BoltDB is an embedded key/value database written in Go. It supports fully serializable transactions, ACID semantics, and lock-free MVCC with multiple readers and a single writer. BoltDB uses a single-level, zero-copy, B+tree data storage, which allows fast read access and does not require recovery in the event of a system crash.
In 2011, Howard Chu introduced MDB, a memory-mapped database backend for OpenLDAP, later renamed to [LMDB] (https://symas.com/lmdb/technical/)(Lightning Memory-Mapped Database). In 2013, BoltDB was initially started by Ben Johnson as a port of LMDB to Go, but then the two projects diverged as the author of Bolt decided to focus on simplicity and providing the easy-to-use Go API. The goal of BoltDB became to provide a simple, fast, and reliable database for projects that don't require a full database server such as Postgres or MySQL. With BoltDB being stable, its API fixed, and its file format fixed, the author considered the project a success. Leaving it in such a state, the project was abandoned by its author in 2017.
Bolt allows only one read-write transaction at a time but allows as many read-only transactions as requested. Each transaction has a consistent view of the data as it existed when the transaction started. BoltDB recommends three types of transactions: read-write transactions with `DB.Update`, read-only transactions with `DB.View` and batch read-write transactions with `DB.Batch`. Along with the three recommended transactions, BoltDB also allows manual transaction management with `DB.Begin`.
BoltDB supports lock-free MVCC using a single writer and multiple readers. It allows only one read-write transaction at a time but allows multiple read-only transactions at the same time. 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.
Bolt saves data into a single memory-mapped file on disk. It doesn’t have a separate journal or write-ahead log.
BoltDB supports fully serializable ACID transactions. It allows only one read-write transaction at a time but allows as many read-only transactions as requested. When opening a database with `bolt.Open()`, BoltDB 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. BoltDB supports the ability to prevent an indefinite wait by passing a timeout option to the `Open()` function.
There are only a few types in BoltDB: `DB`, `Bucket`, `Tx`, and `Cursor`. - The `DB` is a single file represented by pages on disk. Each page is commonly 4096 Bytes. The first two pages of the file store the metadata that keeps track of version, transaction id, and page size of the database, as well as the locations of the `freelist` and the first page id of data. The third page stores a `freelist`that keeps track the page id's of the free pages. The rest of the pages are the collection of buckets that store the key/value paris. - A `Bucket` is a collection of unique keys that are associated with values. Each bucket is represented using a B+ tree. When accessing a B+ tree, the nodes in the corresponding page are fetched into memory. The B+ tree used in BoltDB is different from common B+tree in the following aspects: - While B+ trees typically have n+1 values and n keys in each node, the number of values and the number of keys are equal in BoltDB B+ tree. - In BoltDB B+ tree, the value field in a non-leaf node stores the page id of its child node and the corresponding key stores the first key in the child node. - There are no pointers between sibling nodes in BoltDB B+ tree.
Bolt is a key-value store that provides an ordered map, which allows easy access and lookup. All collections of key/value pairs are stored in buckets, and all keys in a bucket must be unique. A bucket can be created with `DB.CreateBucket` or `Tx.CreateBucketIfNotEXists` so that it can be created only if it doesn't exist.
BoltDB saves data into a single memory-mapped file on disk. Write-ahead log is not necessary since BoltDB only deals with one file at a time. With copy-on-write, when writing to a page, BoltDB makes updates on the copy of the original page and updates the pointer to point at the new page upon commit.
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