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:
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.
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.
* [Consul](https://github.com/hashicorp/consul) - Consul is service discovery and configuration made easy. Distributed, highly available, and datacenter-aware.
* [DVID](https://github.com/janelia-flyem/dvid) - Added Bolt as optional storage engine and testing it against Basho-tuned leveldb.
* [dcrwallet](https://github.com/decred/dcrwallet) - A wallet for the Decred cryptocurrency.
* [drive](https://github.com/odeke-em/drive) - drive is an unofficial Google Drive command line client for \*NIX operating systems.
* [Freehold](http://tshannon.bitbucket.org/freehold/) - An open, secure, and lightweight platform for your files and data.
* [Go Report Card](https://goreportcard.com/) - Go code quality report cards as a (free and open source) service.
* [GoWebApp](https://github.com/josephspurrier/gowebapp) - A basic MVC web application in Go using BoltDB.
* [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.
* [gopherpit](https://github.com/gopherpit/gopherpit) - A web service to manage Go remote import paths with custom domains
* [Ironsmith](https://github.com/timshannon/ironsmith) - A simple, script-driven continuous integration (build - > test -> release) tool, with no external dependencies
* [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.
* [Prometheus Annotation Server](https://github.com/oliver006/prom_annotation_server) - Annotation server for PromDash & Prometheus service monitoring system.
* [reef-pi](https://github.com/reef-pi/reef-pi) - reef-pi is an award winning, modular, DIY reef tank controller using easy to learn electronics based on a Raspberry Pi.
* [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
* [Seaweed File System](https://github.com/chrislusf/seaweedfs) - Highly scalable distributed key~file system with O(1) disk read.
* [torrent](https://github.com/anacrolix/torrent) - Full-featured BitTorrent client package and utilities in Go. BoltDB is a storage backend in development.
