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bbolt is a fork of Ben Johnson's 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 LMDB project. 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.

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. API should not change between patch and minor releases. New minor versions may add additional features to the API.

Table of Contents

Getting Started

Installing

To start using Bolt, install Go and run go get:

$ 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:

$ 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.

$ go install go.etcd.io/bbolt/cmd/bbolt@latest

Importing bbolt

To use bbolt as an embedded key-value store, import as:

import bolt "go.etcd.io/bbolt"

db, err := bolt.Open(path, 0600, 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:

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:

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:

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:

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:

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:

var id uint64
err := db.Batch(func(tx *bolt.Tx) error {
	// Find last key in bucket, decode as bigendian uint64, 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.

// 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:

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 retrieve an existing bucket using the Tx.Bucket() function:

db.Update(func(tx *bolt.Tx) error {
	b := tx.Bucket([]byte("MyBucket"))
	if b == nil {
		return errors.New("bucket does not exist")
	}
	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.

You can also iterate over all existing top-level buckets with Tx.ForEach():

db.View(func(tx *bolt.Tx) error {
	tx.ForEach(func(name []byte, b *bolt.Bucket) error {
		fmt.Println(string(name))
		return nil
	})
	return nil
})

Using key/value pairs

To save a key/value pair to a bucket, use the Bucket.Put() function:

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:

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:

db.Update(func (tx *bolt.Tx) error {
    b := tx.Bucket([]byte("MyBucket"))
    err := b.Delete([]byte("answer"))
    return err
})

This will delete the key answers from the bucket MyBucket.

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.

// 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.PutUint64(b, uint64(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:

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). 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.

When you have iterated to the end of the cursor, then Next() will return a nil key and the cursor still points to the last element if present. When you have iterated to the beginning of the cursor, then Prev() will return a nil key and the cursor still points to the first element if present.

If you remove key/value pairs during iteration, the cursor may automatically move to the next position if present in current node each time removing a key. When you call c.Next() after removing a key, it may skip one key/value pair. Refer to pull/611 to get more detailed info.

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():

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:

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:

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:

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.


// 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 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:

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:

$ 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 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.
		json.NewEncoder(os.Stderr).Encode(diff)

		// Save stats for the next loop.
		prev = stats
	}
}()

It's also useful to pipe these stats to a service such as statsd for monitoring or to provide an HTTP endpoint that will perform a fixed-length sample.

Read-Only Mode

Sometimes it is useful to create a shared, read-only Bolt database. To this, set the Options.ReadOnly flag when opening your database. Read-only mode uses a shared lock to allow multiple processes to read from the database but it will block any processes from opening the database in read-write mode.

db, err := bolt.Open("my.db", 0600, &bolt.Options{ReadOnly: true})
if err != nil {
	log.Fatal(err)
}

Mobile Use (iOS/Android)

Bolt is able to run on mobile devices by leveraging the binding feature of the gomobile tool. Create a struct that will contain your database logic and a reference to a *bolt.DB with a initializing constructor that takes in a filepath where the database file will be stored. Neither Android nor iOS require extra permissions or cleanup from using this method.

func NewBoltDB(filepath string) *BoltDB {
	db, err := bolt.Open(filepath+"/demo.db", 0600, nil)
	if err != nil {
		log.Fatal(err)
	}

	return &BoltDB{db}
}

type BoltDB struct {
	db *bolt.DB
	...
}

func (b *BoltDB) Path() string {
	return b.db.Path()
}

func (b *BoltDB) Close() {
	b.db.Close()
}

Database logic should be defined as methods on this wrapper struct.

To initialize this struct from the native language (both platforms now sync their local storage to the cloud. These snippets disable that functionality for the database file):

Android

String path;
if (android.os.Build.VERSION.SDK_INT >=android.os.Build.VERSION_CODES.LOLLIPOP){
    path = getNoBackupFilesDir().getAbsolutePath();
} else{
    path = getFilesDir().getAbsolutePath();
}
Boltmobiledemo.BoltDB boltDB = Boltmobiledemo.NewBoltDB(path)

iOS

- (void)demo {
    NSString* path = [NSSearchPathForDirectoriesInDomains(NSLibraryDirectory,
                                                          NSUserDomainMask,
                                                          YES) objectAtIndex:0];
	GoBoltmobiledemoBoltDB * demo = GoBoltmobiledemoNewBoltDB(path);
	[self addSkipBackupAttributeToItemAtPath:demo.path];
	//Some DB Logic would go here
	[demo close];
}

- (BOOL)addSkipBackupAttributeToItemAtPath:(NSString *) filePathString
{
    NSURL* URL= [NSURL fileURLWithPath: filePathString];
    assert([[NSFileManager defaultManager] fileExistsAtPath: [URL path]]);

    NSError *error = nil;
    BOOL success = [URL setResourceValue: [NSNumber numberWithBool: YES]
                                  forKey: NSURLIsExcludedFromBackupKey error: &error];
    if(!success){
        NSLog(@"Error excluding %@ from backup %@", [URL lastPathComponent], error);
    }
    return success;
}

Resources

For more information on getting started with Bolt, check out the following articles:

Comparison with other databases

Postgres, MySQL, & other relational databases

Relational databases structure data into rows and are only accessible through the use of SQL. This approach provides flexibility in how you store and query your data but also incurs overhead in parsing and planning SQL statements. Bolt accesses all data by a byte slice key. This makes Bolt fast to read and write data by key but provides no built-in support for joining values together.

Most relational databases (with the exception of SQLite) are standalone servers that run separately from your application. This gives your systems flexibility to connect multiple application servers to a single database server but also adds overhead in serializing and transporting data over the network. Bolt runs as a library included in your application so all data access has to go through your application's process. This brings data closer to your application but limits multi-process access to the data.

LevelDB, RocksDB

LevelDB and its derivatives (RocksDB, HyperLevelDB) are similar to Bolt in that they are libraries bundled into the application, however, their underlying structure is a log-structured merge-tree (LSM tree). An LSM tree optimizes random writes by using a write ahead log and multi-tiered, sorted files called SSTables. Bolt uses a B+tree internally and only a single file. Both approaches have trade-offs.

If you require a high random write throughput (>10,000 w/sec) or you need to use spinning disks then LevelDB could be a good choice. If your application is read-heavy or does a lot of range scans then Bolt could be a good choice.

One other important consideration is that LevelDB does not have transactions. It supports batch writing of key/values pairs and it supports read snapshots but it will not give you the ability to do a compare-and-swap operation safely. Bolt supports fully serializable ACID transactions.

LMDB

Bolt was originally a port of LMDB so it is architecturally similar. Both use a B+tree, have ACID semantics with fully serializable transactions, and support lock-free MVCC using a single writer and multiple readers.

The two projects have somewhat diverged. LMDB heavily focuses on raw performance while Bolt has focused on simplicity and ease of use. For example, LMDB allows several unsafe actions such as direct writes for the sake of performance. Bolt opts to disallow actions which can leave the database in a corrupted state. The only exception to this in Bolt is DB.NoSync.

There are also a few differences in API. LMDB requires a maximum mmap size when opening an mdb_env whereas Bolt will handle incremental mmap resizing automatically. LMDB overloads the getter and setter functions with multiple flags whereas Bolt splits these specialized cases into their own functions.

Caveats & Limitations

It's important to pick the right tool for the job and Bolt is no exception. Here are a few things to note when evaluating and using Bolt:

Reading the Source

Bolt is a relatively small code base (<5KLOC) for an embedded, serializable, transactional key/value database so it can be a good starting point for people interested in how databases work.

The best places to start are the main entry points into Bolt:

If you have additional notes that could be helpful for others, please submit them via pull request.

Known Issues

Other Projects Using Bolt

Below is a list of public, open source projects that use Bolt:

If you are using Bolt in a project please send a pull request to add it to the list.