[erlang-questions] Newbie Question: Module with DB Connection as a "global state"

[Michael Weibel - Amiado Group]

@Gustav: Thanks for your hints, very appreciated

@Garett,
Thanks for your help. I'm currently trying to implement it that way :)

- Michael

2011/10/5 Garrett Smith <g@REDACTED>

> Hi Michael,
>
> On Wed, Oct 5, 2011 at 8:36 AM, Michael Weibel - Amiado Group
> <michael.weibel@REDACTED> wrote:
> > Hi all,
> >
> > I'm new to erlang, new to this list and I hope here's the right place to
> ask this question.
>
> Welcome!
>
> >
> > I'm currently implementing a module for ejabberd where I need to log
> certain packets to a MySQL DB and I'm using the MySQL Native Driver for it.
> >
> > My current implementation is on github:
> >
> https://github.com/amiadogroup/mod_log_chat_mysql5/blob/master/src/mod_log_chat_mysql5.erl
> >
> > In this file you see that I'm opening the MySQL Connection in the
> function "open_mysql_connection" and assign it to a ETS Table because I
> didn't find another way to store the DB Reference (I tried something with
> records but didn't succeed).
> >
> > This worked pretty well but I ran into the problem that after some
> days/weeks the ets table didn't have the DB Reference anymore.
> > To prevent this, I want to create functions which setup the DB Connection
> again, if it doesn't have it anymore.
>
> You typically don't want to do this. There's a much, much better way...
>
> > As I'm trying to do this I run into the problem that the informations
> about how to connect to the db is only in the init-function available and I
> don't really want to store this information also in the ets table.
>
> The "go to" pattern in Erlang for what you're trying to do is a
> supervised process. Using standard OTP facilities, you get this:
>
> - Registration of process IDs using atoms (analogous to you storing a
> connection PID in ets)
> - Well defined "init" logic that's automatically applied when a
> process needs to be restarted
> - Encapsulation of the messy details of handling service provisioning
>
> Fortunately, the MySQL driver (at the more recent ones - there are
> several lineages out there) provides an OTP compliant connection
> process, so you can do something as simple as this:
>
> -module(my_db).
>
> -export([start_link/0, fetch/1]).
>
> start_link() ->
>    Host = "localhost",
>    Port = 3306,
>    Db = "`my-db`",
>    User = "root",
>    Password = "password",
>    mysql:start_link(?MODULE, Host, Port, User, Password, Db, fun
> mysql_log/4).
>
> fetch(Sql) ->
>    mysql:fetch(?MODULE, Sql).
>
> mysql_log(_Module, _Line, _Level, _FormatFun) ->
>    ok.
>
> This is a registered process (i.e. there's one of them at runtime)
> that provides access to your database.
>
> The connection config in start_link/0 would typically come from an
> application config, which you can read using application:get_env/2.
> You can also create a start_link that takes the params.
>
> To use this module as a service, you need to plug it into your
> supervisory hierarchy. Here's what your top-level supervisor might
> look like:
>
> -module(my_app_sup).
>
> -behaviour(supervisor).
>
> -export([start_link/0]).
>
> -export([init/1]).
>
> -define(SERVER, ?MODULE).
>
> start_link() ->
>    supervisor:start_link({local, ?SERVER}, ?MODULE, []).
>
> init([]) ->
>    {ok, {{one_for_one, 5, 5},
>          [{my_db, {my_db, start_link, []},
>            permanent, 5000, worker, [my_db]}]}}.
>
> If your my_db service crashes, the supervisor will restart it. The
> "config" that you'd use for that restart would come from either the
> OTP app config. (You *could* provide it in the supervisor child spec,
> but I wouldn't go that way.)
>
> Erlang gives you very nice facilities to create a mini "service
> oriented architecture" -- you want to look for ways to create very
> opaque services that have drop-dead simple start/restart semantics
> (i.e. simple as pushing a button) and let supervisors monitor and
> recover failed services as needed.
>
> If for some reason you needed to dynamically configure your MySQL
> connection params, you could store the config in a database, external
> file, etc. and force a restart of your my_db process to re-read that
> config on init.
>
> Garrett
>



-- 
Michael Weibel
Entwickler

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