Message Context

When creating a handler for a message, you provide an IHandler<T> implementation that will process the JustSaying message. However, sometimes you want to access the raw SQS message itself, i.e. an Amazon.SQS.Model.Message instance. This can be useful for accessing non-domain concepts, such as message attributes like ApproximateReceiveCount or the message receipt handle.

JustSaying allows you to get access to this through the type MessageContext, which you can access by injecting an IMessageContextAccessor into your handler.

IMessageContextAccessor internally stores the MessageContext for the current message being processed in an AsyncLocal<MessageContext>, so the appropriate instance will be available anywhere in the logical call stack while handling a message.

Example usage:

public class OrderReadyEventHandler : IHandlerAsync<OrderReadyEvent>
{
    private readonly IMessageContextAccessor _contextAccessor;
    private readonly ILogger<OrderReadyEventHandler> _logger;

    public OrderReadyEventHandler(IMessageContextAccessor contextAccessor, ILogger<OrderReadyEventHandler> logger)
    {
        _contextAccessor = contextAccessor;
        _logger = logger;
    }

    public async Task<bool> Handle(OrderReadyEvent message)
    {
        // This is the AWS SDK Message instance.
        var message = _contextAccessor.Context.Message;

        if (message.Attributes.TryGetValue("ApproximateReceiveCount", out var approximateReceiveCount))
        {
            _logger.LogInformation("ApproximateReceiveCount: {ApproximateReceiveCount}", approximateReceiveCount);
        }

        // Do something here
        return true;
    }
}

Attributes

On the MessageContext type, there are the following places to get attributes:

Context.MessageAttributes

Message attributes are user-defined attributes, and the attributes here are retrieved from the message body.

Context.Message.MessageAttributes

Message is the AWS SQS SDK type, the MessageAttributes here are the ones that present in the MessageAttributes XML payload that the message is wrapped in. When using JustSaying, this property will always be empty as we populate Context.MessageAttributes from the message body, so don't request to include any message attributes in the response object.

Context.Message.Attributes

These are the "system attributes" for the message, in JustSaying we request for the ApproximateReceiveCount system attribute to be included, so this should be the only present attribute here, and will always be available.