[erlang-questions] Where to configure & start mnesia in a release?

[Nuku Ameyibor]

 mnesia is not  included as part of the release .
when rebar3 shell command is run  the path to mnesia is added automatically
to the code paths .
in  a release environment however its not copied  over into the release libs
unless specified in one of your .app.src files.
in your app.src  file check to see if mnesia is one  of the applications
which has to  be started before the main application is started.


On Sun, Apr 15, 2018 at 3:50 PM, Igor Clark <igor.clark@REDACTED> wrote:

> Thanks Nuku,
>
> - It's definitely crashing due to the mnesia configuration - it shows a
> {badmatch,undefined} error on the line where I call { ok, MnesiaDir } =
> application:get_env( mnesia, dir ), and I've tried io:format()'ing the
> result of application:get_env( mnesia, dir ), which shows 'undefined'
> when running as a release
>
> - relx section of rebar.config is set up exactly like that - I used rebar3
> to generate the release
>
> - sys.config has exactly that mnesia entry
>
> It all works as expected with 'rebar3 shell', environment variable is
> found, mnesia starts, it all works fine - but when I run 'rebar3 release'
> and try to run the generated startup script, either with 'console' or with
> 'start' and looking in the log file, the OTP application included in the
> release gets 'undefined' from application:get_env( mnesia, dir ).
>
> From your sys.config example I realise I could say for example [ {
> my_app, [ { mnesia_dir, "/tmp/my_db_dir" } ] } ], and use
> application:get_env( my_app, mnesia_dir ) to initialise; this works and
> returns { ok, "/tmp/my_db_dir" }, which is great, but then ok =
> application:ensure_started( mnesia ) fails with {{badmatch, {error,{"no
> such file or directory","mnesia.app"}}}. I have no idea where mnesia.app
> should go, and I haven't been able to find any references to this file in
> docs or online that explain where it should go or what should be in it,
> except that it exists in the mnesia package under the OTP source tree.
>
> So still pretty unclear how to proceed, pointers welcome!
>
> Thanks,
> Igor
>
>
> On 15/04/2018 16:06, Nuku Ameyibor wrote:
>
>
>     - under_build/<profile>/rel/<release>/bin/<release> console it fails
>> (returns undefined), so the app crashes on start, regardless of whether
>> <profile> is prod or default
>
> it may or may not be crashing due the mnesia configuration .
> you may have to check the the erl_crash.dump to see the cause of the crash
> .
>
>
> How do I set mnesia application environment variables in the release
>> project? (Or, maybe, how do I get the OTP application to be able to access
>> the release project's application environment variables?)
>
>
> in the relx section of your rebar.config you can set a sys_config to be
> used
> {relx,
> [
> {sys_config,"config/sys.config"}
>        ]
>
> in that sys.config you can setup environmental variables to be used by
> your applications including an entry for mnesia.
> [
> {mnesia,
> [{dir,"/tmp/my_db_dir"}]
> },
> {app1,[{a,1},{b,2}],
> {app2,[{a,3},{b,4}]
> }
> ]
> this will be used by your release system when the release is created .
> check out http://www.rebar3.org/docs/releases .
>
>
>
>
> On Sat, Apr 14, 2018 at 3:17 PM, Igor Clark <igor.clark@REDACTED> wrote:
>
>> Hi list,
>>
>> I have an OTP application which uses mnesia. It checks/ensures the schema
>> and table files exist and then calls application:ensure_started( mnesia )
>> in its application:start/2 callback. It works fine when I run the
>> application on its own under rebar3 shell, but it doesn't work when I try
>> to run it from an OTP release:
>>
>> - When I run the OTP app under rebar3 shell in its own separate
>> directory, with { mnesia, [ { dir, "/tmp/my_db_dir" } ] } in
>> priv/sys.config, it works fine and application:get_env( mnesia, dir )
>> returns /tmp/my_db_dir as normal/expected
>>
>> - If I set { mnesia, [ { dir, "/tmp/my_db_dir" } ] } in
>> <release>/config/sys.config, then:
>>
>>     - under rebar3 shell --name=myapp@REDACTED, calling
>> application:get_env( mnesia, dir ) succeeds (returns /tmp/my_db_dir) , and
>> the app uses reads/writes mnesia tables successfully
>>
>>     - under_build/<profile>/rel/<release>/bin/<release> console it fails
>> (returns undefined), so the app crashes on start, regardless of whether
>> <profile> is prod or default
>>
>> How do I set mnesia application environment variables in the release
>> project? (Or, maybe, how do I get the OTP application to be able to access
>> the release project's application environment variables?)
>>
>> Also, I realise I could be going about this the wrong way - maybe mnesia
>> should be configured inside the specific OTP app, even under a release?
>> Apart from the fact that that doesn't seem to work (as I have it set up,
>> anyway), I'm aware I should steer clear of "included applications" - not
>> least because I want to be able to use the OTP app in other projects - so I
>> guess there's a broader question of how to configure mnesia in general, in
>> how & when to create schema and tables, if it could be used by multiple OTP
>> apps in a release?
>>
>> Bit puzzled, appreciate any advice!
>>
>> Cheers,
>> Igor
>> _______________________________________________
>> erlang-questions mailing list
>> erlang-questions@REDACTED
>> http://erlang.org/mailman/listinfo/erlang-questions
>>
>
>
>
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