There's Nothing 100% in Computer Science

When it comes to correctness guarantees, there are few communities in Computer Science (software) more obsessed with them than the database and distributed system communities. They better be serious about correctness because everyone's bank account balances depend on it. You have heard about terms like Paxos, consensus, fault tolerance, but…

When would someone use std::enable_shared_from_this

std::enable_shared_from_this allows a class to have a valid shared_ptr of this. Simply adding a member function that returns shared_ptr<T>(this) is susceptible to double-free. But when would you use such a feature? It's a relevant question because, next time when you…

How FoundationDB works and why it works

FoundationDB is a very impressive database. Its paper won the best industry paper award in SIGMOD'21. In this post, I will explain, in detail, how FDB works and discuss a few very interesting design choices they made. It's a dense paper packed with neat ideas. Many details (sometimes even proof…

Paxos vs. Quorum-based Consistency

Paxos, is the famous synonym for consistency in the context of distributed system. Unfortunately, consistency alone, is such an overloaded term, it's often practically meaningless. In this post, I will explain the difference between Paxos-consistency vs. quorum-consistency. I assume you know what Paxos is and what problem it solves. If…

Build `folly::coro` with GCC

You have heard about Coroutine in C++, and you want to use it. There're two coroutine implementations that are considered most mature - cppcoro and folly::coro. They are written by the same guy - Lewis Baker. He's brilliant, and you shoud watch his cppcon talk on structured concurrency if…

How `AUTO_INCREMENT` works on MySQL

You have created tables with AUTO_INCREMENT. Something like CREATE TABLE user ( user_id INT(11) NOT NULL AUTO_INCREMENT, ... ) MySQL manages user_id for you automatically. You get user_id 1, 2, 3, 4, ... But how does it work? InnoDB, one of the most common storage engine MySQL uses,…

MyRocks and Repeatable Read Isolation

tl;dr: Repeatable read isolation on MyRocks is effectively Snapshot isolation (unless it's affecting an unique index). According to MySQL reference, REPEATABLE READ This is the default isolation level for InnoDB. Consistent reads within the same transaction read the snapshot established by the first read. This means that if you…

SeqLock

Sequential lock is a common technique used to protect data that's frequently read and rarely updated. Writers are required to take exclusive lock to mutate the data. Reads are effectively lock free and optimistic. The data protected by a SeqLock usually needs to be trivially_copy_constructible. // reader std::atomoc&…

`folly::Indestructible`

folly::Indestructible<T> is a class template that makes a static variable well, indestructible. Notice that it's meant for static variables in the Meyers Singleton pattern. If it's for heap allocated memory, it would just be called memory leak instead of "indestructible". It boils down to making a…