From 0f8f2bdd5b4eba9694b34e53c4c0a5956b74b67f Mon Sep 17 00:00:00 2001
From: Jeremy Tregunna <jeremy@tregunna.ca>
Date: Sat, 10 May 2025 13:18:17 -0600
Subject: [PATCH] feat: toy implementation of MVCC

---
 go.mod  |   3 +
 main.go | 174 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 177 insertions(+)
 create mode 100644 go.mod
 create mode 100644 main.go

diff --git a/go.mod b/go.mod
new file mode 100644
index 0000000..cee2114
--- /dev/null
+++ b/go.mod
@@ -0,0 +1,3 @@
+module git.canoozie.net/jer/mvcc
+
+go 1.24.2
diff --git a/main.go b/main.go
new file mode 100644
index 0000000..28ed9be
--- /dev/null
+++ b/main.go
@@ -0,0 +1,174 @@
+package main
+
+import (
+	"fmt"
+	"sync"
+	"time"
+)
+
+type Timestamp int64
+
+type VersionedValue struct {
+	Value     string
+	Timestamp Timestamp
+}
+
+type KeyHistory struct {
+	Versions []VersionedValue
+}
+
+type Store struct {
+	mu    sync.RWMutex
+	data  map[string]*KeyHistory
+	clock Timestamp
+}
+
+type Transaction struct {
+	store       *Store
+	readTS      Timestamp
+	writeBuffer map[string]string
+	committed   bool
+}
+
+func NewStore() *Store {
+	return &Store{
+		data:  make(map[string]*KeyHistory),
+		clock: Timestamp(time.Now().UnixNano()),
+	}
+}
+
+func (s *Store) nextTimestamp() Timestamp {
+	s.clock++
+	return s.clock
+}
+
+func (s *Store) Read(key string, ts Timestamp) (string, bool) {
+	s.mu.RLock()
+	defer s.mu.RUnlock()
+
+	history, exists := s.data[key]
+	if !exists {
+		return "", false
+	}
+
+	for i := len(history.Versions) - 1; i >= 0; i-- {
+		if history.Versions[i].Timestamp <= ts {
+			return history.Versions[i].Value, true
+		}
+	}
+
+	return "", false
+}
+
+func (s *Store) Begin() *Transaction {
+	readTS := s.nextTimestamp()
+	return &Transaction{
+		store:       s,
+		readTS:      readTS,
+		writeBuffer: make(map[string]string),
+	}
+}
+
+func (tx *Transaction) Read(key string) (string, bool) {
+	if val, ok := tx.writeBuffer[key]; ok {
+		return val, true
+	}
+	return tx.store.Read(key, tx.readTS)
+}
+
+func (tx *Transaction) Write(key, value string) {
+	tx.writeBuffer[key] = value
+}
+
+func (tx *Transaction) Commit() error {
+	if tx.committed {
+		return fmt.Errorf("transaction already committed")
+	}
+
+	tx.store.mu.Lock()
+	defer tx.store.mu.Unlock()
+
+	// Conflict detection
+	for key := range tx.writeBuffer {
+		history, exists := tx.store.data[key]
+		if !exists {
+			continue
+		}
+		latest := history.Versions[len(history.Versions)-1]
+		if latest.Timestamp > tx.readTS {
+			return fmt.Errorf("write-write conflict on key '%s'", key)
+		}
+	}
+
+	commitTS := tx.store.nextTimestamp()
+	for key, val := range tx.writeBuffer {
+		history, exists := tx.store.data[key]
+		if !exists {
+			history = &KeyHistory{}
+			tx.store.data[key] = history
+		}
+		history.Versions = append(history.Versions, VersionedValue{
+			Value:     val,
+			Timestamp: commitTS,
+		})
+	}
+
+	tx.committed = true
+	return nil
+}
+
+func main() {
+	store := NewStore()
+
+	// Transaction 1: Writes "foo" -> "bar"
+	tx1 := store.Begin()
+	tx1.Write("foo", "bar")
+	err := tx1.Commit()
+	if err != nil {
+		fmt.Println("tx1 commit error:", err)
+	} else {
+		fmt.Println("tx1 committed")
+	}
+
+	// Transaction 2: Reads "foo"
+	tx2 := store.Begin()
+	val, ok := tx2.Read("foo")
+	if ok {
+		fmt.Printf("tx2 reads foo: %s\n", val)
+	} else {
+		fmt.Println("tx2 reads foo: not found")
+	}
+
+	// Transaction 3: Conflicting write
+	tx3 := store.Begin()
+	tx3.Write("foo", "baz")
+	// tx2 committed first, so tx3 will conflict if tx2 also wrote to "foo"
+	err = tx3.Commit()
+	if err != nil {
+		fmt.Println("tx3 commit error:", err)
+	} else {
+		fmt.Println("tx3 committed")
+	}
+
+	// Transaction 4: Should read latest "foo"
+	tx4 := store.Begin()
+	val, _ = tx4.Read("foo")
+	fmt.Printf("tx4 reads foo: %s\n", val)
+
+	tx5 := store.Begin()
+	tx6 := store.Begin()
+
+	tx5.Write("x", "b")
+	if err := tx5.Commit(); err != nil {
+		fmt.Println("tx5 commit error:", err)
+	} else {
+		fmt.Println("tx5 committed")
+	}
+
+	tx6.Write("x", "c")
+	if err := tx6.Commit(); err != nil {
+		fmt.Println("tx6 commit error (expected conflict):", err)
+	} else {
+		fmt.Println("tx6 committed (unexpectedly)")
+	}
+}