SQS Essentials:

  • SQS provides the ability to have hosted/highly available queues that can be used for messages being sent between severs.

  • This allows for the creation of distributed/decoupled application components.

  • SQS is used to create decoupled application environments.

  • Messages between severs are retrieved through polling.

Two types of polling

  • Long Polling (1-20 seconds):
    • Allows the SQS service to wait until a message is available in a queue before sending a response, and will return all messages from all SQS service.
    • Long polling reduces API requests (over using short polling).

  • Short Polling:

    • SQS samples a subset of servers and returns messages from just those servers.

    • Will not return all possible messages in a poll.

    • Increases API requests (over long polling), which increases costs.

Other important SQS facts:

  • Each message can contain up to 256KB of text (in any format).
  • Amazon SQS offer two different types of queues:
    • Standard Queue: Guarantees delivery of each message at least once BUT DOES NOT guarantee the order (best effort) in which they are delivered to the queue.
    • First-in-first-out (FIFO) Queue: Designed for applications where the order of operations and events are critical, or where duplicates can't be tolerated.
  • SQS is also highly available and redundant - means AWS will back up and replicate the messages across multiple AZs within a region.

SQS Workflow

  • Generally a "worker" instance will "poll" a queue to retrieve waiting messages for processing.
  • Auto Scaling can be applied based off of queue size so that if a component of your application has an increase in demand, the number of worker instances can increase.

Using SQS to Build Decoupled Application Architectures:

Decoupled Architecture:

  • Tightly Coupled System:
    • A system architecture of components that are not just linked together but are also dependent upon each other.
    • if one component fails all components fail.
  • Loosely Coupled/Decoupled Systems:
    • Multiple components that can process information without being connected.
    • Components are not connected - if one fails the rest of the system can continue processing (fault tolerant/highly available).
  • AWS Services that are used for distributed/decoupled system architectures:
    • SWF (Simple Work Flow Service)
    • SQS (Simple Queue Service)

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