lhotari commented on code in PR #24704:
URL: https://github.com/apache/pulsar/pull/24704#discussion_r2333389757
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pip/pip-439.md:
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+# PIP-439: Adding Transaction Support to Pulsar Functions Through
Auto-Transaction Wrapping
+
+# Background knowledge
+
+Apache Pulsar transactions enable atomic operations across multiple topics,
allowing producers to send messages and consumers to acknowledge messages as a
single unit
+of work. This provides the foundation for exactly-once processing semantics in
streaming applications.
+
+## Transaction Architecture
+
+Pulsar's transaction system consists of four key components:
+
+1. **Transaction Coordinator (TC)**: A broker module that manages transaction
lifecycles, allocates transaction IDs, and orchestrates the commit/abort
process.
+
+2. **Transaction Log**: A persistent topic storing transaction metadata and
state changes, enabling recovery after failures.
+
+3. **Transaction Buffer**: Temporarily stores messages produced within
transactions, making them visible to consumers only after commit.
+
+4. **Pending Acknowledge State**: Tracks message acknowledgments within
transactions, preventing conflicts between competing transactions.
+
+## Transaction Lifecycle
+
+Transactions follow a defined lifecycle:
+
+1. **OPEN**: Client obtains a transaction ID from the Transaction Coordinator.
+2. **PRODUCING/ACKNOWLEDGING**: Client registers topic
partitions/subscriptions with the TC, then produces/acknowledges messages
within the transaction.
+3. **COMMITTING/ABORTING**: Client requests to end the transaction, TC begins
two-phase commit.
+4. **COMMITTED/ABORTED**: After processing all partitions, TC finalizes the
transaction state.
+5. **TIMED_OUT**: Transactions exceeding their timeout are automatically
aborted.
+
+## Transaction Guarantees
+
+Pulsar transactions provide:
+- Atomic writes across multiple topics
+- Conditional acknowledgment to prevent duplicate processing by "zombie"
instances
+- Visibility control ensuring consumers only see committed transaction messages
+- Support for exactly-once processing in consume-transform-produce patterns
+
+## Pulsar Functions
+
+Pulsar Functions is a lightweight compute framework integrated with Apache
Pulsar that
+enables stream processing without managing infrastructure. Key characteristics
include:
+ - Simple Programming Model: Functions receive messages, process them, and
optionally
+produce output
+ - Processing Patterns: Supports both synchronous and asynchronous message
processing
+ - Context Object: Provides access to message metadata, output production, and
state
+storage
+ - Integration: Natively integrated with Pulsar's pub-sub messaging system
+ - Deployment: Managed by Pulsar with automatic scaling and fault tolerance
+
+Functions operate on a per-message basis, making them ideal for implementing
stream
+processing with exactly-once semantics when combined with transactions.
+
+# Motivation
+
+Currently, Pulsar Functions cannot publish to multiple topics transactionally,
which is a significant limitation for use cases requiring atomic multi-topic
+publishing. For instance, if a function processes an input message and needs
to publish related updates to several output topics, there's no guarantee that
all
+operations will succeed atomically.
+
+This limitation prevents building robust stream processing applications that
require exactly-once semantics across multiple input and output topics. Without
+transaction support in Functions, developers must implement their own error
handling and retry mechanisms, which can be complex and error-prone.
+
+Adding transaction support to Pulsar Functions would finally ensure message
processing atomicity.
+
+# Goals
+
+## In Scope
+
+1. Enable automatic transaction support for Pulsar Functions through
configuration
+2. Allow Functions to publish messages to multiple topics within a single
transaction
+3. Support transactional acknowledgment of input messages
+4. Ensure transactions are committed only if message processing completes
successfully
+5. Provide transaction timeout configuration for Functions
+
+## Out of Scope
+
+1. Exposing explicit transaction management APIs in the Functions interface
+2. Supporting multi-function transactions (transactions spanning multiple
function invocations)
+3. Adding transaction support to Pulsar IO connectors
+4. Changes to the Function interface itself
+
+# High Level Design
+
+The proposed solution introduces managed transaction wrapping for Pulsar
Functions through configuration settings. When enabled, each function execution
will be automatically wrapped in a transaction without requiring code changes
to the function implementation.
+
+The general flow will be:
+1. Function is configured with `transactionMode: MANAGED`
+2. When a message arrives, the function runtime creates a new transaction
+3. The function processes the message with an enhanced Context that uses the
transaction
+4. Any output messages are published using the transaction
+5. Input message acknowledgment is performed within the transaction
+6. If the function completes successfully, the transaction is committed
+7. If the function throws an exception, the transaction is aborted
+
+This approach provides transaction support in a way that is transparent to
function implementers, requiring only configuration changes rather than code
changes.
+
+# Detailed Design
+
+## Design & Implementation Details
+
+### Configuration Classes
+
+We will update the FunctionConfig to include transaction-related settings
through a new `TransactionConfig` class:
+
+```java
+public enum TransactionMode {
+ OFF,
+ MANAGED
+}
+
+public class TransactionConfig {
+ private TransactionMode transactionMode = TransactionMode.OFF;
+ private Long transactionTimeoutMs = 60000L;
+
+ // Getters and setters...
+}
+
+public class FunctionConfig {
+ // Existing fields...
+
+ private TransactionConfig transaction = new TransactionConfig();
+
+ // Getter and setter ...
+}
+```
+
+```java
+We also need to update the protobuf definition for FunctionDetails to include
these fields:
+
+message TransactionSpec {
+ enum TransactionMode {
+ OFF = 0;
+ MANAGED = 1;
+ }
+ TransactionMode transactionMode = 1;
+ int64 transactionTimeoutMs = 2;
+}
+
+message FunctionDetails {
+ // Other existing fields...
+ TransactionSpec transaction = 24;
+}
+```
+
+### Modifications to Context Interface
+
+We will update the Context interface to expose the current transaction:
+
+```java
+public interface Context {
+ // Existing methods...
+
+ /**
+ * Returns the current transaction if function is running in managed
transaction mode.
+ *
+ * <p>IMPORTANT: This method is not async-safe. When writing asynchronous
functions that
+ * return CompletableFuture and need to use the transaction inside
callbacks or chained
+ * operations, you must store a reference to the transaction locally
before returning the future:
+ *
+ * <pre>{@code
+ * public CompletableFuture<String> process(String input, Context context)
{
+ * // Store transaction reference locally before async operations
+ * Transaction txn = context.getCurrentTransaction();
Review Comment:
This shouldn't be exposed at all. It should be possible to handle this also
for asynchronous functions.
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