Documentation ¶
Overview ¶
package dbolt 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
- Variables
- func Compact(dst, src *DB, txMaxSize int64) error
- type Bucket
- func (b *Bucket) Bucket(name []byte) *Bucket
- func (b *Bucket) CreateBucket(key []byte) (*Bucket, error)
- func (b *Bucket) CreateBucketIfNotExists(key []byte) (*Bucket, error)
- func (b *Bucket) Cursor() *Cursor
- func (b *Bucket) Delete(key []byte) error
- func (b *Bucket) DeleteBucket(key []byte) error
- func (b *Bucket) ForEach(fn func(k, v []byte) error) error
- func (b *Bucket) Get(key []byte) []byte
- func (b *Bucket) NextSequence() (uint64, error)
- func (b *Bucket) Put(key []byte, value []byte) error
- func (b *Bucket) Root() pgid
- func (b *Bucket) Sequence() uint64
- func (b *Bucket) SetSequence(v uint64) error
- func (b *Bucket) Stats() BucketStats
- func (b *Bucket) Tx() *Tx
- func (b *Bucket) Writable() bool
- type BucketStats
- type Cursor
- func (c *Cursor) Bucket() *Bucket
- func (c *Cursor) Delete() error
- func (c *Cursor) First() (key []byte, value []byte)
- func (c *Cursor) Last() (key []byte, value []byte)
- func (c *Cursor) Next() (key []byte, value []byte)
- func (c *Cursor) Prev() (key []byte, value []byte)
- func (c *Cursor) Seek(seek []byte) (key []byte, value []byte)
- type DB
- func (db *DB) Batch(fn func(*Tx) error) error
- func (db *DB) Begin(writable bool) (*Tx, error)
- func (db *DB) Close() error
- func (db *DB) GoString() string
- func (db *DB) Info() *Info
- func (db *DB) IsReadOnly() bool
- func (db *DB) Path() string
- func (db *DB) Stats() Stats
- func (db *DB) String() string
- func (db *DB) Sync() error
- func (db *DB) Update(fn func(*Tx) error) error
- func (db *DB) View(fn func(*Tx) error) error
- type FreelistType
- type Info
- type Options
- type PageInfo
- type Stats
- type Tx
- func (tx *Tx) Bucket(name []byte) *Bucket
- func (tx *Tx) Check() <-chan error
- func (tx *Tx) Commit() error
- func (tx *Tx) Copy(w io.Writer) error
- func (tx *Tx) CopyFile(path string, mode os.FileMode) error
- func (tx *Tx) CreateBucket(name []byte) (*Bucket, error)
- func (tx *Tx) CreateBucketIfNotExists(name []byte) (*Bucket, error)
- func (tx *Tx) Cursor() *Cursor
- func (tx *Tx) DB() *DB
- func (tx *Tx) DeleteBucket(name []byte) error
- func (tx *Tx) ForEach(fn func(name []byte, b *Bucket) error) error
- func (tx *Tx) ID() int
- func (tx *Tx) OnCommit(fn func())
- func (tx *Tx) Page(id int) (*PageInfo, error)
- func (tx *Tx) Rollback() error
- func (tx *Tx) Size() int64
- func (tx *Tx) Stats() TxStats
- func (tx *Tx) Writable() bool
- func (tx *Tx) WriteTo(w io.Writer) (n int64, err error)
- type TxStats
Constants ¶
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 )
const ( // FreelistArrayType indicates backend freelist type is array FreelistArrayType = FreelistType("array") // FreelistMapType indicates backend freelist type is hashmap FreelistMapType = FreelistType("hashmap") )
const ( DefaultMaxBatchSize int = 1000 DefaultMaxBatchDelay = 10 * time.Millisecond DefaultAllocSize = 16 * 1024 * 1024 )
Default values if not set in a DB instance.
const DefaultFillPercent = 0.5
DefaultFillPercent is the percentage that split pages are filled. This value can be changed by setting Bucket.FillPercent.
const Magic uint32 = 0xDEADCAFE // changed from reversed deadcode to deadcafe
Magic represents a marker value to indicate that a file is a DBolt DB.
const MaxMmapStep = 1 << 30 // 1GB
The largest step that can be taken when remapping the mmap.
const Version = 1
Version is the data file format version.
Variables ¶
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.
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.
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.
var DefaultOptions = &Options{ Timeout: 0, NoGrowSync: false, FreelistType: FreelistArrayType, OpenFile: os.OpenFile, MaxBatchSize: DefaultMaxBatchSize, MaxBatchDelay: DefaultMaxBatchDelay, AllocSize: DefaultAllocSize, StrictMode: 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 ¶
func Compact ¶
Compact will create a copy of the source DB and in the destination DB. This may reclaim space that the source database no longer has use for. txMaxSize can be used to limit the transactions size of this process and may trigger intermittent commits. A value of zero will ignore transaction sizes. TODO: merge with: https://github.com/etcd-io/etcd/blob/b7f0f52a16dbf83f18ca1d803f7892d750366a94/mvcc/backend/backend.go#L349
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 ¶
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 ¶
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 ¶
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 ¶
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 ¶
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 ¶
DeleteBucket deletes a bucket at the given key. Returns an error if the bucket does not exist, or if the key represents a non-bucket value.
func (*Bucket) ForEach ¶
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 ¶
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 ¶
NextSequence returns an autoincrementing integer for the bucket.
func (*Bucket) Put ¶
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) Sequence ¶
Sequence returns the current integer for the bucket without incrementing it.
func (*Bucket) SetSequence ¶
SetSequence updates the sequence number for the bucket.
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) Delete ¶
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 ¶
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 ¶
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 ¶
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 ¶
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.
type DB ¶
type DB struct { *Options // 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 ¶
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 OpenTemp ¶
Open creates and opens a database at the temporary folder. 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 ¶
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 ¶
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 ¶
Close releases all database resources. It will block waiting for any open transactions to finish before closing the database and returning.
func (*DB) 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) Stats ¶
Stats retrieves ongoing performance stats for the database. This is only updated when a transaction closes.
func (*DB) Sync ¶
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 ¶
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.
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 // Do not sync freelist to disk. This improves the database write performance // under normal operation, but requires a full database re-sync during recovery. NoFreelistSync bool // FreelistType sets the backend freelist type. There are two options. Array which is simple but endures // dramatic performance degradation if database is large and framentation in freelist is common. // The alternative one is using hashmap, it is faster in almost all circumstances // but it doesn't guarantee that it offers the smallest page id available. In normal case it is safe. // The default type is array FreelistType FreelistType // 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 // PageSize overrides the default OS page size. PageSize int // NoSync sets the initial value of DB.NoSync. Normally this can just be // set directly on the DB itself when returned from Open(), but this option // is useful in APIs which expose Options but not the underlying DB. NoSync bool // OpenFile is used to open files. It defaults to os.OpenFile. This option // is useful for writing hermetic tests. OpenFile func(string, int, os.FileMode) (*os.File, error) // Mlock locks database file in memory when set to true. // It prevents potential page faults, however // used memory can't be reclaimed. (UNIX only) Mlock bool MaxBatchSize int MaxBatchDelay time.Duration AllocSize int StrictMode bool }
Options represents the options that can be set when opening a database.
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.
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 ¶
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 ¶
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 ¶
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 ¶
Copy writes the entire database to a writer. This function exists for backwards compatibility.
Deprecated; Use WriteTo() instead.
func (*Tx) CopyFile ¶
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 ¶
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 ¶
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 ¶
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) DeleteBucket ¶
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 ¶
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) OnCommit ¶
func (tx *Tx) OnCommit(fn func())
OnCommit adds a handler function to be executed after the transaction successfully commits.
func (*Tx) Page ¶
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 ¶
Rollback closes the transaction and ignores all previous updates. Read-only transactions must be rolled back and not committed.
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.