What causes "rpc discovery: not supported select mode" in geektutu/7days-golang?

Use only the exported constants: xclient.RandomSelect or xclient.RoundRobinSelect when creating XClient or calling Get

rpc discovery: not supported select mode

Error message

rpc discovery: not supported select mode

What it means

MultiServersDiscovery.Get only supports RandomSelect and RoundRobinSelect; any other SelectMode value falls to the default branch and returns this error. It indicates an invalid load-balancing mode was passed.

Source

Thrown at gee-rpc/day7-registry/xclient/discovery.go:65

}

// Get a server according to mode
func (d *MultiServersDiscovery) Get(mode SelectMode) (string, error) {
	d.mu.Lock()
	defer d.mu.Unlock()
	n := len(d.servers)
	if n == 0 {
		return "", errors.New("rpc discovery: no available servers")
	}
	switch mode {
	case RandomSelect:
		return d.servers[d.r.Intn(n)], nil
	case RoundRobinSelect:
		s := d.servers[d.index%n] // servers could be updated, so mode n to ensure safety
		d.index = (d.index + 1) % n
		return s, nil
	default:
		return "", errors.New("rpc discovery: not supported select mode")
	}
}

// returns all servers in discovery
func (d *MultiServersDiscovery) GetAll() ([]string, error) {
	d.mu.RLock()
	defer d.mu.RUnlock()
	// return a copy of d.servers
	servers := make([]string, len(d.servers), len(d.servers))
	copy(servers, d.servers)
	return servers, nil
}

// NewMultiServerDiscovery creates a MultiServersDiscovery instance
func NewMultiServerDiscovery(servers []string) *MultiServersDiscovery {
	d := &MultiServersDiscovery{
		servers: servers,
		r:       rand.New(rand.NewSource(time.Now().UnixNano())),

View on GitHub (pinned to cf36443821)

Solutions

Use only the exported constants: xclient.RandomSelect or xclient.RoundRobinSelect when creating XClient or calling Get
Fix any hardcoded numeric SelectMode values to the defined enum members
If a new strategy is needed, extend SelectMode and add a case in Get rather than passing an unknown value

Example fix

// before
client := xclient.NewXClient(d, 3, geerpc.GeeOption{}) // unsupported mode
// after
client := xclient.NewXClient(d, xclient.RoundRobinSelect, geerpc.GeeOption{})

Defensive patterns

Strategy: validation

Validate before calling

func validSelectMode(m xclient.SelectMode) bool {
    return m == xclient.RandomSelect || m == xclient.RoundRobinSelect
}
// reject before constructing the client
if !validSelectMode(mode) { return errors.New("unsupported select mode") }

Try / catch

xc, err := xclient.NewXClient(d, mode, opt)
if err != nil { return err }
_ = xc
// and when calling Get directly:
s, err := d.Get(mode)
if err != nil && strings.Contains(err.Error(), "not supported select mode") {
    return fmt.Errorf("mode %d unsupported: %w", mode, err)
}

Prevention

Only use the exported SelectMode constants, never raw numbers
Add a switch exhaustiveness check or unit test enumerating all valid modes
Document supported modes at the call site where mode is configured

When it happens

Trigger: Passing an out-of-range or unknown SelectMode value to Get directly, or constructing XClient with a SelectMode other than the defined constants (e.g. 5) — note that with a single server, XClient may bypass Get, so the error appears once multiple servers exist.

Common situations: Using a numeric literal for SelectMode instead of the exported constants; custom enum values added client-side that the discovery does not know; copy-pasted code from another framework with different mode enums.

Related errors

AI-assisted analysis of geektutu/7days-golang@cf36443821 (2026-09-03). Data as JSON: /api/errors/bcd3262fcafe0b61. Report an issue: GitHub.