CS451/651 - Spring 2020

CS451/651 Lab 2: Raft

Part 2A Due: Friday Feb 14 at 11:59pm

Part 2B Due: Friday Feb 21 at 11:59pm

Part 2C Due: Friday Feb 28 at 11:59pm


Introduction

This is the first in a series of labs in which you'll build a fault-tolerant key/value storage system. In this lab you'll implement 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 over multiple replicated state machines for higher performance.

A replicated service (e.g., key/value database) achieves fault tolerance by storing copies of its data on multiple replica servers. Replication allows the service to continue operating even if some of its servers experience failures (crashes or a broken or flaky network). The challenge is that failures may cause the replicas to hold differing copies of the data.

Raft manages a service's state replicas, and in particular it helps the service sort out what the correct state is after failures. Raft implements a replicated state machine. It 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 replica's local copy of 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 can communicate again.

In this lab you'll implement Raft as a 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 find it useful to look at this advice written for 6.824 students in 2016, and this illustrated guide to Raft. For a wider perspective, have a look at Paxos, Chubby, Paxos Made Live, Spanner, Zookeeper, Harp, Viewstamped Replication, and Bolosky et al.

This lab is due in three parts. You must submit each part on the corresponding due date. This lab does not involve a lot of code, but concurrency makes it potentially challenging to debug; start each part early.

Collaboration Policy

You must write all the code you hand in for CS451/651, 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 anyone else's code, or allow anyone else to look at your code.

Please do not publish your code or make it available to current or future CS451/651 students. github.com repositories are public by default, so please don't put your code there unless you make the repository private. You may find it convenient to use MIT's GitHub, but be sure to create a private repository.

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:

$ export GOROOT=/research/sesa/451/go
$ export PATH=$PATH:$GOROOT/bin
$ export GOPATH=$HOME/451
$ cd $GOPATH
$ git pull
...
$ cd src/raft
$ go test
Test (2A): initial election ...
--- FAIL: TestInitialElection (5.03s)
config.go:270: expected one leader, got 0
Test (2A): election after network failure ...
--- FAIL: TestReElection (5.03s)
config.go:270: expected one leader, got 0
...
$
When you've finished all three parts of the lab, your implementation should pass all the tests in the src/raft directory:
$ go test
Test (2A): initial election ...
... Passed
Test (2A): election after network failure ...
... Passed
Test (2B): basic agreement ...
... Passed
...
PASS
ok  	raft	162.413s

The code

Implement Raft by adding code to raft/raft.go. In that file you'll find a bit of skeleton code, plus examples of how to send and receive RPCs.

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()). The reason you must use labrpc instead of Go's RPC package is that the tester tells labrpc to delay RPCs, re-order them, and delete them to simulate challenging networks conditions under which your code should work correctly. Don't modify labrpc because we will test your code with the labrpc as handed out.

This lab may be your first exposure to writing challenging concurrent code and your first implementation may not be clean enough that you can easily reason about its correctness. Give yourself enough time to rewrite your implementation so that you can easily reason about its correctness. Subsequent labs will build on this lab, so it is important to do a good job on your implementation.

Part 2A

Implement leader election and heartbeats (AppendEntries RPCs with no log entries). The goal for Part 2A is for a single leader to be elected, for the leader to remain the leader if there are no failures, and for a new leader to take over if the old leader fails or if packets to/from the old leader are lost. Run go test -run 2A to test your 2A code.

Be sure you pass the 2A tests before submitting Part 2A. Note that the 2A tests test the basic operation of leader election. Parts B and C will test leader election in more challenging settings and may expose bugs in your leader election code for which the 2A tests do not test.

Use submission instructions for Part 2A.

Part 2B

We want Raft to keep a consistent, replicated log of operations. A call to Start() at the leader starts the process of adding a new operation to the log; the leader sends the new operation to the other servers in 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, fleshing out the AppendEntry RPC handler, and advancing the commitIndex at the leader. Your first goal should be to pass the TestBasicAgree() test (in test_test.go). Once you have that working, you should get all the 2B tests to pass (go test -run 2B).

Be sure you pass the 2A and 2B tests before submitting Part 2B.

Use submission instructions for Part 2B.

Part 2C

If a Raft-based server reboots it should resume service where it left off. This requires that Raft keep persistent state that survives a reboot. The paper's Figure 2 mentions which state should be persistent, and raft.go contains examples of how to save and restore persistent state.

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 Raft.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. Use the Persister's ReadRaftState() and SaveRaftState() methods.

Implement persistence by first adding code that saves and restores persistent state to persist() and readPersist() in raft.go. You will need to encode (or "serialize") the state as an array of bytes in order to pass it to the Persister. Use Go's gob encoder to do this; see the comments in in persist() and 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. You must also load persisted state in Raft.Make(). Once you've done this, you should pass the remaining tests. You may want to first try to pass the "basic persistence" test (go test -run 'TestPersist1$'), and then tackle the remaining ones (go test -run 2C).

In order to avoid running out of memory, Raft must periodically discard old log entries, but you do not have to worry about this until the next lab.

Be sure you pass all the tests before submitting Part 2C.

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 gsubmit command using the provided makefile Depending on which part you are submitting choose the right make target for the example below

$ cd ~/451
$ make lab2a|lab2b|lab2c
You can use the gsubmit command to check that it was submitted correctly.

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.