I got an SQS FIFO queue, I want to know if there is a way to trigger an AWS lambda once the queue is not empty. For example, if my queue is empty and a new message enters trigger lambda, but if the queue is already containing at least one message and a new message enters no lambda will be triggered. Is it possible?
There is an Amazon CloudWatch metric called ApproximateNumberOfMessagesVisible that shows the number of messages in the queue. The documentation says that "For FIFO queues, the result is exact."
You could create a CloudWatch Alarm that triggers when the number of messages drops to zero for a period of time. The Alarm can send a message to an Amazon SNS topic. If you subscribe your AWS Lambda function to this topic, it will be triggered when the queue is empty for the specified duration (eg over a period of 5 minutes). It will only be triggered when the alarm enters the 'Alarm' state and it will not trigger again until the alarm exits the state and enters the state again.
Important: When configuring the alarm, go to the Additional configuration and set Missing data treatment to "Treat missing data as bad (breaching threshold)". This is required because the SQS queue will not send metrics if the queue is empty. (Many queues are empty, so this saves a lot of metric storage!)