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你很可能从某种途径听说过 Go 语言。它越来越受欢迎,并且有充分的理由可以证明。 Go 快速、简单,有强大的社区支持。学习这门语言最令人兴奋的一点是它的并发模型。 Go 的并发原语使创建多线程并发程序变得简单而有趣。我将通过插图介绍 Go 的并发原语,希望能点透相关概念以方便后续学习。本文是写给 Go 语言编程新手以及准备开始学习 Go 并发原语 (goroutines 和 channels) 的同学。 云时代,并发和集群化,是服务标配,
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. ## 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.
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