# Message Correlation


Correlation is the process of finding saga instances based on data in the incoming message. For example, an `OrderId ` property of a `CompleteOrder` message can be used to find the existing saga instance for that order.

> [!NOTE]
> The correlation property may be case-sensitive, depending on the persistence and datastore settings.

To declare this, use the `ConfigureHowToFindSaga` method and use the `Mapper` to specify which saga property each message maps to.

> [!NOTE]
> All correlated properties must have a non-null value when the saga instance is persisted.
>
> Changing the value of correlated properties for existing instances is **not** supported.


<!-- snippet: saga-find-by-property -->

```cs
protected override void ConfigureHowToFindSaga(SagaPropertyMapper<MySagaData> mapper)
{
    mapper.MapSaga(sagaData => sagaData.SomeId)
        .ToMessage<MyMessage>(message => message.SomeId);
}
```

<!-- endsnippet -->

When an instance of `MyMessage` arrives, NServiceBus asks the saga persistence infrastructure to find an object of the type `MySagaData` that has a property `SomeId` whose value is the same as the `SomeId` property of the message. If found, the saga instance is loaded and the `Handle` method for the `MyMessage` message is invoked. Should the saga instance not be found, the saga is not started and the [saga not found](saga-not-found.md) handlers will be invoked.



## Message property expression

If correlating on more than one saga property is necessary or matched properties are of different types use a [custom saga finder](#custom-saga-finder).

It is possible to specify the mapping to the message using expressions if the correlation information is split between multiple fields.

<!-- snippet: saga-find-by-expression -->

```cs
protected override void ConfigureHowToFindSaga(SagaPropertyMapper<MySagaData> mapper)
{
    mapper.MapSaga(sagaData => sagaData.SomeId)
        .ToMessage<MyMessage>(message => $"{message.Part1}_{message.Part2}");
}
```

<!-- endsnippet -->

## Message header correlation

Sagas can be correlated to messages using a message header instead of a message property.

<!-- snippet: saga-find-by-message-header -->

```cs
protected override void ConfigureHowToFindSaga(SagaPropertyMapper<MySagaData> mapper)
{
    mapper.MapSaga(saga => saga.SomeId)
        .ToMessageHeader<MyMessage>("HeaderName");
}
```

<!-- endsnippet -->

> [!NOTE]
> An exception is thrown if an incoming message does not contain a header with the configured key, or the value cannot be converted into the saga correlation property type.

## Auto-correlation

A common usage of sagas is to have them send out a request message to get some work done and receive a response message back when the work is complete. To make this easier NServiceBus automatically correlates those response messages back to the correct saga instance.

> [!NOTE]
> If it's not clear if the message can be auto-correlated, it's better to provide the mappings. In cases where the message is auto-correlated, the mappings are ignored.

> [!NOTE]
> A limitation of this feature is that it doesn't support auto-correlation between sagas. If the request is handled by another saga, relevant message properties must be added and mapped to the requesting saga using the syntax described above.

## Custom saga finder

Full control over how a message is correlated can be achieved by creating a custom [saga finder](/nservicebus/sagas/saga-finding.md).

## Uniqueness

NServiceBus makes sure that all properties used for correlation are unique across all instances of the given saga type. How this is enforced is up to each persister but will most likely translate to a unique key constraint in the database.

Mapping a single message to multiple saga instances is not supported. This can be simulated by using a message handler that looks up all saga instances affected and sends a separate message targeting each of those instances using the regular correlation described above.