Document the concurrency model for subscribers

docs/docs/en/getting-started/subscription/concurrency.md

@@ -0,0 +1,69 @@
+---
+# 0.5 - API
+# 2 - Release
+# 3 - Contributing
+# 5 - Template Page
+# 10 - Default
+search:
+  boost: 10
+---
+
+# Concurrency
+
+**FastStream** gives you two independent levers for processing messages concurrently. Mixing them up is a common source of confusion, so it helps to keep their roles straight.
+
+## `max_workers` — handler-level parallelism
+
+`max_workers` is the **number of concurrent handler invocations** a single subscriber may run. It is available on every broker's subscriber decorator:
+
+```python hl_lines="3"
+@broker.subscriber(
+    "test",
+    max_workers=4,
+)
+async def handle(msg): ...
+```
+
+With `max_workers=1` (the default) **FastStream** processes one message at a time per subscriber. With `max_workers=N`, up to `N` invocations of the handler can run at the same time.
+
+## `prefetch_count` — RabbitMQ flow control
+
+`prefetch_count` is **RabbitMQ-specific**. It comes from AMQP's `basic.qos` and tells the broker how many unacknowledged messages it may have outstanding on a given channel at any time.
+
+It is set at the channel level:
+
+```python hl_lines="6"
+from faststream.rabbit import RabbitBroker
+from faststream.rabbit.schemas import Channel
+
+broker = RabbitBroker(
+    "amqp://guest:guest@localhost:5672/",
+    channel=Channel(prefetch_count=10),
+)
+```
+
+`prefetch_count=N` does **not** mean "process `N` messages at the same time". It means "let the broker push up to `N` messages to this consumer before requiring an ack". It is a buffer / flow-control window — not parallelism.
+
+## How they combine
+
+The two settings are orthogonal:
+
+| `max_workers` | `prefetch_count` | Behavior                                                                                |
+|---------------|------------------|-----------------------------------------------------------------------------------------|
+| `1`           | `1`              | One message in flight, processed sequentially. The classic per-message round trip.       |
+| `1`           | `N`              | Up to `N` messages held locally by the consumer, but **still processed one at a time**.  |
+| `K`           | `N` (≥ `K`)      | Up to `K` invocations of the handler run concurrently; up to `N` may be buffered.        |
+| `K`           | `< K`            | Concurrency is throttled by `prefetch_count`: the broker will not push enough messages to keep all workers busy. |
+
+A useful rule of thumb for RabbitMQ subscribers: set `prefetch_count` at least as large as `max_workers`, otherwise extra workers sit idle waiting for messages.
+
+## Other brokers
+
+- **Kafka / Confluent.** There is no `prefetch_count`. The consumer pulls in batches; tune `max_poll_records` (and friends) to control how many records are fetched per poll, and use `max_workers` for handler parallelism. See [Kafka Subscriber concurrent processing](../../kafka/Subscriber/index.md#concurrent-processing) for the interaction with `AckPolicy`.
+- **Redis, NATS, MQTT.** Only `max_workers` applies. There is no broker-side prefetch knob equivalent to RabbitMQ's.
+
+## The common confusion
+
+> Does `prefetch_count=10` mean ten messages are processed at the same time?
+
+No. It means the **broker** may deliver up to ten messages before it expects acknowledgments. How many are *processed in parallel* is determined by `max_workers`. With the default `max_workers=1`, the ten prefetched messages are processed one after another — `prefetch_count` only changes how aggressively they are pulled from the broker.

docs/docs/navigation_template.txt

@@ -12,6 +12,7 @@ search:
         - [Filtering](getting-started/subscription/filtering.md)
         - [Testing](getting-started/subscription/test.md)
         - [Dynamic Subscribers](getting-started/subscription/dynamic.md)
+        - [Concurrency](getting-started/subscription/concurrency.md)
     - [Publishing](getting-started/publishing/index.md)
         - [Broker Publish](getting-started/publishing/broker.md)
         - [Decorator](getting-started/publishing/decorator.md)