Add transactional topic writer API for atomic table and topic operations

[Copilot]

Pull request type

• Feature

What is the current behavior?

No built-in support for atomic operations spanning table modifications and topic publications. Developers must implement manual outbox patterns with background workers, adding complexity and operational burden.

What is the new behavior?

Introduces CreateTxWriter<T>() on YdbConnection to publish messages to topics within an ACID transaction. Messages become visible atomically with table changes after successful commit.

API Surface

using var connection = new YdbConnection(connectionString);
await connection.OpenAsync();

using var transaction = connection.BeginTransaction();

// Table operations
await connection.ExecuteAsync(
    "INSERT INTO Users(Id, Name) VALUES (@Id, @Name)",
    new { Id = 1, Name = "John" });

// Topic writes (non-blocking, buffered)
var writer = connection.CreateTxWriter<string>("notifications");
writer.Write("User created: John");

// Atomic commit: both table and topic succeed or fail together
await transaction.CommitAsync();

Implementation

• ITxTopicWriter<T>: Public interface with void Write(T) method
• TxWriterOptions: Configurable buffer size (default 64MB), producer ID, codec, partition
• TxTopicWriter<T>: Internal buffering implementation with ConcurrentQueue
• YdbConnection.CreateTxWriter<T>(): Factory requiring active transaction
• YdbTransaction.CommitAsync(): Flushes all writers via IInternalTxWriter interface before commit
• TxTopicWriterException: Thrown on buffer overflow (non-blocking design)

Buffer Overflow Strategy

Throws exception when buffer is full (default 64MB) rather than blocking producer thread. Safe for server workloads; caller controls backpressure through flush/retry/batch logic.

Current Limitations

Topic sending to YDB service is placeholder simulation. Production implementation requires WriterSession pooling, transaction ID binding in WriteRequest, and acknowledgement handling. API contract and integration points are production-ready.

Other information

• 9 files changed: +859, -1
• Comprehensive test coverage (requires YDB instance)
• CodeQL: 0 alerts
• No breaking changes

Original prompt

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This section details on the original issue you should resolve

<issue_title>Transactional Write to YDB Topic and Table (TxWriter API)</issue_title>
<issue_description># Summary

Introduces an API for publishing messages to a YDB topic within the same ACID transaction that modifies tables.

All messages written via TxWriter are bound to the current transaction and become visible atomically together with table changes after a successful commit. Message sending is performed in the background while the application continues to work with tables or other topics. Before committing, the YdbConnection waits for acknowledgements of all pending messages, and then commits.

Motivation

• Today, it is difficult to guarantee atomicity between updates to tables and publications on topics. Failures in the middle of the process cause divergence.
• Developers often implement an outbox table and a background worker to approximate atomicity; this is complex and adds operational burden.
• We want a simple, built-in way to publish to a topic as part of a DB transaction, with background sending and an explicit flush-before-commit step.

Proposed API (names are indicative)

Interfaces

public interface ITxTopicWriter<T>
{
    // Enqueue a message for this transaction.
    void Write(T value);
}

Maybe unused interface:

internal interface IBufferedTxTopicWriter<T> : ITxTopicWriter<T>
{
    // Wait for all enqueued messages to be durably accepted in the context of the current transaction.
    // CommitAsync will call this automatically.
    Task FlushAsync(CancellationToken cancellationToken = default);
}

YdbConnection

public sealed class YdbConnection : DbConnection
{
    // Validates an active transaction, acquires a WriterSession from the pool that is compatible
    // with the current QuerySession, and binds all writes to this transaction's ID.
    public ITxTopicWriter<T> CreateTxWriter<T>(string topicName, TxWriterOptions? options = null);
}

Usage example

await ydbDataSource.ExecuteInTransactionAsync(async (ydbConnection, ct) =>
{
    var txWriter1 = ydbConnection.CreateTxWriter<string>("topic_1");
    var txWriter2 = ydbConnection.CreateTxWriter<string>("topic_another");

    for (var i = 0; i < selectedCount; i++)
        txWriter1.Write("Example payload: " + i);

    for (var i = 0; i < selectedCount; i++)
        txWriter2.Write("Example payload: " + i);

    await ydbConnection.ExecuteAsync(
        "INSERT INTO Users(Id, Name, Email) VALUES (@Id, @Name, @Email)",
        new User { Id = 1, Name = "Name, Email = "Email" }, ct);

    await ydbConnection.ExecuteAsync(
        "INSERT INTO Users(Id, Name, Email) VALUES (@Id, @Name, @Email)",
        new User { Id = 2, Name = "New_Name", Email = "New_Email" }, ct);

    // No explicit Flush needed; the transaction will Flush both writers, then Commit.
});

Reference

• Go / Python topic SDK.
• Java is not a stable solution. There will be a new API in the next versions.

Core Concepts

• Transaction source: The TopicService has no native transactions. Transactional publishing uses a QueryService transaction, which is created by a QuerySession and has a transaction ID.
• Prerequisite for Write: transactional topic Write requires an active QueryService transaction. If none exists, BeginTransaction must be executed before sending (by the library).
• Placement: CreateTxWriter is on YdbConnection (it owns QuerySession and the transaction context) and binds the writer to the current txId.
• Lazy start: Prefer starting the transaction on CreateTxWriter if none is active; starting on first Write is acceptable but must remain single and idempotent under concurrency.
• Attach if exists: If a transaction is already active (e.g., after table reads/writes), CreateTxWriter attaches to it; no new transaction is started.
• Non-blocking writes: Write enqueues to a buffer and triggers background sending bound to the txId; messages stay invisible until commit and are dropped on rollback.
• Flush before commit: YdbConnection tracks all TxWriters in the scope and Flushes them before Commit. If any Flush fails or is canceled, commit is not attempted and the tx is rolled back.
• Thread-safety: One active transaction per connection; concurrent CreateTxWriter/Write calls must coalesce to a single BeginTransaction.
• WriterSession pool: Rent WriterSession from the writer pool, rebind to a new txId as needed, clear buffers after aborted tx, and quarantine unhealthy sessions. A WriterSession is returned to the pool only after a successful commit; on rollback it is cleaned and either quarantined or disposed per policy.

TxWriter: API design with void Write and buffer backpressure/overflow handling

Goal and context

• Provide a simple API: void Write(TValue value) with background sending; Commit waits for confirmed de...

• Fixes Transactional Write to YDB Topic and Table (TxWriter API) #545

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