[Davide Montanari (davidemontanari)]

Summary

MessageSender.SendAsync(IEnumerable<Message>) adds every message to a single ServiceBusMessageBatch and ignores the result of TryAddMessage. Any message that doesn't fit in the batch is never sent, and nothing records it.

This PR makes these drops visible.

Changes

• When one or more messages don't fit in the batch, an Error trace is written with event type MessageSender-SendAsync-MessagesDropped. It includes:
  • the entity path
  • the number of dropped messages and their total body size in bytes
  • the number of messages added to the batch
  • the maximum batch size in bytes
• The trace has a fixed size, so it is never truncated by the event source's field size limit, however many messages are dropped.
• The message batch is now disposed after it is sent.

Behaviour

• Messages that fit in the batch are sent in a single batch with one send call.
• Messages that don't fit are not sent; the trace records how many were dropped.
• If every message fits, no trace is written.

Notes

All messages in one call share a single batch (256 KB on Standard, 1 MB on Premium), even though each message on its own stays below MessageOverflowThresholdInBytes (170 KB by default). Large fan-outs can exceed the batch size: many activities scheduled in one episode, or the matching history tracking messages. When that happens, the extra messages are dropped and the new trace records them.

Testing

• DurableTask.ServiceBus builds with no warnings or errors.

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[Copilot]

🟡 Changes recommended

Large dropped-ID lists are truncated by the tracing layer, leaving the intended diagnostics incomplete.

2 open issues

• Chunk dropped-ID traces to avoid silent truncation · New
• Avoid claiming batch messages were sent before delivery · New

What changed in this PR

This PR adds visibility into messages dropped by the Service Bus provider when they exceed a single batch’s capacity, without changing the existing single-batch send behavior.

Changes:

• Adds error tracing with dropped-message counts, body bytes, IDs, and batch context.
• Disposes the message batch after use.
• Documents that messages exceeding batch capacity are traced but not sent.

File	Description
src/​DurableTask.ServiceBus/​Common/​Abstraction/​ServiceBusAbstraction.cs	Adds overflow diagnostics, batch disposal, and method documentation.

_{Review effort: Balanced}

[Copilot]

🟢 Approval recommended

The focused changes preserve existing sending behavior and introduce no identified blocking issues.

No open issues 🎉

2 resolved since last review

• Chunk dropped-ID traces to avoid silent truncation
• Avoid claiming batch messages were sent before delivery

_{Review effort: Balanced}