// Campaign handles leader election. Basically this just acquires a lock on the LeadershipKey and whoever gets the lock
// is the leader. A re-election occurs when there are changes in the LeadershipKey.
func (c *Candidate) campaign() {
c.retrieveNode()
c.retrieveSession()
consul := c.consulClient()
logrus.Debugf("%s is running for election with session %s.", c.node, c.session)
kvpair := &consulapi.KVPair{
Key: c.LeadershipKey,
Value: []byte(c.node),
Session: c.session,
}
acquired, _, err := consul.KV().Acquire(kvpair, nil)
if err != nil {
logrus.Errorln("Failed to run Consul KV Acquire:", err)
}
if acquired {
logrus.Infof("%s has become the leader.", c.node)
}
kv, _, _ := consul.KV().Get(c.LeadershipKey, nil)
if kv != nil && kv.Session != "" {
logrus.Debugf("%s is the current leader.", string(kv.Value))
logrus.Debugf("%s is waiting for changes in '%s'.", c.node, c.LeadershipKey)
latestIndex := kv.ModifyIndex
options := &consulapi.QueryOptions{
WaitIndex: latestIndex,
}
consul.KV().Get(c.LeadershipKey, options)
}
time.Sleep(15 * time.Second)
c.campaign()
}
// IsLeader returns true if the current agent is the leader.
func (c *Candidate) IsLeader() bool {
consul := c.consulClient()
c.retrieveNode()
c.retrieveSession()
kv, _, err := consul.KV().Get(c.LeadershipKey, nil)
if err != nil {
logrus.Errorln("Unable to check for leadership:", err)
return false
}
if kv == nil {
logrus.Warnf("Leadership key '%s' is missing in Consuk KV.", c.LeadershipKey)
return false
}
return c.node == string(kv.Value) && c.session == kv.Session
}
// RetrieveSession retrieves the existing session needed to run leader election. If a session does not exist, a new
// session is created with the LeadershipKey as the name.
func (c *Candidate) retrieveSession() {
consul := c.consulClient()
if sessions, _, err := consul.Session().List(nil); err != nil {
logrus.Warnln("Unable to retrieve list of sessions.")
} else {
for _, session := range sessions {
if session.Name == c.LeadershipKey && session.Node == c.node {
c.session = session.ID
return
}
}
}
newSession := &consulapi.SessionEntry{
Name: c.LeadershipKey,
}
if sessionId, _, err := consul.Session().Create(newSession, nil); err != nil {
logrus.Errorln("Unable to create new sessions:", err)
} else {
c.session = sessionId
}
}