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While the core concepts remain similar to those in the CAP theorem, PACELC extends the idea by introducing a second decision point. Let’s explore what the theorem is about and why it matters.

Systems can be PA or PC, and they can be EL or EC. So, instead of making a single trade-off as in the CAP theorem, PACELC requires us to consider two decisions. These decisions impact how a system behaves under partition and normal conditions. For example:

Google Spanner: Prioritizes consistency over availability during partitions ( PC) and consistency over latency otherwise ( EC). Spanner is known as PC/EC.

Amazon DynamoDB: Prioritizes availability over consistency during partitions ( PA) and latency over consistency otherwise ( EL). DynamoDB is known as PA/EL.

But wait… we introduced the PACELC theorem, but we haven’t discussed yet why this extension of the CAP theorem was needed in the first place.

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