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package zk
import (
"fmt"
"net"
"sync"
)
// DNSHostProvider is the default HostProvider. It currently matches
// the Java StaticHostProvider, resolving hosts from DNS once during
// the call to Init. It could be easily extended to re-query DNS
// periodically or if there is trouble connecting.
type DNSHostProvider struct {
mu sync.Mutex // Protects everything, so we can add asynchronous updates later.
servers []string
curr int
last int
lookupHost func(string) ([]string, error) // Override of net.LookupHost, for testing.
}
// Init is called first, with the servers specified in the connection
// string. It uses DNS to look up addresses for each server, then
// shuffles them all together.
func (hp *DNSHostProvider) Init(servers []string) error {
hp.mu.Lock()
defer hp.mu.Unlock()
lookupHost := hp.lookupHost
if lookupHost == nil {
lookupHost = net.LookupHost
}
found := []string{}
for _, server := range servers {
host, port, err := net.SplitHostPort(server)
if err != nil {
return err
}
addrs, err := lookupHost(host)
if err != nil {
return err
}
for _, addr := range addrs {
found = append(found, net.JoinHostPort(addr, port))
}
}
if len(found) == 0 {
return fmt.Errorf("No hosts found for addresses %q", servers)
}
// Randomize the order of the servers to avoid creating hotspots
stringShuffle(found)
hp.servers = found
hp.curr = -1
hp.last = -1
return nil
}
// Len returns the number of servers available
func (hp *DNSHostProvider) Len() int {
hp.mu.Lock()
defer hp.mu.Unlock()
return len(hp.servers)
}
// Next returns the next server to connect to. retryStart will be true
// if we've looped through all known servers without Connected() being
// called.
func (hp *DNSHostProvider) Next() (server string, retryStart bool) {
hp.mu.Lock()
defer hp.mu.Unlock()
hp.curr = (hp.curr + 1) % len(hp.servers)
retryStart = hp.curr == hp.last
if hp.last == -1 {
hp.last = 0
}
return hp.servers[hp.curr], retryStart
}
// Connected notifies the HostProvider of a successful connection.
func (hp *DNSHostProvider) Connected() {
hp.mu.Lock()
defer hp.mu.Unlock()
hp.last = hp.curr
}
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