Introduction

This project, whose starting point was the flask-redis repository, is used as an educational material to teach and demonstrate in practice the following topics:

• The main traits and benefits of a fault-tolerant application.
• The advantages of using containers to ship products.
• The benefits of using an AI pair programming technique with GitHub Copilot via IDE.
• Introduction to the Circuit Breaker pattern.
• Basics of the following technologies:
  • Flask based RESTful service
  • Redis database
  • Docker ecosystem
• How to utilize GitHub Dependabot to receive notifications and pull requests regarding version updates and vulnerabilities.
• How to create an effective visualization of actions inside a terminal window using the Terminalizer tool.

Setup

You must create the .env file (inside the root project folder) having the following structure:

NAME=<insert any text here>

Demostration of a Fault-Tolerant Behavior

You should run the commands below from the root folder of this project.

Step	Description
docker compose up -d	Start up the application stack in the background.
docker compose ps	List running containers of this stack.
docker compose exec web id	Show that the web service is running under a non-root user.
docker compose logs	Show generated logs inside running containers (option -f may be used to follow the logs).
docker compose port web 8080	Display the port on host that is mapped to port 8080 inside a web container.
curl localhost:"port on host"	Execute couple of times this command on host to see the application in action.
docker compose pause redis	Pause the database to simulate a connectivity problem.
curl localhost:"port on host"	Execute couple of times this command on host to see fault-tolerance in action. Observe that the first attempt takes a bit longer to run, since the circuit breaker waits for a timeout to happen on a socket. Subsequent requests immediately fail. Occasionally the circuit will try to close again and that also demands waiting for a timeout.
docker compose unpause redis	Unpause the database to simulate that everything is OK again.
curl localhost:"port on host"	Execute couple of times this command on host to verify that everything fully works.
docker compose down	Stop the application stack.

Hint: Click on the image to watch it enlarged.

Monitoring Redis Keys

Open a new terminal window positioned in the same project folder and run docker compose exec redis redis-cli. Execute the monitor command and watch what happens inside the database.

127.0.0.1:6379> monitor
OK
1702666552.931740 [0 172.28.0.3:55674] "HELLO" "3"
1702666552.932190 [0 172.28.0.3:55674] "CLIENT" "SETINFO" "LIB-NAME" "redis-py"
1702666552.932670 [0 172.28.0.3:55674] "CLIENT" "SETINFO" "LIB-VER" "5.0.1"
1702666552.933153 [0 172.28.0.3:55674] "INCRBY" "visits" "1"
1702666554.167246 [0 172.28.0.3:55674] "INCRBY" "visits" "1"

Conclusion

A distibuted system must incorporate some form of fault-tolerance, since many things may go wrong during its operation. This project sheds light on the essential idea pertaining to tolerating fault by providing a limited functionality instead of completely failing with an error message. There are numerous patterns available for building resilient applications, circuit breaker being one of them, which was presented here.