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Add comments in db/dbformat.h

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Summary: Pull Request resolved: https://github.com/facebook/rocksdb/pull/5342

Differential Revision: D15485238

Pulled By: siying

fbshipit-source-id: a56b374584cb1d815c1173907a807d90b37d4dd6
This commit is contained in:
Siying Dong 2019-05-23 16:16:38 -07:00 committed by Facebook Github Bot
parent dc30a9b69b
commit 02830a20f8
1 changed files with 18 additions and 3 deletions
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21
db/dbformat.h
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@ -28,6 +28,12 @@
namespace rocksdb {
// The file declares data structures and functions that deal with internal
// keys.
// Each internal key contains a user key, a sequence number (SequenceNumber)
// and a type (ValueType), and they are usually encoded together.
// There are some related helper classes here.
class InternalKey;
// Value types encoded as the last component of internal keys.
@ -88,6 +94,8 @@ static const SequenceNumber kMaxSequenceNumber = ((0x1ull << 56) - 1);
static const SequenceNumber kDisableGlobalSequenceNumber = port::kMaxUint64;
// The data structure that represents an internal key in the way that user_key,
// sequence number and type are stored in separated forms.
struct ParsedInternalKey {
Slice user_key;
SequenceNumber sequence;
@ -192,9 +200,7 @@ class InternalKeyComparator
}
};
// Modules in this directory should keep internal keys wrapped inside
// the following class instead of plain strings so that we do not
// incorrectly use string comparisons instead of an InternalKeyComparator.
// The class represent the internal key in encoded form.
class InternalKey {
private:
std::string rep_;
@ -295,6 +301,12 @@ inline uint64_t GetInternalKeySeqno(const Slice& internal_key) {
return num >> 8;
}
// The class to store keys in an efficient way. It allows:
// 1. Users can either copy the key into it, or have it point to an unowned
// address.
// 2. For copied key, a short inline buffer is kept to reduce memory
// allocation for smaller keys.
// 3. It tracks user key or internal key, and allow conversion between them.
class IterKey {
public:
IterKey()
@ -506,6 +518,8 @@ class IterKey {
void operator=(const IterKey&) = delete;
};
// Convert from a SliceTranform of user keys, to a SliceTransform of
// user keys.
class InternalKeySliceTransform : public SliceTransform {
public:
explicit InternalKeySliceTransform(const SliceTransform* transform)
@ -631,6 +645,7 @@ inline int InternalKeyComparator::CompareKeySeq(const Slice& akey,
return r;
}
// Wrap InternalKeyComparator as a comparator class for ParsedInternalKey.
struct ParsedInternalKeyComparator {
explicit ParsedInternalKeyComparator(const InternalKeyComparator* c)
: cmp(c) {}