How to specify directory for mnesia in cowboy application?

Question

Is there ways to specify path to schema in cowboy app? Maybe it's possible to set in my_app.app.src or any config file?

Answer 1

The path to the mnesia directory has to be provided to erlang VM before mnesia application is started through application configuration parameters. In Mnesia tutorial, this is done with the -Application par val VM arguments syntax.

What you call a cowboy application is probably an Erlang OTP release (built by relx as per cowboy tutorial). The solutions, quickly described in Cowboy issue #595, are as follows.

The choice between solutions really depends on style as well as some constraints. Any sufficiently complex release would use a configuration file, so it would be a good choice. vm.args seems easier to deal with. Eventually, you might need to alter the start script (for example to run several nodes from a single deployment), and include some logic to define the mnesia directory.

Provide relx with a configuration file (sys_config option)

To do so, add the following term to relx.config as documented.

{sys_config, "./config/sys.config"}.

sys.config actually is a standard Erlang configuration file, also documented. Specifying mnesia dir is done by adding a section for mnesia application. If no other configuration is required, the file would be:

[{mnesia, [{dir, "/path/to/dir"}]}].

Get relx to pass arguments to the vm (vm_args option)

The vm.args file is actually passed to the VM through -args_file option. This is a simple text file with arguments.

You would add the following term to relx.config as documented.

{vm_args, "./config/vm.args"}.

And put the following content in the vm.args file:

-mnesia dir foo

Write your own start script

relx actually creates a start script, which passes -config sys.config and args_file vm.args to the VM as required. You could modify this script or roll your own to actually pass the -mnesia dir argument to the VM.

Answer 2

in case of vm.args, planit text -mnesia dir foo is invalid, please use format: -mnesia {dir,foo}