Achieving Write Concurrency in SQLite Applications (Without Breaking Everything)

• Reading: Multiple connections can access the database for reading data simultaneously. This means several processes can query the database at the same time without affecting each other.
• Writing: Only one connection can write to the database at a time. If another connection tries to write while one is already in progress, it will have to wait until the first write finishes.

This design ensures data integrity. If multiple writes happened concurrently, it could lead to conflicts and inconsistencies in the data.

Here's an analogy: Think of the database as a library book. Multiple people can read the book at once, but only one person can write notes in the margins at a time.

Additional points:

• If you need to manage concurrent writes, you'll need to implement mechanisms like semaphores or mutexes in your programing language to control access. These are tools that ensure only one process writes at a time.

import sqlite3
import time

def write_data(conn, data):
  """
  This function attempts to write data to the database.
  If another write is ongoing, it waits with a small delay.
  """
  while True:
    try:
      cursor = conn.cursor()
      cursor.execute("INSERT INTO your_table (data) VALUES (?)", (data,))
      conn.commit()
      break  # Successful write, exit loop
    except sqlite3.OperationalError as e:
      if "locked" in str(e):  # Check for write lock error
        time.sleep(0.1)  # Wait for a short time

# Example usage
conn = sqlite3.connect("your_database.db")

# Simulate multiple write attempts (can be separate threads)
data_list = ["data1", "data2", "data3"]
for data in data_list:
  write_data(conn, data)

conn.close()

This code defines a write_data function that attempts to insert data into the database. It uses a loop to keep trying until successful. Inside the loop:

1. It attempts to execute an insert query.
2. If an OperationalError occurs with "locked" in the message, it indicates another write is happening.
3. The code waits for a short time (0.1 seconds here) using time.sleep before retrying.

1. Single Writer with Queue:

   • Maintain a queue (like a list or channel) to store write requests in your program.
   • Have a single background process (thread or coroutine) that continuously dequeues requests and writes them to the database.
   • This ensures only one write happens at a time while allowing concurrent requests to be queued.

2. Separate Read/Write Connections:

   • Use two separate connections:
     • One connection set to read-only mode for concurrent reads.
     • Another connection for writes, ensuring only one is open at a time.
   • This allows concurrent reads but requires managing write access through your program.

3. Alternative Database:

Choosing the best method depends on your specific requirements:

• If occasional writes occur with mostly reads, the queueing approach within your program might suffice.
• For stricter separation of read/write workloads, separate connections could be useful.
• If high write concurrency is crucial, explore alternative database engines.