Datos distribuidos con ETS y Mnesia
Obtenga una introducción a ETS para tablas distribuidas en memoria y a Mnesia para capacidades ligeras de bases de datos distribuidas.
Datos distribuidos con ETS y Mnesia es una lección gratuita de Erlang OTP: Distributed & Fault-Tolerant Systems Programming en CoddyKit. Esta es la lección 2 de 4. Puedes leer la lección completa abajo gratuitamente — luego la practicas en el navegador con un editor de código integrado y un tutor de IA 24/7. Forma parte de la ruta de aprendizaje de Erlang OTP: Distributed & Fault-Tolerant Systems Programming, y tu progreso se sincroniza en la web y la app de CoddyKit. El curso de Erlang OTP: Distributed & Fault-Tolerant Systems Programming incluye 4 lecciones en total.
Partes de esta lección aún no han sido traducidas y se muestran en inglés.
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 toset, 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_Ais not automatically available onnode_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 adisc_copiesis 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.
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¿La lección «Datos distribuidos con ETS y Mnesia» es gratis?
Sí — el texto completo de «Datos distribuidos con ETS y Mnesia» es gratis para leer aquí en la web. Para practicarla de forma interactiva (editor de código integrado y tutor de IA 24/7) y desbloquear el resto del curso de Erlang OTP: Distributed & Fault-Tolerant Systems Programming, actualiza a CoddyKit PRO. El curso de Erlang OTP: Distributed & Fault-Tolerant Systems Programming incluye 4 lecciones en total.
¿Qué aprenderé en «Datos distribuidos con ETS y Mnesia»?
Obtenga una introducción a ETS para tablas distribuidas en memoria y a Mnesia para capacidades ligeras de bases de datos distribuidas. Practicas Erlang OTP: Distributed & Fault-Tolerant Systems Programming con código real que ejecutas directamente en el navegador, y un tutor de IA 24/7 responde tus preguntas mientras trabajas en la lección.
¿Necesito experiencia previa para empezar Erlang OTP: Distributed & Fault-Tolerant Systems Programming?
No se requiere experiencia previa. Erlang OTP: Distributed & Fault-Tolerant Systems Programming en CoddyKit está estructurado para principiantes hasta estudiantes avanzados, así que puedes empezar aquí o desde el inicio y avanzar a tu ritmo. Esta es la lección 2 de 4.
¿Cuánto tiempo toma la lección «Datos distribuidos con ETS y Mnesia»?
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¿Puedo escribir y ejecutar código en esta lección de Erlang OTP: Distributed & Fault-Tolerant Systems Programming?
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Todas las lecciones de este curso
- Gestión de particiones de red
- Datos distribuidos con ETS y Mnesia
- Diseño de escalabilidad y resiliencia
- Balanceo de carga y failover entre nodos