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

Distributed Data with ETS & Mnesia

Get an introduction to ETS for in-memory distributed tables and Mnesia for lightweight distributed database capabilities.

Distributed Data with ETS & Mnesia is a free Erlang OTP: Distributed & Fault-Tolerant Systems Programming lesson on CoddyKit — lesson 2 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.

Why Distributed Data?

In large-scale Erlang systems, data often needs to be shared and synchronized across multiple nodes. This is crucial for building fault-tolerant and scalable applications.

Imagine a chat application where user sessions or message queues need to be accessible even if one server fails. Distributed data stores make this possible by allowing data to live beyond a single process or node.

Erlang Term Storage (ETS)

ETS stands for Erlang Term Storage. It's a powerful, built-in in-memory key-value store. Think of it as a super-fast hash table directly integrated into the Erlang runtime.

Extremely fast for read/write operations.
Can store any Erlang term (integers, atoms, lists, tuples, PIDs, etc.).
Process-safe: multiple processes can access the same table concurrently.

Exploring ETS Table Types

ETS offers different table types, each with unique properties:

set: The default. Each key can only have one value. Like a dictionary.
ordered_set: Similar to set, but keys are kept sorted. Slower writes, faster range lookups.
bag: Allows multiple objects with the same key.
duplicate_bag: Allows multiple objects with the same key AND value.

Choosing the right type depends on your data's structure and access patterns.

Basic ETS Operations

Let's see how to create an ETS table and perform basic operations like inserting and looking up data. We'll use a set table for simplicity.

-module(ets_example).
-export([run/0, add_user/2, get_user/1, cleanup/0]).

% Function to run the example sequence
run() ->
    io:format("--- ETS Example Start ---~n"),
    % Create a named public set table
    ets:new(users_table, [set, public, named_table]),
    io:format("Table 'users_table' created.~n"),

    add_user(1, "Alice"),
    add_user(2, "Bob"),
    add_user(1, "Alicia"), % This will update Alice to Alicia

    get_user(1),
    get_user(2),
    get_user(3), % User 3 doesn't exist

    cleanup(),
    io:format("--- ETS Example End ---~n").

add_user(Id, Name) ->
    ets:insert(users_table, {Id, Name}),
    io:format("Inserted/updated user {~p, ~p}~n", [Id, Name]).

get_user(Id) ->
    case ets:lookup(users_table, Id) of
        [{Id, Name}] ->
            io:format("Lookup result for ~p: {~p, ~p}~n", [Id, Name]);
        [] ->
            io:format("User ~p not found in table.~n", [Id])
    end.

cleanup() ->
    ets:delete(users_table),
    io:format("Table 'users_table' deleted.~n").

ETS: Local Storage Only

While ETS is incredibly fast and concurrent, it's important to remember a key limitation: ETS tables are local to the Erlang node where they are created.

Data in an ETS table on node_A is not automatically available on node_B.
To share ETS data across nodes, you'd need to implement custom replication or remote access mechanisms, often using message passing or RPC.

This is where Mnesia comes in to simplify distributed data management.

Mnesia: Erlang's Distributed DB

Mnesia is a powerful, distributed, fault-tolerant, real-time database management system built into Erlang/OTP. It leverages ETS and DETS (Disk Erlang Term Storage) for its storage.

Mnesia simplifies the challenges of managing data across multiple Erlang nodes, providing transactional guarantees and automatic replication, making it ideal for robust distributed applications.

Mnesia Setup & Schema

Before using Mnesia, you need to start it and define your data schema. The schema describes your tables and their properties (e.g., what node hosts which copy).

mnesia:start(): Initializes the Mnesia system.
mnesia:create_schema([node()]): Creates the Mnesia directory and schema on the specified nodes. This is typically a one-time setup per cluster.
mnesia:create_table(Name, Props): Defines a new table with its attributes and storage type.

Mnesia Transactions

Mnesia operations are typically performed within transactions to ensure atomicity, consistency, isolation, and durability (ACID properties). This means all operations within a transaction either succeed or fail together.

Here's how to create a table and interact with it using transactions:

-module(mnesia_example).
-export([init_mnesia/0, add_product/2, get_product/1, cleanup/0]).

% Define a record for our product data
-record(product, {id, name}).

init_mnesia() ->
    io:format("--- Mnesia Init Start ---~n"),
    % Start Mnesia
    mnesia:start(),
    io:format("Mnesia started.~n"),

    % Uncomment and run 'mnesia_example:create_schema().' once for a new Mnesia setup:
    % mnesia:create_schema([node()]),
    % io:format("Mnesia schema created.~n"),

    % Create product table with disc_copies on this node
    % Uncomment to delete an existing table before creating a new one:
    % mnesia:delete_table(product),
    TableProps = [{disc_copies, [node()]}, {attributes, record_info(fields, product)}],
    case mnesia:create_table(product, TableProps) of
        {atomic, ok} -> io:format("Table 'product' created.~n");
        {aborted, {already_exists, product}} -> io:format("Table 'product' already exists.~n");
        Other -> io:format("Error creating table: ~p~n", [Other])
    end,
    io:format("--- Mnesia Init End ---~n").

add_product(Id, Name) ->
    Product = #product{id = Id, name = Name},
    F = fun() -> mnesia:write(Product) end,
    case mnesia:transaction(F) of
        {atomic, ok} -> io:format("Added product {~p, ~p}~n", [Id, Name]);
        {aborted, Reason} -> io:format("Failed to add product: ~p~n", [Reason])
    end.

get_product(Id) ->
    F = fun() -> mnesia:read({product, Id}) end,
    case mnesia:transaction(F) of
        {atomic, [ProductRecord]} ->
            io:format("Found product ~p: {~p, ~p}~n", [Id, ProductRecord#product.name]);
        {atomic, []} ->
            io:format("Product ~p not found.~n", [Id]);
        {aborted, Reason} ->
            io:format("Failed to get product: ~p~n", [Reason])
    end.

cleanup() ->
    io:format("--- Mnesia Cleanup Start ---~n"),
    mnesia:stop(),
    io:format("Mnesia stopped.~n"),
    io:format("--- Mnesia Cleanup End ---~n").

Mnesia Data Replication

Mnesia achieves distribution and fault tolerance through data replication. When creating a table, you specify where its copies should reside:

disc_copies: Data is stored on disk and in RAM on the specified nodes. Best for persistence and fault tolerance.
ram_copies: Data is stored only in RAM on the specified nodes. Faster access but data is lost on node crash unless a disc_copies is also present elsewhere.
disc_only_copies: Data is stored only on disk. Slowest access but lowest RAM footprint.

Mnesia automatically handles replication and synchronization across these copies, ensuring data consistency.

Quick Check: ETS vs Mnesia

Both ETS and Mnesia are powerful tools for managing data in Erlang, but they serve different purposes. Select all statements that correctly describe the key differences or characteristics.

Recap: Distributed Data Tools

In this lesson, we explored two essential tools for managing data in Erlang: ETS and Mnesia.

ETS is a super-fast, in-memory key-value store, excellent for local caches and lookup tables. Remember, it's local to a node.
Mnesia is Erlang's built-in distributed, fault-tolerant database. It offers transactional safety, schema definition, and automatic data replication across nodes, built upon ETS.

Understanding when to use each is key to building resilient and scalable Erlang applications.

Frequently asked questions

Is the “Distributed Data with ETS & Mnesia” lesson free?

Yes — the full text of “Distributed Data with ETS & Mnesia” 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 “Distributed Data with ETS & Mnesia”?

Get an introduction to ETS for in-memory distributed tables and Mnesia for lightweight distributed database capabilities. 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 2 of 4, so you can start here or from the beginning and move at your own pace.

How long does the “Distributed Data with ETS & Mnesia” 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?

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