SQS队列/可见性超时/消息组

Question

I am new to AWS. I am trying to understand SQS here. I have gone over a few trainings also but I still could not get some answers there in the discussion forum. I am re-iterating my question here. Note that I know that a few questions below have obvious answers and are therefore more of a rhetoric. My confusion stems from the fact that my understanding of the topic at present leads me to give conflicting answers to the follow up questions that spring up in my mind after the obvious known ones and takes away the confidence of whatever I think I understand alright.

If I have a Standard queue named MyQueue and there are 100 messages, and if there are 2 completely separate applications (as consumers; note they are not a consumer group of the same applications like you have in Kafka; instead they are 2 separate applications) for this queue, then the consumers may receive

(i) out of order messages and

(ii) multiple copies of the messages

Both of my applications do not need to bother about the order of the messages. But for the sake of the question lets say we have a perfect order of delivery, no multiple copies and no network issues and both consumers finish their processing if each message well within the Visibility Timeout window.

Q1: Will both the applications individually receive 100 messages each or will a message that is made available to one consumer won't ever be delivered to the other consumer? If the latter is true ( with no network issues, out of order delivery, multiple deliveries), then:

1. Is SNS-SQS fanout the way to ensure that the same message is processed by multiple consumers?
2. Is the consumer supposed to delete the message from the queue after processing? So, if a message is picked up by a processor, and it goes into visibility timeout while the processing happens and then is not deleted by the consumer even after the processing is complete before the visibility timeout, then will the message appear back for other consumers possibly to consume it? If that is the case, then won't the same thing apply to a FIFO queue as well?

Other Questions:

Q2: Is the Visibility timeout applicable to both Standard Queue and FIFO Queue? If it is also applicable to FIFO Queue which promises exactly once delivery, then, if the Visibility Timeout appears before the consumer ends processing a message, then it reappears in the queue only to be delivered again thereby going back to at least once processing. Can someone confirm?

Q3: What are multiple message Groups within a FIFO Queue? Are they like partitions of a queue?

Answer 1

Q: Will both the applications individually receive 100 messages each?

A consumer can request up to 10 messages per API call. These will become 'invisible' and will not be provided to other consumers. (Well, there actually is a small possibility that a message might be provided to multiple consumers. It is rare, but it can happen. If this is bad for your use-case, then you should track the messages in a database to ensure they are only processed once each.)

Q: Is SNS-SQS fanout the way to ensure that the same message is processed by multiple consumers?

It is very strange to want to want a single message consumed by 'multiple consumers'. The normal desire is to process each message once. If you do want a message processed by multiple consumers then, yes, you could send the message to SNS, which could then send it to multiple queues.

Q:Is the consumer supposed to delete the message from the queue after processing?

Yes. Amazon SQS does not know when a message is processed. The consumer must delete the message via the ReceiptHandle provided when the message was received. If a message times-out and another consumer receives it, SQS will provide a different ReceiptHandle so it knows which process requested the delete.

This also applies to FIFO queues.

Q: Is the Visibility timeout applicable to both Standard Queue and FIFO Queue?

Yes. If the visibility timeout expires, the message will be provided to another consumer. The "exactly once delivery" avoids the rare situation mentioned above when a message in a Standard queue might be provided more than once. However, if visibility times-out, even in a FIFO queue, then it will intentionally be visible on the queue again.

Q: What are multiple message Groups within a FIFO Queue? Are they like partitions of a queue?

A message group is a way of grouping messages that must be delivered in-order.

Let's say there are two message groups, A and B , and they send messages in this order: A1 , B1 , A2 , B2

Message B1 can be provided even if A1 is not yet deleted. However, message A2 will not be provided until A1 is deleted. Think of them as 'mini-queues'. This allows processing of lots of messages are are unrelated, without having to wait for all previous messages to be deleted.

See: Using the Amazon SQS Message Group ID - Amazon Simple Queue Service

Answer 2

Q1: Will both the applications individually receive 100 messages each or will a message that is made available to one consumer won't ever be delivered to the other consumer?

Neither of these is quite accurate.

Standard queues never intentionally deliver a message more than once. It is possible that messages may occasionally be delivered more than once -- but this is the exception and is an artifact of the fact that SQS is a distributed system and situations could arise where, for example, the queue had a message stored in multiple replicas and the fact that a message was not known to all replicas due to an internal failure.

If a message is inadvertently delivered more than once, it could be to multiple consumers or the same consumer. The consumer "connections" to SQS are actually stateless, resetting each time a list of messages is delivered, so SQS does not have a sense of which consumer it delivered each message to.

Consumers delete their messages after processing, otherwise their visibilitt timeout expires and they are delivered again and again -- to whichever consumer the luck of the draw delivers them to, each time. As noted, SQS has no concept of consumer identity or state. (In high volume applications, a single consumer may actually have multiple connections to SQS, all receiving messages in parallel, because the network round-trips and cycle of receive/delete will otherwise limit a single consumer to a few hundred messages per second. Whether these connections are handled using asynchronous I/O, threads, etc., is unimportant to SQS, which doesn't care which consumer is on a given connection.)

If you want all messages sent to all consumers, you need fan-out from SNS to SQS.

Q2: Is the Visibility timeout applicable to both Standard Queue and FIFO Queue?

Yes. Because (noted above) the connection to SQS is not a persistent, stateful connection, SQS uses visibility timeout as the indication that a consumer has lost the message or failed ungracefully, so the message needs to be made accessible again. (Dead letter queues prevent this from happening endlessly, moving a message to a different queue, since repeated failures indicate a problem with a consumer, or a "poison pill" message.)

FIFO queues retain in-order delivery, here, and you could argue that they revert to "at least once" delivery, but the idea is that this should never happen. If it does, then your visibility timeout is too short or your consumer is crashing or otherwise misplacing messages.

Q3: What are multiple message Groups within a FIFO Queue?

Message groups allow FIFO queues to support in-order, parallel processing of groups messages whose ordering relative to each other across group boundaries doesn't matter. Messages are delivered in order, within each group.

If a FIFO queue, if all messages are sent with the same group ID, then only one consumer can be working at a time.

In-order delivery (simple illustration) means that message 2 will not be delivered to any consumer until message 1 has been received and deleted -- finished -- by a consumer. In order delivery includes all processing (not merely the initial "delivery"). Or if 20 messages in the queue have the same group ID and two consumers request 10 messages each, one consumer gets 10 and the other gets nothing -- yet -- because those second 10 messages have to be sequestered, until the first 10 have been processed (else we are no longer "in order").

In the 20 messages scenario, if 14 were in group A and 6 were in group B, one consumer would receive A1-A10, A11-A14 would be sequestered until A1-A10 were complete, but while the first consumer is busy, another consumer could have B1-B6 at the same time.

Note again that there is no consumer affinity. If A1-A10 and B1-B6 were deleted at the same instant, A11-A14 would next be delivered to one consumer, but not necessarily the one that handled A1-A10.