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We are using Lambda subscribed to SNS to process the messages. Lambda has a safety limit of 100 concurrent invocations at any time. So what happens to the throttled events ? Are they rejected or they are added to the queue to process them later ?
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At the highest level, throttling just means that Lambda will intentionally reject one of your requests and so what we see from the user side is that when making a client call, Lambda will throw a throttling exception, which you need to handle. Typically, people handle this by backing off for some time and retrying.
If you're doing some sort of long running processing then your other option may be to run this task on an EC2 instance. If this long running process can be broken down in to multiple steps then you could look in to Lambda Step Functions. 15 Minutes is the max and this max can not be extended.
With increased concurrent execution limit, there is still one more limit the Burst Concurrency limit. This will limit lambda to serve only 3000 concurrent request at time. If it receives more than 3000 concurrent requests some of them will be throttled until lambda scales by 500 per minute.
You can increase your concurrent limit by requesting an increase. I would assume that a throttled event would be treated like any failed event. Per the documentation it is retired twice then discarded. You can setup a Dead Letter Queue (DLQ). Failed events will be added to the configured queue and they can be re-picked up for processing. I'm also not sure if you get an error reason or not in the message in the queue so a throttled event may look the same as a failed event (not sure if that would matter to you or not). You can also monitor throttles via CloudWatch, but this would not contain the event information necessary to replay the event.
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