raft

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 Go to latest Published: Mar 28, 2014 License: MIT, MIT Imports: 20 Imported by: 0

README

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Overview

This is a Go implementation of the Raft distributed consensus protocol. Raft is a protocol by which a cluster of nodes can maintain a replicated state machine. The state machine is kept in sync through the use of a replicated log.

For more details on Raft, you can read In Search of an Understandable Consensus Algorithm by Diego Ongaro and John Ousterhout.

Project Status

This library is feature complete but should be considered experimental until it has seen more usage. If you have any questions on implementing go-raft in your project please file an issue. There is an active community of developers who can help. go-raft is under the MIT license.

Features
  • Leader election
  • Log replication
  • Configuration changes
  • Log compaction
  • Unit tests
  • Fast Protobuf Log Encoding
  • HTTP transport
Projects

These projects are built on go-raft:

  • coreos/etcd - A highly-available key value store for shared configuration and service discovery.
  • goraft/raftd - A reference implementation for using the go-raft library for distributed consensus.
  • skynetservices/skydns - DNS for skynet or any other service discovery.
  • influxdb/influxdb - An open-source, distributed, time series, events, and metrics database.
  • Weed File System - A scalable distributed key-to-file system with O(1) disk access for each read.

If you have a project that you're using go-raft in, please add it to this README so others can see implementation examples.

Contact and Resources

  • raft-dev is a mailing list for discussion about best practices and implementation of Raft. Not goraft specific but helpful if you have questions.
  • Slides from Ben's talk which includes easy to understand diagrams of leader election and replication
  • The Raft Consensus homepage has links to additional raft implementations, slides to talks on Raft and general information

The Raft Protocol

This section provides a summary of the Raft protocol from a high level. For a more detailed explanation on the failover process and election terms please see the full paper describing the protocol: In Search of an Understandable Consensus Algorithm.

Overview

Maintaining state in a single process on a single server is easy. Your process is a single point of authority so there are no conflicts when reading and writing state. Even multi-threaded processes can rely on locks or coroutines to serialize access to the data.

However, in a distributed system there is no single point of authority. Servers can crash or the network between two machines can become unavailable or any number of other problems can occur.

A distributed consensus protocol is used for maintaining a consistent state across multiple servers in a cluster. Many distributed systems are built upon the Paxos protocol but Paxos can be difficult to understand and there are many gaps between Paxos and real world implementation.

An alternative is the Raft distributed consensus protocol by Diego Ongaro and John Ousterhout. Raft is a protocol built with understandability as a primary tenet and it centers around two things:

  1. Leader Election
  2. Replicated Log

With these two constructs, you can build a system that can maintain state across multiple servers -- even in the event of multiple failures.

Leader Election

The Raft protocol effectively works as a master-slave system whereby state changes are written to a single server in the cluster and are distributed out to the rest of the servers in the cluster. This simplifies the protocol since there is only one data authority and conflicts will not have to be resolved.

Raft ensures that there is only one leader at a time. It does this by performing elections among the nodes in the cluster and requiring that a node must receive a majority of the votes in order to become leader. For example, if you have 3 nodes in your cluster then a single node would need 2 votes in order to become the leader. For a 5 node cluster, a server would need 3 votes to become leader.

Replicated Log

To maintain state, a log of commands is maintained. Each command makes a change to the state of the server and the command is deterministic. By ensuring that this log is replicated identically between all the nodes in the cluster we can replicate the state at any point in time in the log by running each command sequentially.

Replicating the log under normal conditions is done by sending an AppendEntries RPC from the leader to each of the other servers in the cluster (called Peers). Each peer will append the entries from the leader through a 2-phase commit process which ensure that a majority of servers in the cluster have entries written to log.

Raft in Practice

Optimal Cluster Size

The primary consideration when choosing the node count in your Raft cluster is the number of nodes that can simultaneously fail. Because Raft requires a majority of nodes to be available to make progress, the number of node failures the cluster can tolerate is (n / 2) - 1.

This means that a 3-node cluster can tolerate 1 node failure. If 2 nodes fail then the cluster cannot commit entries or elect a new leader so progress stops. A 5-node cluster can tolerate 2 node failures. A 9-node cluster can tolerate 4 node failures. It is unlikely that 4 nodes will simultaneously fail so clusters larger than 9 nodes are not common.

Another consideration is performance. The leader must replicate log entries for each follower node so CPU and networking resources can quickly be bottlenecked under stress in a large cluster.

Scaling Raft

Once you grow beyond the maximum size of your cluster there are a few options for scaling Raft:

  1. Core nodes with dumb replication. This option requires you to maintain a small cluster (e.g. 5 nodes) that is involved in the Raft process and then replicate only committed log entries to the remaining nodes in the cluster. This works well if you have reads in your system that can be stale.

  2. Sharding. This option requires that you segment your data into different clusters. This option works well if you need very strong consistency and therefore need to read and write heavily from the leader.

If you have a very large cluster that you need to replicate to using Option 1 then you may want to look at performing hierarchical replication so that nodes can better share the load.

History

Ben Johnson started this library for use in his behavioral analytics database called Sky. He put it under the MIT license in the hopes that it would be useful for other projects too.

Documentation

Index

Constants

View Source 
const (
	Debug = 1
	Trace = 2
)
View Source 
const (
	StateChangeEventType  = "stateChange"
	LeaderChangeEventType = "leaderChange"
	TermChangeEventType   = "termChange"
	CommitEventType       = "commit"
	AddPeerEventType      = "addPeer"
	RemovePeerEventType   = "removePeer"

	HeartbeatIntervalEventType        = "heartbeatInterval"
	ElectionTimeoutThresholdEventType = "electionTimeoutThreshold"

	HeartbeatEventType = "heartbeat"
)
View Source 
const (
	Stopped      = "stopped"
	Follower     = "follower"
	Candidate    = "candidate"
	Leader       = "leader"
	Snapshotting = "snapshotting"
)
View Source 
const (
	MaxLogEntriesPerRequest         = 2000
	NumberOfLogEntriesAfterSnapshot = 200
)
View Source 
const (
	// DefaultHeartbeatInterval is the interval that the leader will send
	// AppendEntriesRequests to followers to maintain leadership.
	DefaultHeartbeatInterval = 50 * time.Millisecond

	DefaultElectionTimeout = 150 * time.Millisecond
)
View Source 
const ElectionTimeoutThresholdPercent = 0.8

ElectionTimeoutThresholdPercent specifies the threshold at which the server will dispatch warning events that the heartbeat RTT is too close to the election timeout.

Variables

View Source 
var CommandTimeoutError = errors.New("raft: Command timeout")
View Source 
var DuplicatePeerError = errors.New("raft.Server: Duplicate peer")
View Source 
var NotLeaderError = errors.New("raft.Server: Not current leader")

Functions

func LogLevel 

func LogLevel() int

func RegisterCommand 

func RegisterCommand(command Command)

Registers a command by storing a reference to an instance of it.

func SetLogLevel 

func SetLogLevel(level int)

Types

type AppendEntriesRequest 

type AppendEntriesRequest struct {
	Term         uint64
	PrevLogIndex uint64
	PrevLogTerm  uint64
	CommitIndex  uint64
	LeaderName   string
	Entries      []*protobuf.LogEntry
}

The request sent to a server to append entries to the log.

func (*AppendEntriesRequest) Decode 

func (req *AppendEntriesRequest) Decode(r io.Reader) (int, error)

Decodes the AppendEntriesRequest from a buffer. Returns the number of bytes read and any error that occurs.

func (*AppendEntriesRequest) Encode 

func (req *AppendEntriesRequest) Encode(w io.Writer) (int, error)

Encodes the AppendEntriesRequest to a buffer. Returns the number of bytes written and any error that may have occurred.

type AppendEntriesResponse 

type AppendEntriesResponse struct {
	// contains filtered or unexported fields
}

The response returned from a server appending entries to the log.

func (*AppendEntriesResponse) CommitIndex 

func (aer *AppendEntriesResponse) CommitIndex() uint64

func (*AppendEntriesResponse) Decode 

func (resp *AppendEntriesResponse) Decode(r io.Reader) (int, error)

Decodes the AppendEntriesResponse from a buffer. Returns the number of bytes read and any error that occurs.

func (*AppendEntriesResponse) Encode 

func (resp *AppendEntriesResponse) Encode(w io.Writer) (int, error)

Encodes the AppendEntriesResponse to a buffer. Returns the number of bytes written and any error that may have occurred.

func (*AppendEntriesResponse) Index 

func (aer *AppendEntriesResponse) Index() uint64

func (*AppendEntriesResponse) Success 

func (aer *AppendEntriesResponse) Success() bool

func (*AppendEntriesResponse) Term 

func (aer *AppendEntriesResponse) Term() uint64

type Command 

type Command interface {
	CommandName() string
}

Command represents an action to be taken on the replicated state machine.

type CommandApply 

type CommandApply interface {
	Apply(Context) (interface{}, error)
}

CommandApply represents the interface to apply a command to the server.

type CommandEncoder 

type CommandEncoder interface {
	Encode(w io.Writer) error
	Decode(r io.Reader) error
}

type Config 

type Config struct {
	CommitIndex uint64 `json:"commitIndex"`
	// TODO decide what we need to store in peer struct
	Peers []*Peer `json:"peers"`
}

type Context 

type Context interface {
	Server() Server
	CurrentTerm() uint64
	CurrentIndex() uint64
	CommitIndex() uint64
}

Context represents the current state of the server. It is passed into a command when the command is being applied since the server methods are locked.

type DefaultJoinCommand 

type DefaultJoinCommand struct {
	Name             string `json:"name"`
	ConnectionString string `json:"connectionString"`
}

Join command

func (*DefaultJoinCommand) Apply 

func (c *DefaultJoinCommand) Apply(server Server) (interface{}, error)

func (*DefaultJoinCommand) CommandName 

func (c *DefaultJoinCommand) CommandName() string

The name of the Join command in the log

func (*DefaultJoinCommand) NodeName 

func (c *DefaultJoinCommand) NodeName() string

type DefaultLeaveCommand 

type DefaultLeaveCommand struct {
	Name string `json:"name"`
}

Leave command

func (*DefaultLeaveCommand) Apply 

func (c *DefaultLeaveCommand) Apply(server Server) (interface{}, error)

func (*DefaultLeaveCommand) CommandName 

func (c *DefaultLeaveCommand) CommandName() string

The name of the Leave command in the log

func (*DefaultLeaveCommand) NodeName 

func (c *DefaultLeaveCommand) NodeName() string

type Event 

type Event interface {
	Type() string
	Source() interface{}
	Value() interface{}
	PrevValue() interface{}
}

Event represents an action that occurred within the Raft library. Listeners can subscribe to event types by using the Server.AddEventListener() function.

type EventListener 

type EventListener func(Event)

EventListener is a function that can receive event notifications.

type HTTPMuxer 

type HTTPMuxer interface {
	HandleFunc(string, func(http.ResponseWriter, *http.Request))
}

type HTTPTransporter 

type HTTPTransporter struct {
	DisableKeepAlives bool

	Transport *http.Transport
	// contains filtered or unexported fields
}

An HTTPTransporter is a default transport layer used to communicate between multiple servers.

func NewHTTPTransporter 

func NewHTTPTransporter(prefix string) *HTTPTransporter

Creates a new HTTP transporter with the given path prefix.

func (*HTTPTransporter) AppendEntriesPath 

func (t *HTTPTransporter) AppendEntriesPath() string

Retrieves the AppendEntries path.

func (*HTTPTransporter) Install 

func (t *HTTPTransporter) Install(server Server, mux HTTPMuxer)

Applies Raft routes to an HTTP router for a given server.

func (*HTTPTransporter) Prefix 

func (t *HTTPTransporter) Prefix() string

Retrieves the path prefix used by the transporter.

func (*HTTPTransporter) RequestVotePath 

func (t *HTTPTransporter) RequestVotePath() string

Retrieves the RequestVote path.

func (*HTTPTransporter) SendAppendEntriesRequest 

func (t *HTTPTransporter) SendAppendEntriesRequest(server Server, peer *Peer, req *AppendEntriesRequest) *AppendEntriesResponse

Sends an AppendEntries RPC to a peer.

func (*HTTPTransporter) SendSnapshotRecoveryRequest 

func (t *HTTPTransporter) SendSnapshotRecoveryRequest(server Server, peer *Peer, req *SnapshotRecoveryRequest) *SnapshotRecoveryResponse

Sends a SnapshotRequest RPC to a peer.

func (*HTTPTransporter) SendSnapshotRequest 

func (t *HTTPTransporter) SendSnapshotRequest(server Server, peer *Peer, req *SnapshotRequest) *SnapshotResponse

Sends a SnapshotRequest RPC to a peer.

func (*HTTPTransporter) SendVoteRequest 

func (t *HTTPTransporter) SendVoteRequest(server Server, peer *Peer, req *RequestVoteRequest) *RequestVoteResponse

Sends a RequestVote RPC to a peer.

func (*HTTPTransporter) SnapshotPath 

func (t *HTTPTransporter) SnapshotPath() string

Retrieves the Snapshot path.

func (*HTTPTransporter) SnapshotRecoveryPath 

func (t *HTTPTransporter) SnapshotRecoveryPath() string

Retrieves the SnapshotRecovery path.

type JoinCommand 

type JoinCommand interface {
	Command
	NodeName() string
}

Join command interface

type LeaveCommand 

type LeaveCommand interface {
	Command
	NodeName() string
}

Leave command interface

type Log 

type Log struct {
	ApplyFunc func(*LogEntry, Command) (interface{}, error)
	// contains filtered or unexported fields
}

A log is a collection of log entries that are persisted to durable storage.

func (*Log) CommitIndex 

func (l *Log) CommitIndex() uint64

The last committed index in the log.

type LogEntry 

type LogEntry struct {
	Position int64 // position in the log file
	// contains filtered or unexported fields
}

A log entry stores a single item in the log.

func (*LogEntry) Command 

func (e *LogEntry) Command() []byte

func (*LogEntry) CommandName 

func (e *LogEntry) CommandName() string

func (*LogEntry) Decode 

func (e *LogEntry) Decode(r io.Reader) (int, error)

Decodes the log entry from a buffer. Returns the number of bytes read and any error that occurs.

func (*LogEntry) Encode 

func (e *LogEntry) Encode(w io.Writer) (int, error)

Encodes the log entry to a buffer. Returns the number of bytes written and any error that may have occurred.

func (*LogEntry) Index 

func (e *LogEntry) Index() uint64

func (*LogEntry) Term 

func (e *LogEntry) Term() uint64

type NOPCommand 

type NOPCommand struct {
}

NOP command

func (NOPCommand) Apply 

func (c NOPCommand) Apply(server Server) (interface{}, error)

func (NOPCommand) CommandName 

func (c NOPCommand) CommandName() string

The name of the NOP command in the log

func (NOPCommand) Decode 

func (c NOPCommand) Decode(r io.Reader) error

func (NOPCommand) Encode 

func (c NOPCommand) Encode(w io.Writer) error

type Peer 

type Peer struct {
	Name             string `json:"name"`
	ConnectionString string `json:"connectionString"`
	// contains filtered or unexported fields
}

A peer is a reference to another server involved in the consensus protocol.

func (*Peer) LastActivity 

func (p *Peer) LastActivity() time.Time

LastActivity returns the last time any response was received from the peer.

type RequestVoteRequest 

type RequestVoteRequest struct {
	Term          uint64
	LastLogIndex  uint64
	LastLogTerm   uint64
	CandidateName string
	// contains filtered or unexported fields
}

The request sent to a server to vote for a candidate to become a leader.

func (*RequestVoteRequest) Decode 

func (req *RequestVoteRequest) Decode(r io.Reader) (int, error)

Decodes the RequestVoteRequest from a buffer. Returns the number of bytes read and any error that occurs.

func (*RequestVoteRequest) Encode 

func (req *RequestVoteRequest) Encode(w io.Writer) (int, error)

Encodes the RequestVoteRequest to a buffer. Returns the number of bytes written and any error that may have occurred.

type RequestVoteResponse 

type RequestVoteResponse struct {
	Term        uint64
	VoteGranted bool
	// contains filtered or unexported fields
}

The response returned from a server after a vote for a candidate to become a leader.

func (*RequestVoteResponse) Decode 

func (resp *RequestVoteResponse) Decode(r io.Reader) (int, error)

Decodes the RequestVoteResponse from a buffer. Returns the number of bytes read and any error that occurs.

func (*RequestVoteResponse) Encode 

func (resp *RequestVoteResponse) Encode(w io.Writer) (int, error)

Encodes the RequestVoteResponse to a buffer. Returns the number of bytes written and any error that may have occurred.

type Server 

type Server interface {
	Name() string
	Context() interface{}
	StateMachine() StateMachine
	Leader() string
	State() string
	Path() string
	LogPath() string
	SnapshotPath(lastIndex uint64, lastTerm uint64) string
	Term() uint64
	CommitIndex() uint64
	VotedFor() string
	MemberCount() int
	QuorumSize() int
	IsLogEmpty() bool
	LogEntries() []*LogEntry
	LastCommandName() string
	GetState() string
	ElectionTimeout() time.Duration
	SetElectionTimeout(duration time.Duration)
	HeartbeatInterval() time.Duration
	SetHeartbeatInterval(duration time.Duration)
	Transporter() Transporter
	SetTransporter(t Transporter)
	AppendEntries(req *AppendEntriesRequest) *AppendEntriesResponse
	RequestVote(req *RequestVoteRequest) *RequestVoteResponse
	RequestSnapshot(req *SnapshotRequest) *SnapshotResponse
	SnapshotRecoveryRequest(req *SnapshotRecoveryRequest) *SnapshotRecoveryResponse
	AddPeer(name string, connectiongString string) error
	RemovePeer(name string) error
	Peers() map[string]*Peer
	Start() error
	Stop()
	Running() bool
	Do(command Command) (interface{}, error)
	TakeSnapshot() error
	LoadSnapshot() error
	AddEventListener(string, EventListener)
	FlushCommitIndex()
}

A server is involved in the consensus protocol and can act as a follower, candidate or a leader.

func NewServer 

func NewServer(name string, path string, transporter Transporter, stateMachine StateMachine, ctx interface{}, connectionString string) (Server, error)

Creates a new server with a log at the given path. transporter must not be nil. stateMachine can be nil if snapshotting and log compaction is to be disabled. context can be anything (including nil) and is not used by the raft package except returned by Server.Context(). connectionString can be anything.

type Snapshot 

type Snapshot struct {
	LastIndex uint64 `json:"lastIndex"`
	LastTerm  uint64 `json:"lastTerm"`

	// Cluster configuration.
	Peers []*Peer `json:"peers"`
	State []byte  `json:"state"`
	Path  string  `json:"path"`
}

Snapshot represents an in-memory representation of the current state of the system.

type SnapshotRecoveryRequest 

type SnapshotRecoveryRequest struct {
	LeaderName string
	LastIndex  uint64
	LastTerm   uint64
	Peers      []*Peer
	State      []byte
}

The request sent to a server to start from the snapshot.

func (*SnapshotRecoveryRequest) Decode 

func (req *SnapshotRecoveryRequest) Decode(r io.Reader) (int, error)

Decodes the SnapshotRecoveryRequest from a buffer. Returns the number of bytes read and any error that occurs.

func (*SnapshotRecoveryRequest) Encode 

func (req *SnapshotRecoveryRequest) Encode(w io.Writer) (int, error)

Encodes the SnapshotRecoveryRequest to a buffer. Returns the number of bytes written and any error that may have occurred.

type SnapshotRecoveryResponse 

type SnapshotRecoveryResponse struct {
	Term        uint64
	Success     bool
	CommitIndex uint64
}

The response returned from a server appending entries to the log.

func (*SnapshotRecoveryResponse) Decode 

func (req *SnapshotRecoveryResponse) Decode(r io.Reader) (int, error)

Decodes the SnapshotRecoveryResponse from a buffer.

func (*SnapshotRecoveryResponse) Encode 

func (req *SnapshotRecoveryResponse) Encode(w io.Writer) (int, error)

Encode writes the response to a writer. Returns the number of bytes written and any error that occurs.

type SnapshotRequest 

type SnapshotRequest struct {
	LeaderName string
	LastIndex  uint64
	LastTerm   uint64
}

The request sent to a server to start from the snapshot.

func (*SnapshotRequest) Decode 

func (req *SnapshotRequest) Decode(r io.Reader) (int, error)

Decodes the SnapshotRequest from a buffer. Returns the number of bytes read and any error that occurs.

func (*SnapshotRequest) Encode 

func (req *SnapshotRequest) Encode(w io.Writer) (int, error)

Encodes the SnapshotRequest to a buffer. Returns the number of bytes written and any error that may have occurred.

type SnapshotResponse 

type SnapshotResponse struct {
	Success bool `json:"success"`
}

The response returned if the follower entered snapshot state

func (*SnapshotResponse) Decode 

func (resp *SnapshotResponse) Decode(r io.Reader) (int, error)

Decodes the SnapshotResponse from a buffer. Returns the number of bytes read and any error that occurs.

func (*SnapshotResponse) Encode 

func (resp *SnapshotResponse) Encode(w io.Writer) (int, error)

Encodes the SnapshotResponse to a buffer. Returns the number of bytes written and any error that may have occurred.

type StateMachine 

type StateMachine interface {
	Save() ([]byte, error)
	Recovery([]byte) error
}

StateMachine is the interface for allowing the host application to save and recovery the state machine. This makes it possible to make snapshots and compact the log.

type Transporter 

type Transporter interface {
	SendVoteRequest(server Server, peer *Peer, req *RequestVoteRequest) *RequestVoteResponse
	SendAppendEntriesRequest(server Server, peer *Peer, req *AppendEntriesRequest) *AppendEntriesResponse
	SendSnapshotRequest(server Server, peer *Peer, req *SnapshotRequest) *SnapshotResponse
	SendSnapshotRecoveryRequest(server Server, peer *Peer, req *SnapshotRecoveryRequest) *SnapshotRecoveryResponse
}

Transporter is the interface for allowing the host application to transport requests to other nodes.

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