An AWS SQS FIFO queue has a batch setting of 1 and a delay of 1 second. Every item received is associated with a MessageGroup.
All at once the queue receives 30 messages across 10 different message groups with each message group accounting for 3 messages...
Will the delay of one second apply at the queue level i.e. the 30 messages will take an elapsed time of 30 seconds to deliver?
Or will the queue spin up 10 consumers, one for each message group, emptying the queue in 3 seconds?
Will the delay of one second apply at the queue level i.e. the 30 messages will take an elapsed time of 30 seconds to deliver?
For FIFO, the delay is applied at queue level:
FIFO queues don't support per-message delays, only per-queue delays. If your application sets the same value of the DelaySeconds parameter on each message, you must modify your application to remove the per-message delay and set DelaySeconds on the entire queue instead.
Number of comsumer lambdas working in parallel does not need to be 10 as written bellow:
In SQS FIFO queues, using more than one MessageGroupId enables Lambda to scale up and process more items in the queue using a greater concurrency limit. Total concurrency is equal to or less than the number of unique MessageGroupIds in the SQS FIFO queue.
Thus you can have still fewer consumer lambdas than groups. But in ideal situation you would have 10 lambdas working in parallel. How it works with lambda is explained in the following AWS blog post:
