6.824 - Spring 2016

6.824 Lab 2: Raft

Due: Fri Feb 26, 11:59pm


This is the first in a series of labs in which you'll build a fault-tolerant key/value storage system. You'll start in this lab by implementing Raft, a replicated state machine protocol. In the next lab lab you'll build a key/value service on top of Raft. Then you will “shard” your service for higher performance, and finally implement transactional operations across shards.

A replicated service (e.g., key/value database) uses Raft to help manage its replica servers. The point of having replicas is so that the service can continue operating even if some of the replicas experience failures (crashes or a broken or flaky network). The challenge is that, due to these failures, the replicas won't always hold identical data; Raft helps the service sort out what the correct data is.

Raft's basic approach is to implement a replicated state machine. Raft organizes client requests into a sequence, called the log, and ensures that all the replicas agree on the the contents of the log. Each replica executes the client requests in the log in the order they appear in the log, applying those requests to the service's state. Since all the live replicas see the same log contents, they all execute the same requests in the same order, and thus continue to have identical service state. If a server fails but later recovers, Raft takes care of bringing its log up to date. Raft will continue to operate as long as at least a majority of the servers are alive and can talk to each other. If there is no such majority, Raft will make no progress, but will pick up where it left off as soon as a majority is alive again.

In this lab you'll implement Raft in the form of Go object type with associated methods, meant to be used as a module in a larger service. A set of Raft instances talk to each other with RPC to maintain replicated logs. Your Raft interface will support an indefinite sequence of numbered commands, also called log entries. The entries are numbered with index numbers. The log entry with a given index will eventually be committed. At that point, your Raft should send the log entry to the larger service for it to execute.

Only RPC may be used for interaction between different Raft instances. For example, different instances of your Raft implementation are not allowed to share Go variables. Your implementation should not use files at all.

In this lab you'll implement most of the Raft design described in the extended paper, including saving persistent state and reading it after a node fails and then restarts. You will not implement cluster membership changes (Section 6) or log compaction / snapshotting (Section 7).

You should consult the extended Raft paper and the Raft lecture notes. You may also find this illustrated Raft guide useful to get a sense of the high-level workings of Raft. For a wider perspective, have a look at Paxos, Chubby, Paxos Made Live, Spanner, Zookeeper, Harp, Viewstamped Replication, and Bolosky et al.

Collaboration Policy

You must write all the code you hand in for 6.824, except for code that we give you as part of the assignment. You are not allowed to look at anyone else's solution, you are not allowed to look at code from previous years, and you are not allowed to look at other Raft implementations. You may discuss the assignments with other students, but you may not look at or copy each others' code. Please do not publish your code or make it available to future 6.824 students -- for example, please do not make your code visible on GitHub (instead, create a private repository on MIT's GitHub deployment).

Getting Started

Do a git pull to get the latest lab software. We supply you with skeleton code and tests in src/raft, and a simple RPC-like system in src/labrpc.

To get up and running, execute the following commands:

$ setup ggo_v1.5
$ cd ~/6.824
$ git pull
$ cd src/raft
$ GOPATH=~/6.824
$ export GOPATH
$ go test
Test: initial election ...
--- FAIL: TestInitialElection (5.03s)
	config.go:270: expected one leader, got 0
Test: election after network failure ...
--- FAIL: TestReElection (5.03s)
	config.go:270: expected one leader, got 0
When you're done, your implementation should pass all the tests in the src/raft directory:
$ go test
Test: initial election ...
  ... Passed
Test: election after network failure ...
  ... Passed
ok  	raft	162.413s

Your Job

You should implement Raft by adding code to raft/raft.go. In that file you'll find a bit of skeleton code, plus some examples of how to send and receive RPCs, and examples of how to save and restore persistent state.

Your implementation must support the following interface, which the tester and (eventually) your key/value server will use. You'll find more details in comments in raft.go.

// create a new Raft server instance:
rf := Make(peers, me, persister, applyCh)

// start agreement on a new log entry:
rf.Start(command interface{}) (index, term, isleader)

// ask a Raft for its current term, and whether it thinks it is leader
rf.GetState() (term, isLeader)

// each time a new entry is committed to the log, each Raft peer
// should send an ApplyMsg to the service (or tester).
type ApplyMsg

A service calls Make(peers,me,…) to create a Raft peer. The peers argument is an array of established RPC connections, one to each Raft peer (including this one). The me argument is the index of this peer in the peers array. Start(command) asks Raft to start the processing to append the command to the replicated log. Start() should return immediately, without waiting for for this process to complete. The service expects your implementation to send an ApplyMsg for each new committed log entry to the applyCh argument to Make().

Your Raft peers should exchange RPCs using the labrpc Go package that we provide to you. It is modeled after Go's rpc library, but internally uses Go channels rather than sockets. raft.go contains some example code that sends an RPC (sendRequestVote()) and that handles an incoming RPC (RequestVote()).

Implement leader election and heartbeats (empty AppendEntries calls). This should be sufficient for a single leader to be elected, and to stay the leader, in the absence of failures. Once you have this working, you should be able to pass the first two "go test" tests.

While being able to elect a leader is useful, we want to use Raft to keep a consistent, replicated log of operations. To do so, we need to have the servers accept client operations through Start(), and insert them into the log. In Raft, only the leader is allowed to append to the log, and should disseminate new entries to other servers by including them in its outgoing AppendEntries RPCs.

Implement the leader and follower code to append new log entries. This will involve implementing Start(), completing the AppendEntries RPC structs, sending them, and fleshing out the AppendEntry RPC handler. Your goal should first be to pass the TestBasicAgree() test (in test_test.go). Once you have that working, you should try to get all the tests before the "basic persistence" test to pass.

A Raft-based server must be able to pick up where it left off, and continue if the computer it's on reboots. This requires that Raft keep persistent state that survives a reboot (the paper's Figure 2 mentions which state should be persistent).

A “real” implementation would do this by writing Raft's persistent state to disk each time it changes, and reading the latest saved state from disk when restarting after a reboot. Your implementation won't use the disk; instead, it will save and restore persistent state from a Persister object (see persister.go). Whoever calls Make() supplies a Persister that initially holds Raft's most recently persisted state (if any). Raft should initialize its state from that Persister, and should use it to save its persistent state each time the state changes. You can use the ReadRaftState() and SaveRaftState() methods for this respectively.

Implement persistence by first adding code to serialize any state that needs persisting in persist(), and to unserialize that same state in readPersist(). You now need to determine at what points in the Raft protocol your servers are required to persist their state, and insert calls to persist() in those places. Once this code is complete, you should pass the remaining tests. You may want to first try and pass the "basic persistence" test (go test -run 'TestPersist1$'), and then tackle the remaining ones.

You will need to encode the state as an array of bytes in order to pass it to the Persister; raft.go contains some example code for this in persist() and readPersist().

In order to avoid running out of memory, Raft must periodically discard old log entries, but you do not have to worry about garbage collecting the log in this lab. You will implement that in the next lab by using snapshotting (Section 7 in the paper).

Handin procedure

Before submitting, please run all the tests one final time. You are responsible for making sure your code works. Keep in mind that the more obscure corner cases may not appear on every run, so it's a good idea to run the tests multiple times.

$ go test

Submit your code via the class's submission website, located at https://6824.scripts.mit.edu:444/submit/handin.py/.

You may use your MIT Certificate or request an API key via email to log in for the first time. Your API key (XXX) is displayed once you logged in, which can be used to upload the lab from the console as follows.

$ cd "$GOPATH"
$ echo "XXX" > api.key
$ make lab2

Check the submission website to make sure you submitted a working lab!

You may submit multiple times. We will use the timestamp of your last submission for the purpose of calculating late days. Your grade is determined by the score your solution reliably achieves when we run the tester on our test machines.

Please post questions on Piazza.