Why is the `maxHeaderSize` 21 and not 22?

liangzheng · August 11, 2022, 4:30pm

dgraph-io/badger/blob/2de177900e3ced14fdab91aa7c6a7e3e441f0192/structs.go#L76

    
      
          	klen      uint32
          	vlen      uint32
          	expiresAt uint64
          	meta      byte
          	userMeta  byte
          }
          
          
const (
          	// Maximum possible size of the header. The maximum size of header struct will be 18 but the
          	// maximum size of varint encoded header will be 21.
          	maxHeaderSize = 21
          )
          
          
// Encode encodes the header into []byte. The provided []byte should be atleast 5 bytes. The
          // function will panic if out []byte isn't large enough to hold all the values.
          // The encoded header looks like
          // +------+----------+------------+--------------+-----------+
          // | Meta | UserMeta | Key Length | Value Length | ExpiresAt |
          // +------+----------+------------+--------------+-----------+
          func (h header) Encode(out []byte) int {
          	out[0], out[1] = h.meta, h.userMeta

Why is the maxHeaderSize 21 and not 22=5+5+10+1+1?

MichelDiz · August 11, 2022, 6:20pm

Sorry, but no one will be able to address your question in short or mid term. Who kept that part of the code is not 100% available. Maybe he will appear in the future.

cc @ibrahim

jarifibrahim · August 13, 2022, 9:20am

I don’t remember how 21 was calculated but it looks like it should be 22.

Based on binary package - encoding/binary - Go Packages
We will have

type header struct {
	klen      uint32 // min 1 byte, max 5 bytes
	vlen      uint32 // min 1 byte, max 5 bytes
	expiresAt uint64 // min 1 byte, max 10 bytes
	meta      byte   // min 1 byte, max 1 byte
	userMeta  byte   // min 1 byte, max 1 byte
}

So we get maxHeaderSize=22

liangzheng · August 13, 2022, 2:30pm

Yes, I also think the value of maxHeaderSize is 22, not 21. Could you help to confirm it? Thanks.

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Why is the `maxHeaderSize` 21 and not 22?

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