When using AWS Lambda with a SQS queue (as event source), it is written in the doc
If messages are still available, Lambda increases the number of processes that are reading batches by up to 60 more instances per minute.
CodePudding user response：
One of the calls to the SQS API is to get queue attributes (Java API, others similar). This returns a response and one of the attributes of the response is "approximate number of messages". With this you or AWS can determine about how many messages are in the queue.
From this, AWS can determine if it's worth spinning up additional instances. You too can get this information from the queue.
CodePudding user response：
I imagine it uses the
ApproximateNumberOfMessagesVisiblemetric on the SQS queue to check how many messages are available, and uses that number, plus your batch size configuration, to determine how many more Lambda instances your function needs to be scaled out to.CodePudding user response：
I believe the documentation refers to Lambda polling the queue to know whether there are still messages. Read more about it here.
Lambda polls the queue and invokes your Lambda function synchronously with an event that contains queue messages. Lambda reads messages in batches and invokes your function once for each batch. When your function successfully processes a batch, Lambda deletes its messages from the queue.
Event Source Mapping:
Lambda only sees messages that are visible, via the visibility timeout setting on the SQS queue. This is to prevent other queue consumers processing the message. I believe as an event-source, Lambda receives messages from the SQS queue, via being mapped to it.
CodePudding user response：
As per the documentation you shared,for standard queues, Long Polling is in effect. Long polling basically waits for a certain amount of time to verify if there is a message in the queue. refer to the following docs :
