Architecture

The entire premise of the project is the ability to handle crashes at any time, making it a durable database. The idea I implemented was a simple logical Write Ahead Log, that logged each request action into a byte file. Requests had to be created into a specific file format, and handed off to a controller.

The entire architecture was revolved around the handling of requests, which became the "Front-end" of the application.

I followed a MVC pattern for this assignment.

The views is the user interface, and the actions that come along with it, such as viewing a specific city, or deleting a city.

The controller is what modifies the data.

The models are stored as classes and located inside a datarepository.

Each request from the view to the controller goes through a router. This follows the mediator design pattern, where the frontend does not need to understand the inner workings of the backend, such as "Backends for Frontends".

Services are usuable code that is throughout the application, creating greater seperation of business logic that is not directly related to the models.

Write-Ahead Log

This project uses a write-ahead log (WAL) to ensure data durability. All state-changing operations (CREATE_CITY, UPDATE_CITY, DELETE_CITY) are logged to a binary file before being applied to the data models. On startup, the application replays all logged requests to reconstruct the previous state. This guarantees no data loss even if the application crashes unexpectedly.

Running the code:

make; ./exec.exe

Usage Guide

After running the application, you'll see a db > prompt. Enter commands to interact with the city database.

Available Commands

add <city_name>

• Creates a new city. You'll be prompted to enter city history, population, year of establishment, coordinates, and mayor information.
• Example: add London

update <city_name>

• Modifies an existing city's information including history, population, year, coordinates, and mayor details.
• Example: update London

delete <city_name>

• Removes a city and all its associated data from the database.
• Example: delete London

display city <city_name>

• Shows all details for a specific city.
• Example: display city London

display cities

• Shows all cities in the database.
• Example: display cities

display mayor <city_name>

• Shows the mayor information for a specific city.
• Example: display mayor London

distance <city_name_1> <city_name_2>

• Calculates the geographic distance between two cities based on their coordinates.
• Example: distance London Paris

history <city_name>

• Displays the history of a specific city.
• Example: history London

population <city_name>

• Displays the population of a specific city.
• Example: population London

year <city_name>

• Displays the year a city was established.
• Example: year London

coordinates <city_name>

• Displays the latitude and longitude coordinates of a city.
• Example: coordinates London

sorted cities

• Displays all cities in sorted order.
• Example: sorted cities

seed

• Generates sample city data for testing. Useful for populating the database quickly.
• Example: seed 5

save

• Manually saves all pending changes to the write-ahead log.
• Example: save

help

• Shows all available commands.
• Example: help

exit

• Exits the application and saves all data.
• Example: exit