Skip to content
Permalink
Branch: master
Find file Copy path
Find file Copy path
Fetching contributors…
Cannot retrieve contributors at this time
364 lines (330 sloc) 11.8 KB
package raft
import "fmt"
// ServerSuffrage determines whether a Server in a Configuration gets a vote.
type ServerSuffrage int
// Note: Don't renumber these, since the numbers are written into the log.
const (
// Voter is a server whose vote is counted in elections and whose match index
// is used in advancing the leader's commit index.
Voter ServerSuffrage = iota
// Nonvoter is a server that receives log entries but is not considered for
// elections or commitment purposes.
Nonvoter
// Staging is a server that acts like a nonvoter with one exception: once a
// staging server receives enough log entries to be sufficiently caught up to
// the leader's log, the leader will invoke a membership change to change
// the Staging server to a Voter.
Staging
)
func (s ServerSuffrage) String() string {
switch s {
case Voter:
return "Voter"
case Nonvoter:
return "Nonvoter"
case Staging:
return "Staging"
}
return "ServerSuffrage"
}
// ConfigurationStore provides an interface that can optionally be implemented by FSMs
// to store configuration updates made in the replicated log. In general this is only
// necessary for FSMs that mutate durable state directly instead of applying changes
// in memory and snapshotting periodically. By storing configuration changes, the
// persistent FSM state can behave as a complete snapshot, and be able to recover
// without an external snapshot just for persisting the raft configuration.
type ConfigurationStore interface {
// ConfigurationStore is a superset of the FSM functionality
FSM
// StoreConfiguration is invoked once a log entry containing a configuration
// change is committed. It takes the index at which the configuration was
// written and the configuration value.
StoreConfiguration(index uint64, configuration Configuration)
}
type nopConfigurationStore struct{}
func (s nopConfigurationStore) StoreConfiguration(_ uint64, _ Configuration) {}
// ServerID is a unique string identifying a server for all time.
type ServerID string
// ServerAddress is a network address for a server that a transport can contact.
type ServerAddress string
// Server tracks the information about a single server in a configuration.
type Server struct {
// Suffrage determines whether the server gets a vote.
Suffrage ServerSuffrage
// ID is a unique string identifying this server for all time.
ID ServerID
// Address is its network address that a transport can contact.
Address ServerAddress
}
// Configuration tracks which servers are in the cluster, and whether they have
// votes. This should include the local server, if it's a member of the cluster.
// The servers are listed no particular order, but each should only appear once.
// These entries are appended to the log during membership changes.
type Configuration struct {
Servers []Server
}
// Clone makes a deep copy of a Configuration.
func (c *Configuration) Clone() (copy Configuration) {
copy.Servers = append(copy.Servers, c.Servers...)
return
}
// ConfigurationChangeCommand is the different ways to change the cluster
// configuration.
type ConfigurationChangeCommand uint8
const (
// AddStaging makes a server Staging unless its Voter.
AddStaging ConfigurationChangeCommand = iota
// AddNonvoter makes a server Nonvoter unless its Staging or Voter.
AddNonvoter
// DemoteVoter makes a server Nonvoter unless its absent.
DemoteVoter
// RemoveServer removes a server entirely from the cluster membership.
RemoveServer
// Promote is created automatically by a leader; it turns a Staging server
// into a Voter.
Promote
)
func (c ConfigurationChangeCommand) String() string {
switch c {
case AddStaging:
return "AddStaging"
case AddNonvoter:
return "AddNonvoter"
case DemoteVoter:
return "DemoteVoter"
case RemoveServer:
return "RemoveServer"
case Promote:
return "Promote"
}
return "ConfigurationChangeCommand"
}
// configurationChangeRequest describes a change that a leader would like to
// make to its current configuration. It's used only within a single server
// (never serialized into the log), as part of `configurationChangeFuture`.
type configurationChangeRequest struct {
command ConfigurationChangeCommand
serverID ServerID
serverAddress ServerAddress // only present for AddStaging, AddNonvoter
// prevIndex, if nonzero, is the index of the only configuration upon which
// this change may be applied; if another configuration entry has been
// added in the meantime, this request will fail.
prevIndex uint64
}
// configurations is state tracked on every server about its Configurations.
// Note that, per Diego's dissertation, there can be at most one uncommitted
// configuration at a time (the next configuration may not be created until the
// prior one has been committed).
//
// One downside to storing just two configurations is that if you try to take a
// snapshot when your state machine hasn't yet applied the committedIndex, we
// have no record of the configuration that would logically fit into that
// snapshot. We disallow snapshots in that case now. An alternative approach,
// which LogCabin uses, is to track every configuration change in the
// log.
type configurations struct {
// committed is the latest configuration in the log/snapshot that has been
// committed (the one with the largest index).
committed Configuration
// committedIndex is the log index where 'committed' was written.
committedIndex uint64
// latest is the latest configuration in the log/snapshot (may be committed
// or uncommitted)
latest Configuration
// latestIndex is the log index where 'latest' was written.
latestIndex uint64
}
// Clone makes a deep copy of a configurations object.
func (c *configurations) Clone() (copy configurations) {
copy.committed = c.committed.Clone()
copy.committedIndex = c.committedIndex
copy.latest = c.latest.Clone()
copy.latestIndex = c.latestIndex
return
}
// hasVote returns true if the server identified by 'id' is a Voter in the
// provided Configuration.
func hasVote(configuration Configuration, id ServerID) bool {
for _, server := range configuration.Servers {
if server.ID == id {
return server.Suffrage == Voter
}
}
return false
}
// checkConfiguration tests a cluster membership configuration for common
// errors.
func checkConfiguration(configuration Configuration) error {
idSet := make(map[ServerID]bool)
addressSet := make(map[ServerAddress]bool)
var voters int
for _, server := range configuration.Servers {
if server.ID == "" {
return fmt.Errorf("Empty ID in configuration: %v", configuration)
}
if server.Address == "" {
return fmt.Errorf("Empty address in configuration: %v", server)
}
if idSet[server.ID] {
return fmt.Errorf("Found duplicate ID in configuration: %v", server.ID)
}
idSet[server.ID] = true
if addressSet[server.Address] {
return fmt.Errorf("Found duplicate address in configuration: %v", server.Address)
}
addressSet[server.Address] = true
if server.Suffrage == Voter {
voters++
}
}
if voters == 0 {
return fmt.Errorf("Need at least one voter in configuration: %v", configuration)
}
return nil
}
// nextConfiguration generates a new Configuration from the current one and a
// configuration change request. It's split from appendConfigurationEntry so
// that it can be unit tested easily.
func nextConfiguration(current Configuration, currentIndex uint64, change configurationChangeRequest) (Configuration, error) {
if change.prevIndex > 0 && change.prevIndex != currentIndex {
return Configuration{}, fmt.Errorf("Configuration changed since %v (latest is %v)", change.prevIndex, currentIndex)
}
configuration := current.Clone()
switch change.command {
case AddStaging:
// TODO: barf on new address?
newServer := Server{
// TODO: This should add the server as Staging, to be automatically
// promoted to Voter later. However, the promotion to Voter is not yet
// implemented, and doing so is not trivial with the way the leader loop
// coordinates with the replication goroutines today. So, for now, the
// server will have a vote right away, and the Promote case below is
// unused.
Suffrage: Voter,
ID: change.serverID,
Address: change.serverAddress,
}
found := false
for i, server := range configuration.Servers {
if server.ID == change.serverID {
if server.Suffrage == Voter {
configuration.Servers[i].Address = change.serverAddress
} else {
configuration.Servers[i] = newServer
}
found = true
break
}
}
if !found {
configuration.Servers = append(configuration.Servers, newServer)
}
case AddNonvoter:
newServer := Server{
Suffrage: Nonvoter,
ID: change.serverID,
Address: change.serverAddress,
}
found := false
for i, server := range configuration.Servers {
if server.ID == change.serverID {
if server.Suffrage != Nonvoter {
configuration.Servers[i].Address = change.serverAddress
} else {
configuration.Servers[i] = newServer
}
found = true
break
}
}
if !found {
configuration.Servers = append(configuration.Servers, newServer)
}
case DemoteVoter:
for i, server := range configuration.Servers {
if server.ID == change.serverID {
configuration.Servers[i].Suffrage = Nonvoter
break
}
}
case RemoveServer:
for i, server := range configuration.Servers {
if server.ID == change.serverID {
configuration.Servers = append(configuration.Servers[:i], configuration.Servers[i+1:]...)
break
}
}
case Promote:
for i, server := range configuration.Servers {
if server.ID == change.serverID && server.Suffrage == Staging {
configuration.Servers[i].Suffrage = Voter
break
}
}
}
// Make sure we didn't do something bad like remove the last voter
if err := checkConfiguration(configuration); err != nil {
return Configuration{}, err
}
return configuration, nil
}
// encodePeers is used to serialize a Configuration into the old peers format.
// This is here for backwards compatibility when operating with a mix of old
// servers and should be removed once we deprecate support for protocol version 1.
func encodePeers(configuration Configuration, trans Transport) []byte {
// Gather up all the voters, other suffrage types are not supported by
// this data format.
var encPeers [][]byte
for _, server := range configuration.Servers {
if server.Suffrage == Voter {
encPeers = append(encPeers, trans.EncodePeer(server.ID, server.Address))
}
}
// Encode the entire array.
buf, err := encodeMsgPack(encPeers)
if err != nil {
panic(fmt.Errorf("failed to encode peers: %v", err))
}
return buf.Bytes()
}
// decodePeers is used to deserialize an old list of peers into a Configuration.
// This is here for backwards compatibility with old log entries and snapshots;
// it should be removed eventually.
func decodePeers(buf []byte, trans Transport) Configuration {
// Decode the buffer first.
var encPeers [][]byte
if err := decodeMsgPack(buf, &encPeers); err != nil {
panic(fmt.Errorf("failed to decode peers: %v", err))
}
// Deserialize each peer.
var servers []Server
for _, enc := range encPeers {
p := trans.DecodePeer(enc)
servers = append(servers, Server{
Suffrage: Voter,
ID: ServerID(p),
Address: ServerAddress(p),
})
}
return Configuration{
Servers: servers,
}
}
// encodeConfiguration serializes a Configuration using MsgPack, or panics on
// errors.
func encodeConfiguration(configuration Configuration) []byte {
buf, err := encodeMsgPack(configuration)
if err != nil {
panic(fmt.Errorf("failed to encode configuration: %v", err))
}
return buf.Bytes()
}
// decodeConfiguration deserializes a Configuration using MsgPack, or panics on
// errors.
func decodeConfiguration(buf []byte) Configuration {
var configuration Configuration
if err := decodeMsgPack(buf, &configuration); err != nil {
panic(fmt.Errorf("failed to decode configuration: %v", err))
}
return configuration
}
You can’t perform that action at this time.