第一节 boltdb的Bucket结构

先来看官方文档的一段描述Bucket的话。

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

下面是Bucket的详细定义，本节我们先暂时忽略事务Tx，后面章节会详细介绍事务

// 16 byte
const bucketHeaderSize = int(unsafe.Sizeof(bucket{}))

const (
    minFillPercent = 0.1
    maxFillPercent = 1.0
)

// DefaultFillPercent is the percentage that split pages are filled.
// This value can be changed by setting Bucket.FillPercent.
const DefaultFillPercent = 0.5

// Bucket represents a collection of key/value pairs inside the database.
// 一组key/value的集合，也就是一个b+树
type Bucket struct {
    *bucket //在内联时bucket主要用来存储其桶的value并在后面拼接所有的元素，即所谓的内联
    tx       *Tx                // the associated transaction
    buckets  map[string]*Bucket // subbucket cache
    page     *page              // inline page reference，内联页引用
    rootNode *node              // materialized node for the root page.
    nodes    map[pgid]*node     // node cache

    // 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.
    // 填充率
    FillPercent float64
}

// bucket represents the on-file representation of a bucket.
// This is stored as the "value" of a bucket key. If the bucket is small enough,
// then its root page can be stored inline in the "value", after the bucket
// header. In the case of inline buckets, the "root" will be 0.
type bucket struct {
    root     pgid   // page id of the bucket's root-level page
    sequence uint64 // monotonically incrementing, used by NextSequence()
}

// newBucket returns a new bucket associated with a transaction.
func newBucket(tx *Tx) Bucket {
    var b = Bucket{tx: tx, FillPercent: DefaultFillPercent}
    if tx.writable {
        b.buckets = make(map[string]*Bucket)
        b.nodes = make(map[pgid]*node)
    }
    return b
}

下图展现的是数据在bucket中的存储方式。

上面是一个Bucket的定义，在开始下面的内容前，我们先提前介绍一下另一个角色Cursor，因为后面会频繁的用到它。大家在这里先知道，一个Bucket就是一个b+树就可以了。我们后面会对其进行详细的分析。