When Consistency is King
When money is on the line
Boltu Bhai looked at Montu. "Let's say you're ordering a bulk shipment of premium tea from Sylhet for everyone at your company. You need to send the supplier 10,000 Taka. You open your banking app and hit 'Send Money'."
"Right," Montu nodded.
"The exact millisecond you hit 'Send', the network cable between the bank's Dhaka server and their Sylhet server gets sliced! Boom, a Partition (P) just happened. The Dhaka server has absolutely no way of talking to the Sylhet server."
Montu leaned in, listening intently.
"Now, if the system prioritized Availability (A) and just blindly processed writes without coordinating across nodes during a partition, it would spell disaster. For instance, the Dhaka server might deduct the money from your account, but because the Sylhet server is offline, it wouldn't know to credit the supplier. As a result, the ledgers on both servers would completely drift apart—your money would vanish into the void, and the supplier wouldn't get paid!"
Montu gasped, "Are you kidding me? I'd sue the bank!"
"Exactly why," Boltu Bhai smiled, "when it comes to financial transactions, Consistency (C) is way more important than Availability. In a proper CP system, the moment a Network Partition (P) hits, the system refuses to take any chances. If the Dhaka server realizes it can't talk to the Sylhet server, it cancels the transaction right then and there—aborting before any final commit—and throws a 'Service Unavailable' error on your screen. No split data, no messed-up ledgers, and your money stays safe. Because the system chooses Consistency (C) over Availability (A) during a partition, banking is a classic CP system."
"Makes sense, Bhai. This sounds like the perfect setup, no messed-up data. Why would anyone even need an AP system then?" Montu asked, feeling pretty pleased with his new understanding.