第二节 boltdb事务Tx定义

// txid represents the internal transaction identifier.
type txid uint64

// 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.
// Tx 主要封装了读事务和写事务。其中通过writable来区分是读事务还是写事务
type Tx struct {
    writable       bool
    managed        bool
    db             *DB
    meta           *meta
    root           Bucket
    pages          map[pgid]*page
    stats          TxStats
    // 提交时执行的动作
    commitHandlers []func()

    // 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
}

// init initializes the transaction.
func (tx *Tx) init(db *DB) {
    tx.db = db
    tx.pages = nil

    // Copy the meta page since it can be changed by the writer.
    // 拷贝元信息
    tx.meta = &meta{}
    db.meta().copy(tx.meta)

    // Copy over the root bucket.
    // 拷贝根节点
    tx.root = newBucket(tx)
    tx.root.bucket = &bucket{}
    // meta.root=bucket{root:3}
    *tx.root.bucket = tx.meta.root

    // Increment the transaction id and add a page cache for writable transactions.
    if tx.writable {
        tx.pages = make(map[pgid]*page)
        tx.meta.txid += txid(1)
    }
}