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bbolt ===== [](https://goreportcard.com/report/github.com/etcd-io/bbolt) [](https://codecov.io/gh/etcd-io/bbolt) [](https://travis-ci.com/etcd-io/bbolt) [](https://godoc.org/github.com/etcd-io/bbolt) [](https://github.com/etcd-io/bbolt/releases) [](https://github.com/etcd-io/bbolt/blob/master/LICENSE) bbolt is a fork of [Ben Johnson's][gh_ben] [Bolt][bolt] key/value store. The purpose of this fork is to provide the Go community with an active maintenance and development target for Bolt; the goal is improved reliability and stability. bbolt includes bug fixes, performance enhancements, and features not found in Bolt while preserving backwards compatibility with the Bolt API. Bolt is a pure Go key/value store inspired by [Howard Chu's][hyc_symas] [LMDB project][lmdb]. The goal of the project is to provide a simple, fast, and reliable database for projects that don't require a full database server such as Postgres or MySQL. Since Bolt is meant to be used as such a low-level piece of functionality, simplicity is key. The API will be small and only focus on getting values and setting values. That's it. [gh_ben]: https://github.com/benbjohnson [bolt]: https://github.com/boltdb/bolt [hyc_symas]: https://twitter.com/hyc_symas [lmdb]: https://www.symas.com/symas-embedded-database-lmdb ## Project Status Bolt is stable, the API is fixed, and the file format is fixed. Full unit test coverage and randomized black box testing are used to ensure database consistency and thread safety. Bolt is currently used in high-load production environments serving databases as large as 1TB. Many companies such as Shopify and Heroku use Bolt-backed services every day. ## Project versioning bbolt uses [semantic versioning](http://semver.org). API should not change between patch and minor releases. New minor versions may add additional features to the API. ## Table of Contents - [Getting Started](#getting-started) - [Installing](#installing) - [Opening a database](#opening-a-database) - [Transactions](#transactions) - [Read-write transactions](#read-write-transactions) - [Read-only transactions](#read-only-transactions) - [Batch read-write transactions](#batch-read-write-transactions) - [Managing transactions manually](#managing-transactions-manually) - [Using buckets](#using-buckets) - [Using key/value pairs](#using-keyvalue-pairs) - [Autoincrementing integer for the bucket](#autoincrementing-integer-for-the-bucket) - [Iterating over keys](#iterating-over-keys) - [Prefix scans](#prefix-scans) - [Range scans](#range-scans) - [ForEach()](#foreach) - [Nested buckets](#nested-buckets) - [Database backups](#database-backups) - [Statistics](#statistics) - [Read-Only Mode](#read-only-mode) - [Mobile Use (iOS/Android)](#mobile-use-iosandroid) - [Resources](#resources) - [Comparison with other databases](#comparison-with-other-databases) - [Postgres, MySQL, & other relational databases](#postgres-mysql--other-relational-databases) - [LevelDB, RocksDB](#leveldb-rocksdb) - [LMDB](#lmdb) - [Caveats & Limitations](#caveats--limitations) - [Reading the Source](#reading-the-source) - [Other Projects Using Bolt](#other-projects-using-bolt) ## Getting Started ### Installing To start using Bolt, install Go and run `go get`: ```sh $ go get go.etcd.io/bbolt@latest ``` This will retrieve the library and update your `go.mod` and `go.sum` files. To run the command line utility, execute: ```sh $ go run go.etcd.io/bbolt/cmd/bbolt@latest ``` Run `go install` to install the `bbolt` command line utility into your `$GOBIN` path, which defaults to `$GOPATH/bin` or `$HOME/go/bin` if the `GOPATH` environment variable is not set. ```sh $ go install go.etcd.io/bbolt/cmd/bbolt@latest ``` ### Importing bbolt To use bbolt as an embedded key-value store, import as: ```go import bolt "go.etcd.io/bbolt" db, err := bolt.Open(path, 0666, nil) if err != nil { return err } defer db.Close() ``` ### Opening a database The top-level object in Bolt is a `DB`. It is represented as a single file on your disk and represents a consistent snapshot of your data. To open your database, simply use the `bolt.Open()` function: ```go package main import ( "log" bolt "go.etcd.io/bbolt" ) func main() { // Open the my.db data file in your current directory. // It will be created if it doesn't exist. db, err := bolt.Open("my.db", 0600, nil) if err != nil { log.Fatal(err) } defer db.Close() ... } ``` Please note that Bolt obtains a file lock on the data file so multiple processes cannot open the same database at the same time. Opening an already open Bolt database will cause it to hang until the other process closes it. To prevent an indefinite wait you can pass a timeout option to the `Open()` function: ```go db, err := bolt.Open("my.db", 0600, &bolt.Options{Timeout: 1 * time.Second}) ``` ### Transactions Bolt allows only one read-write transaction at a time but allows as many read-only transactions as you want at a time. Each transaction has a consistent view of the data as it existed when the transaction started. Individual transactions and all objects created from them (e.g. buckets, keys) are not thread safe. To work with data in multiple goroutines you must start a transaction for each one or use locking to ensure only one goroutine accesses a transaction at a time. Creating transaction from the `DB` is thread safe. Transactions should not depend on one another and generally shouldn't be opened simultaneously in the same goroutine. This can cause a deadlock as the read-write transaction needs to periodically re-map the data file but it cannot do so while any read-only transaction is open. Even a nested read-only transaction can cause a deadlock, as the child transaction can block the parent transaction from releasing its resources. #### Read-write transactions To start a read-write transaction, you can use the `DB.Update()` function: ```go err := db.Update(func(tx *bolt.Tx) error { ... return nil }) ``` Inside the closure, you have a consistent view of the database. You commit the transaction by returning `nil` at the end. You can also rollback the transaction at any point by returning an error. All database operations are allowed inside a read-write transaction. Always check the return error as it will report any disk failures that can cause your transaction to not complete. If you return an error within your closure it will be passed through. #### Read-only transactions To start a read-only transaction, you can use the `DB.View()` function: ```go err := db.View(func(tx *bolt.Tx) error { ... return nil }) ``` You also get a consistent view of the database within this closure, however, no mutating operations are allowed within a read-only transaction. You can only retrieve buckets, retrieve values, and copy the database within a read-only transaction. #### Batch read-write transactions Each `DB.Update()` waits for disk to commit the writes. This overhead can be minimized by combining multiple updates with the `DB.Batch()` function: ```go err := db.Batch(func(tx *bolt.Tx) error { ... return nil }) ``` Concurrent Batch calls are opportunistically combined into larger transactions. Batch is only useful when there are multiple goroutines calling it. The trade-off is that `Batch` can call the given function multiple times, if parts of the transaction fail. The function must be idempotent and side effects must take effect only after a successful return from `DB.Batch()`. For example: don't display messages from inside the function, instead set variables in the enclosing scope: ```go var id uint*** err := db.Batch(func(tx *bolt.Tx) error { // Find last key in bucket, decode as bigendian uint***, increment // by one, encode back to []byte, and add new key. ... id = newValue return nil }) if err != nil { return ... } fmt.Println("Allocated ID %d", id) ``` #### Managing transactions manually The `DB.View()` and `DB.Update()` functions are wrappers around the `DB.Begin()` function. These helper functions will start the transaction, execute a function, and then safely close your transaction if an error is returned. This is the recommended way to use Bolt transactions. However, sometimes you may want to manually start and end your transactions. You can use the `DB.Begin()` function directly but **please** be sure to close the transaction. ```go // Start a writable transaction. tx, err := db.Begin(true) if err != nil { return err } defer tx.Rollback() // Use the transaction... _, err := tx.CreateBucket([]byte("MyBucket")) if err != nil { return err } // Commit the transaction and check for error. if err := tx.Commit(); err != nil { return err } ``` The first argument to `DB.Begin()` is a boolean stating if the transaction should be writable. ### Using buckets Buckets are collections of key/value pairs within the database. All keys in a bucket must be unique. You can create a bucket using the `Tx.CreateBucket()` function: ```go db.Update(func(tx *bolt.Tx) error { b, err := tx.CreateBucket([]byte("MyBucket")) if err != nil { return fmt.Errorf("create bucket: %s", err) } return nil }) ``` You can also create a bucket only if it doesn't exist by using the `Tx.CreateBucketIfNotExists()` function. It's a common pattern to call this function for all your top-level buckets after you open your database so you can guarantee that they exist for future transactions. To delete a bucket, simply call the `Tx.DeleteBucket()` function. ### Using key/value pairs To save a key/value pair to a bucket, use the `Bucket.Put()` function: ```go db.Update(func(tx *bolt.Tx) error { b := tx.Bucket([]byte("MyBucket")) err := b.Put([]byte("answer"), []byte("42")) return err }) ``` This will set the value of the `"answer"` key to `"42"` in the `MyBucket` bucket. To retrieve this value, we can use the `Bucket.Get()` function: ```go db.View(func(tx *bolt.Tx) error { b := tx.Bucket([]byte("MyBucket")) v := b.Get([]byte("answer")) fmt.Printf("The answer is: %s\n", v) return nil }) ``` The `Get()` function does not return an error because its operation is guaranteed to work (unless there is some kind of system failure). If the key exists then it will return its byte slice value. If it doesn't exist then it will return `nil`. It's important to note that you can have a zero-length value set to a key which is different than the key not existing. Use the `Bucket.Delete()` function to delete a key from the bucket. Please note that values returned from `Get()` are only valid while the transaction is open. If you need to use a value outside of the transaction then you must use `copy()` to copy it to another byte slice. ### Autoincrementing integer for the bucket By using the `NextSequence()` function, you can let Bolt determine a sequence which can be used as the unique identifier for your key/value pairs. See the example below. ```go // CreateUser saves u to the store. The new user ID is set on u once the data is persisted. func (s *Store) CreateUser(u *User) error { return s.db.Update(func(tx *bolt.Tx) error { // Retrieve the users bucket. // This should be created when the DB is first opened. b := tx.Bucket([]byte("users")) // Generate ID for the user. // This returns an error only if the Tx is closed or not writeable. // That can't happen in an Update() call so I ignore the error check. id, _ := b.NextSequence() u.ID = int(id) // Marshal user data into bytes. buf, err := json.Marshal(u) if err != nil { return err } // Persist bytes to users bucket. return b.Put(itob(u.ID), buf) }) } // itob returns an 8-byte big endian representation of v. func itob(v int) []byte { b := make([]byte, 8) binary.BigEndian.PutUint***(b, uint***(v)) return b } type User struct { ID int ... } ``` ### Iterating over keys Bolt stores its keys in byte-sorted order within a bucket. This makes sequential iteration over these keys extremely fast. To iterate over keys we'll use a `Cursor`: ```go db.View(func(tx *bolt.Tx) error { // Assume bucket exists and has keys b := tx.Bucket([]byte("MyBucket")) c := b.Cursor() for k, v := c.First(); k != nil; k, v = c.Next() { fmt.Printf("key=%s, value=%s\n", k, v) } return nil }) ``` The cursor allows you to move to a specific point in the list of keys and move forward or backward through the keys one at a time. The following functions are available on the cursor: ``` First() Move to the first key. Last() Move to the last key. Seek() Move to a specific key. Next() Move to the next key. Prev() Move to the previous key. ``` Each of those functions has a return signature of `(key []byte, value []byte)`. When you have iterated to the end of the cursor then `Next()` will return a `nil` key. You must seek to a position using `First()`, `Last()`, or `Seek()` before calling `Next()` or `Prev()`. If you do not seek to a position then these functions will return a `nil` key. During iteration, if the key is non-`nil` but the value is `nil`, that means the key refers to a bucket rather than a value. Use `Bucket.Bucket()` to access the sub-bucket. #### Prefix scans To iterate over a key prefix, you can combine `Seek()` and `bytes.HasPrefix()`: ```go db.View(func(tx *bolt.Tx) error { // Assume bucket exists and has keys c := tx.Bucket([]byte("MyBucket")).Cursor() prefix := []byte("1234") for k, v := c.Seek(prefix); k != nil && bytes.HasPrefix(k, prefix); k, v = c.Next() { fmt.Printf("key=%s, value=%s\n", k, v) } return nil }) ``` #### Range scans Another common use case is scanning over a range such as a time range. If you use a sortable time encoding such as RFC3339 then you can query a specific date range like this: ```go db.View(func(tx *bolt.Tx) error { // Assume our events bucket exists and has RFC3339 encoded time keys. c := tx.Bucket([]byte("Events")).Cursor() // Our time range spans the 90's decade. min := []byte("1990-01-01T00:00:00Z") max := []byte("2000-01-01T00:00:00Z") // Iterate over the 90's. for k, v := c.Seek(min); k != nil && bytes.Compare(k, max) <= 0; k, v = c.Next() { fmt.Printf("%s: %s\n", k, v) } return nil }) ``` Note that, while RFC3339 is sortable, the Golang implementation of RFC3339Nano does not use a fixed number of digits after the decimal point and is therefore not sortable. #### ForEach() You can also use the function `ForEach()` if you know you'll be iterating over all the keys in a bucket: ```go db.View(func(tx *bolt.Tx) error { // Assume bucket exists and has keys b := tx.Bucket([]byte("MyBucket")) b.ForEach(func(k, v []byte) error { fmt.Printf("key=%s, value=%s\n", k, v) return nil }) return nil }) ``` Please note that keys and values in `ForEach()` are only valid while the transaction is open. If you need to use a key or value outside of the transaction, you must use `copy()` to copy it to another byte slice. ### Nested buckets You can also store a bucket in a key to create nested buckets. The API is the same as the bucket management API on the `DB` object: ```go func (*Bucket) CreateBucket(key []byte) (*Bucket, error) func (*Bucket) CreateBucketIfNotExists(key []byte) (*Bucket, error) func (*Bucket) DeleteBucket(key []byte) error ``` Say you had a multi-tenant application where the root level bucket was the account bucket. Inside of this bucket was a sequence of accounts which themselves are buckets. And inside the sequence bucket you could have many buckets pertaining to the Account itself (Users, Notes, etc) isolating the information into logical groupings. ```go // createUser creates a new user in the given account. func createUser(accountID int, u *User) error { // Start the transaction. tx, err := db.Begin(true) if err != nil { return err } defer tx.Rollback() // Retrieve the root bucket for the account. // Assume this has already been created when the account was set up. root := tx.Bucket([]byte(strconv.FormatUint(accountID, 10))) // Setup the users bucket. bkt, err := root.CreateBucketIfNotExists([]byte("USERS")) if err != nil { return err } // Generate an ID for the new user. userID, err := bkt.NextSequence() if err != nil { return err } u.ID = userID // Marshal and save the encoded user. if buf, err := json.Marshal(u); err != nil { return err } else if err := bkt.Put([]byte(strconv.FormatUint(u.ID, 10)), buf); err != nil { return err } // Commit the transaction. if err := tx.Commit(); err != nil { return err } return nil } ``` ### Database backups Bolt is a single file so it's easy to backup. You can use the `Tx.WriteTo()` function to write a consistent view of the database to a writer. If you call this from a read-only transaction, it will perform a hot backup and not block your other database reads and writes. By default, it will use a regular file handle which will utilize the operating system's page cache. See the [`Tx`](https://godoc.org/go.etcd.io/bbolt#Tx) documentation for information about optimizing for larger-than-RAM datasets. One common use case is to backup over HTTP so you can use tools like `cURL` to do database backups: ```go func BackupHandleFunc(w http.ResponseWriter, req *http.Request) { err := db.View(func(tx *bolt.Tx) error { w.Header().Set("Content-Type", "application/octet-stream") w.Header().Set("Content-Disposition", `attachment; filename="my.db"`) w.Header().Set("Content-Length", strconv.Itoa(int(tx.Size()))) _, err := tx.WriteTo(w) return err }) if err != nil { http.Error(w, err.Error(), http.StatusInternalServerError) } } ``` Then you can backup using this command: ```sh $ curl http://localhost/backup > my.db ``` Or you can open your browser to `http://localhost/backup` and it will download automatically. If you want to backup to another file you can use the `Tx.CopyFile()` helper function. ### Statistics The database keeps a running count of many of the internal operations it performs so you can better understand what's going on. By grabbing a snapshot of these stats at two points in time we can see what operations were performed in that time range. For example, we could start a goroutine to log stats every 10 seconds: ```go go func() { // Grab the initial stats. prev := db.Stats() for { // Wait for 10s. time.Sleep(10 * time.Second) // Grab the current stats and diff them. stats := db.Stats() diff := stats.Sub(&prev) // Encode stats to JSON and print to STDERR. ... ...

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