พื้นฐานและสคีมาของ 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 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.

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:

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.

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:

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.

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.

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.

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!

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!