How do I approach Software Engineering Fundamentals interview questions?

Software Engineering Fundamentals questions require understanding of core concepts and practice. PracHub provides solutions with explanations to help you master software engineering fundamentals interviews.

What difficulty level is this interview question?

This is a medium difficulty Software Engineering Fundamentals question, commonly asked during Technical Screen rounds at Airtable.

What role is this question designed for?

This question is commonly asked for Software Engineer candidates at Airtable during technical interviews.

Implement a Connection Pool | Airtable Interview Question

Q: Implement a Connection Pool

This question evaluates a candidate's competence in resource management, object ownership and lifecycle handling, and safe reuse of pooled database connections.

You are given an existing DBConnection class that simulates a physical database connection. It exposes the following methods and must not be modified:

initialize()
query(sql: str)
close()

Implement the missing behavior for ConnectionPool and PoolConnection.

Skeleton:

class DBConnection:

__init__(self)
initialize(self)
query(self, sql: str) -> str
close(self)
Destroys the physical connection.

class ConnectionPool:

__init__(self, max_conn: int)
get_connection(self) -> PoolConnection | None

class PoolConnection:

__init__(self, parent: ConnectionPool, db_conn: DBConnection)
query(self, sql: str)
close(self)

Requirements:

ConnectionPool(max_conn)

Throw an error if max_conn <= 0 .
Track how many connections are currently checked out.
Track previously used DBConnection instances that have been returned to the pool.

ConnectionPool.get_connection()

If the number of active checked-out connections has reached max_conn , return None .
If an idle DBConnection is available in the pool, reuse it.
Otherwise create a new DBConnection , initialize it, and return it wrapped in a PoolConnection .

PoolConnection.query(sql)

Forward the call to the underlying DBConnection.query(sql) .
If this PoolConnection has already been closed, throw an error.

PoolConnection.close()

Return the underlying DBConnection to the parent ConnectionPool so it can be reused later.
Do not destroy the underlying DBConnection when a PoolConnection is closed.
Prevent a closed PoolConnection from being queried again.
Prevent double-closing, or treat it as an error.

The goal is to implement a simple reusable connection pool with correct ownership and lifecycle handling.