Bolt [![Coverage Status](https://coveralls.io/repos/boltdb/bolt/badge.svg?branch=master)](https://coveralls.io/r/boltdb/bolt?branch=master) [![GoDoc](https://godoc.org/github.com/boltdb/bolt?status.svg)](https://godoc.org/github.com/boltdb/bolt) ![Version](https://img.shields.io/badge/version-1.2.1-green.svg)
====

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.

[hyc_symas]: https://twitter.com/hyc_symas
[lmdb]: http://symas.com/mdb/

## 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.

## A message from the author

> The original goal of Bolt was to provide a simple pure Go key/value store and to
> not bloat the code with extraneous features. To that end, the project has been
> a success. However, this limited scope also means that the project is complete.
> 
> Maintaining an open source database requires an immense amount of time and energy.
> Changes to the code can have unintended and sometimes catastrophic effects so
> even simple changes require hours and hours of careful testing and validation.
>
> Unfortunately I no longer have the time or energy to continue this work. Bolt is
> in a stable state and has years of successful production use. As such, I feel that
> leaving it in its current state is the most prudent course of action.
>
> If you are interested in using a more featureful version of Bolt, I suggest that
> you look at the CoreOS fork called [bbolt](https://github.com/coreos/bbolt).

- Ben Johnson ([@benbjohnson](https://twitter.com/benbjohnson))

## 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 github.com/boltdb/bolt/...
```

This will retrieve the library and install the `bolt` command line utility into
your `$GOBIN` path.


### 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"

	"github.com/boltdb/bolt"
)

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.

Read-only transactions and read-write 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 a read-only transaction is open.


#### 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 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.

```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 `DB.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.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`:

```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/github.com/boltdb/bolt#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.
		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.

```go
db, err := bolt.Open("my.db", 0666, &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](https://github.com/golang/mobile) 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.

```go
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

```java
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

```objc
- (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:

* [Intro to BoltDB: Painless Performant Persistence](http://npf.io/2014/07/intro-to-boltdb-painless-performant-persistence/) by [Nate Finch](https://github.com/natefinch).
* [Bolt -- an embedded key/value database for Go](https://www.progville.com/go/bolt-embedded-db-golang/) by Progville


## 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:

* Bolt is good for read intensive workloads. Sequential write performance is
  also fast but random writes can be slow. You can use `DB.Batch()` or add a
  write-ahead log to help mitigate this issue.

* Bolt uses a B+tree internally so there can be a lot of random page access.
  SSDs provide a significant performance boost over spinning disks.

* Try to avoid long running read transactions. Bolt uses copy-on-write so
  old pages cannot be reclaimed while an old transaction is using them.

* Byte slices returned from Bolt are only valid during a transaction. Once the
  transaction has been committed or rolled back then the memory they point to
  can be reused by a new page or can be unmapped from virtual memory and you'll
  see an `unexpected fault address` panic when accessing it.

* Bolt uses an exclusive write lock on the database file so it cannot be
  shared by multiple processes.

* Be careful when using `Bucket.FillPercent`. Setting a high fill percent for
  buckets that have random inserts will cause your database to have very poor
  page utilization.

* Use larger buckets in general. Smaller buckets causes poor page utilization
  once they become larger than the page size (typically 4KB).

* Bulk loading a lot of random writes into a new bucket can be slow as the
  page will not split until the transaction is committed. Randomly inserting
  more than 100,000 key/value pairs into a single new bucket in a single
  transaction is not advised.

* Bolt uses a memory-mapped file so the underlying operating system handles the
  caching of the data. Typically, the OS will cache as much of the file as it
  can in memory and will release memory as needed to other processes. This means
  that Bolt can show very high memory usage when working with large databases.
  However, this is expected and the OS will release memory as needed. Bolt can
  handle databases much larger than the available physical RAM, provided its
  memory-map fits in the process virtual address space. It may be problematic
  on 32-bits systems.

* The data structures in the Bolt database are memory mapped so the data file
  will be endian specific. This means that you cannot copy a Bolt file from a
  little endian machine to a big endian machine and have it work. For most
  users this is not a concern since most modern CPUs are little endian.

* Because of the way pages are laid out on disk, Bolt cannot truncate data files
  and return free pages back to the disk. Instead, Bolt maintains a free list
  of unused pages within its data file. These free pages can be reused by later
  transactions. This works well for many use cases as databases generally tend
  to grow. However, it's important to note that deleting large chunks of data
  will not allow you to reclaim that space on disk.

  For more information on page allocation, [see this comment][page-allocation].

[page-allocation]: https://github.com/boltdb/bolt/issues/308#issuecomment-74811638


## Reading the Source

Bolt is a relatively small code base (<3KLOC) 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:

- `Open()` - Initializes the reference to the database. It's responsible for
  creating the database if it doesn't exist, obtaining an exclusive lock on the
  file, reading the meta pages, & memory-mapping the file.

- `DB.Begin()` - Starts a read-only or read-write transaction depending on the
  value of the `writable` argument. This requires briefly obtaining the "meta"
  lock to keep track of open transactions. Only one read-write transaction can
  exist at a time so the "rwlock" is acquired during the life of a read-write
  transaction.

- `Bucket.Put()` - Writes a key/value pair into a bucket. After validating the
  arguments, a cursor is used to traverse the B+tree to the page and position
  where they key & value will be written. Once the position is found, the bucket
  materializes the underlying page and the page's parent pages into memory as
  "nodes". These nodes are where mutations occur during read-write transactions.
  These changes get flushed to disk during commit.

- `Bucket.Get()` - Retrieves a key/value pair from a bucket. This uses a cursor
  to move to the page & position of a key/value pair. During a read-only
  transaction, the key and value data is returned as a direct reference to the
  underlying mmap file so there's no allocation overhead. For read-write
  transactions, this data may reference the mmap file or one of the in-memory
  node values.

- `Cursor` - This object is simply for traversing the B+tree of on-disk pages
  or in-memory nodes. It can seek to a specific key, move to the first or last
  value, or it can move forward or backward. The cursor handles the movement up
  and down the B+tree transparently to the end user.

- `Tx.Commit()` - Converts the in-memory dirty nodes and the list of free pages
  into pages to be written to disk. Writing to disk then occurs in two phases.
  First, the dirty pages are written to disk and an `fsync()` occurs. Second, a
  new meta page with an incremented transaction ID is written and another
  `fsync()` occurs. This two phase write ensures that partially written data
  pages are ignored in the event of a crash since the meta page pointing to them
  is never written. Partially written meta pages are invalidated because they
  are written with a checksum.

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


## Other Projects Using Bolt

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

* [BoltDbWeb](https://github.com/evnix/boltdbweb) - A web based GUI for BoltDB files.
* [Operation Go: A Routine Mission](http://gocode.io) - An online programming game for Golang using Bolt for user accounts and a leaderboard.
* [Bazil](https://bazil.org/) - A file system that lets your data reside where it is most convenient for it to reside.
* [DVID](https://github.com/janelia-flyem/dvid) - Added Bolt as optional storage engine and testing it against Basho-tuned leveldb.
* [Skybox Analytics](https://github.com/skybox/skybox) - A standalone funnel analysis tool for web analytics.
* [Scuttlebutt](https://github.com/benbjohnson/scuttlebutt) - Uses Bolt to store and process all Twitter mentions of GitHub projects.
* [Wiki](https://github.com/peterhellberg/wiki) - A tiny wiki using Goji, BoltDB and Blackfriday.
* [ChainStore](https://github.com/pressly/chainstore) - Simple key-value interface to a variety of storage engines organized as a chain of operations.
* [MetricBase](https://github.com/msiebuhr/MetricBase) - Single-binary version of Graphite.
* [Gitchain](https://github.com/gitchain/gitchain) - Decentralized, peer-to-peer Git repositories aka "Git meets Bitcoin".
* [event-shuttle](https://github.com/sclasen/event-shuttle) - A Unix system service to collect and reliably deliver messages to Kafka.
* [ipxed](https://github.com/kelseyhightower/ipxed) - Web interface and api for ipxed.
* [BoltStore](https://github.com/yosssi/boltstore) - Session store using Bolt.
* [photosite/session](https://godoc.org/bitbucket.org/kardianos/photosite/session) - Sessions for a photo viewing site.
* [LedisDB](https://github.com/siddontang/ledisdb) - A high performance NoSQL, using Bolt as optional storage.
* [ipLocator](https://github.com/AndreasBriese/ipLocator) - A fast ip-geo-location-server using bolt with bloom filters.
* [cayley](https://github.com/google/cayley) - Cayley is an open-source graph database using Bolt as optional backend.
* [bleve](http://www.blevesearch.com/) - A pure Go search engine similar to ElasticSearch that uses Bolt as the default storage backend.
* [tentacool](https://github.com/optiflows/tentacool) - REST api server to manage system stuff (IP, DNS, Gateway...) on a linux server.
* [Seaweed File System](https://github.com/chrislusf/seaweedfs) - Highly scalable distributed key~file system with O(1) disk read.
* [InfluxDB](https://influxdata.com) - Scalable datastore for metrics, events, and real-time analytics.
* [Freehold](http://tshannon.bitbucket.org/freehold/) - An open, secure, and lightweight platform for your files and data.
* [Prometheus Annotation Server](https://github.com/oliver006/prom_annotation_server) - Annotation server for PromDash & Prometheus service monitoring system.
* [Consul](https://github.com/hashicorp/consul) - Consul is service discovery and configuration made easy. Distributed, highly available, and datacenter-aware.
* [Kala](https://github.com/ajvb/kala) - Kala is a modern job scheduler optimized to run on a single node. It is persistent, JSON over HTTP API, ISO 8601 duration notation, and dependent jobs.
* [drive](https://github.com/odeke-em/drive) - drive is an unofficial Google Drive command line client for \*NIX operating systems.
* [stow](https://github.com/djherbis/stow) -  a persistence manager for objects
  backed by boltdb.
* [buckets](https://github.com/joyrexus/buckets) - a bolt wrapper streamlining
  simple tx and key scans.
* [mbuckets](https://github.com/abhigupta912/mbuckets) - A Bolt wrapper that allows easy operations on multi level (nested) buckets.
* [Request Baskets](https://github.com/darklynx/request-baskets) - A web service to collect arbitrary HTTP requests and inspect them via REST API or simple web UI, similar to [RequestBin](http://requestb.in/) service
* [Go Report Card](https://goreportcard.com/) - Go code quality report cards as a (free and open source) service.
* [Boltdb Boilerplate](https://github.com/bobintornado/boltdb-boilerplate) - Boilerplate wrapper around bolt aiming to make simple calls one-liners.
* [lru](https://github.com/crowdriff/lru) - Easy to use Bolt-backed Least-Recently-Used (LRU) read-through cache with chainable remote stores.
* [Storm](https://github.com/asdine/storm) - Simple and powerful ORM for BoltDB.
* [GoWebApp](https://github.com/josephspurrier/gowebapp) - A basic MVC web application in Go using BoltDB.
* [SimpleBolt](https://github.com/xyproto/simplebolt) - A simple way to use BoltDB. Deals mainly with strings.
* [Algernon](https://github.com/xyproto/algernon) - A HTTP/2 web server with built-in support for Lua. Uses BoltDB as the default database backend.
* [MuLiFS](https://github.com/dankomiocevic/mulifs) - Music Library Filesystem creates a filesystem to organise your music files.
* [GoShort](https://github.com/pankajkhairnar/goShort) - GoShort is a URL shortener written in Golang and BoltDB for persistent key/value storage and for routing it's using high performent HTTPRouter.
* [torrent](https://github.com/anacrolix/torrent) - Full-featured BitTorrent client package and utilities in Go. BoltDB is a storage backend in development.
* [gopherpit](https://github.com/gopherpit/gopherpit) - A web service to manage Go remote import paths with custom domains
* [bolter](https://github.com/hasit/bolter) - Command-line app for viewing BoltDB file in your terminal.
* [btcwallet](https://github.com/btcsuite/btcwallet) - A bitcoin wallet.
* [dcrwallet](https://github.com/decred/dcrwallet) - A wallet for the Decred cryptocurrency.
* [Ironsmith](https://github.com/timshannon/ironsmith) - A simple, script-driven continuous integration (build - > test -> release) tool, with no external dependencies
* [BoltHold](https://github.com/timshannon/bolthold) - An embeddable NoSQL store for Go types built on BoltDB
* [Ponzu CMS](https://ponzu-cms.org) - Headless CMS + automatic JSON API with auto-HTTPS, HTTP/2 Server Push, and flexible server framework.

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