SQS

Question 1

What does SQS stand for?

Answer 1

Simple Queue Service

Question 2

What does AWS SQS do?

Answer 2

SQS is a pull-based, distributed queue system forproducer/consumer-typesystems

• _{some applications add requests to the queue, other applications poll the queue for requests and execute them}
• _{Amazon SQS is a web service that gives you access to a message queue that can be used to store messages while waiting for a computer to process them.​}

Question 3

In the context of SQS, what is a queue?

Answer 3

A temporary repository of messages that are awaiting processing

Question 4

On the exam, when you see the word decouple, what should you immediately think about? Why?

Answer 4

SQS

SQS is used to decouple components of an application and ease message management btween them.

(SQS has been using terminology with decouple for a long time)

Question 5

What was the very first service offered by AWS?

Answer 5

SQS

Question 6

What is the maximum size of an SQS message?

Answer 6

256kb BUT you can go over this by storing payloads in S3

Question 7

What are the two types of SQS queues?

Answer 7

• Standard
• FIFO

Question 8

What does TPS stand for?

Answer 8

Transactions Per Second

Question 9

What is the TPS limit on an SQS standard queue?

Answer 9

nearly unlimited transactions per second

Question 10

What are the main drawbacks of standard queues vs. FIFO queues?

Answer 10

Occassionally, becuase of the highly-distributed nature architechture that gives the high throughput:

• (duplicates) more than one copy of a message might be delivered
• (best-effort ordering vs. exactly-once processing) messages might be delivered slightly out of order, but are generally delivered in the same order in which they are sent

(With FIFO queues, you don’t get duplicates, and you get exactly-once processing and ordering is strictly preserved)

Question 11

What is the TPS limit on FIFO queues?

Answer 11

300 TPS

Question 12

How long can messages be kept in SQS?

Answer 12

• Configurable from 1 minute to 14 days.
• Default value is 4 days

Question 13

What are message groups?

Answer 13

• Messages are grouped into distinct, ordered “bundles” within a FIFO queue.
• For each message group ID, all messages are sent and received in strict order. However, messages with different message group ID values might be sent and received out of order.
• Every message must have a message group ID

_{(Source: https://aws.amazon.com/sqs/faqs/)}

Question 14

Can you have multiple producers for an SQS FIFO queue?

Answer 14

Yes. Messages are stored in the order that they were successfully received by Amazon SQS.

_{(Source: https://aws.amazon.com/sqs/faqs/)}

Question 15

Can you have multiple consumers for an SQS FIFO queue?

Answer 15

Yes, but you only get a speedup if you are using multiple message groups.

_{(See: https://aws.amazon.com/sqs/faqs/)}

Question 16

What is visibility timeout?

Answer 16

In SQS, Visibility Timeout is the amount of time that a message is invisible in the SQS queue after a reader picks up the message.

• If the job is processed within that time, it’s deleted from the queue
• If not, the message becomes visible again and another reader can process it

Question 17

When using SQS, a message that you only meant to have processed once seems to have been processed twice! What might have happened? How might we be able to remediate?

Answer 17

• You are using a standard queue, which means you could have duplicates. To remediate: Upgrade to a FIFO queue

OR

• The visibility timeout of your message is too small, and it became visible again before the first reader was able to finish processing. To remediate: raise the visibility timeout

Question 18

What are the max/min/default values for visibility timeout?

Answer 18

• Max: 12 hours
• Min: 0 seconds
• Default: 30 seconds

_{(Source: https://docs.aws.amazon.com/AWSSimpleQueueService/latest/SQSDeveloperGuide/sqs-visibility-timeout.html)}

Question 19

In general, does SQS guarantee that your message will be processed exactly once?

Answer 19

No. They guarantee that your message will be processed at least once

Question 20

What is the difference between SQS Short Polling and Long Polling?

Answer 20

The key difference is in how they handle the empty queue case

• Short Polling returns immediately
• Long Polling waits until a message arrives or it times out

Question 21

Does SQS automatically manage application-level tracking?

Answer 21

No, you need to set this up yourself

Question 22

What does the “DelaySeconds” attribute in SQS control?

Answer 22

When a new message is added to the SQS queue, it will be hidden from consumer instances for a fixed period.

Question 23

In the context of SQS, what is horizontal scaling?

Answer 23

Horizontal scaling involves increasing the number of message producers and consumers in order to increase your overall queue throughput

Question 24

You have an SQS FIFO Queue with multiple message groups and batch retrieval. Can one poll from one consumer get multiple messages from different message groups?

Answer 24

Yes