Configure Prefetch Count

PrefetchCount controls how many unacknowledged messages RabbitMQ can deliver to a subscriber.

It is configured through SubscriberBoundDescriptor.

---

Descriptor

var descriptor = new SubscriberBoundDescriptor
{
    PrefetchCount = 10,

    Queue = new QueueDescriptor
    {
        Name = "orders.created",
        Durable = true
    },

    Exchange = new ExchangeDescriptor
    {
        Name = "orders",
        Type = ExchangeTypes.Direct,
        Durable = true
    },

    Binder = new BinderDescriptor
    {
        RoutingKey = "orders.created"
    }
};

---

Runtime Behavior

graph LR
    RabbitMQ -->|"message 1"| Subscriber
    RabbitMQ -->|"message 2"| Subscriber
    RabbitMQ -->|"message N"| Subscriber

RabbitMQ will not deliver more than the configured number of unacknowledged messages to the subscriber.

---

Low Prefetch Count

A low value is useful when:

• message processing is expensive;
• each message uses significant memory;
• processing time is unpredictable;
• strict workload distribution is required.

Example:

PrefetchCount = 1

---

Higher Prefetch Count

A higher value is useful when:

• messages are small;
• handlers are fast;
• throughput is more important than fairness;
• processing is mostly I/O bound.

Example:

PrefetchCount = 50

---

Guidance

Start with a conservative value and tune based on:

• processing latency;
• message size;
• handler concurrency;
• memory usage;
• consumer count.