A common requirement is to process messages from a sender in the correct order. This is achieved using Message Groups.
The message group ID is the tag that specifies that a message belongs to a specific message group. Messages that belong to the same message group are always processed one by one, in a strict order relative to the message group (however, messages that belong to different message groups might be processed out of order).
For FIFO queues, there can be a maximum of 20,000 inflight messages (received from a queue by a consumer, but not yet deleted from the queue). If you reach this quota, Amazon SQS returns no error messages. If your queue has a large backlog of 20,000 or more messages with the same message group ID, FIFO queues might be unable to return the messages that have a different message group ID but were sent to the queue at a later time until you successfully consume the messages from the backlog.
A backlog of messages that have the same message group ID might build up because of a consumer that can't successfully process a message. Message processing issues can occur because of an issue with the content of a message or because of a technical issue with the consumer.
To move away messages that can't be processed repeatedly, and to unblock the processing of other messages that have the same message group ID, consider setting up a dead-letter queue policy.