mvcc/main.go

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
	readSet     map[string]struct{}
	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,
		readSet:     make(map[string]struct{}),
		writeBuffer: make(map[string]string),
	}
}

func (tx *Transaction) Read(key string) (string, bool) {
	tx.readSet[key] = struct{}{}
	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")
	}

	s := tx.store
	s.mu.Lock()
	defer s.mu.Unlock()

	getLatestTS := func(key string) Timestamp {
		if h, ok := s.data[key]; ok && len(h.Versions) > 0 {
			return h.Versions[len(h.Versions)-1].Timestamp
		}
		return 0
	}

	// Write-write Conflict detection
	for key := range tx.writeBuffer {
		if getLatestTS(key) > tx.readTS {
			return fmt.Errorf("write-write conflict on key: %s", key)
		}
	}

	// Read-write conflict detection (excluding keys we're writing)
	for key := range tx.readSet {
		if _, isWriting := tx.writeBuffer[key]; isWriting {
			continue
		}
		if getLatestTS(key) > tx.readTS {
			return fmt.Errorf("read-write conflict on key: %s", key)
		}
	}

	commitTS := tx.store.nextTimestamp()
	for key, val := range tx.writeBuffer {
		history, exists := s.data[key]
		if !exists {
			history = &KeyHistory{}
			s.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 — modifies "foo" after tx2 read it
	tx3 := store.Begin()
	tx3.Write("foo", "baz")
	err = tx3.Commit()
	if err != nil {
		fmt.Println("tx3 commit error:", err)
	} else {
		fmt.Println("tx3 committed")
	}

	// Now tx2 tries to commit after reading a value that's been modified — should conflict
	tx2.Write("bar", "new") // tx2 tries to do something unrelated, but it read "foo"
	err = tx2.Commit()
	if err != nil {
		fmt.Println("tx2 commit error (expected read-write conflict):", err)
	} else {
		fmt.Println("tx2 committed (unexpected!)")
	}

	// Transaction 4: Should read latest "foo"
	tx4 := store.Begin()
	val, _ = tx4.Read("foo")
	fmt.Printf("tx4 reads foo: %s\n", val)

	// Additional write-write conflict test (unchanged)
	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)")
	}
}