Cloud basics

Few interviewers ask "what is the cloud?" on its own. But most designs run on rented machines, so they often ask "where does this run, and what does it cost?"

A weak answer says "I will use the cloud" and stops. A strong answer names the part it needs, such as a rented server or a store for photos, and says why.

Think of a cab app. If you travel a few times a month, booking a cab is cheaper than buying a car. You pay only for each ride. If you drive all day, every day, owning a car may be cheaper.

The cloud works the same way. A cloud provider owns huge buildings full of computers, called data centres. You rent what you need over the internet and pay for what you use, like the cab.

You rent three kinds of building blocks:

1. Compute: a virtual machine (VM), which is a rented computer. It is usually a slice of a big physical computer. You choose its CPU and memory, and pay by the hour or less. In this lesson, a server is one such rented computer.
2. Storage: a disk attached to your VM, and a file store for files like photos and PDFs. There is also the managed database: the provider runs the database for you, with backups and updates.
3. Network: a private network that connects your machines. You choose which of them the internet can reach. You also choose a region: the area where your machines live. Pick one near your users.

There are three levels of help. With a VM (infrastructure as a service, IaaS), you manage the operating system. With a managed service (platform as a service, PaaS), the provider runs it for you. With finished software (software as a service, SaaS), such as web email, you just use it.

The three big providers, Amazon Web Services (AWS), Microsoft Azure and Google Cloud, use different names:

Assumptions (made up to teach with; this is not a real price):

• Renting one server costs Rs 10 per hour.
• A year has 365 days x 24 hours = 8,760 hours.
• Results day lasts 24 hours.

Option 1: 11 servers all year.

• Server-hours = 11 x 8,760 = 96,360
• Cost = 96,360 x Rs 10 = Rs 963,600 (about 9.6 lakh)

Option 2: 1 server all year, plus 10 extra servers for one day.

• All-year server = 1 x 8,760 = 8,760 server-hours
• Extra servers = 10 x 24 = 240 server-hours
• Total = 8,760 + 240 = 9,000 server-hours
• Cost = 9,000 x Rs 10 = Rs 90,000

Answer to the predict box: 963,600 / 90,000 = 10.7. Option 2 is about 11 times cheaper. Option 1 pays for 10 servers that do no work for 364 days.

• It solves: buying and repairing machines, and paying for unused machines on quiet days.
• It costs: steady, all-day use can cost more than owning, as with the car. You also depend on the provider. If you use one region and it has a problem, your system has one too.
• The bill has many parts. Disks and data sent out to the internet are often charged separately.
• I would change it when usage is high and steady all year. Then compare the cloud bill with the cost of owning servers.

"The cloud is always cheaper." Only for uneven use. In the example, renting won because the extra servers ran one day a year. A server busy all day, every day, is the car you drive daily.

"On the cloud, the provider takes care of everything." With a VM, you still update the operating system and install your software. A managed database does more for you, but you still choose its size, region and who may connect.

1. Name the three building blocks you rent from a cloud provider, with one example of each.
2. A shopping app normally needs 2 servers. During a 3-day sale it needs 20. A server costs Rs 10 per hour (assumption). What do the extra servers cost for the sale?
3. A teammate says, "The cloud is always the cheapest way to run our 50 servers." The servers are busy all day, every day. What would you ask?