gen_server:enter_loop

2018-06-12 14:52:00 +0000

When you use gen_server:start/3,4 or gen_server:start_link/3,4, the call blocks until the other process has finished running init/1.

In particular, this can be a problem when your process is being started from a supervisor. If it takes a long time to start (or restart) the child process, the supervisor won’t handle any messages until it completes. This means that even seemingly simple calls such as supervisor:which_children/1 will block until some unknown child process finishes (re-)starting.

Don’t do this

Occasionally, you’ll see people suggest that you send yourself a message from init/1, like this:

start_link() ->
    gen_server:start_link(?MODULE, [], []).

init([]) ->
    self() ! continue_init,
    {ok, #state{}}.

handle_info(continue_init, State) ->
    {noreply, continue_init_slowly()}.

Most of the time, you’ll get away with this, because there’s no way for anyone else to send your process a message, and continue_init will be the first message your process sees.

But: if your process registers a name, either through gen_server:start_link(Name, Module, Args, Opts) or explicitly by calling (e.g.) register, there opens a window where someone could send you a message which jumps the queue, arriving before continue_init, and before you’ve finished initialising.

gen_server:enter_loop/3,4,5

The correct way to do this is to use gen_server:enter_loop, combined with either proc_lib:spawn_link or proc_lib:start_link.

proc_lib:spawn_link returns immediately, and leaves the specified function to run in the new process. In order to turn your new process into a gen_server, you need to call gen_server:enter_loop at the end of init.

For example:

start_link() ->
    Pid = proc_lib:spawn_link(?MODULE, init, [[]]),
    {ok, Pid}.

init([]) ->
    io:format("async init~n"),
    timer:sleep(1000),
    io:format("async init done~n"),
    gen_server:enter_loop(?MODULE, [], #state{}).

If you want to do some synchronous initialisation, you can use proc_lib:start_link:

start_link() ->
    % Returns the value passed to proc_lib:init_ack, below.
    proc_lib:start_link(?MODULE, init, [[]]).

init([]) ->
    io:format("sync init~n"),
    timer:sleep(1000),
    io:format("sync init done~n"),

    proc_lib:init_ack({ok, self()}),

    io:format("async init~n"),
    timer:sleep(1000),
    io:format("async init done~n"),

    gen_server:enter_loop(?MODULE, [], #state{}).