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

Mnesia 基础与架构

了解 Mnesia 的架构和表类型，以及如何为分布式数据定义架构。

第 1 / 4 课11 个步骤

Mnesia 基础与架构 是 CoddyKit 上的免费 Erlang OTP: Distributed & Fault-Tolerant Systems Programming 课时。这是第 1 节课，共 4 节。你可以在下方免费阅读本课时的完整内容 — 然后在浏览器中使用内置代码编辑器和全天候 AI 导师进行实践。这是 Erlang OTP: Distributed & Fault-Tolerant Systems Programming 学习路径的一部分，你的进度在网页和 CoddyKit 应用中同步。 Erlang OTP: Distributed & Fault-Tolerant Systems Programming 课程共包含 4 节课。

本课时的部分内容尚未翻译，以英文显示。

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?

在交互式课程中回答

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!

免费开始

用 AI 导师学习 Erlang — 免费

在浏览器中编写并运行真实代码，获得全天候 AI 导师的即时帮助，并在网页或应用中继续学习。

课程: 12
课程: 48

免费开始学习查看完整课程

常见问题解答

「Mnesia 基础与架构」课时是免费的吗？

是的 — 「Mnesia 基础与架构」的完整文本可在网页上免费阅读。要进行交互式练习（内置代码编辑器和全天候 AI 导师）并解锁 Erlang OTP: Distributed & Fault-Tolerant Systems Programming 课程的其余内容，请升级到 CoddyKit PRO。 Erlang OTP: Distributed & Fault-Tolerant Systems Programming 课程共包含 4 节课。

「Mnesia 基础与架构」这节课中我会学到什么？

了解 Mnesia 的架构和表类型，以及如何为分布式数据定义架构。你通过在浏览器中直接运行的动手代码来练习 Erlang OTP: Distributed & Fault-Tolerant Systems Programming，全天候 AI 导师会在你学习这节课的过程中回答你的问题。

学习 Erlang OTP: Distributed & Fault-Tolerant Systems Programming 需要有经验吗？

无需任何先前经验。CoddyKit 上的 Erlang OTP: Distributed & Fault-Tolerant Systems Programming 课程适合初学者到高级学习者，你可以从这里开始或从头开始，按照自己的节奏学习。这是第 1 节课，共 4 节。

「Mnesia 基础与架构」课时需要多长时间？

大多数 CoddyKit 课程大约需要 5–10 分钟。每节课都很精短且互动，所以你能稳步进步，并在网页和应用中从离开的地方继续。

我能在这节 Erlang OTP: Distributed & Fault-Tolerant Systems Programming 课中编写并运行代码吗？

能。每节 Erlang OTP: Distributed & Fault-Tolerant Systems Programming 课都包含内置代码编辑器，你可以在浏览器中直接编写并运行真实代码，并获得即时 AI 反馈 — 无需本地设置。

此课程中的所有课时

← 返回 Erlang OTP: Distributed & Fault-Tolerant Systems Programming