leveldb WriteBatch

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Introduction

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// WriteBatch holds a collection of updates to apply atomically to a DB.
//
// The updates are applied in the order in which they are added
// to the WriteBatch.  For example, the value of "key" will be "v3"
// after the following batch is written:
//
//    batch.Put("key", "v1");
//    batch.Delete("key");
//    batch.Put("key", "v2");
//    batch.Put("key", "v3");
//
// Multiple threads can invoke const methods on a WriteBatch without
// external synchronization, but if any of the threads may call a
// non-const method, all threads accessing the same WriteBatch must use
// external synchronization.

doc解释的很清楚了， 重点a collection of updates， 至于保证原子操作， 与db write操作有关， 保证中间不会插入其他update。

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// WriteBatchInternal provides static methods for manipulating a
// WriteBatch that we don't want in the public WriteBatch interface.

对WriteBatch的操作是由WriteBatchInternal提供的， 比较重点的是InsertInto， 通过这个方法， 会把writebatch里的updates写到memtable里

Implementation Details

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// WriteBatch::rep_ :=
//    sequence: fixed64
//    count: fixed32
//    data: record[count]
// record :=
//    kTypeValue varstring varstring         |
//    kTypeDeletion varstring
// varstring :=
//    len: varint32
//    data: uint8[len]

WriteBatch内部保存updates的格式