# Non-Durable Messaging

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Relaxed message delivery guarantees to improve performance at the cost of reliability.

## Overview

Non-durable messaging trades delivery guarantees for better performance, increasing the risk of message loss during server crashes or restarts.

> [!WARNING]
> This approach significantly increases the risk of message loss when servers crash or restart. See [Effect on Transports](#effect-on-transports) for details.

## Enabling non-durable messaging

This feature can be enabled in two ways


### For specific message types

#### Via an Express attribute

A message can be configured to be non-durable via the use of an `[ExpressAttribute]`.

<!-- snippet: ExpressMessageAttribute -->

```cs
[Express]
public class MessageWithExpress :
    IMessage
{
}
```

<!-- endsnippet -->


#### Using the configuration API

A subset of messages can be configured to be non-durable by using a convention.

<!-- snippet: ExpressMessageConvention -->

```cs
var conventions = endpointConfiguration.Conventions();
conventions.DefiningExpressMessagesAs(
    type =>
    {
        return type.Name.EndsWith("Express");
    });
```

<!-- endsnippet -->


### Global for the endpoint

Allows messages to be non-durable via the configuration API.

<!-- snippet: set-to-non-durable -->

```cs
endpointConfiguration.DisableDurableMessages();
```

<!-- endsnippet -->


## Effect on Transports

Each transport implements non-durable messaging differently based on its underlying technology.

### MSMQ

MSMQ typically uses _Store and Forward_ by default, storing messages durably on disk before delivery. With non-durable messaging, MSMQ sends messages in [Express Mode](https://msdn.microsoft.com/en-us/library/ms704130) by setting [`Message.Recoverable`](https://msdn.microsoft.com/en-us/library/system.messaging.message.recoverable) to `false`.

Non-durable messages require [non-transactional queues](https://msdn.microsoft.com/en-us/library/ms704006). Configure this by setting `useTransactionalQueues` to `false` in the [transport connection string](/transports/msmq/connection-strings.md).

> [!NOTE]
> When using non-transactional queues, endpoints must be [configured not to use transactions](/transports/transactions.md#transaction-modes-unreliable-transactions-disabled).

### RabbitMQ

Non-durable messages are sent using RabbitMQ's non-persistent delivery mode, which means the messages are not persisted to disk by the broker.

See [controlling delivery mode](/transports/rabbitmq/index.md#controlling-delivery-mode) in the RabbitMQ transport documentation.


### SQL Server

The SQL Server transport has no support for this setting and it is ignored.

### Azure Service Bus

The Azure Service Bus transport has no support for this setting and it is ignored.

### Azure Storage Queues

The Azure Storage Queues transport has no support for this setting and it is ignored.

### Amazon SQS

The Amazon SQS transport has no support for this setting and it is ignored.

### IBM MQ

The IBM MQ transports supports non-durable messaging with all supported transaction modes.