar}]             } %% implied debug level here
  ]}
]}.

In this example, we have three traces. One using the console backend, and two using the file backend. If the message severity level is left out, it defaults to debug as in the last file backend example.

The traces keyword works on alternative sinks too but the same limitations and caveats noted above apply.

IMPORTANT: You must define a severity level in all lager releases up to and including 3.1.0 or previous. The 2-tuple form wasn't added until 3.2.0.

Lager supports supplying metadata from external sources by registering a callback function. This metadata is also persistent across processes even if the process dies.

In general use you won't need to use this feature. However it is useful in situations such as:

• Tracing information provided by seq_trace
• Contextual information about your application
• Persistent information which isn't provided by the default placeholders
• Situations where you would have to set the metadata before every logging call

You can add the callbacks by using the {lager_parse_transform_functions, X} option. It is only available when using parse_transform. In rebar, you can add it to erl_opts as below:

{erl_opts, [{parse_transform, lager_transform}, 
            {lager_function_transforms, 
              [
                 %% Placeholder              Resolve type  Callback tuple
                {metadata_placeholder,       on_emit,      {module_name, function_name}},
                {other_metadata_placeholder, on_log,       {module_name, function_name}}
              ]}]}.

The first atom is the placeholder atom used for the substitution in your custom formatter. See Custom Formatting for more information.

The second atom is the resolve type. This specify the callback to resolve at the time of the message being emitted or at the time of the logging call. You have to specify either the atom on_emit or on_log. There is not a 'right' resolve type to use, so please read the uses/caveats of each and pick the option which fits your requirements best.

on_emit:

• The callback functions are not resolved until the message is emitted by the backend.
• If the callback function cannot be resolved, not loaded or produces unhandled errors then undefined will be returned.
• Since the callback function is dependent on a process, there is the chance that message will be emitted after the dependent process has died resulting in undefined being returned. This process can also be your own process

on_log:

• The callback functions are resolved regardless whether the message is
  emitted or not
• If the callback function cannot be resolved or not loaded the errors are not handled by lager itself.
• Any potential errors in callback should be handled in the callback function itself.
• Because the function is resolved at log time there should be less chance of the dependent process dying before you can resolve it, especially if you are logging from the app which contains the callback.

The third element is the callback to your function consisting of a tuple in the form {Module Function}. The callback should look like the following regardless if using on_emit or on_log:

• It should be exported
• It should takes no arguments e.g. has an arity of 0
• It should return any traditional iolist elements or the atom undefined
• For errors generated within your callback see the resolve type documentation above.

If the callback returns undefined then it will follow the same fallback and conditional operator rules as documented in the Custom Formatting section.

This example would work with on_emit but could be unsafe to use with on_log. If the call failed in on_emit it would default to undefined, however with on_log it would error.

-export([my_callback/0]).

my_callback() ->
  my_app_serv:call('some options').

This example would be to safe to work with both on_emit and on_log

-export([my_callback/0]).

my_callback() ->
  try my_app_serv:call('some options') of
    Result ->
      Result
  catch
    _ ->
      %% You could define any traditional iolist elements you wanted here
      undefined
  end.

Note that the callback can be any Module:Function/0. It does not have be part of your application. For example you could use cpu_sup:avg1/0 as your
callback function like so {cpu_avg1, on_emit, {cpu_sup, avg1}}

Examples:

-export([reductions/0]).

reductions() ->
  proplists:get_value(reductions, erlang:process_info(self())).

-export([seq_trace/0]).

seq_trace() ->
  case seq_trace:get_token(label) of
    {label, TraceLabel} ->
      TraceLabel;
    _ ->
      undefined
  end.

IMPORTANT: Since on_emit relies on function calls injected at the point where a log message is emitted, your logging performance (ops/sec) will be impacted by what the functions you call do and how much latency they may introduce. This impact will even greater with on_log since the calls are injected at the point a message is logged.

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

If you don't want to see supervisors and applications start/stop logs in debug level of your application, you can use these configs to turn it off:

{lager, [{suppress_application_start_stop, true},
         {suppress_supervisor_start_stop, true}]}

Lager provides an integrated way to use sys 'debug functions'. You can install a debug function in a target process by doing

lager:install_trace(Pid, notice).

You can also customize the tracing somewhat:

lager:install_trace(Pid, notice, [{count, 100}, {timeout, 5000}, {format_string, "my trace event ~p ~p"]}).

The trace options are currently:

• timeout - how long the trace stays installed: infinity (the default) or a millisecond timeout
• count - how many trace events to log: infinity (default) or a positive number
• format_string - the format string to log the event with. Must have 2 format specifiers for the 2 parameters supplied.

This will, on every 'system event' for an OTP process (usually inbound messages, replies and state changes) generate a lager message at the specified log level.

You can remove the trace when you're done by doing:

lager:remove_trace(Pid).

If you want to start an OTP process with tracing enabled from the very beginning, you can do something like this:

gen_server:start_link(mymodule, [], [{debug, [{install, {fun lager:trace_func/3, lager:trace_state(undefined, notice, [])}}]}]).

The third argument to the trace_state function is the Option list documented above.

If you want to send your console output to another group_leader (typically on another node) you can provide a {group_leader, Pid} argument to the console backend. This can be combined with another console config option, id and gen_event's {Module, ID} to allow remote tracing of a node to standard out via nodetool:

    GL = erlang:group_leader(),
    Node = node(GL),
    lager_app:start_handler(lager_event, {lager_console_backend, Node},
         [{group_leader, GL}, {level, none}, {id, {lager_console_backend, Node}}]),
    case lager:trace({lager_console_backend, Node}, Filter, Level) of
         ...

In the above example, the code is assumed to be running via a nodetool rpc invocation so that the code is executing on the Erlang node, but the group_leader is that of the reltool node (eg. appname_maint_12345@127.0.0.1).

If you intend to use tracing with this feature, make sure the second parameter to start_handler and the id parameter match. Thus when the custom group_leader process exits, lager will remove any associated traces for that handler.

There are 2 ways in which Lager can be leveraged in an Elixir project:

1. Lager Backend for Elixir Logger
2. Directly

Elixir's Logger is the idiomatic way to add logging into elixir code. Logger has a plug-in model, allowing for different logging Backends to be used without the need to change the logging code within your project.

This approach will benefit from the fact that most elixir libs and frameworks are likely to use the elixir Logger and as such logging will all flow via the same logging mechanism.

In elixir 1.5 support for parse transforms was deprecated. Taking the "Lager as a Logger Backend" approach is likely bypass any related regression issues that would be introduced into a project which is using lager directly when updating to elixir 1.5.

There are open source elixir Logger backends for Lager available:

• LagerLogger
• LoggerLagerBackend

It is fully possible prior to elixir 1.5 to use lager and all its features directly.

After elixir 1.5 there is no support for parse transforms, and it is recommended to use an elixir wrapper for the lager api that provides compile time log level exclusion via elixir macros when opting for direct use of lager.

Including Lager as a dependency:

# mix.exs
def application do
  [
    applications: [:lager],
    erl_opts: [parse_transform: "lager_transform"]
  ]
end

defp deps do
  [{:lager, "~> 3.2"}]
end

Example Configuration:

# config.exs
use Mix.Config

# Stop lager writing a crash log
config :lager, :crash_log, false

config :lager,
  log_root: '/var/log/hello',
  handlers: [
    lager_console_backend: :info,
    lager_file_backend: [file: "error.log", level: :error],
    lager_file_backend: [file: "console.log", level: :info]
  ]

There is a known issue where Elixir's Logger and Lager both contest for the Erlang error_logger handle if used side by side.

If using both add the following to your config.exs:

# config.exs
use Mix.Config

# Stop lager redirecting :error_logger messages
config :lager, :error_logger_redirect, false

# Stop lager removing Logger's :error_logger handler
config :lager, :error_logger_whitelist, [Logger.ErrorHandler]

Example Usage:

:lager.error('Some message')
:lager.warning('Some message with a term: ~p', [term])

3.9.2 - 14 May 2021

• Bugfix: Prevent "a term is constructed but never used" warnings (#547)
• Bugfix: Update CI test matrix to include OTP 24 (#551)

3.9.1 - 2 March 2021

• Bugfix: More log_root fun (#543)
• Bugfix: Use GHA for all the CIs

3.9.0 - 24 February 2021

* Bugfix: Try to make a log root of "log" more sensible (#540)
* Feature: Further adapt to OTP 24 (also remove pre OTP 21 code),
           adopt Github Actions for tests

3.8.2 - 4 February 2021

* Bugfix: Make directory expansion return an absolute path (#535)
* Feature: Write crash.log under the log_root location (#536)
* Bugfix: Handle line numbering correctly in forthcoming OTP 24 release (#537)

3.8.1 - 28 August 2020

* Feature: Allow metadata fields to be whitelisted in log formatting (#514)
* Feature: Enable a persistent_term log formatter (#530) (#531)
* Bugfix: Handle gen_statem crashes in newer OTP releases correctly (#523)
* Cleanup: Add a hex badge (#525)
* Cleanup: Fix Travis CI badge link
* Policy: Officially ending support for OTP 20 (Support OTP 21, 22, 23)

3.8.0 - 9 August 2019

* Breaking API change: Modify the `lager_rotator_behaviour` to pass in a
  file's creation time to `ensure_logfile/5` to be used to determine if
  file has changed on systems where inodes are not available (i.e.
  `win32`). The return value from `create_logfile/2`, `open_logfile/2` and
  `ensure_logfile/5` now requires ctime to be returned (#509)
* Bugfix: ensure log file rotation works on `win32` (#509)
* Bugfix: ensure test suite passes on `win32` (#509)
* Bugfix: ensure file paths with Unicode are formatted properly (#510)

3.7.0 - 24 May 2019

* Policy: Officially ending support for OTP 19 (Support OTP 20, 21, 22)
* Cleanup: Fix all dialyzer errors
* Bugfix: Minor changes to FSM/statem exits in OTP 22.

3.6.10 - 30 April 2019

* Documentation: Fix pr_stacktrace invocation example (#494)
* Bugfix: Do not count suppressed messages for message drop counts (#499)

3.6.9 - 13 March 2019

* Bugfix: Fix file rotation on windows (#493)

3.6.8 - 21 December 2018

* Documentation: Document the error_logger_whitelist environment variable. (#489)
* Bugfix: Remove the built in handler inside of OTP 21 `logger` system. (#488)
* Bugfix: Cleanup unneeded check for is_map (#486)
* Bugfix: Cleanup ranch errors treated as cowboy errors (#485)
* Testing: Remove OTP 18 from TravisCI testing matrix

3.6.7 - 14 October 2018

* Bugfix: fix tracing to work with OTP21 #480

3.6.6 - 24 September 2018

* Bugfix: When printing records, handle an improper list correctly. #478
* Bugfix: Fix various tests and make some rotation code more explicit. #476
* Bugfix: Make sure not to miscount messages during high-water mark check. #475

3.6.5 - 3 September 2018

* Feature: Allow the console backend to redirect output to a remote node #469
* Feature: is_loggble - support for severity as atom #472
* Bugfix: Prevent silent dropping of messages when hwm is exceeded #467
* Bugfix: rotation - default log file not deleted #474
* Bugfix: Handle strange crash report from gen_statem #473
* Documentation: Various markup fixes: #468 #470

3.6.4 - 11 July 2018

* Bugfix: Reinstall handlers after a sink is killed #459
* Bugfix: Fix platform_define matching not to break on OSX Mojave #461
* Feature: Add support for installing a sys trace function #462

3.6.3 - 6 June 2018

* OTP 21 support

3.6.2 - 26 April 2018

* Bugfix: flush_threshold not working (#449)
* Feature: Add `node` as a formatting option (#447)
* Documentation: Update Elixir section with information about parse_transform (#446)
* Bugfix: Correct default console configuration to use "[{level,info}]" instead (#445)
* Feature: Pretty print lists of records at top level and field values with lager:pr (#442)
* Bugfix: Ignore return value of lager:dispatch_log in lager.hrl (#441)

3.6.1 - 1 February 2018

* Bugfix: Make a few corrections to the recent mailbox flushing changes (#436)
* Bugfix: add flush options to proplist validation (#439)
* Bugfix: Don't log when we dropped 0 messages (#440)

3.6.0 - 16 January 2018

* Feature: Support logging with macros per level (#419)
* Feature: Support custom file rotation handler; support hourly file
           rotation (#420)
* Feature: Optionally reverse pretty stacktraces (so errors are
           at the top and the failed function call is at the bottom.)
           (#424)
* Bugfix:  Handle OTP 20 gen_server failure where client pid
           is dead. (#426)
* Feature: Optionally don't flush notify messages at
           high water mark. (#427)
* Bugfix:  Handle another stacktrace format (#429)
* Bugfix:  Fix test failure using macros on OTP 18 (#430)
* Policy:  Remove all code which supports R15 (#432)

3.5.2 - 19 October 2017

* Bugfix: Properly check for unicode characters in potentially deep
          character list. (#417)

3.5.1 - 15 June 2017

* Doc fix: Missed a curly brace in an example. (#412)
* Feature: Dynamic metadata functions (#392) - It is now possible to
           dynamically add metadata to lager messages. See the "dynamic
           metadata" section above for more information.
* Doc fix: Add information about the "application" placeholder. (#414)

3.5.0 - 28 May 2017

* Bugfix: Support OTP 20 gen_event messages (#410)
* Feature: Enable console output to standard_error.
           Convert to proplist configuration style (like file handler)
           Deprecate previous configuration directives (#409)
* Bugfix: Enable the event shaper to filter messages before they're
          counted; do not count application/supervisor start/stops
          toward high water mark. (#411)
* Docs: Add PR guidelines; add info about the #lager chat room on freenode.

3.4.2 - 26 April 2017

* Docs: Document how to make lager use UTC timestamps (#405)
* Docs: Add a note about our triage cadence.
* Docs: Update lager_syslog URL
* Docs: Document placeholders for error_logger integration (#404)
* Feature: Add hex.pm metadata and full rebar3 support.

3.4.1 - 28 March 2017

* Docs: Added documentation around using lager in the context of elixir applications (#398)
* Bugfix: Properly expand paths when log_root is set. (#386)
* Policy: Removed R15 from Travis configuration

3.4.0 - 16 March 2017

* Policy: Adopt official OTP support policy. (This is the **last** lager 3.x release
  that will support R15.)
* Test: Fix timeouts, R15 missing functions on possibly long-running tests in Travis. (#394, #395)
* Feature: capture and log metadata from error_logger messages (#397)
* Feature: Expose new trace filters and enable filter composition (#389)
* Feature: Log crashes from gen_fsm and gen_statem correctly (#391)
* Docs: Typo in badge URL (#390)

3.3.0 - 16 February 2017

* Docs: Fix documentation to make 'it' unambiguous when discussing asynchronous
  operation. (#387)
* Test: Fix test flappiness due to insufficient sanitation between test runs (#384, #385)
* Feature: Allow metadata only logging. (#380)
* Feature: Add an upper case severity formatter (#372)
* Feature: Add support for suppressing start/stop messages from supervisors (#368)
* Bugfix: Fix ranch crash messages (#366)
* Test: Update Travis config for 18.3 and 19.0 (#365)

3.2.4 - 11 October 2016

* Test: Fix dialyzer warnings.

3.2.3 - 29 September 2016

* Dependency: Update to goldrush 0.19

3.2.2 - 22 September 2016

* Bugfix: Backwards-compatibility fix for `{crash_log, undefined}` (#371)
* Fix documentation/README to reflect the preference for using `false`
  as the `crash_log` setting value rather than `undefined` to indicate
  that the crash log should not be written (#364)
* Bugfix: Backwards-compatibility fix for `lager_file_backend` "legacy"
  configuration format (#374)

3.2.1 - 10 June 2016

* Bugfix: Recent `get_env` changes resulted in launch failure (#355)
* OTP: Support typed records for Erlang 19.0 (#361)

3.2.0 - 08 April 2016

* Feature: Optional sink killer to shed load when mailbox size exceeds a
  configurable high water mark (#346)
* Feature: Export `configure_sink/2` so users may dynamically configure
  previously setup and parse transformed sinks from their own code. (#342)
* Feature: Re-enable Travis CI and update .travis.yml (#340)
* Bugfix: Fix test race conditions for Travis CI (#344)
* Bugfix: Add the atom 'none' to the log_level() type so downstream
  users won't get dialyzer failures if they use the 'none' log level. (#343)
* Bugfix: Fix typo in documentation. (#341)
* Bugfix: Fix OTP 18 test failures due to `warning_map/0` response
  change. (#337)
* Bugfix: Make sure traces that use the file backend work correctly
  when specified in lager configuration. (#336)
* Bugfix: Use `lager_app:get_env/3` for R15 compatibility. (#335)
* Bugfix: Make sure lager uses `id` instead of `name` when reporting
  supervisor children failures. (The atom changed in OTP in 2014.) (#334)
* Bugfix: Make lager handle improper iolists (#327)

3.1.0 - 27 January 2016

* Feature: API calls to a rotate handler, sink or all.  This change
  introduces a new `rotate` message for 3rd party lager backends; that's
  why this is released as a new minor version number. (#311)

3.0.3 - 27 January 2016

* Feature: Pretty printer for human readable stack traces (#298)
* Feature: Make error reformatting optional (#305)
* Feature: Optional and explicit sink for error_logger messages (#303)
* Bugfix: Always explicitly close a file after its been rotated (#316)
* Bugfix: If a relative path already contains the log root, do not add it again (#317)
* Bugfix: Configure and start extra sinks before traces are evaluated (#307)
* Bugfix: Stop and remove traces correctly (#306)
* Bugfix: A byte value of 255 is valid for Unicode (#300)
* Dependency: Bump to goldrush 0.1.8 (#313)