Database Connection Pooling in Python

Building robust and scalable applications often requires efficient management of database connections. Manually opening and closing connections for every database operation can be slow and resource-intensive. This challenge focuses on creating a database connection pool in Python, a fundamental technique for optimizing database interactions.

Problem Description

Your task is to implement a DatabasePool class in Python that manages a pool of database connections. This pool should allow applications to acquire a connection when needed, use it for database operations, and then return it to the pool for reuse.

Key Requirements:

1. Connection Management: The pool should maintain a fixed number of database connections.
2. Acquisition: Provide a method to acquire a connection from the pool.
   • If a connection is available, return it immediately.
   • If all connections are in use, the acquire method should block until a connection becomes available (or raise an exception if a timeout is implemented, though not strictly required for this initial version).
3. Release: Provide a method to release a connection back to the pool. This connection should then be available for subsequent acquire calls.
4. Connection Creation: The pool should be able to create new connections up to its maximum capacity. For this challenge, assume you are working with a conceptual database connection object (you don't need to implement actual database interaction logic, but your pool should simulate this).
5. Pool Size: The pool should have a configurable maximum size.

Expected Behavior:

• When the pool is initialized, it should create the specified number of connections.
• When acquire is called and a connection is free, it should return a connection object.
• When acquire is called and all connections are busy, it should wait.
• When release is called, the connection should be marked as available.

Edge Cases:

• What happens if release is called with a connection not managed by the pool? (For this challenge, you can assume valid releases or implement basic checks).
• What happens if the pool is empty and acquire is called?

Examples

Example 1: Basic Acquisition and Release

# Assume a mock connection class exists
class MockConnection:
    def __init__(self, id):
        self.id = id
        self.is_closed = False

    def close(self):
        self.is_closed = True

# Initialization of the pool
pool = DatabasePool(max_connections=3)

# Acquire connections
conn1 = pool.acquire()
conn2 = pool.acquire()

print(f"Acquired conn1 (ID: {conn1.id})")
print(f"Acquired conn2 (ID: {conn2.id})")

# Release connections
pool.release(conn1)
pool.release(conn2)

print("Connections released.")

# Acquire again
conn3 = pool.acquire()
print(f"Acquired conn3 (ID: {conn3.id})") # Should be conn1 or conn2

Output:

Acquired conn1 (ID: 1)
Acquired conn2 (ID: 2)
Connections released.
Acquired conn3 (ID: 1) # or ID: 2

Explanation: The pool is initialized with 3 connections. Two connections are acquired. When released, they become available. The third acquisition reuses one of the previously released connections.

Example 2: Pool Exhaustion and Subsequent Availability

# Assume MockConnection as above
pool = DatabasePool(max_connections=2)

# Acquire all connections
conn_a = pool.acquire()
conn_b = pool.acquire()

print(f"Acquired conn_a (ID: {conn_a.id})")
print(f"Acquired conn_b (ID: {conn_b.id})")

# Attempt to acquire a third connection (this would block if not for simulated timeout or immediate release)
# For this example, we'll simulate the outcome directly.
# In a real scenario, this acquire might hang or raise an exception if a timeout is set.

# Release one connection
pool.release(conn_a)
print("conn_a released.")

# Now acquire a new connection
conn_c = pool.acquire()
print(f"Acquired conn_c (ID: {conn_c.id})") # Should be conn_a if it was released first

# Release remaining connections
pool.release(conn_b)
pool.release(conn_c)
print("All connections released.")

Output:

Acquired conn_a (ID: 1)
Acquired conn_b (ID: 2)
conn_a released.
Acquired conn_c (ID: 1)
All connections released.

Explanation: The pool with a capacity of 2 is filled. When conn_a is released, it becomes available, and conn_c can acquire it.

Constraints

• The max_connections parameter must be a positive integer.
• Your DatabasePool class should be implemented using standard Python libraries. No external database drivers are required for this simulation.
• The implementation should be thread-safe, meaning it can handle concurrent acquire and release calls from multiple threads without data corruption.

Notes

• You'll need to simulate database connections. A simple class with an ID and a close method would suffice.
• Consider using Python's queue.Queue or threading.Lock and threading.Condition to manage the availability of connections and handle waiting when the pool is empty.
• The core challenge is managing the collection of available and in-use connections and synchronizing access to them.
• Think about how to assign unique identifiers to your mock connections for easier tracking in the examples.