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

Transactions et manipulation des données

Effectuez des transactions atomiques sur les tables Mnesia afin de garantir la cohérence des données entre les nœuds distribués

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Why Mnesia Needs Transactions

Imagine managing important data like bank transfers. You wouldn't want money to leave one account without arriving in another, right?

This "all or nothing" principle is crucial for data consistency, and it's where transactions come in. Mnesia uses transactions to group multiple database operations into a single, atomic unit.

An atomic transaction either fully completes all its operations, or if any part fails, all changes are rolled back. This ensures your data remains consistent and reliable.

The Core: `mnesia:transaction`

In Mnesia, you perform transactional operations using the mnesia:transaction/1 function. It takes an anonymous function (a fun) as its argument.

All Mnesia read and write operations that need to be atomic must be placed inside this fun. If the fun executes successfully, the changes are committed. If an error occurs or mnesia:abort/1 is called, all changes are rolled back.

Let's see its basic structure:

-module(example_transaction).
-export([main/0]).

main() ->
    mnesia:start(),
    {atomic, _Result} = mnesia:transaction(fun() ->
        % Mnesia operations go here
        io:format("Inside the transaction!~n"),
        ok
    end),
    io:format("Transaction completed.~n"),
    mnesia:stop().

Preparing Data for Transactions

Before we can manipulate data, we need a Mnesia table. Let's define a simple person record and create a table for it. This setup will be used in our transaction examples.

Remember, mnesia:create_table/2 is usually called once when your application starts up for the first time.

-module(mnesia_table_setup).
-export([main/0]).
-record(person, {id, name, age}).

main() ->
    % Ensure Mnesia is started on this node
    mnesia:create_schema([node()]),
    mnesia:start(),
    % Create table if it doesn't exist
    case mnesia:create_table(person, [{attributes, record_info(fields, person)}]) of
        {atomic, ok} -> io:format("Table 'person' created successfully.~n");
        {aborted, {already_exists, person}} -> io:format("Table 'person' already exists.~n");
        Error -> io:format("Error creating table: ~p~n", [Error])
    end,
    mnesia:stop().

Adding & Updating Records

Inside a transaction, you use mnesia:write/1 to store or update records. If a record with the same primary key (the first field in our person record, id) already exists, it will be updated. Otherwise, a new record is inserted.

Let's add a new person to our table. Remember to start Mnesia first!

-module(write_example).
-export([main/0]).
-record(person, {id, name, age}).

main() ->
    % Ensure Mnesia is started and table exists (simplified setup)
    mnesia:create_schema([node()]),
    mnesia:start(),
    mnesia:create_table(person, [{attributes, record_info(fields, person)}]),
    mnesia:wait_for_tables([person], 5000),

    % Write a new person record
    Person1 = #person{id = 1, name = "Alice", age = 30},
    {atomic, ok} = mnesia:transaction(fun() ->
        mnesia:write(Person1)
    end),
    io:format("Wrote: ~p~n", [Person1]),

    mnesia:stop().

Fetching Records Transactionally

To retrieve data inside a transaction, use mnesia:read/1. It takes a record or a record key (like {person, 1}) and returns a list of matching records. If no record is found, it returns [] (an empty list).

Let's read the person we just added. We'll combine writing and reading in a single transaction for a fuller example.

-module(read_example).
-export([main/0]).
-record(person, {id, name, age}).

main() ->
    mnesia:create_schema([node()]),
    mnesia:start(),
    mnesia:create_table(person, [{attributes, record_info(fields, person)}]),
    mnesia:wait_for_tables([person], 5000),

    Person1 = #person{id = 1, name = "Alice", age = 30},
    Person2 = #person{id = 2, name = "Bob", age = 25},

    {atomic, Result} = mnesia:transaction(fun() ->
        % Write two records
        mnesia:write(Person1),
        mnesia:write(Person2),
        % Read one of them
        mnesia:read({person, 1})
    end),
    io:format("Transaction result (read data): ~p~n", [Result]),

    mnesia:stop().

Modifying Data with Transactions

mnesia:write/1 handles both inserting new records and updating existing ones. To remove a record entirely, you use mnesia:delete/1, providing the full record or just its key.

Let's update Alice's age and then delete Bob from our Mnesia table, all within a single, consistent transaction.

-module(update_delete_example).
-export([main/0]).
-record(person, {id, name, age}).

main() ->
    mnesia:create_schema([node()]),
    mnesia:start(),
    mnesia:create_table(person, [{attributes, record_info(fields, person)}]),
    mnesia:wait_for_tables([person], 5000),

    % Ensure initial data for update/delete
    mnesia:transaction(fun() ->
        mnesia:write(#person{id = 1, name = "Alice", age = 30}),
        mnesia:write(#person{id = 2, name = "Bob", age = 25})
    end),

    io:format("--- Before transaction ---~n"),
    io:format("Alice: ~p~n", [mnesia:dirty_read({person, 1})]),
    io:format("Bob: ~p~n", [mnesia:dirty_read({person, 2})]),

    {atomic, _} = mnesia:transaction(fun() ->
        % Update Alice's age
        UpdatedAlice = #person{id = 1, name = "Alice", age = 31},
        mnesia:write(UpdatedAlice),
        % Delete Bob
        mnesia:delete({person, 2})
    end),

    io:format("--- After transaction ---~n"),
    io:format("Alice: ~p~n", [mnesia:dirty_read({person, 1})]),
    io:format("Bob: ~p~n", [mnesia:dirty_read({person, 2})]), % Should be []

    mnesia:stop().

Transaction Rollbacks in Action

The power of transactions lies in their atomicity. If any operation within the fun fails or you explicitly call mnesia:abort/1, Mnesia will roll back all changes made during that transaction.

This means your database state will revert to how it was before the transaction started, preventing partial updates and maintaining consistency.

-module(rollback_example).
-export([main/0]).
-record(person, {id, name, age}).

main() ->
    mnesia:create_schema([node()]),
    mnesia:start(),
    mnesia:create_table(person, [{attributes, record_info(fields, person)}]),
    mnesia:wait_for_tables([person], 5000),

    % Ensure Alice exists initially
    mnesia:transaction(fun() ->
        mnesia:write(#person{id = 1, name = "Alice", age = 30})
    end),
    io:format("Initial Alice: ~p~n", [mnesia:dirty_read({person, 1})]),

    % Attempt a transaction that will abort
    Result = mnesia:transaction(fun() ->
        mnesia:write(#person{id = 1, name = "Alice", age = 35}), % Update
        mnesia:write(#person{id = 3, name = "Charlie", age = 22}), % New record
        mnesia:abort("Something went wrong!") % Abort the transaction
    end),
    io:format("Transaction result: ~p~n", [Result]),

    io:format("Alice after aborted transaction: ~p~n", [mnesia:dirty_read({person, 1})]),
    io:format("Charlie after aborted transaction: ~p~n", [mnesia:dirty_read({person, 3})]),

    mnesia:stop().

`mnesia:sync_transaction/1` for Global Commit

While mnesia:transaction/1 ensures local atomicity, its return doesn't guarantee the transaction has committed on all Mnesia replicas in a distributed setup. For that, you use mnesia:sync_transaction/1.

mnesia:sync_transaction/1 blocks the caller until the transaction has been successfully committed on all nodes where the affected tables are resident. This is crucial for strong consistency guarantees in distributed systems.

Use it when you absolutely need to know that data is consistent across your entire cluster before proceeding.

-module(sync_transaction_example).
-export([main/0]).
-record(item, {id, name}).

main() ->
    % This example assumes a distributed Mnesia setup.
    % For a single node, it behaves like mnesia:transaction/1.
    mnesia:create_schema([node()]),
    mnesia:start(),
    mnesia:create_table(item, [{attributes, record_info(fields, item)}]),
    mnesia:wait_for_tables([item], 5000),

    Item = #item{id = 101, name = "Widget A"},

    io:format("Attempting sync_transaction...~n"),
    {atomic, ok} = mnesia:sync_transaction(fun() ->
        mnesia:write(Item)
    end),
    io:format("Item written and committed across all replicas: ~p~n", [Item]),

    mnesia:stop().

Transaction Knowledge Check

You've learned about Mnesia transactions and how to manipulate data atomically. Let's test your understanding!

Transactions: Your Data's Safety Net

In this lesson, you mastered Mnesia transactions, a cornerstone for building robust and reliable applications.

We learned that transactions provide atomicity, ensuring "all or nothing" data operations.
You practiced using mnesia:transaction/1 to group Mnesia operations.
We covered mnesia:write/1 for adding/updating, mnesia:read/1 for fetching, and mnesia:delete/1 for removing records.
We briefly touched upon mnesia:sync_transaction/1 for strong distributed consistency.

Next, we'll dive into configuring Mnesia for distributed operation, including data replication and fault-tolerant storage across a cluster.

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Toutes les leçons de ce cours

Fondamentaux et schéma de Mnesia
Transactions et manipulation des données
Mnesia distribuée et réplication
Indexation Mnesia et optimisation des requêtes

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