Configuration

Nova uses standard OTP application configuration via sys.config. This chapter covers organizing configuration across environments, using environment variables, and the key settings you'll need.

Configuration files

The generated project includes two config files:

config/dev_sys.config.src — development settings (used by rebar3 shell and rebar3 nova serve)
config/prod_sys.config.src — production settings (used when building releases)

The .src suffix means rebar3 processes the file as a template — ${VAR} references are replaced with environment variables at build time.

Development config

[
  {kernel, [
    {logger_level, debug},
    {logger, [
      {handler, default, logger_std_h,
        #{formatter => {flatlog, #{
            map_depth => 3,
            term_depth => 50,
            colored => true,
            template => [colored_start, "[\033[1m", level, "\033[0m",
                         colored_start, "] ", msg, "\n", colored_end]
          }}}}
    ]}
  ]},
  {nova, [
         {use_stacktrace, true},
         {environment, dev},
         {cowboy_configuration, #{port => 8080}},
         {dev_mode, true},
         {bootstrap_application, blog},
         {plugins, [
                    {pre_request, nova_request_plugin, #{
                        read_urlencoded_body => true,
                        decode_json_body => true,
                        parse_qs => true
                    }}
                   ]}
        ]},
  {blog, [
         {database, <<"blog_dev">>}
        ]}
].

Production config

[
  {kernel, [
    {logger_level, info},
    {logger, [
      {handler, default, logger_std_h,
        #{config => #{file => "log/erlang.log"},
          formatter => {flatlog, #{
            map_depth => 3,
            term_depth => 50,
            colored => false,
            template => ["[", level, "] ", msg, "\n"]
          }}}}
    ]}
  ]},
  {nova, [
         {use_stacktrace, false},
         {environment, prod},
         {cowboy_configuration, #{port => 8080}},
         {dev_mode, false},
         {bootstrap_application, blog},
         {plugins, [
                    {pre_request, nova_correlation_plugin, #{
                        request_correlation_header => <<"x-correlation-id">>,
                        logger_metadata_key => correlation_id
                    }},
                    {pre_request, nova_request_plugin, #{
                        decode_json_body => true,
                        read_urlencoded_body => true,
                        parse_qs => true
                    }},
                    {pre_request, nova_csrf_plugin, #{
                        excluded_paths => [<<"/api/">>]
                    }}
                   ]}
        ]},
  {blog, [
      {database, <<"${DB_NAME}">>},
      {db_host, <<"${DB_HOST}">>},
      {db_user, <<"${DB_USER}">>},
      {db_password, <<"${DB_PASSWORD}">>},
      {sendgrid_api_key, <<"${SENDGRID_API_KEY}">>}
  ]}
].

Key differences from development:

Logger level is info instead of debug
use_stacktrace is false — don't leak stack traces to users
Correlation plugin is enabled for request tracing
CSRF plugin is enabled
Secrets use ${VAR} environment variable substitution
sendgrid_api_key is stored under the blog app — the blog_mailer module reads it via application:get_env/3 (see Sending Email)

Nova configuration reference

Key	Default	Description
`bootstrap_application`	(required)	Main application to bootstrap
`environment`	`dev`	Current environment (`dev` or `prod`)
`cowboy_configuration`	`#{port => 8080}`	Cowboy listener settings
`plugins`	`[]`	Global middleware plugins
`json_lib`	`thoas`	JSON encoding library
`use_stacktrace`	`false`	Include stacktraces in error responses
`use_sessions`	`true`	Enable session management
`session_manager`	`nova_session_ets`	Session backend module
`dev_mode`	`false`	Enable development features
`render_error_pages`	`true`	Use custom error page controllers
`dispatch_backend`	`persistent_term`	Route dispatch storage backend

Environment-based routing

The routes/1 function receives the environment atom:

routes(prod) -> prod_routes();
routes(dev) -> prod_routes() ++ dev_routes().

This lets you add development-only routes (debug tools, test endpoints) without them leaking into production.

VM arguments

config/vm.args.src controls Erlang VM settings:

-name blog@${HOSTNAME}
-setcookie ${RELEASE_COOKIE}
+K true
+A30
+sbwt very_long
+swt very_low

-name — full node name (needed for clustering)
-setcookie — cluster security cookie
+K — enable kernel poll
+A — async thread pool size
+sbwt / +swt — scheduler busy-wait tuning

Next: Observability — tracing, metrics, and logging in production.