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// Package rascaldb is a key-value log-structured storage engine with hash map index.
package rascaldb
import (
// DB represents RascalDB database on disk, created by Open.
type DB struct {
// name is a dir where segment files are stored.
name string
// segmentNamer is a function that returns random segment names.
segmentNamer func() string
// mu mutex is used only to modify segments slice.
mu sync.Mutex
// segments is a slice of segment files where records are stored.
// Oldest segments are in the beginning of the slice.
segments atomic.Value
// DB is a state machine which requires various concurrent actions, for example,
// put new keys, rotate segments when the current one becomes too big.
// Actions are executed in run method which acts as a serialization point;
// have a look at Actor stuff
actionsc chan func()
// quitc signals the actor to stop.
quitc chan struct{}
// Open opens a database with the specified name.
// If a database doesn't exist, it will be created. Database is a dir where segment files are kept.
func Open(name string) (*DB, error) {
db := DB{
name: name,
segmentNamer: newSegmentNamer(),
actionsc: make(chan func()),
quitc: make(chan struct{}),
if err := os.MkdirAll(, 0700); err != nil {
return nil, err
path := filepath.Join(, trunk)
filenames, err := readSegmentNames(path)
// Since there is no trunk file, this is a new database.
// Let's create the first segment and store its name in the trunk.
if os.IsNotExist(err) {
filenames = append(filenames, db.segmentNamer())
err = writeSegmentNames(path, filenames)
if err != nil {
return nil, err
ss := make([]*segment, 0, len(filenames))
var s *segment
// Open segments for reads, load indexes. The last segment is opened for writes.
for i, segName := range filenames {
isLast := i == len(filenames)-1
path = filepath.Join(, segName)
if s, err = openSegment(path, isLast); err != nil {
return nil, err
if err = s.loadIndex(); err != nil {
return nil, err
ss = append(ss, s)
return &db, nil
// Close closes database resources.
func (db *DB) Close() {
// The state machine's loop is stopped.
// All segment files are closed.
ss := db.segments.Load().([]*segment)
for _, s := range ss {
// run executes every function from actionsc and acts as a serialization point.
// It doesn't know about business logic.
func (db *DB) run() {
for {
select {
case f := <-db.actionsc:
case <-db.quitc:
// Set puts a key in database. You can call it concurrently.
func (db *DB) Set(key string, value []byte) error {
errc := make(chan error)
db.actionsc <- func() {
ss := db.segments.Load().([]*segment)
current := ss[len(ss)-1]
errc <- current.write(key, value)
return <-errc
// Get retrieves a key from database. You can call it concurrently.
func (db *DB) Get(key string) ([]byte, error) {
ss := db.segments.Load().([]*segment)
var ok bool
var offset int64
for i := len(ss) - 1; i >= 0; i-- {
if offset, ok = ss[i].index[key]; ok {
_, value, err := ss[i].read(offset)
return value, err
return nil, ErrKeyNotFound