Web Applications

Question 1

Client tier

Answer 1

Client tier is composed by:
* Web clients: HTML + Javascript
* Mobile apps: Android, iOS, etc

Question 2

Application tier

Answer 2

Application tier is composed by:
* REST servers
* Static and dynamic web pages
* Popular application servers
* E.g.: Tomcat, Wildfly, ASP.NET, etc

Question 3

Data tier

Answer 3

Data tier is composed by:
* Filesystems, BLOB stores, key-value stores, SQL databases, etc.
* Other services: caches, message queue, etc.

Question 4

Why multiple instances of web apps?

Answer 4

1. A single server might not be sufficient to handle all client
   requests;
2. A single server is a single point of failure: if it fails the
   service becomes unavailable;
3. A single server may be too far from some clients: latency
   too high.

Insuffient, failure, latency

Question 5

What is simplicfied by having multiple instances?

Answer 5

No internal state in server (state is maintained in the data tier)

Question 6

App service

Answer 6

handles HTTP/REST requests.

Question 7

database/storage

Answer 7

durability

Question 8

cache

Answer 8

improved performance in the servers

Question 9

CDN

Answer 9

reducing load time and save bandwidth

Question 10

How to avoid downtime in deploying new versions? (Staging)

Answer 10

Basic approach:
1. Keep the old version running;
2. Deploy the new version to a staging slot (using Azure
terminology) – instances are created but requests from
clients keep being processed by the old version;
3. (Optionally) warm-up application;
4. Hotswap from the old to the new version, by start
forwarding client requests to the new version.