所謂snapshot就是一個快照，我們可以從快照中讀到舊的數據。
先寫一個測試程序來看看snapshot的使用：
#include <iostream>
#include "leveldb/db.h"
using namespace std;
using namespace leveldb;
int main() {
DB *db ;
Options op;
op.create_if_missing = true;
Status s = DB::Open(op,"/tmp/testdb",&db);
if(s.ok()){
cout << "create successfully" << endl;
s = db->Put(WriteOptions(),"abcd","1234");
if(s.ok()){
cout << "put successfully" << endl;
string value;
s = db->Get(ReadOptions(),"abcd",&value);
if(s.ok()){
cout << "get successfully,value:" << value << endl;
}
}
if(s.ok()){
string value;
const Snapshot * ss =db->GetSnapshot();
ReadOptions rop;
db->Put(WriteOptions(),"abcd","123456");
db->Get(rop,"abcd",&value);
if(s.ok()){
cout << "get successfully,value:" << value << endl;
}
rop.snapshot = ss;
db->Get(rop,"abcd",&value);
if(s.ok()){
cout << "get from snapshot successfully,value:" << value << endl;
}
db->ReleaseSnapshot(ss);
}
}
delete db;
return 0;
}
程序運行的輸出結果是：
create successfully
put successfully
get successfully,value:1234
get successfully,value:123456
get from snapshot successfully,value:1234
可以看出，即使在數據更新后，我們仍然可以從snapshot中讀到舊的數據。
下面我們來分析leveldb中snapshot的實現。
SequenceNumber(db/dbformat.h)
SequenceNumber是leveldb很重要的東西，每次對數據庫進行更新操作，都會生成一個新的SequenceNumber,64bits，其中高8位為0，可以跟key的類型(8bits)進行合并成64bits。
typedef uint64_t SequenceNumber;
// We leave eight bits empty at the bottom so a type and sequence#
// can be packed together into 64-bits.
static const SequenceNumber kMaxSequenceNumber =
((0x1ull << 56) - 1);
SnapShot(db/snapshot.h),，可以看出snapshot其實就是一個sequence number
class SnapshotImpl : public Snapshot {
public:
//創建后保持不變
SequenceNumber number_;
private:
friend class SnapshotList;
//雙向循環鏈表
SnapshotImpl* prev_;
SnapshotImpl* next_;
SnapshotList* list_; // just for sanity checks
};
創建snapshot:
const Snapshot* DBImpl::GetSnapshot() {
MutexLock l(&mutex_);
return snapshots_.New(versions_->LastSequence());
}
刪除snapshot:
void DBImpl::ReleaseSnapshot(const Snapshot* s) {
MutexLock l(&mutex_);
snapshots_.Delete(reinterpret_cast<const SnapshotImpl*>(s));
}