Concurrency Control

Multi-version Concurrency Control (MVCC)

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. Allowing read-only transactions and read-write transactions depend on one another can cause deadlocks since if there exists a reader, another writer cannot periodically remap the file as needed by copy-on-write. Similarly, the co-occurrence of read-only transactions and read-write transactions in the same goroutine should also be avoided due to the same reason.

Views

Virtual Views

Bolt allows only one read-write transaction at a time but allows as many read-only transactions as requested. The view obtained by each transaction is consistent with the data present at the beginning of the transaction. 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.

Checkpoints

Not Supported

Joins

Not Supported

Data Model

Key/Value

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 within which all keys must be unique. The keys are stored in byte-sorted order within a bucket.

A bucket can be created with Tx.CreateBucket or Tx.CreateBucketIfNotEXists so that it can be created only if it doesn't exist.

Indexes

B+Tree

BoltDB stores key/value pairs in a B+ tree. The B+ tree has a cursor that is able to find a specific key or move freely up and down the tree.

Storage Model

Custom

BoltDB uses key/value storage, which does not involve rows or columns.

Storage Architecture

Hybrid

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 freelistthat 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 leaf nodes in BoltDB B+ tree.

Isolation Levels

Serializable

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, BoltDB obtains a file lock on the data file which prevents multiple processes from accessing the same database at the same time. A process will hang upon attempting to open an already open Bolt database until the process that opens the database closes it. BoltDB supports the ability to prevent an indefinite wait by passing a timeout option upon opening a database.

Storage Organization

Copy-on-Write / Shadow Paging

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.

Logging

Not Supported

Bolt saves data into a single memory-mapped file on disk. It doesn’t have a separate journal or write-ahead log.

Query Interface

Custom API

BoltDB supports basic key/value queries such as insert, delete and update with its own APIs. All bucket operations are organized and monitored by transactions Tx. For example, transactions have the ability to create a bucket or remove a bucket. Buckets support basic key/value query operations that involve read, write and delete.

Since BoltDB stores its key in byte-sorted order within a bucket, it supports sequential iteration over a bucket with a cursor. The cursor allows moving to the first key, the last key, the previous key, the next key, and a specific key in a bucket.

BoltDB performs a prefix scan by using the cursor to locate specific keys with the required prefix. It performs a range scan by using the cursor to find keys that satisfy a comparison predicate.

Besides the cursor, the bucket also supports self-iterating over all key/value pairs in a bucket with a foreach function.

Foreign Keys

Not Supported

System Architecture

Embedded