"Add Message Acknowledgment Mechanism for Exactly-Once Processing"

[Sambhram1]

Add Message Acknowledgment for Exactly-Once Processing

Summary

Implements message acknowledgment mechanism to support exactly-once processing guarantees, addressing the autoscaling feature (PR #202) where actors with pending messages could be dropped.

Changes

Core Implementation

• SignedMessage: Added ack(), acknowledge(), and is_acknowledged() methods
• BastionContext: Added message_processed(&SignedMessage) convenience method
• ContextState: Added unacknowledged message counter for autoscaling integration

API Usage

Option 1 - Direct acknowledgment:

let msg = ctx.recv().await?;
msg.ack();

Option 2 - Context-based:

let msg = ctx.recv().await?;
ctx.message_processed(&msg);

Benefits

✅ Exactly-once message processing
✅ Safer autoscaling (prevents dropping actors with pending work)
✅ Thread-safe atomic operations
✅ Backward compatible (opt-in feature)
✅ Comprehensive tests and examples

Files Changed

• src/bastion/src/envelope.rs - Added acknowledgment to SignedMessage
• src/bastion/src/context.rs - Added tracking and context method
• src/bastion/src/message.rs - Pattern matching updates
• src/bastion/examples/message_acknowledgment.rs - Working example
• src/bastion/tests/message_acknowledgment.rs - Test suite
• Documentation files

Testing

cargo test message_acknowledgment
cargo run --example message_acknowledgment

Autoscaling Integration

The unacknowledged message counter enables autoscaling to check if actors are safe to remove:

// Only remove actors with no pending messages
if state.unacknowledged_count() == 0 {
    // Safe to remove actor
}

Breaking Changes

None. Fully backward compatible - existing code works unchanged.

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Ready for review ✨