Welcome to the SLB or Server Load Balancer portion of the course. We'll get started by introducing some basic server load balancer concepts. So when you first start building a web service or web application, you have a relatively small number of users. And at that point it's okay to put your site or application on a single server. You might not want to do that for liability reasons, but let's just start our example by assuming that your site is deployed on a single server, of course, eventually, if your web service becomes popular, the number of users grows and then you need to start splitting your service up across multiple servers.

But the problem then becomes how do you allow your users to seamlessly access these multiple servers? You can't give each of them a different URL. You can't give each of them a different domain name and then expect your users to remember that. That's too much work. That also doesn't help you deal with failure. What if you did give each machine a separate domain name and then one of them failed, while the users who are accustomed to using that server, wouldn't be able to reach it anymore and might not know how to reach the other servers.

So you need to have some kind of an interface in front of these multiple backend servers. That is what server load balancer is. It's a machine that sits in front of your backend servers and distributes incoming requests evenly across them. That's what a load balancer does. And that's what Alibaba cloud's server load balancer does on the Ali Baba cloud platform. So server load balancer is a traffic distribution control service. That's what it is, that distributes incoming traffic among multiple ECS instances according to a set of forwarding rules that you configure. It has built in high availability. It has 99.95% service uptime across multiple zones. It's scalable.

So you can scale your services without interrupting service continuity because the server load balancer is sitting in front of your backend servers, if you need to replace or upgrade some of them, server load balancer can route traffic away from those machines that are being upgraded or replaced. Therefore you can avoid downtime. It's low cost compared to building your own load balancer. There's a price reduction of around 60% and it has built in security. So of course like our other Alibaba cloud products, it defends against DDoS attacks of up to 5 Gigabits per second. Like ECS OSS on our other core services, SLB is available across all 22 regions and 66 zones that Alibaba cloud has worldwide. So no matter what region you choose, you will be able to deploy SLB. Next, we'll take a look at some of SLBs key features.