Elixir Project Structure

Each of the application server code of the various Components, Subsystems, and Platforms making up the MuseBMS is implemented as independent Elixir projects, each of which is organized along lines which support our development philosophy. While our Elixir projects largely conform to the standard patterns used by most Elixir projects, there are nonetheless departures from convention which are worthy of discussion.

Central Motivation

As has been said, the directory organization is based on the standard Elixir project organization as created by running mix new myapp. Our Components are also started this way and then modified to match our requirements.

A core development principle for our Elixir related projects includes the idea of strictly separating out API definitions from the business logic of the Component. It follows that this separation of interface from logic impacts our project structure. Within the lib directory which typically holds Elixir source files and possibly other directories, we allow for three directories:

  • api - for Elixir source files defining the public API of the Component

  • impl - for Elixir source files implementing business logic concerns of the Component.

  • runtime - for Elixir source files implementing Component runtime concerns such as defining GenServers.

Under our structure, there are no source files directly homed in the lib directory itself.

Directory Structure and Common Files

Below is an example of a typical MuseBMS Elixir project followed by a detailed description. Depending on the specific features and scope of the Component not all directories or common files displayed below will be present.

├── README.md
├── config
├── database_utils
│   └── reset_dev_database.psql
├── dev_support
│   └── dev_support.ex
├── lib
│   ├── api
│   │   ├── msdata
│   │   ├── msform
│   │   ├── types
│   │   ├── types.ex
│   │   └── <module_name>.ex
│   ├── impl
│   │   ├── msdata
│   │   └── msform
│   └── runtime
├── mix.exs
├── priv
│   ├── database
│   └── plts
└── test
    ├── support
    └── test_helper.exs

Top Level Entry Descriptions

What follows are the important details regarding each of the organizational elements at the top level, with additional details as relevant.


The README.md files serves the typical purpose of describing the Component’s purpose and basics about usage. An important extra role for the README.md is that it is incorporated into the project’s technical documentation as the opening text.

Given this important opening role, the README.md file should contain a solid overview as well as an introduction to the Component at the conceptual and definitional levels.


The config directory is the standard Elixir project directory containing compile time related configurations. We do not use the config directory or its files in any non-standard way and actively avoid having our Components depend on these configuration files. There are exceptions, such as when third party libraries require configuration using this method, but even then our usage is in keeping with Elixir standard practices.


This directory contains psql scripts, specifically written for the needs of this component, which are useful during development and testing. Only Components which have a database requirement will have this directory.

  • reset_dev_database.psql

    This script is run at the end of mix test execution to reset the testing database back to a clean state. It is specific to the Component where it is defined, though this script will be similar across all components where found.


The dev_support directory contains Elixir code which is use to aid development activities. For the most part, this involves building migrations and deploying them to a development database server. Other dependencies which require services to be started may also be started via code sourced from this directory.

  • dev_support.ex

    Typically this is the only file in this directory. While that’s not a hard and fast rule that future Components will follow, there are few needs for development runtime support and so a single module makes sense.


This is the traditional home of Elixir application source code and this doesn’t change for this project. As previously discussed, where we depart from convention is in how the source code inside this directory is organized.

  • api

    All source files in this directory should be focused on defining and documenting the APIs used to interact with the Component. Business logic implementation should not, as a practice, be included in the API defining source files.

    Files in api will typically consist of defdelegate calls, struct definitions, and type definitions, all of which should be documented.

    Some of the favored practices just discussed are possible sources of performance degradation. The first choice is to follow the standard established here and assume performance penalties are inconsequential. If performance issues arise, then we break ti standard documented here with a focus on preserving the API, but moving implementation closer to the API defining functions. We don’t expect material performance impacts in all but a few edge cases.

    • msdata

      This is special sub-directory containing the definitions of Ecto.Schema structs which establish database table representations. In addition there will be defdelegate references to Ecto.Changeset processing functions. Components which do not define database schema will not have an api/msdata directory.

      The structs created in msdata are namespaced directly to the Msdata namespace, so for example a users table may be in a struct module named Msdata.Users. Naturally, this means that all Ecto Schemas in msdata must be uniquely named across all Components in the project.

    • msform

      This is special sub-directory containing the source files which define MscmpSystForms implementing modules. More information about the organization of the api/msforms directory can be found in the MscmpSystForms Developing Forms documentation.

      This directory will only exist for Components implementing MscmpSystForms based user interfaces.

    • types

      The types directory hosts source files which principally define public structs which are usable outside of the Component and which aren’t defined in a more dedicated part of the api directory hierarchy (e.g. msdata database related structs). Public struct source files will include the struct definition and defdelegate calls to impl or runtime hosted function implementations. This directory will not exist if there are no module-level type definitions.

    • types.ex

      This source file holds typespecs for simpler types which don’t require a full module to be created. Again, these types should be documented and considered useful outside of the Component. Typically this fill will list all its defined types in alphabetical order, though other organization is acceptable if there are enough defined types to make more topical organization useful.

    • <module name>.ex

      A source file containing the Component’s principle API definition and API documentation. This file carries the same name as the Elixir project and will front the majority of the Component’s functionality on offer to the outside world. In a standard Elixir project, a source file with roughly the same purpose and name would be found directly in lib; we simply move that file to api and more strictly define its purpose.

  • impl

    This directory provides a home for the application’s business logic implementation. The source files in this directory will contain the vast majority of code, though there should be no ExDoc related doc tags in these files aside from @moduledoc false. Comments describing feature/function intention are allowed and, indeed, welcomed.

    • msdata

      This is the implementation side of previously discussed api/msdata directory. If a data struct requires business logic implementation, such as the definition of Ecto.Changeset processing or protocol implementation, a sub-directory will be created here with the same name as the corresponding struct in api/msdata. Other, more general changeset validation or helper modules which implement data related logic may host their source files directly in the root of impl/msdata. Standard names for these sorts of files include the following:

      • impl/msdata/helpers.ex

        Helper functions which work across multiple structs’ logic.

      • impl/msdata/general_validators.ex

        Validation and Changeset processing functions which are reusable across multiple data defining structs in api/msdata.

      • impl/msdata/<data struct name>/validators.ex

        Source files hosting Changeset processing functions which are delegated to from struct modules defined in api/msdata.

      • impl/msdata/<data struct name>/protocol.ex

        Source files implementing various protocols for a given struct as needed.

    • msform

      This is the implementation oriented compliment to the api/msform directory. For more complete documentation on the use of this directory please see the MscmpSystForms Developing Forms documentation.

  • runtime

    Source files which define and implement runtime services, such as GenServers. This directory will only exist if the Component defines such runtime services. Note that code in runtime source files is limited to the runtime concerns. Business logic implementations continue to be hosted in the impl directory structure and referenced from the runtime service as necessary.


This is the typical Elixir mix.exs defining project file. There are some organizational differences between our approach and the typical generated Elixir mix.exs, but there are no differences that go further than trivial stylist differences.


This is the standard Elixir priv directory, though there are a few special sub-directories to be aware of.

  • priv/database

    In Component level Elixir projects, this directory may be visible and hosts database migrations built during the testing process. When tests are not actually running, the directory will exist but will be empty as the testing cleanup processes will typically delete this database related testing artifacts.

    At the Subsystem level the priv/database directory will host sub-directories named after their corresponding database type name (see the MscmpSystDb documentation for more about database types). The database migrations built for the database type will be hosted by the appropriate sub-directory. Note that these database migration files are persistent and drive the release as opposed to the transient nature of Component level migrations.

  • priv/plts

    This directory hosts generated PLT files for Dialyzer analysis during development and the Continuous Integration process. There is some historical precedence for doing handling PLTs and we’re just following this community convention.

    PLT files should not be part of releases and so this directory should not be a factor outside of development and testing processes.


This is the standard Elixir project testing directory. We follow standard conventions for the most part in regard to testing. We do define two different kinds of testing:

  • Unit Testing

    Unit tests are aimed at testing non-private impl and runtime related code and functions at a granular level. We are not confirming the public API in unit testing, but rather the internal implementations for correct results and typing.

  • Integration Testing

    Integration testing works to test the public API of the Component in a fashion which mimics the end-to-end business process as much as possible, including the Component’s interaction with any dependencies.

These two testing types will not be run together and will typically use different testing database seed data (if database interactions are part of the component’s functionality.)

Sub-directories and files to the test directory include:

  • support

    For testing support related files. It is typical for this directory to host implementations of ExUnit.CaseTemplate and logic for building, migrating, and seeding test databases, runtime services defined by dependencies and making good on other testing pre- conditions.

  • test_helper.exs

    This is the standard Elixir/ExUnit test_helper.exs file, typically modified to help in supporting testing related database operations and post-testing cleanup.