go libp2p kad的value可用于存放任意数据，目前kad默认在value中存放了ipns和pk这两种数据（pk也是为ipns服务的，详情请阅读我的另一篇博文《ipns实现机制解读》）。

Record

Record proto:

// Record represents a dht record that contains a value
// for a key value pair
type Record struct {// The key that references this recordKey []byte `protobuf:"bytes,1,opt,name=key,proto3" json:"key,omitempty"`// The actual value this record is storingValue []byte `protobuf:"bytes,2,opt,name=value,proto3" json:"value,omitempty"`// Time the record was received, set by receiverTimeReceived         string   `protobuf:"bytes,5,opt,name=timeReceived,proto3" json:"timeReceived,omitempty"`
}

具体到ipns，key形式为为/ipns/{pid}

// RecordKey returns the libp2p record key for a given peer ID.
func RecordKey(pid peer.ID) string {return "/ipns/" + string(pid)
}

handlePutValue处理接收到的record，在保存之前，会先检查它，具体步骤如下：

validate：检查record是否有效
select：比较收到的record和本地保存的record的新旧
保存：将TimeReceived设置为当前的时间，然后保存

// Store a value in this peer local storage
func (dht *IpfsDHT) handlePutValue(ctx context.Context, p peer.ID, pmes *pb.Message) (_ *pb.Message, err error) {if len(pmes.GetKey()) == 0 {return nil, errors.New("handleGetValue but no key was provided")}rec := pmes.GetRecord()if rec == nil {logger.Debugw("got nil record from", "from", p)return nil, errors.New("nil record")}if !bytes.Equal(pmes.GetKey(), rec.GetKey()) {return nil, errors.New("put key doesn't match record key")}cleanRecord(rec)// Make sure the record is valid (not expired, valid signature etc)if err = dht.Validator.Validate(string(rec.GetKey()), rec.GetValue()); err != nil {logger.Infow("bad dht record in PUT", "from", p, "key", loggableRecordKeyBytes(rec.GetKey()), "error", err)return nil, err}dskey := convertToDsKey(rec.GetKey())// fetch the striped lock for this keyvar indexForLock byteif len(rec.GetKey()) == 0 {indexForLock = 0} else {indexForLock = rec.GetKey()[len(rec.GetKey())-1]}lk := &dht.stripedPutLocks[indexForLock]lk.Lock()defer lk.Unlock()// Make sure the new record is "better" than the record we have locally.// This prevents a record with for example a lower sequence number from// overwriting a record with a higher sequence number.existing, err := dht.getRecordFromDatastore(dskey)if err != nil {return nil, err}if existing != nil {recs := [][]byte{rec.GetValue(), existing.GetValue()}i, err := dht.Validator.Select(string(rec.GetKey()), recs)if err != nil {logger.Warnw("dht record passed validation but failed select", "from", p, "key", loggableRecordKeyBytes(rec.GetKey()), "error", err)return nil, err}if i != 0 {logger.Infow("DHT record in PUT older than existing record (ignoring)", "peer", p, "key", loggableRecordKeyBytes(rec.GetKey()))return nil, errors.New("old record")}}// record the time we receive every recordrec.TimeReceived = u.FormatRFC3339(time.Now())data, err := proto.Marshal(rec)if err != nil {return nil, err}err = dht.datastore.Put(dskey, data)return pmes, err
}

保存时，将record key的base32，作为key保存到datastore。

// putLocal stores the key value pair in the datastore
func (dht *IpfsDHT) putLocal(key string, rec *recpb.Record) error {data, err := proto.Marshal(rec)if err != nil {logger.Warnw("failed to put marshal record for local put", "error", err, "key", loggableRecordKeyString(key))return err}return dht.datastore.Put(mkDsKey(key), data)
}

Validator

kad使用Validator验证value的有效性。

// Validator is an interface that should be implemented by record validators.
type Validator interface {// Validate validates the given record, returning an error if it's// invalid (e.g., expired, signed by the wrong key, etc.).Validate(key string, value []byte) error// Select selects the best record from the set of records (e.g., the// newest).//// Decisions made by select should be stable.Select(key string, values [][]byte) (int, error)
}

目前Validator主要有两种实现：

ipns::Validator
PublicKeyValidator

如前所述，pk是为ipns服务的，它的存在有些历史原因，新版的ipfs已经不需要它了，为简单起见，下面只讨论ipns。
IpnsEntry是ipns记录的proto结构。

type IpnsEntry struct {Value        []byte                  `protobuf:"bytes,1,req,name=value" json:"value,omitempty"`Signature    []byte                  `protobuf:"bytes,2,req,name=signature" json:"signature,omitempty"`ValidityType *IpnsEntry_ValidityType `protobuf:"varint,3,opt,name=validityType,enum=ipns.pb.IpnsEntry_ValidityType" json:"validityType,omitempty"`Validity     []byte                  `protobuf:"bytes,4,opt,name=validity" json:"validity,omitempty"`Sequence     *uint64                 `protobuf:"varint,5,opt,name=sequence" json:"sequence,omitempty"`Ttl          *uint64                 `protobuf:"varint,6,opt,name=ttl" json:"ttl,omitempty"`// in order for nodes to properly validate a record upon receipt, they need the public// key associated with it. For old RSA keys, its easiest if we just send this as part of// the record itself. For newer ed25519 keys, the public key can be embedded in the// peerID, making this field unnecessary.PubKey               []byte   `protobuf:"bytes,7,opt,name=pubKey" json:"pubKey,omitempty"`
}

Validator 用于校验ipns记录，keyBook主要用于在peerstore里面（通过peerID）查找公钥，如果节点的公私钥是由ed25519生成的，则可以直接通过peerID反推出公钥。

// Validator is an IPNS record validator that satisfies the libp2p record
// validator interface.
type Validator struct {// KeyBook, if non-nil, will be used to lookup keys for validating IPNS// records.KeyBook pstore.KeyBook
}

Validate用于检查ipns记录的有效性，具体是先检查签名，后检查有效时间。

// Validate validates an IPNS record.
func (v Validator) Validate(key string, value []byte) error {ns, pidString, err := record.SplitKey(key)if err != nil || ns != "ipns" {return ErrInvalidPath}// Parse the value into an IpnsEntryentry := new(pb.IpnsEntry)err = proto.Unmarshal(value, entry)if err != nil {return ErrBadRecord}// Get the public key defined by the ipns pathpid, err := peer.IDFromString(pidString)if err != nil {log.Debugf("failed to parse ipns record key %s into peer ID", pidString)return ErrKeyFormat}pubk, err := v.getPublicKey(pid, entry)if err != nil {return err}return Validate(pubk, entry)
}// Validates validates the given IPNS entry against the given public key.
func Validate(pk ic.PubKey, entry *pb.IpnsEntry) error {// Check the ipns record signature with the public keyif ok, err := pk.Verify(ipnsEntryDataForSig(entry), entry.GetSignature()); err != nil || !ok {return ErrSignature}eol, err := GetEOL(entry)if err != nil {return err}if time.Now().After(eol) {return ErrExpiredRecord}return nil
}

Select用于挑选最佳的ipns记录，具体步骤是：

比较sequence，sequence越大的记录越新
比较validity，validity越大的记录越新
比较value，字节码越大的记录越新

// Select selects the best record by checking which has the highest sequence
// number and latest EOL.
//
// This function returns an error if any of the records fail to parse. Validate
// your records first!
func (v Validator) Select(k string, vals [][]byte) (int, error) {var recs []*pb.IpnsEntryfor _, v := range vals {e := new(pb.IpnsEntry)if err := proto.Unmarshal(v, e); err != nil {return -1, err}recs = append(recs, e)}return selectRecord(recs, vals)
}func selectRecord(recs []*pb.IpnsEntry, vals [][]byte) (int, error) {switch len(recs) {case 0:return -1, errors.New("no usable records in given set")case 1:return 0, nil}var i intfor j := 1; j < len(recs); j++ {cmp, err := Compare(recs[i], recs[j])if err != nil {return -1, err}if cmp == 0 {cmp = bytes.Compare(vals[i], vals[j])}if cmp < 0 {i = j}}return i, nil
}// Compare compares two IPNS entries. It returns:
//
// * -1 if a is older than b
// * 0 if a and b cannot be ordered (this doesn't mean that they are equal)
// * +1 if a is newer than b
func Compare(a, b *pb.IpnsEntry) (int, error) {as := a.GetSequence()bs := b.GetSequence()if as > bs {return 1, nil} else if as < bs {return -1, nil}at, err := u.ParseRFC3339(string(a.GetValidity()))if err != nil {return 0, err}bt, err := u.ParseRFC3339(string(b.GetValidity()))if err != nil {return 0, err}if at.After(bt) {return 1, nil} else if bt.After(at) {return -1, nil}return 0, nil
}

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go libp2p kad record详解

Record

Validator

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