第五节 key/value的插入、获取、删除

上面一节我们介绍了一下如何创建一个Bucket、如何获取一个Bucket。有了Bucket，我们就可以对我们最关心的key/value键值对进行增删改查了。其实本质上，对key/value的所有操作最终都要表现在底层的node上。因为node节点就是用来存储真实数据的。

3.5.1 插入一个key/value对

// Put sets the value for a key in the bucket.
// If the key exist then its previous value will be overwritten.
// Supplied value must remain valid for the life of the transaction.
// Returns an error if the bucket was created from a read-only transaction,
// if the key is blank, if the key is too large, or if the value is too large.
func (b *Bucket) Put(key []byte, value []byte) error {
    if b.tx.db == nil {
        return ErrTxClosed
    } else if !b.Writable() {
        return ErrTxNotWritable
    } else if len(key) == 0 {
        return ErrKeyRequired
    } else if len(key) > MaxKeySize {
        return ErrKeyTooLarge
    } else if int64(len(value)) > MaxValueSize {
        return ErrValueTooLarge
    }

    // Move cursor to correct position.
    c := b.Cursor()
    k, _, flags := c.seek(key)

    // Return an error if there is an existing key with a bucket value.
    if bytes.Equal(key, k) && (flags&bucketLeafFlag) != 0 {
        return ErrIncompatibleValue
    }

    // Insert into node.
    key = cloneBytes(key)
    c.node().put(key, key, value, 0, 0)

    return nil
}

3.5.2 获取一个key/value对

// Get retrieves the value for a key in the bucket.
// Returns a nil value if the key does not exist or if the key is a nested bucket.
// The returned value is only valid for the life of the transaction.
func (b *Bucket) Get(key []byte) []byte {
    k, v, flags := b.Cursor().seek(key)

    // Return nil if this is a bucket.
    if (flags & bucketLeafFlag) != 0 {
        return nil
    }

    // If our target node isn't the same key as what's passed in then return nil.
    if !bytes.Equal(key, k) {
        return nil
    }
    return v
}

3.5.3 删除一个key/value对

// Delete removes a key from the bucket.
// If the key does not exist then nothing is done and a nil error is returned.
// Returns an error if the bucket was created from a read-only transaction.
func (b *Bucket) Delete(key []byte) error {
    if b.tx.db == nil {
        return ErrTxClosed
    } else if !b.Writable() {
        return ErrTxNotWritable
    }

    // Move cursor to correct position.
    c := b.Cursor()
    _, _, flags := c.seek(key)

    // Return an error if there is already existing bucket value.
    if (flags & bucketLeafFlag) != 0 {
        return ErrIncompatibleValue
    }

    // Delete the node if we have a matching key.
    c.node().del(key)

    return nil
}

3.5.4 遍历Bucket中所有的键值对

// ForEach executes a function for each key/value pair in a bucket.
// If the provided function returns an error then the iteration is stopped and
// the error is returned to the caller. The provided function must not modify
// the bucket; this will result in undefined behavior.
func (b *Bucket) ForEach(fn func(k, v []byte) error) error {
    if b.tx.db == nil {
        return ErrTxClosed
    }
    c := b.Cursor()
    // 遍历键值对
    for k, v := c.First(); k != nil; k, v = c.Next() {
        if err := fn(k, v); err != nil {
            return err
        }
    }
    return nil
}