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Tomer Ben David
Tomer Ben David

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Consistency isn’t consistent at all!


In the world of databases, the term “consistency” isn’t consistent at all! It’s a word that wears many hats and can leave you scratching your head. But fear not, fellow coders, because we’re about to break down the confusing world of consistency in databases, so let’s dive in!

The Double Life of Consistency

Picture this — You’re dealing with a SQL relational database, and you hear the word “consistency.” What does it mean? Well, traditionally, it’s been part of the ACID (Atomicity, Consistency, Isolation, Durability) package. But that’s not the only consistency in town!

The SQL Standard Consistency

Imagine you have a table that tracks how many people attended a walk with a church, and it says five people came wearing red shirts. Cool, right? But then you notice that for each user, their shirt color is stored separately. Uh-oh! User one has a virtual resurrection, and everyone else is in blue. Now, you’ve got an inconsistency! Two places, different data. That’s inconsistency for you.

Eventual Consistency

Enter “eventual consistency.” When we talk about databases, especially at scale, we often have read replicas. The question is, is the data synchronized from the primary (where you write) to the replicas? Well, it depends. If it’s synchronized, it takes time, but everything’s in sync. If not, you might get old data. That’s “eventual” because it says, “Hey, you’ll get the new data eventually, just not right now.”

The Clash of Consistencies

These two consistencies are like cousins. If you read from an inconsistent replica (eventual consistency), you might get outdated data, but it’s fast! But there’s a catch — it can lead to inconsistencies elsewhere if you use that outdated data elsewhere. In some cases, this is fine (like seeing 300 likes instead of 299), but not in others (imagine your bank account balance being off).

Consistency Matters, Except When It Doesn’t

In the wild world of databases, consistency is a crucial concept, but it’s not one-size-fits-all. Sometimes, you want that hardcore transaction-level consistency, like when you’re moving money around. Other times, a bit of inconsistency is acceptable as long as it eventually sorts itself out.

So, the next time someone drops the “C” word in a database conversation, remember the two faces of consistency and when it’s okay to embrace the chaos (or not). Happy coding, my fellow data wranglers! 🤓

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