- Elastic Load Balancing: automatically distributes incoming application traffic across multiple Amazon EC2 instances.
- Auto Scaling: automatically scale your Amazon EC2 capacity up or down according to conditions you define.
- CloudWatch: is a web service that provides monitoring for AWS cloud resources. Auto Scaling is enabled by CloudWatch.
Let's say you have a tremendous business idea like selling binoculars on the web. You wouldn't believe it (as I also didn't) but one of my college told me a story about his friend making a living by selling telescopes on the web. So you developed a simple web application, it's tested on your laptop, and you want to go public. First you don't want to invest too much into hardware and licenses so you just create a Amazon Web Services account, and start up just a small instance with jetty or Tomcat serving, for a reasonable 0.10 usd/hour fee. This is the way that AWS is selling like hot cakes: no upfront costs / pay as you go.
After a couple of weeks you realize, that binoculars sell like hot cakes, and your lonely instance can't serve all the request. You have 2 choices: either change to a bigger instance (a large instance costs 0.40 usd/hour), or you start up a second or third small instance. Let's say you went on the second way (a large instance will be saturated soon, so you will need more instances anyway) and you face a couple of problems:
- You need to distribute the load between the web servers.
- In the peak hours you need 3 instances while during the night one server is sufficient to handle the request, you don't want babysit the application and start up and shut down instances regarding the load.
To distribute the load between several web servers, you'll need some sort of load balancer. Hardware load balancers are out of scope as they are quite expensive, and anyway, you decided to use Amazon's virtual environment. You could use round robin DNS (setting multiple ip address for the same DNS name), but then it gets tricky when you scale up or down: you have to refresh the DNS entries (A-records) and you have to choose a reasonable TTL (time to live) value which influences how quick your changes will propagate on the net.
Most probably you would go with the software load balancing approach, and you end up choosing Apache with mod_proxy_balancer. Then you face another decision: if you co-locate Apache with your Java web server, then you increase the load on the web server, you have the same problem with maintaining changing numbers, or changing ip (in case of a restart of apache) in the DNS entry. Or if you use dedicated instances for Apache, you almost double the costs: you pay 2-3 x 10 cents hourly for the web server instances, and 2x10 cents for Apaches (if you want to eliminate a single point of failure)
This is where you can introduce Elastic Load Balancing which costs just 2.5 cents/hour. (When I'm talking about costs, it's just a rough estimation, as I calculate only the box usage, and not the network traffic, but let's say the network traffic is about the same for the different scenarios I have described above.)
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