What is SQS | SQS Queue Types

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What is SQS?

• SQS stands for Simple Queue Service.
• SQS was the first service available in AWS.
• 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.
• Amazon SQS is a distributed queue
  system that enables web service
  applications to quickly and reliably queue
  messages that one component in the
  application generates to be consumed by
  another component where a queue is a temporary
  repository for messages that are awaiting
  processing.
• With the help of SQS, you can send, store
  and receive messages between software
  components at any volume without losing messages.
• Using Amazon sqs, you can separate the components
  of an application so that they can run
  independently, easing message management
  between components.
• Any component of a distributed application
  can store the messages in the queue.
• Messages can contain up to 256 KB of text in any
  format such as json, xml, etc.
• Any component of an application
  can later retrieve the messages
  programmatically using the Amazon SQS API.
• The queue acts as a buffer between the
  component producing and saving data, and the
  component receives the data for processing. This
  means that the queue resolves issues that arise
  if the producer is producing work faster than the
  consumer can process it, or if the
  producer or consumer is only
  intermittently connected to the network.
• If you got two EC2 instances which are
  pulling the SQS Queue. You can configure
  the autoscaling group if a number of
  messages go over a certain limit. Suppose
  the number of messages exceeds 10,
  then you can add additional EC2 instance
  to process the job faster. In this way,
  SQS provides elasticity.
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Let's look at a website that generates a Meme.
Suppose the user wants to upload a photo and
wants to convert into Meme. User uploads a photo
on a website and website might
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store a photo in s3. As soon as it finished
uploads, it triggers a Lambda function. Lambda
analyzes the data about this particular image to
SQS, and this data can be "what the top of the
meme should say", "what the bottom of the meme
should say", the location of the S3 bucket, etc.
The data sits inside the SQS as a message. An EC2
instance looks at the message and performs its
job. An EC2 instance creates a Meme and stores it
in S3 bucket. Once the EC2 instance completed its
job, it moves back to the SQS. The best thing is
that if you lose your EC2 instance, then also you
would not lose the job as the job sits inside the
S3 bucket. Suppose the user wants to look for a
package holiday and wants to look at the best
possible flight. AUser types a query in a
browser, it then hits the EC2 instance. An EC2
instance looks "What the user is looking for?",
it then puts the message in a queue to the SQS.
An EC2 instance pulls queue. An EC2 instance
continuously pulling the queue and looking for
the jobs to do. Once it gets the job, it then
processes it. It interrogates the Airline service
to get all the best possible flights. It sends
the result to the web server, and the web server
sends back the result to the user. A User then
selects the best flight according to his or her
budget. If we didn't have SQS, then what
happened? A web server passes the
information to an application server and
then application server queried an Airline
service. If an Application server crashes, then a
user loses its query. One of the great thing
about SQS is that data is queued in the SQS even
if the application server crashes, the
message in the queue is marked as an
invisible in a timeout interval window. When
the timeout runs out, message reappears in
the queue then a new EC2 instance can use this
message to perform its job. Therefore, we can say
that SQS removes the application server
dependency. Queue Types

• There are two types of Queue
• Standard Queues (default)
• FIFO Queues (First-In-First-Out)
• Standard Queue

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• SQS offers a standard queue as the default queue
  type.
• It allows you to have an unlimited number of
  transactions per second.
• It guarantees that a message is delivered
  at least once. However, sometime, more than
  one copy of a message might be delivered out of
  order.
• It provides best-effort ordering which ensures
  that messages are generally delivered in the same
  order as they are sent but it does not provide a
  guarantee.
• FIFO Queue

• The FIFO Queue complements the standard Queue.
• It guarantees ordering, i.e., the order in
  which they are sent is also received in
  the same order.
• The most important features of a queue are FIFO
  Queue and exactly-once processing, i.e., a
  message is delivered once and remains available
  until consumer processes and deletes it.
• FIFO Queue does not allow duplicates to be
  introduced into the Queue.
• It also supports message groups that allow
  multiple ordered message groups within a single
  Queue.
• FIFO Queues are limited to 300 transactions per
  second but have all the capabilities of standard
  queues.
• SQS Visibility Timeout
• The visibility timeout is the amount of time that
  the message is invisible in the SQS Queue after a
  reader picks up that message.
• If the provided job is processed before the
  visibility time out expires, the message will
  then be deleted from the Queue. If the
  job is not processed within that time, the
  message will become visible again and another
  reader will process it. This could result in the
  same message being delivered twice.

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• The Default Visibility Timeout is 30 seconds.
• Visibility Timeout can be increased if your task
  takes more than 30 seconds.
• The maximum Visibility Timeout is 12 hours.
• Important points to remember
• SQS is pull-based, not push-based.
• Messages are 256 KB in size.
• Messages are kept in a queue from 1 minute to 14
  days.
• The default retention period is 4 days.
• It guarantees that your messages will be
  processed at least once.