The project is a distributed system for a library. It allows you to view available books and reserve them. Later system allows you to manage your reservations, you can cancel (update) them. System consists of cassandra cluster where all the data is stored and a cli client script to communicate with the cluster.
When working with databases like cassandra it is a good idea to start with the application and only then design the schema (in contrast to relational DBs where we usually start with a schema). So I began by writting the CLI script using a fake database. While doing that I saw which operations are the most needed in my system and was able to come up with the optimal schema for Cassandra which balances efficiency and redundancy.
---
title: library keyspace
---
classDiagram
direction LR
class books {
id: UUID (PRIMARY KEY)
title: str
author: str
}
class reservations {
id: UUID (PRIMARY KEY)
book_id: UUID
email: str
}
class reservations_by_book {
book_id: UUID (PRIMARY KEY)
id: UUID
email: str
}
I implemented the cassandra database connection by implementing the same interface as in the fake database. Then I needed to populate the DB. I wrote a script to obtain the data from web [fetch_data.py](The database) And the seeding script to setup keyspace, tables and insert the data into the cluster. Cluster is being started by the docker compose file (I am starting 3 nodes).
After writing stress tests utilizing concurrency I ran into the following problem: multiple reservations were created for the same book
364bac5a-b005-478f-af24-899060af980d - Killers of the Flower Moon: The Osage Murders and the Birth of the FBI - reserved by tester
4d427ab1-f87d-43e3-850c-db987a728fde - Killers of the Flower Moon: The Osage Murders and the Birth of the FBI - reserved by tester
522485ec-56e3-4a38-9006-8822914a15e5 - Killers of the Flower Moon: The Osage Murders and the Birth of the FBI - reserved by tester
...
295f31c8-ef7b-4865-804b-bddda400c2eb - Killers of the Flower Moon: The Osage Murders and the Birth of the FBI - reserved by bar
df24ec03-2025-46c2-938b-b25b265faa52 - Killers of the Flower Moon: The Osage Murders and the Birth of the FBI - reserved by bar
ca40d808-e504-4ef7-8c40-00f68e243a38 - Origin: A Novel (Robert Langdon) - reserved by foo
09392399-3200-4758-a7ff-8fee1ec138b9 - The Life-Changing Magic of Tidying Up: The Japanese Art of Decluttering and Organizing - reserved by foo
ab0cd372-0a95-4a8b-846d-d07b351014e9 - The Martian - reserved by bar
d0c1b0b0-f6b0-41ce-b879-78fe458d43cc - The Martian - reserved by foo
...
878da01c-e5d4-4d2b-8b5b-fffd8d5e8ac3 - The 5 Love Languages: The Secret to Love That Lasts - reserved by bar
d255d537-7814-4f87-9d70-1fbabdca6048 - The 5 Love Languages: The Secret to Love That Lasts - reserved by foo
507291a9-0dc6-406f-8e57-9c5a792ec357 - The Hunger Games (Book 1) - reserved by foo
b1ccd76d-c982-4000-9190-d887aaa439d4 - The Hunger Games (Book 1) - reserved by bar
...
I used lightweight transactions to fix this problem.
INSERT INTO library.reservations_by_book (book_id, id, email) VALUES (%s, %s, %s) IF NOT EXISTS
book_id is primary key in this table and this query fails if there is already some record
with this book_id. We can check if it was successfull with property was_applied of the
result of the query. This was enough to fix test 1 and test 2:
This time I only received one row (as expected) the whole test takes around 20 seconds
e7b49fc1-c5fd-4bba-841b-331936ca8c76 - National Geographic Little Kids First Big Book of Why (National Geographic Little Kids First Big Books) - reserved by tester
Second test was also fixed, we can see mix of the users in the reservations (they made a lot of random requests)
4ef40565-6183-4b76-afe7-31b3a5cf5473 - Killers of the Flower Moon: The Osage Murders and the Birth of the FBI - reserved by foo
d132ecaa-7f0c-40b8-9ce8-8467042579b9 - JOURNEY TO THE ICE P - reserved by foo
5ffbc8da-17d1-4630-967e-7aa019af0a10 - The Nightingale: A Novel - reserved by bar
7e616a17-310a-46ba-ad45-c4732c5e1c5b - Pokémon Deluxe Essential Handbook: The Need-to-Know Stats and Facts on Over 700 Pokémon - reserved by foo
c3eb1f0e-6a9f-4a80-bd1f-f9eca4a7366d - Uninvited: Living Loved When You Feel Less Than, Left Out, and Lonely - reserved by bar
...
Third test also worked, now both parties get roughly the same number of reservations (non takes all)
9106cbc0-2f17-4b65-8af7-906b287cfe63 - The Book Thief - reserved by bar
c4936b37-77cb-4616-b196-de889832c3df - Option B: Facing Adversity, Building Resilience, and Finding Joy - reserved by foo
42146ab7-0528-4179-9022-c48563be2afa - Goodnight, Goodnight Construction Site (Hardcover Books for Toddlers, Preschool Books for Kids) - reserved by foo
abc87f73-c491-4546-8f09-f2505257eff9 - The Life-Changing Magic of Tidying Up: The Japanese Art of Decluttering and Organizing - reserved by bar
...
Also I needed to count only the reservations that succedeed to show number of books reserved to the user:
success = db.add_new_reservation(key, user_email)
if success:
reserved += 1