第二节 boltdb事务Tx定义

  1. // txid represents the internal transaction identifier.
  2. type txid uint64
  4. // Tx represents a read-only or read/write transaction on the database.
  5. // Read-only transactions can be used for retrieving values for keys and creating cursors.
  6. // Read/write transactions can create and remove buckets and create and remove keys.
  7. //
  8. // IMPORTANT: You must commit or rollback transactions when you are done with
  9. // them. Pages can not be reclaimed by the writer until no more transactions
  10. // are using them. A long running read transaction can cause the database to
  11. // quickly grow.
  12. // 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
  13. type Tx struct {
  14.     writable       bool
  15.     managed        bool
  16.     db             *DB
  17.     meta           *meta
  18.     root           Bucket
  19.     pages          map[pgid]*page
  20.     stats          TxStats
  21.     // 提交时执行的动作
  22.     commitHandlers []func()
  24.     // WriteFlag specifies the flag for write-related methods like WriteTo().
  25.     // Tx opens the database file with the specified flag to copy the data.
  26.     //
  27.     // By default, the flag is unset, which works well for mostly in-memory
  28.     // workloads. For databases that are much larger than available RAM,
  29.     // set the flag to syscall.O_DIRECT to avoid trashing the page cache.
  30.     WriteFlag int
  31. }
  33. // init initializes the transaction.
  34. func (tx *Tx) init(db *DB) {
  35.     tx.db = db
  36.     tx.pages = nil
  38.     // Copy the meta page since it can be changed by the writer.
  39.     // 拷贝元信息
  40.     tx.meta = &meta{}
  41.     db.meta().copy(tx.meta)
  43.     // Copy over the root bucket.
  44.     // 拷贝根节点
  45.     tx.root = newBucket(tx)
  46.     tx.root.bucket = &bucket{}
  47.     // meta.root=bucket{root:3}
  48.     *tx.root.bucket = tx.meta.root
  50.     // Increment the transaction id and add a page cache for writable transactions.
  51.     if tx.writable {
  52.         tx.pages = make(map[pgid]*page)
  53.         tx.meta.txid += txid(1)
  54.     }
  55. }