lager formatters

Overview

If you’re using lager for your Erlang logging, you’ve probably left the default formatter well alone, by doing something like this in your configuration file:

{lager_console_backend, [info, true]},

However, lager’s built-in lager_default_formatter defines a mini-DSL that allows you to customise the formatting. Let’s take a look at the default format, and break it down.

Default Format

The default format is defined in lager_default_formatter.erl and looks like this:

[date, " ", time, " ", color, "[", severity, "] ",
            {pid, ""},
            {module, [
                    {pid, ["@"], ""},
                    module,
                    {function, [":", function], ""},
                    {line, [":",line], ""}], ""},
            " ", message, EOL]

What we have here looks a bit like an Erlang iolist, where certain atoms (date, time, pid, etc.) are replaced with values at runtime.

Actually, apart from a couple of specific atoms (message, date, time, severity – see the source), these are simply looked up in the metadata associated with the call to lager:log.

You can set some of this metadata by using lager:md/1; some of it is set by lager’s parse transform when it sees a call to (e.g.) lager:warning.

Default Values

The language also supports default values, for example:

{pid, ""}

…states that the process ID should be output. If it’s not defined, then output an empty string instead.

Conditionals

There’s also support for conditionals. These are specified as {Prop, Present, Absent}. For example:

{function, [":", function], ""}

…states that if function is present in the metadata, then output [":", function]; otherwise output an empty string.

This is taken to an extreme in the module clause:

{module, [
    {pid, ["@"], ""},
    module,
    {function, [":", function], ""},
    {line, [":",line], ""}], ""}

…which says: if module is not defined, output an empty string. If module is defined, then output @ (if pid is defined – to terminate the pid we might already have printed), followed by the module name, followed by the function and line number (both with prefixes and defaults).

Conclusion

You can probably do all of the custom log formatting without having to write a custom backend or a custom formatter, just by using lager_default_formatter and its formatting DSL.