Erlang OTP: Distributed & Fault-Tolerant Systems Programming · Lesson

Mnesia Fundamentals & Schema

Understand Mnesia's architecture, table types, and how to define schemas for your distributed data.

Mnesia Fundamentals & Schema is a free Erlang OTP: Distributed & Fault-Tolerant Systems Programming lesson on CoddyKit — lesson 1 of 4. You can read the complete lesson below for free — then practise it hands-on in the browser with a built-in code editor and a 24/7 AI tutor. It is part of the Erlang OTP: Distributed & Fault-Tolerant Systems Programming learning path, one of 4 lessons in the course, and your progress syncs across the web and the CoddyKit app.

Welcome to Mnesia!

Mnesia is Erlang's built-in distributed database. It's designed for high availability, fault tolerance, and soft real-time systems, making it perfect for concurrent Erlang applications.

Think of Mnesia as a powerful, embedded database that seamlessly integrates with Erlang's concurrency model.

Mnesia's Core Strengths

Mnesia offers key features that align perfectly with Erlang's philosophy:

Distributed: It can span across multiple Erlang nodes.
Transactional: Guarantees atomicity, consistency, isolation, and durability (ACID) for data operations.
Fault-Tolerant: Automatically handles node failures and data replication.
Hybrid Storage: Supports both in-memory and disk-based tables.

Starting Mnesia

Before you can use Mnesia, you need to start the Mnesia application and then the Mnesia database manager. This is a common first step when working with Mnesia.

Try running this simple example:

-module(mnesia_lesson_start).
-export([run/0]).

run() ->
    % 1. Start the Mnesia application
    application:start(mnesia),
    io:format("Mnesia application started.~n"),

    % 2. Start the Mnesia database manager
    mnesia:start(),
    io:format("Mnesia database started successfully!~n"),

    % Clean up (important for simple runnable examples)
    mnesia:stop(),
    application:stop(mnesia),
    ok.

What is a Mnesia Schema?

A Mnesia schema defines the overall structure of your Mnesia database. It specifies which tables exist, their properties, and how they are stored across different nodes.

The schema itself is stored in a special Mnesia table called mnesia_schema.

Creating the Schema

The schema must be created once for the entire Mnesia system (cluster of nodes). You use mnesia:create_schema/1, passing a list of node names that will participate in the Mnesia system.

Here's how to create a schema for a single node:

-module(mnesia_lesson_schema).
-export([run/0]).

run() ->
    application:start(mnesia),

    % Create schema for the current node
    case mnesia:create_schema([node()]) of
        ok -> io:format("Mnesia schema created.~n");
        {error, {already_exists, _}} ->
            io:format("Schema already exists. (This is fine if run before)~n");
        Error -> io:format("Error creating schema: ~p~n", [Error])
    end,

    application:stop(mnesia),
    ok.

Mnesia Table Fundamentals

Mnesia stores data in tables, similar to a relational database. Each table has a name and a set of attributes (columns).

Tables are dynamic and can be created or deleted at runtime.
They can be replicated across multiple nodes for fault tolerance.
Mnesia supports different storage types for tables.

Understanding Table Types

Mnesia offers various table types to suit different needs:

ram_copies: Data is stored only in RAM. Fastest access, but data is lost if the node crashes.
disc_copies: Data is stored in RAM and also replicated to disk. Provides persistence and good performance.
disc_only_copies: Data is stored only on disk. Slower access, but uses less RAM and is persistent.

Choosing the right type depends on your data's persistence and performance requirements.

Defining Tables with Records

In Erlang, Mnesia tables are typically defined using records. A record declaration specifies the table's name (the record's name) and its attributes (the record's fields).

For example, a #user{} record would define a user table with specific fields like id, name, and email.

Creating Your First Table

Let's combine what we've learned! This example will start Mnesia, ensure a schema exists, define a user record, and then create a user table with disc_copies storage.

Run this to see a complete Mnesia setup for a basic table!

-module(mnesia_lesson_create_table).
-export([run/0]).

% Define the user record for our table
-record(user, {id, name, email}).

run() ->
    % 1. Start the Mnesia application
    application:start(mnesia),
    io:format("Mnesia application started.~n"),

    % 2. Create schema (if it doesn't exist)
    case mnesia:create_schema([node()]) of
        ok -> io:format("Mnesia schema created.~n");
        {error, {already_exists, _}} ->
            io:format("Schema already exists (skipping creation).~n");
        Error -> io:format("Error creating schema: ~p~n", [Error])
    end,

    % 3. Start the Mnesia database manager
    mnesia:start(),
    io:format("Mnesia database started.~n"),

    % 4. Create the 'user' table
    % Options: [{disc_copies, [node()]}] means persistent table on this node
    case mnesia:create_table(user, [{disc_copies, [node()]}]) of
        {atomic, ok} ->
            io:format("Table 'user' created successfully (disc_copies).~n");
        {aborted, {already_exists, user}} ->
            io:format("Table 'user' already exists (skipping creation).~n");
        Error ->
            io:format("Error creating table: ~p~n", [Error])
    end,

    % Give Mnesia a moment
    timer:sleep(100),

    % 5. Stop Mnesia and the application
    mnesia:stop(),
    application:stop(mnesia),
    io:format("Mnesia stopped.~n"),
    ok.

Quick Check

Which Mnesia table type stores data only in memory and loses it if the node crashes or restarts?

Mnesia Fundamentals Recap

Great job! You've taken your first steps into Mnesia. We covered:

What Mnesia is and its core features.
How to start Mnesia and initialize its schema.
The concept of Mnesia tables and their definition using Erlang records.
The different table types: ram_copies, disc_copies, and disc_only_copies.

Next, we'll dive into performing atomic transactions and manipulating data within these tables!

Frequently asked questions

Is the “Mnesia Fundamentals & Schema” lesson free?

Yes — the full text of “Mnesia Fundamentals & Schema” is free to read here on the web, and the Erlang OTP: Distributed & Fault-Tolerant Systems Programming course includes 4 lessons in total. To practise it interactively (a built-in code editor and a 24/7 AI tutor) and unlock the rest of the Erlang OTP: Distributed & Fault-Tolerant Systems Programming course, upgrade to CoddyKit PRO.

What will I learn in “Mnesia Fundamentals & Schema”?

Understand Mnesia's architecture, table types, and how to define schemas for your distributed data. You practise Erlang OTP: Distributed & Fault-Tolerant Systems Programming with hands-on code you run directly in the browser, and a 24/7 AI tutor answers your questions as you work through the lesson.

Do I need any experience to start Erlang OTP: Distributed & Fault-Tolerant Systems Programming?

No prior experience is required. Erlang OTP: Distributed & Fault-Tolerant Systems Programming on CoddyKit is structured for beginners through advanced learners; this is — lesson 1 of 4, so you can start here or from the beginning and move at your own pace.

How long does the “Mnesia Fundamentals & Schema” lesson take?

Most CoddyKit lessons take about 5–10 minutes. Each one is bite-sized and interactive, so you make steady progress and pick up exactly where you left off across the web and the app.

Can I write and run code in this Erlang OTP: Distributed & Fault-Tolerant Systems Programming lesson?

Yes. Every Erlang OTP: Distributed & Fault-Tolerant Systems Programming lesson includes a built-in code editor, so you write and run real code right in your browser and get instant AI feedback — no local setup required.

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