Every storage engine starts as a thing you can open and ask questions of. Today you build the smallest possible key-value store - one that holds nothing yet - so the public API exists from day one and every later lesson thickens it.
Create an empty in-memory store whose Get reports that any key is missing.
An LSM engine is, from the outside, a key-value store: you put keys in and get them back. Before any of the log-structured machinery matters, the shape of that surface has to exist - something you can construct and query. Today it holds nothing, so every lookup misses.
The one decision that pays off for the rest of the project is making missing its own answer. A key that was never written is different from a key whose value is empty, and later a deleted key will be different again. Returning a value plus a found flag keeps those cases separate from the very first line of code, so nothing downstream has to guess what an empty result means.
// the whole engine will grow inside this typetype Memtable struct { /* storage comes next lesson */ }func New() *Memtable { return &Memtable{} }// return a value AND a found flag - "missing" must be// distinguishable from "present but empty"func (m *Memtable) Get(key string) ([]byte, bool) {return nil, false}