Releases & Hot Code Upgrades

A release bundles the ERTS, your applications, and dependencies into a self-contained, deployable package.

36.1 Building releases

A release bundles the ERTS, your applications, and dependencies into a self-contained, deployable package.

Tools: relx (inside rebar3), systools, reltool.

rebar3 release                 # _build/default/rel/myapp
rebar3 as prod tar             # tarball for deployment
_build/default/rel/myapp/bin/myapp foreground   # run in foreground
_build/default/rel/myapp/bin/myapp daemon       # background
_build/default/rel/myapp/bin/myapp remote_console
_build/default/rel/myapp/bin/myapp stop
_build/default/rel/myapp/bin/myapp eval "myapp:migrate()."

36.2 Code loading basics

The BEAM can hold two versions of a module: current and old.

Fully qualified calls (?MODULE:loop(State)) jump to the newest version; local calls stay in the current one.

Loading a third version kills processes still running the oldest code.

code:load_file(mod), code:purge(mod), code:soft_purge(mod),
code:is_loaded(mod), code:get_path(), code:add_patha("ebin"),
code:which(mod), l(mod).

loop(State) ->
    receive
        upgrade -> ?MODULE:loop(State);      % switch to new code
        Msg -> loop(handle(Msg, State))
    end.

36.3 Release upgrades: appup & relup

code_change/3 transforms process state between versions.

systools:make_relup/3 (or rebar3 relup / rebar3_appup_plugin) generates relup.

release_handler:unpack_release/1, install_release/1, make_permanent/1 apply upgrades live.

Instructions: {load_module, M}, {update, M, {advanced, Extra}}, {add_module, M}, {delete_module, M}, {apply, {M, F, A}}, {restart_application, App}.

%% myapp.appup
{"0.2.0",
 [{"0.1.0", [{update, kv_store, {advanced, []}}]}],   % upgrade instructions
 [{"0.1.0", [{update, kv_store, {advanced, []}}]}]}.  % downgrade instructions

36.4 Deployment today

Many teams use rolling deploys with Docker/Kubernetes; hot upgrades remain valuable for telecom-grade systems that must never drop connections.