чт 07 мая 2020, tags: ErlangLagerN2Ologging

Re-posting from 2014-10-17

In this tutorial we will show you how to create a custom logging backend for N2O. We will use Lager as a logging library.

As we described in the previous (N2O Logging) tutorial, the logging backend should be implementing 3 methods:

• warning/3
• info/3
• error/3

So, let's get started.

Note: For this example, we will use code from (N2O Logging) tutorial.

As a first step we should add lager to the dependencies list in rebar.config.

vim rebar.config

{deps, [
  {n2o, ".*", {git, "git://github.com/5HT/n2o.git", {tag, "HEAD"}}},
  {lager, ".*", {git, "git://github.com/basho/lager", {tag, "2.0.3"}}}  % added lager
]}.

And add to the applications list in web.app.src for an automatic lager starting

vim apps/web/src/web.app.src

{application, web,
 [
  {description, ""},
  {vsn, "1"},
  {registered, []},
  {applications, [
                  kernel,
                  stdlib,
                  n2o,
                  lager   % added lager
                 ]},
  {mod, { web_app, []}},
  {env, []}
 ]}.

Next, create a new file where we will implement the required functions.

cd apps/web/src      % go to the app directory
touch n2o_lager.erl  % create an empty file

Open n2o_lager.erl file

vim n2o_lager.erl

And write the implementation. The code is pretty straight forward. You can extend this code also. How? I we will show you later.

-module(n2o_lager).
-export([info/3, warning/3, error/3]).
-compile([{parse_transform, lager_transform}]).            % force compile

format_message(Module, String) ->                          % log formatter "module:message"
    wf:to_list([Module, ":", String]).

info(Module, String, Args) ->
    lager:info(format_message(Module, String), Args).

warning(Module, String, Args) ->
    lager:warning(format_message(Module, String), Args).

error(Module, String, Args) ->
    lager:error(format_message(Module, String), Args).

Now, when the implementation is ready, let's configure N2O for using our module.

Open web_app.erl and edit start/0 function. We should set up out module as the N2O logging backend.

vim web_app.erl

start() ->
   application:ensure_all_started(web),
   application:set_env(n2o, route, routes),
   application:set_env(n2o, log_modules, web_sup),
   application:set_env(n2o, log_backend, n2o_lager),  % setup a backend for logging
   application:start(web).

The backend is ready. Let's add some logs to the view files.

cd ../../../      % go to rood directory
./rebar get-deps  % get dependencies
./run.sh          % start app

And make a call

curl localhost:9002