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Implement a modified round robin for server selection #21

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Merged
stapelberg merged 1 commit into from
May 27, 2025
Merged

Implement a modified round robin for server selection #21

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33 changes: 21 additions & 12 deletions robustsession/robustsession.go
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Original file line number Diff line number Diff line change
Expand Up @@ -95,9 +95,10 @@ func CopyNetworks() []*Network {
// This type is only exported so that you can expose internal network state
// for debugging via CopyNetworks().
type Network struct {
servers []string
mu sync.RWMutex
backoff map[string]backoffState
servers []string
idxOffset int
mu sync.RWMutex
backoff map[string]backoffState
}

func (n *Network) String() string {
Expand All @@ -112,9 +113,9 @@ func (n *Network) String() string {
}
lines = append(lines, fmt.Sprintf("\tserver %v (backoff: next possible reconnect: %v)", srv, reconnect))
}
return fmt.Sprintf("[network %p with %d servers]\n",
n,
len(n.servers)) + strings.Join(lines, "\n")
return fmt.Sprintf("[network %p with %d servers]\n", n, len(n.servers)) +
fmt.Sprintf("index offset to the next server=%v\n", n.idxOffset) +
strings.Join(lines, "\n")
}

func newNetwork(networkname string) (*Network, error) {
Expand Down Expand Up @@ -166,8 +167,9 @@ func newNetwork(networkname string) (*Network, error) {
}

return &Network{
servers: servers,
backoff: make(map[string]backoffState),
servers: servers,
idxOffset: 0,
backoff: make(map[string]backoffState),
}, nil
}

Expand All @@ -180,7 +182,8 @@ func (n *Network) server(random bool) string {

for {
soonest := time.Duration(math.MaxInt64)
// Try to use a random server, but fall back to using the next

// If random, try to use a random server, but fall back to using the next
// available server in case the randomly picked server is unhealthy.
if random {
server := n.servers[rand.Intn(len(n.servers))]
Expand All @@ -189,10 +192,14 @@ func (n *Network) server(random bool) string {
return server
}
}
for _, server := range n.servers {
wait := n.backoff[server].next.Sub(time.Now())
// Try to use the next available server, searching in offset order
// (modified round robin).
for i := 0; i < len(n.servers); i++ {
idx := (i + n.idxOffset) % len(n.servers)
wait := n.backoff[n.servers[idx]].next.Sub(time.Now())
if wait <= 0 {
return server
n.idxOffset = (idx + 1) % len(n.servers)
return n.servers[idx]
}
if wait < soonest {
soonest = wait
Expand All @@ -212,6 +219,7 @@ func (n *Network) setServers(servers []string) {

// TODO(secure): we should clean up n.backoff from servers which no longer exist
n.servers = servers
n.idxOffset = 0
}

// prefer moves (or adds, if it doesn't already exist) the specified server to
Expand All @@ -229,6 +237,7 @@ func (n *Network) prefer(server string) {
}
}
n.servers = servers
n.idxOffset = 0
}

func (n *Network) failed(server string) {
Expand Down