[LevelDB] 写批处理过程详解

1 Status DBImpl::Write(const WriteOptions& options,WriteBatch* my_batch) {
 2 　// -----A begin-------  
 3   Writer w(&mutex_);  
 4   w.batch = my_batch;  
 5   w.sync = options.sync;  
 6   w.done = false;  
 7   // -----A end  --------  
 8 
 9 
10   // -----B begin-------  
11   MutexLock l(&mutex_);  
12   writers_.push_back(&w);  
13   while (!w.done && &w != writers_.front()) {  
14     w.cv.Wait();  
15   }  
16   if (w.done) {  
17     return w.status;  
18   }  
19   // -----B end  -------  
20 
21   // May temporarily unlock and wait.  
22   Status status = MakeRoomForWrite(my_batch == NULL);  
23   uint64_t last_sequence = versions_->LastSequence();  
24   Writer* last_writer = &w;  
25   if (status.ok() && my_batch != NULL) {  // NULL batch is for compactions  
26     WriteBatch* updates = BuildBatchGroup(&last_writer);  
27     WriteBatchInternal::SetSequence(updates,last_sequence + 1);  
28     last_sequence += WriteBatchInternal::Count(updates);  
29 
30     // Add to log and apply to memtable.  We can release the lock  
31     // during this phase since &w is currently responsible for logging  
32     // and protects against concurrent loggers and concurrent writes  
33     // into mem_.  
34     {
35       // -----C begin-------  
36       mutex_.Unlock();  
37       // -----C end  -------  
38       status = log_->AddRecord(WriteBatchInternal::Contents(updates));  
39       if (status.ok() && options.sync) {  
40         status = logfile_->Sync();  
41       }  
42       if (status.ok()) {  
43         status = WriteBatchInternal::InsertInto(updates,mem_);  
44       }  
45       // -----D begin-------  
46       mutex_.Lock();  
47       // -----D end  -------  
48     }  
49     if (updates == tmp_batch_) tmp_batch_->Clear();  
50 
51     versions_->SetLastSequence(last_sequence);  
52   }  
53 
54   // -----E begin-------  
55   while (true) {  
56     Writer* ready = writers_.front();  
57     writers_.pop_front();  
58     if (ready != &w) {  
59       ready->status = status;  
60       ready->done = true;  
61       ready->cv.Signal();  
62     }  
63     if (ready == last_writer) break;  
64   }  
65   // -----E end -------  
66 
67 
68   // -----F begin-------  
69   // Notify new head of write queue  
70   if (!writers_.empty()) {  
71     writers_.front()->cv.Signal();  
72   }  
73   // -----F end-------  
74 
75   return status;  
76 }