improve comments in db_impl_secondary

Summary: Pull Request resolved: https://github.com/facebook/rocksdb/pull/5360

Differential Revision: D15502973

Pulled By: miasantreble

fbshipit-source-id: 15b7f9d7928e771a6fac0643861173be8ba6b37a

db/db_impl_secondary.cc

@@ -120,8 +120,6 @@ Status DBImplSecondary::FindNewLogNumbers(std::vector<uint64_t>* logs) {
   return s;
 }
 
-// try to find log reader using log_number from log_readers_ map, initialize
-// if it doesn't exist
 Status DBImplSecondary::MaybeInitLogReader(
     uint64_t log_number, log::FragmentBufferedReader** log_reader) {
   auto iter = log_readers_.find(log_number);

db/db_impl_secondary.h

@@ -13,6 +13,7 @@
 
 namespace rocksdb {
 
+// A wrapper class to hold log reader, log reporter, log status.
 class LogReaderContainer {
  public:
   LogReaderContainer()
@@ -62,11 +63,19 @@ class LogReaderContainer {
   };
 };
 
+// The secondary instance shares access to the storage as the primary.
+// The secondary is able to read and replay changes described in both the
+// MANIFEST and the WAL files without coordination with the primary.
+// The secondary instance can be opened using `DB::OpenAsSecondary`. After
+// that, it can call `DBImplSecondary::TryCatchUpWithPrimary` to make best
+// effort attempts to catch up with the primary.
 class DBImplSecondary : public DBImpl {
  public:
   DBImplSecondary(const DBOptions& options, const std::string& dbname);
   ~DBImplSecondary() override;
 
+  // Recover by replaying MANIFEST and WAL. Also initialize manifest_reader_
+  // and log_readers_ to facilitate future operations.
   Status Recover(const std::vector<ColumnFamilyDescriptor>& column_families,
                  bool read_only, bool error_if_log_file_exist,
                  bool error_if_data_exists_in_logs) override;
@@ -182,10 +191,15 @@ class DBImplSecondary : public DBImpl {
   // method can take long time due to all the I/O and CPU costs.
   Status TryCatchUpWithPrimary() override;
 
+
+  // Try to find log reader using log_number from log_readers_ map, initialize
+  // if it doesn't exist
   Status MaybeInitLogReader(uint64_t log_number,
                             log::FragmentBufferedReader** log_reader);
 
  protected:
+  // ColumnFamilyCollector is a write batch handler which does nothing
+  // except recording unique column family IDs
   class ColumnFamilyCollector : public WriteBatch::Handler {
     std::unordered_set<uint32_t> column_family_ids_;
 
@@ -262,6 +276,8 @@ class DBImplSecondary : public DBImpl {
       std::unordered_set<ColumnFamilyData*>* cfds_changed,
       JobContext* job_context);
   Status FindNewLogNumbers(std::vector<uint64_t>* logs);
+  // After manifest recovery, replay WALs and refresh log_readers_ if necessary
+  // REQUIRES: log_numbers are sorted in ascending order
   Status RecoverLogFiles(const std::vector<uint64_t>& log_numbers,
                          SequenceNumber* next_sequence,
                          std::unordered_set<ColumnFamilyData*>* cfds_changed,