bolt

package
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 Go to latest Published: Nov 13, 2017 License: Apache-2.0, MIT Imports: 15 Imported by: 0

Documentation

Overview

Package bolt implements a low-level key/value store in pure Go. It supports fully serializable transactions, ACID semantics, and lock-free MVCC with multiple readers and a single writer. Bolt can be used for projects that want a simple data store without the need to add large dependencies such as Postgres or MySQL.

Bolt is a single-level, zero-copy, B+tree data store. This means that Bolt is optimized for fast read access and does not require recovery in the event of a system crash. Transactions which have not finished committing will simply be rolled back in the event of a crash.

The design of Bolt is based on Howard Chu's LMDB database project.

Bolt currently works on Windows, Mac OS X, and Linux.

Basics

There are only a few types in Bolt: DB, Bucket, Tx, and Cursor. The DB is a collection of buckets and is represented by a single file on disk. A bucket is a collection of unique keys that are associated with values.

Transactions provide either read-only or read-write access to the database. Read-only transactions can retrieve key/value pairs and can use Cursors to iterate over the dataset sequentially. Read-write transactions can create and delete buckets and can insert and remove keys. Only one read-write transaction is allowed at a time.

Caveats

The database uses a read-only, memory-mapped data file to ensure that applications cannot corrupt the database, however, this means that keys and values returned from Bolt cannot be changed. Writing to a read-only byte slice will cause Go to panic.

Keys and values retrieved from the database are only valid for the life of the transaction. When used outside the transaction, these byte slices can point to different data or can point to invalid memory which will cause a panic.

Index

Constants

View Source 
const (
	// MaxKeySize is the maximum length of a key, in bytes.
	MaxKeySize = 32768

	// MaxValueSize is the maximum length of a value, in bytes.
	MaxValueSize = (1 << 31) - 2
)
View Source 
const (
	DefaultMaxBatchSize  int = 1000
	DefaultMaxBatchDelay     = 10 * time.Millisecond
	DefaultAllocSize         = 16 * 1024 * 1024
)

Default values if not set in a DB instance.

View Source 
const DefaultFillPercent = 0.5

DefaultFillPercent is the percentage that split pages are filled. This value can be changed by setting Bucket.FillPercent.

View Source 
const IgnoreNoSync = runtime.GOOS == "openbsd"

IgnoreNoSync specifies whether the NoSync field of a DB is ignored when syncing changes to a file. This is required as some operating systems, such as OpenBSD, do not have a unified buffer cache (UBC) and writes must be synchronized using the msync(2) syscall.

Variables

View Source 
var (
	// ErrDatabaseNotOpen is returned when a DB instance is accessed before it
	// is opened or after it is closed.
	ErrDatabaseNotOpen = errors.New("database not open")

	// ErrDatabaseOpen is returned when opening a database that is
	// already open.
	ErrDatabaseOpen = errors.New("database already open")

	// ErrInvalid is returned when both meta pages on a database are invalid.
	// This typically occurs when a file is not a bolt database.
	ErrInvalid = errors.New("invalid database")

	// ErrVersionMismatch is returned when the data file was created with a
	// different version of Bolt.
	ErrVersionMismatch = errors.New("version mismatch")

	// ErrChecksum is returned when either meta page checksum does not match.
	ErrChecksum = errors.New("checksum error")

	// ErrTimeout is returned when a database cannot obtain an exclusive lock
	// on the data file after the timeout passed to Open().
	ErrTimeout = errors.New("timeout")
)

These errors can be returned when opening or calling methods on a DB.

View Source 
var (
	// ErrTxNotWritable is returned when performing a write operation on a
	// read-only transaction.
	ErrTxNotWritable = errors.New("tx not writable")

	// ErrTxClosed is returned when committing or rolling back a transaction
	// that has already been committed or rolled back.
	ErrTxClosed = errors.New("tx closed")

	// ErrDatabaseReadOnly is returned when a mutating transaction is started on a
	// read-only database.
	ErrDatabaseReadOnly = errors.New("database is in read-only mode")
)

These errors can occur when beginning or committing a Tx.

View Source 
var (
	// ErrBucketNotFound is returned when trying to access a bucket that has
	// not been created yet.
	ErrBucketNotFound = errors.New("bucket not found")

	// ErrBucketExists is returned when creating a bucket that already exists.
	ErrBucketExists = errors.New("bucket already exists")

	// ErrBucketNameRequired is returned when creating a bucket with a blank name.
	ErrBucketNameRequired = errors.New("bucket name required")

	// ErrKeyRequired is returned when inserting a zero-length key.
	ErrKeyRequired = errors.New("key required")

	// ErrKeyTooLarge is returned when inserting a key that is larger than MaxKeySize.
	ErrKeyTooLarge = errors.New("key too large")

	// ErrValueTooLarge is returned when inserting a value that is larger than MaxValueSize.
	ErrValueTooLarge = errors.New("value too large")

	// ErrIncompatibleValue is returned when trying create or delete a bucket
	// on an existing non-bucket key or when trying to create or delete a
	// non-bucket key on an existing bucket key.
	ErrIncompatibleValue = errors.New("incompatible value")
)

These errors can occur when putting or deleting a value or a bucket.

View Source 
var DefaultOptions = &Options{
	Timeout:    0,
	NoGrowSync: false,
}

DefaultOptions represent the options used if nil options are passed into Open(). No timeout is used which will cause Bolt to wait indefinitely for a lock.

Functions

This section is empty.

Types

type Bucket 

type Bucket struct {

	// Sets the threshold for filling nodes when they split. By default,
	// the bucket will fill to 50% but it can be useful to increase this
	// amount if you know that your write workloads are mostly append-only.
	//
	// This is non-persisted across transactions so it must be set in every Tx.
	FillPercent float64
	// contains filtered or unexported fields
}

Bucket represents a collection of key/value pairs inside the database.

func (*Bucket) Bucket 

func (b *Bucket) Bucket(name []byte) *Bucket

Bucket retrieves a nested bucket by name. Returns nil if the bucket does not exist. The bucket instance is only valid for the lifetime of the transaction.

func (*Bucket) CreateBucket 

func (b *Bucket) CreateBucket(key []byte) (*Bucket, error)

CreateBucket creates a new bucket at the given key and returns the new bucket. Returns an error if the key already exists, if the bucket name is blank, or if the bucket name is too long. The bucket instance is only valid for the lifetime of the transaction.

func (*Bucket) CreateBucketIfNotExists 

func (b *Bucket) CreateBucketIfNotExists(key []byte) (*Bucket, error)

CreateBucketIfNotExists creates a new bucket if it doesn't already exist and returns a reference to it. Returns an error if the bucket name is blank, or if the bucket name is too long. The bucket instance is only valid for the lifetime of the transaction.

func (*Bucket) Cursor 

func (b *Bucket) Cursor() *Cursor

Cursor creates a cursor associated with the bucket. The cursor is only valid as long as the transaction is open. Do not use a cursor after the transaction is closed.

func (*Bucket) Delete 

func (b *Bucket) Delete(key []byte) error

Delete removes a key from the bucket. If the key does not exist then nothing is done and a nil error is returned. Returns an error if the bucket was created from a read-only transaction.

func (*Bucket) DeleteBucket 

func (b *Bucket) DeleteBucket(key []byte) error

DeleteBucket deletes a bucket at the given key. Returns an error if the bucket does not exists, or if the key represents a non-bucket value.

func (*Bucket) ForEach 

func (b *Bucket) ForEach(fn func(k, v []byte) error) error

ForEach executes a function for each key/value pair in a bucket. If the provided function returns an error then the iteration is stopped and the error is returned to the caller. The provided function must not modify the bucket; this will result in undefined behavior.

func (*Bucket) Get 

func (b *Bucket) Get(key []byte) []byte

Get retrieves the value for a key in the bucket. Returns a nil value if the key does not exist or if the key is a nested bucket. The returned value is only valid for the life of the transaction.

func (*Bucket) NextSequence 

func (b *Bucket) NextSequence() (uint64, error)

NextSequence returns an autoincrementing integer for the bucket.

func (*Bucket) Put 

func (b *Bucket) Put(key []byte, value []byte) error

Put sets the value for a key in the bucket. If the key exist then its previous value will be overwritten. Supplied value must remain valid for the life of the transaction. Returns an error if the bucket was created from a read-only transaction, if the key is blank, if the key is too large, or if the value is too large.

func (*Bucket) Root 

func (b *Bucket) Root() pgid

Root returns the root of the bucket.

func (*Bucket) Stats 

func (b *Bucket) Stats() BucketStats

Stat returns stats on a bucket.

func (*Bucket) Tx 

func (b *Bucket) Tx() *Tx

Tx returns the tx of the bucket.

func (*Bucket) Writable 

func (b *Bucket) Writable() bool

Writable returns whether the bucket is writable.

type BucketStats 

type BucketStats struct {
	// Page count statistics.
	BranchPageN     int // number of logical branch pages
	BranchOverflowN int // number of physical branch overflow pages
	LeafPageN       int // number of logical leaf pages
	LeafOverflowN   int // number of physical leaf overflow pages

	// Tree statistics.
	KeyN  int // number of keys/value pairs
	Depth int // number of levels in B+tree

	// Page size utilization.
	BranchAlloc int // bytes allocated for physical branch pages
	BranchInuse int // bytes actually used for branch data
	LeafAlloc   int // bytes allocated for physical leaf pages
	LeafInuse   int // bytes actually used for leaf data

	// Bucket statistics
	BucketN           int // total number of buckets including the top bucket
	InlineBucketN     int // total number on inlined buckets
	InlineBucketInuse int // bytes used for inlined buckets (also accounted for in LeafInuse)
}

BucketStats records statistics about resources used by a bucket.

func (*BucketStats) Add 

func (s *BucketStats) Add(other BucketStats)

type Cursor 

type Cursor struct {
	// contains filtered or unexported fields
}

Cursor represents an iterator that can traverse over all key/value pairs in a bucket in sorted order. Cursors see nested buckets with value == nil. Cursors can be obtained from a transaction and are valid as long as the transaction is open.

Keys and values returned from the cursor are only valid for the life of the transaction.

Changing data while traversing with a cursor may cause it to be invalidated and return unexpected keys and/or values. You must reposition your cursor after mutating data.

func (*Cursor) Bucket 

func (c *Cursor) Bucket() *Bucket

Bucket returns the bucket that this cursor was created from.

func (*Cursor) Delete 

func (c *Cursor) Delete() error

Delete removes the current key/value under the cursor from the bucket. Delete fails if current key/value is a bucket or if the transaction is not writable.

func (*Cursor) First 

func (c *Cursor) First() (key []byte, value []byte)

First moves the cursor to the first item in the bucket and returns its key and value. If the bucket is empty then a nil key and value are returned. The returned key and value are only valid for the life of the transaction.

func (*Cursor) Last 

func (c *Cursor) Last() (key []byte, value []byte)

Last moves the cursor to the last item in the bucket and returns its key and value. If the bucket is empty then a nil key and value are returned. The returned key and value are only valid for the life of the transaction.

func (*Cursor) Next 

func (c *Cursor) Next() (key []byte, value []byte)

Next moves the cursor to the next item in the bucket and returns its key and value. If the cursor is at the end of the bucket then a nil key and value are returned. The returned key and value are only valid for the life of the transaction.

func (*Cursor) Prev 

func (c *Cursor) Prev() (key []byte, value []byte)

Prev moves the cursor to the previous item in the bucket and returns its key and value. If the cursor is at the beginning of the bucket then a nil key and value are returned. The returned key and value are only valid for the life of the transaction.

func (*Cursor) Seek 

func (c *Cursor) Seek(seek []byte) (key []byte, value []byte)

Seek moves the cursor to a given key and returns it. If the key does not exist then the next key is used. If no keys follow, a nil key is returned. The returned key and value are only valid for the life of the transaction.

type DB 

type DB struct {
	// When enabled, the database will perform a Check() after every commit.
	// A panic is issued if the database is in an inconsistent state. This
	// flag has a large performance impact so it should only be used for
	// debugging purposes.
	StrictMode bool

	// Setting the NoSync flag will cause the database to skip fsync()
	// calls after each commit. This can be useful when bulk loading data
	// into a database and you can restart the bulk load in the event of
	// a system failure or database corruption. Do not set this flag for
	// normal use.
	//
	// If the package global IgnoreNoSync constant is true, this value is
	// ignored.  See the comment on that constant for more details.
	//
	// THIS IS UNSAFE. PLEASE USE WITH CAUTION.
	NoSync bool

	// When true, skips the truncate call when growing the database.
	// Setting this to true is only safe on non-ext3/ext4 systems.
	// Skipping truncation avoids preallocation of hard drive space and
	// bypasses a truncate() and fsync() syscall on remapping.
	//
	// https://github.com/boltdb/bolt/issues/284
	NoGrowSync bool

	// If you want to read the entire database fast, you can set MmapFlag to
	// syscall.MAP_POPULATE on Linux 2.6.23+ for sequential read-ahead.
	MmapFlags int

	// MaxBatchSize is the maximum size of a batch. Default value is
	// copied from DefaultMaxBatchSize in Open.
	//
	// If <=0, disables batching.
	//
	// Do not change concurrently with calls to Batch.
	MaxBatchSize int

	// MaxBatchDelay is the maximum delay before a batch starts.
	// Default value is copied from DefaultMaxBatchDelay in Open.
	//
	// If <=0, effectively disables batching.
	//
	// Do not change concurrently with calls to Batch.
	MaxBatchDelay time.Duration

	// AllocSize is the amount of space allocated when the database
	// needs to create new pages. This is done to amortize the cost
	// of truncate() and fsync() when growing the data file.
	AllocSize int
	// contains filtered or unexported fields
}

DB represents a collection of buckets persisted to a file on disk. All data access is performed through transactions which can be obtained through the DB. All the functions on DB will return a ErrDatabaseNotOpen if accessed before Open() is called.

func Open 

func Open(path string, mode os.FileMode, options *Options) (*DB, error)

Open creates and opens a database at the given path. If the file does not exist then it will be created automatically. Passing in nil options will cause Bolt to open the database with the default options.

func (*DB) Batch 

func (db *DB) Batch(fn func(*Tx) error) error

Batch calls fn as part of a batch. It behaves similar to Update, except:

1. concurrent Batch calls can be combined into a single Bolt transaction.

2. the function passed to Batch may be called multiple times, regardless of whether it returns error or not.

This means that Batch function side effects must be idempotent and take permanent effect only after a successful return is seen in caller.

The maximum batch size and delay can be adjusted with DB.MaxBatchSize and DB.MaxBatchDelay, respectively.

Batch is only useful when there are multiple goroutines calling it.

func (*DB) Begin 

func (db *DB) Begin(writable bool) (*Tx, error)

Begin starts a new transaction. Multiple read-only transactions can be used concurrently but only one write transaction can be used at a time. Starting multiple write transactions will cause the calls to block and be serialized until the current write transaction finishes.

Transactions should not be dependent on one another. Opening a read transaction and a write transaction in the same goroutine can cause the writer to deadlock because the database periodically needs to re-mmap itself as it grows and it cannot do that while a read transaction is open.

If a long running read transaction (for example, a snapshot transaction) is needed, you might want to set DB.InitialMmapSize to a large enough value to avoid potential blocking of write transaction.

IMPORTANT: You must close read-only transactions after you are finished or else the database will not reclaim old pages.

func (*DB) Close 

func (db *DB) Close() error

Close releases all database resources. All transactions must be closed before closing the database.

func (*DB) GoString 

func (db *DB) GoString() string

GoString returns the Go string representation of the database.

func (*DB) Info 

func (db *DB) Info() *Info

This is for internal access to the raw data bytes from the C cursor, use carefully, or not at all.

func (*DB) IsReadOnly 

func (db *DB) IsReadOnly() bool

func (*DB) Path 

func (db *DB) Path() string

Path returns the path to currently open database file.

func (*DB) Stats 

func (db *DB) Stats() Stats

Stats retrieves ongoing performance stats for the database. This is only updated when a transaction closes.

func (*DB) String 

func (db *DB) String() string

String returns the string representation of the database.

func (*DB) Sync 

func (db *DB) Sync() error

Sync executes fdatasync() against the database file handle.

This is not necessary under normal operation, however, if you use NoSync then it allows you to force the database file to sync against the disk.

func (*DB) Update 

func (db *DB) Update(fn func(*Tx) error) error

Update executes a function within the context of a read-write managed transaction. If no error is returned from the function then the transaction is committed. If an error is returned then the entire transaction is rolled back. Any error that is returned from the function or returned from the commit is returned from the Update() method.

Attempting to manually commit or rollback within the function will cause a panic.

func (*DB) View 

func (db *DB) View(fn func(*Tx) error) error

View executes a function within the context of a managed read-only transaction. Any error that is returned from the function is returned from the View() method.

Attempting to manually rollback within the function will cause a panic.

type Info 

type Info struct {
	Data     uintptr
	PageSize int
}

type Options 

type Options struct {
	// Timeout is the amount of time to wait to obtain a file lock.
	// When set to zero it will wait indefinitely. This option is only
	// available on Darwin and Linux.
	Timeout time.Duration

	// Sets the DB.NoGrowSync flag before memory mapping the file.
	NoGrowSync bool

	// Open database in read-only mode. Uses flock(..., LOCK_SH |LOCK_NB) to
	// grab a shared lock (UNIX).
	ReadOnly bool

	// Sets the DB.MmapFlags flag before memory mapping the file.
	MmapFlags int

	// InitialMmapSize is the initial mmap size of the database
	// in bytes. Read transactions won't block write transaction
	// if the InitialMmapSize is large enough to hold database mmap
	// size. (See DB.Begin for more information)
	//
	// If <=0, the initial map size is 0.
	// If initialMmapSize is smaller than the previous database size,
	// it takes no effect.
	InitialMmapSize int
}

Options represents the options that can be set when opening a database.

type PageInfo 

type PageInfo struct {
	ID            int
	Type          string
	Count         int
	OverflowCount int
}

PageInfo represents human readable information about a page.

type Stats 

type Stats struct {
	// Freelist stats
	FreePageN     int // total number of free pages on the freelist
	PendingPageN  int // total number of pending pages on the freelist
	FreeAlloc     int // total bytes allocated in free pages
	FreelistInuse int // total bytes used by the freelist

	// Transaction stats
	TxN     int // total number of started read transactions
	OpenTxN int // number of currently open read transactions

	TxStats TxStats // global, ongoing stats.
}

Stats represents statistics about the database.

func (*Stats) Sub 

func (s *Stats) Sub(other *Stats) Stats

Sub calculates and returns the difference between two sets of database stats. This is useful when obtaining stats at two different points and time and you need the performance counters that occurred within that time span.

type Tx 

type Tx struct {

	// WriteFlag specifies the flag for write-related methods like WriteTo().
	// Tx opens the database file with the specified flag to copy the data.
	//
	// By default, the flag is unset, which works well for mostly in-memory
	// workloads. For databases that are much larger than available RAM,
	// set the flag to syscall.O_DIRECT to avoid trashing the page cache.
	WriteFlag int
	// contains filtered or unexported fields
}

Tx represents a read-only or read/write transaction on the database. Read-only transactions can be used for retrieving values for keys and creating cursors. Read/write transactions can create and remove buckets and create and remove keys.

IMPORTANT: You must commit or rollback transactions when you are done with them. Pages can not be reclaimed by the writer until no more transactions are using them. A long running read transaction can cause the database to quickly grow.

func (*Tx) Bucket 

func (tx *Tx) Bucket(name []byte) *Bucket

Bucket retrieves a bucket by name. Returns nil if the bucket does not exist. The bucket instance is only valid for the lifetime of the transaction.

func (*Tx) Check 

func (tx *Tx) Check() <-chan error

Check performs several consistency checks on the database for this transaction. An error is returned if any inconsistency is found.

It can be safely run concurrently on a writable transaction. However, this incurs a high cost for large databases and databases with a lot of subbuckets because of caching. This overhead can be removed if running on a read-only transaction, however, it is not safe to execute other writer transactions at the same time.

func (*Tx) Commit 

func (tx *Tx) Commit() error

Commit writes all changes to disk and updates the meta page. Returns an error if a disk write error occurs, or if Commit is called on a read-only transaction.

func (*Tx) Copy 

func (tx *Tx) Copy(w io.Writer) error

Copy writes the entire database to a writer. This function exists for backwards compatibility. Use WriteTo() instead.

func (*Tx) CopyFile 

func (tx *Tx) CopyFile(path string, mode os.FileMode) error

CopyFile copies the entire database to file at the given path. A reader transaction is maintained during the copy so it is safe to continue using the database while a copy is in progress.

func (*Tx) CreateBucket 

func (tx *Tx) CreateBucket(name []byte) (*Bucket, error)

CreateBucket creates a new bucket. Returns an error if the bucket already exists, if the bucket name is blank, or if the bucket name is too long. The bucket instance is only valid for the lifetime of the transaction.

func (*Tx) CreateBucketIfNotExists 

func (tx *Tx) CreateBucketIfNotExists(name []byte) (*Bucket, error)

CreateBucketIfNotExists creates a new bucket if it doesn't already exist. Returns an error if the bucket name is blank, or if the bucket name is too long. The bucket instance is only valid for the lifetime of the transaction.

func (*Tx) Cursor 

func (tx *Tx) Cursor() *Cursor

Cursor creates a cursor associated with the root bucket. All items in the cursor will return a nil value because all root bucket keys point to buckets. The cursor is only valid as long as the transaction is open. Do not use a cursor after the transaction is closed.

func (*Tx) DB 

func (tx *Tx) DB() *DB

DB returns a reference to the database that created the transaction.

func (*Tx) DeleteBucket 

func (tx *Tx) DeleteBucket(name []byte) error

DeleteBucket deletes a bucket. Returns an error if the bucket cannot be found or if the key represents a non-bucket value.

func (*Tx) ForEach 

func (tx *Tx) ForEach(fn func(name []byte, b *Bucket) error) error

ForEach executes a function for each bucket in the root. If the provided function returns an error then the iteration is stopped and the error is returned to the caller.

func (*Tx) ID 

func (tx *Tx) ID() int

ID returns the transaction id.

func (*Tx) OnCommit 

func (tx *Tx) OnCommit(fn func())

OnCommit adds a handler function to be executed after the transaction successfully commits.

func (*Tx) Page 

func (tx *Tx) Page(id int) (*PageInfo, error)

Page returns page information for a given page number. This is only safe for concurrent use when used by a writable transaction.

func (*Tx) Rollback 

func (tx *Tx) Rollback() error

Rollback closes the transaction and ignores all previous updates. Read-only transactions must be rolled back and not committed.

func (*Tx) Size 

func (tx *Tx) Size() int64

Size returns current database size in bytes as seen by this transaction.

func (*Tx) Stats 

func (tx *Tx) Stats() TxStats

Stats retrieves a copy of the current transaction statistics.

func (*Tx) Writable 

func (tx *Tx) Writable() bool

Writable returns whether the transaction can perform write operations.

func (*Tx) WriteTo 

func (tx *Tx) WriteTo(w io.Writer) (n int64, err error)

WriteTo writes the entire database to a writer. If err == nil then exactly tx.Size() bytes will be written into the writer.

type TxStats 

type TxStats struct {
	// Page statistics.
	PageCount int // number of page allocations
	PageAlloc int // total bytes allocated

	// Cursor statistics.
	CursorCount int // number of cursors created

	// Node statistics
	NodeCount int // number of node allocations
	NodeDeref int // number of node dereferences

	// Rebalance statistics.
	Rebalance     int           // number of node rebalances
	RebalanceTime time.Duration // total time spent rebalancing

	// Split/Spill statistics.
	Split     int           // number of nodes split
	Spill     int           // number of nodes spilled
	SpillTime time.Duration // total time spent spilling

	// Write statistics.
	Write     int           // number of writes performed
	WriteTime time.Duration // total time spent writing to disk
}

TxStats represents statistics about the actions performed by the transaction.

func (*TxStats) Sub 

func (s *TxStats) Sub(other *TxStats) TxStats

Sub calculates and returns the difference between two sets of transaction stats. This is useful when obtaining stats at two different points and time and you need the performance counters that occurred within that time span.

Source Files

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