What causes "invalid codec type %s" in geektutu/7days-golang?

Set opt.CodecType to a registered type such as codec.GobType, or leave it empty to use the default

invalid codec type %s

Error message

invalid codec type %s

What it means

NewClient in gee-rpc/day2-client looks up the codec constructor in codec.NewCodecFuncMap by opt.CodecType; if the name is not registered it returns "invalid codec type %s" and refuses to connect. Only codecs registered in NewCodecFuncMap (e.g. Gob "application/gob") are supported.

Source

Thrown at gee-rpc/day2-client/client.go:205

	// if opts is nil or pass nil as parameter
	if len(opts) == 0 || opts[0] == nil {
		return DefaultOption, nil
	}
	if len(opts) != 1 {
		return nil, errors.New("number of options is more than 1")
	}
	opt := opts[0]
	opt.MagicNumber = DefaultOption.MagicNumber
	if opt.CodecType == "" {
		opt.CodecType = DefaultOption.CodecType
	}
	return opt, nil
}

func NewClient(conn net.Conn, opt *Option) (*Client, error) {
	f := codec.NewCodecFuncMap[opt.CodecType]
	if f == nil {
		err := fmt.Errorf("invalid codec type %s", opt.CodecType)
		log.Println("rpc client: codec error:", err)
		return nil, err
	}
	// send options with server
	if err := json.NewEncoder(conn).Encode(opt); err != nil {
		log.Println("rpc client: options error: ", err)
		_ = conn.Close()
		return nil, err
	}
	return newClientCodec(f(conn), opt), nil
}

func newClientCodec(cc codec.Codec, opt *Option) *Client {
	client := &Client{
		seq:     1, // seq starts with 1, 0 means invalid call
		cc:      cc,
		opt:     opt,
		pending: make(map[uint64]*Call),

View on GitHub (pinned to cf36443821)

Solutions

Set opt.CodecType to a registered type such as codec.GobType, or leave it empty to use the default
Use geerpc.DefaultOption or omit the *Option argument to Dial
If a custom codec is needed, register it in codec.NewCodecFuncMap under the exact CodecType string on both client and server
Print codec.NewCodecFuncMap keys to confirm the exact accepted name

Example fix

// before
opt := &geerpc.Option{CodecType: "json"}
client, err := geerpc.Dial("tcp", "localhost:9000", opt)
// after
opt := &geerpc.Option{CodecType: codec.GobType} // "application/gob"
client, err := geerpc.Dial("tcp", "localhost:9000", opt)

Defensive patterns

Strategy: validation

Validate before calling

if _, ok := codec.NewCodecFuncMap[opt.CodecType]; !ok {
	return fmt.Errorf("codec %q not registered; use codec.GobType", opt.CodecType)
}
client, err := geerpc.Dial("tcp", addr, opt)

Type guard

func codecSupported(t string) bool {
	_, ok := codec.NewCodecFuncMap[t]
	return ok
}

Try / catch

client, err := geerpc.Dial("tcp", addr, opt)
if err != nil {
	if strings.Contains(err.Error(), "invalid codec type") {
		opt.CodecType = codec.GobType // fall back to default codec
		client, err = geerpc.Dial("tcp", addr, opt)
	}
	if err != nil { return err }
}

Prevention

Always build Option from codec constants, never string literals
Prefer geerpc.DefaultOption or passing nil to Dial
Register custom codecs in init() on both client and server
Validate CodecType against NewCodecFuncMap before dialing

When it happens

Trigger: Dial(addr, opt) / NewClient(conn, opt) is called with an Option whose CodecType is a non-empty string that is not a registered key, e.g. opt.CodecType = "json" when no JSON codec was registered on the client side.

Common situations: Copy-pasting an Option with a made-up CodecType; server registers a custom codec type but the client build does not; typo like "gob" vs "application/gob"; forgetting that parseOptions only defaults CodecType when it is empty, not when it is wrong.

Related errors

AI-assisted analysis of geektutu/7days-golang@cf36443821 (2026-09-03). Data as JSON: /api/errors/c9886f76b3363d80. Report an issue: GitHub.