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September 17, 2010

Amazon Web Services Part 2 – Scaling Services Provided

by AlanSBPerkins

Here is the second of three posts on Amazon’s Web Services. The first post provided a look at the key foundational services. The next post will talk about how Altium is leveraging Amazon’s offerings. Meanwhile, in this post I promised to write about some of the ways Amazon takes elasticity to the extreme through a range of services aimed at addressing scalability as an issue.

At its most fundamental level, Amazon’s Web Services are aimed at elasticity – this is reflected in the technology as well as the pricing. The pricing is a pay as you go model – by the hour, by the storage used and/or by the bandwidth consumed. Many of the services reflect this elasticity in their name.

So what tools does Amazon provide you to scale up and down? (Remember scalability isn’t just about being able to scale up – it is about being able to scale in either direction – start small and go big, start big and go small, or start big, grow then shrink again. Or vice versa.

The Elastic Compute Cloud (EC2)  enables you to create a machine image that you can turn on or off whenever you need it. This can be done via an API or via a management console. So a program can turn on a computer if it needs it.

EC2 also allows you to define parameters on a computer so that it effectively clones itself or kills off clones based on the demand. For example if a certain number of requests threshold is hit or the time taken to respond to requests, then a new machine instance can start up automatically. And once it is no longer needed, it can just shut down again.

Imagine a business that provides sports statistics to a subscriber base of sporting tragics. Normally the demand requires a four servers to meet the demand the company’s subscriber base. But once every four years, come Olympics, everybody wants to be a sporting expert and so perhaps 400 servers are required for a period of about six weeks. Amazon’s Auto Scaling feature allows for this.

This is just the simple stuff – there are sophisiticated models provided including Elastic Load Balancing, Elastic Map Reduce, Simple Queuing Services and Simple Notification Services. Let’s take a look at these:

  • Elastic Load Balancing allows you to automatically distribute incoming traffic across multiple EC2 instances. It automatically detects failed machines and bypasses them so that no requests are being sent to oblivion. The load balancer can handle things such as automatically assuring specific user sessions remain on one instance.
  • Elastic Map Reduce allows for large compute assignments to be split up into small units so the work can be shared across multiple EC2 instances, offering potentially massive parallelism. If there is a task to handle huge amounts of data for analysis, simulation or artificial intelligence, Elastic Map Reduce can manage the splitting and the pulling back together of the project components. Any failed tasks are rerun, failed instances are automatically shut down.
  • Simple Queue Service (SQS) provides a means for hosting messages travelling between computers. This is an important part of any significant scalable architecture. Messages are posted by one computer without thought for, or knowledge of, what machine is going to pick up the information and process it. Once received, the message is locked to prevent any other computer from trying to read the message, so it is guaranteed that only one computer will process it. Messages can remain in an unread state between machines for up to 14 days. Queues can be shared or kept private and they can be restricted by IP or time. This means that the systems can designed with separate components where each component is loosely coupled from each other component – even different vendors can be responsible for each component. Different systems can work together in a safe and reliable way.
  • Simple Notification Service (SNS). Whereas SQS is asynchronous, ie each post to the queue can be done without thought about when it will be picked up by the other other end, SNS is designed to be handled at the other end immediately. Messages are pushed out to be handled using HTTP requests, emails or other protocols, which means it can be used to build instantaneous feedback communities using a range of different architectures. So long as they support standard web integration architectures, the systems will talk to each other.  SNS can be used to handle automatic notifications by SMS or Email or API calls when some event has taken place, or if written correctly, when some expected event has not taken place.

I will endeavour to provide example applications over time for each of these scenarios, but for now I hope this has provided a sense of understanding of how highly scalable systems can be built using Amazon’s Web Services.

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