Adding lager handlers at runtime

You’re using lager for logging in your Erlang program, and you discover that your configuration isn’t logging anything more verbose than info-level messages. How do you bump that up to debug?

Like this, for lager_file_backend:

Module = lager_file_backend,
Path = "log/debug.log",
Id = Path,
Config = [{file, Path}, {level,debug}],

supervisor:start_child(lager_handler_watcher_sup, [lager_event, {Module, Id}, Config]).

Or like this, for lager_syslog_backend, where the configuration is given differently:

Module = lager_syslog_backend,
Ident = "ident",
Facility = local7,
Id = {Ident, Facility},
Config = [Ident, Facility, debug],

supervisor:start_child(lager_handler_watcher_sup, [lager_event, {Module, Id}, Config]).

Note that the {Module, Id} term is usually derived by calling Module:config_to_id(Config), but that’s not a required export, which makes things tricky.

Updated 2014-11-28: This page used to document adding the event handler directly, but given what I’ve learned about supervised event handlers, it now shows the correct way to do it.