6.824 - Spring 2020

6.824 Lab 2: Raft

Part 2A Due: Feb 21 23:59

Part 2B Due: Feb 28 23:59

Part 2C Due: Mar 6 23:59


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 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 achieves fault tolerance by storing complete copies of its state (i.e., 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 organizes client requests into a sequence, called the log, and ensures that all the replica servers see the same log. Each replica executes client requests in log order, applying them to its 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.

You should follow the design in the extended Raft paper, with particular attention to Figure 2. You'll implement most of what's in the paper, including saving persistent state and reading it after a node fails and then restarts. You will not implement cluster membership changes (Section 6). You'll implement log compaction / snapshotting (Section 7) in a later lab.

You may find this guide useful, as well as this advice about locking and structure for concurrency. 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.

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 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 6.824 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

If you have done Lab 1, you already have a copy of the lab source code. If not, you can find directions for obtaining the source via git in the Lab 1 instructions.

We supply you with skeleton code src/raft/raft.go. We also supply a set of tests, which you should use to drive your implementation efforts, and which we'll use to grade your submitted lab. The tests are in src/raft/test_test.go.

To get up and running, execute the following commands. Don't forget the git pull to get the latest software.

$ cd ~/6.824
$ git pull
$ cd src/raft
$ go test
Test (2A): initial election ...
--- FAIL: TestInitialElection2A (5.04s)
        config.go:326: expected one leader, got none
Test (2A): election after network failure ...
--- FAIL: TestReElection2A (5.03s)
        config.go:326: expected one leader, got none

The code

Implement Raft by adding code to raft/raft.go. In that file you'll find 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 network identifiers of the Raft peers (including this one), for use with RPC. 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 the log appends to complete. The service expects your implementation to send an ApplyMsg for each newly committed log entry to the applyCh channel argument to Make().

raft.go contains example code that sends an RPC (sendRequestVote()) and that handles an incoming RPC (RequestVote()). Your Raft peers should exchange RPCs using the labrpc Go package (source in src/labrpc). The tester can tell labrpc to delay RPCs, re-order them, and discard them to simulate various network failures. While you can temporarily modify labrpc, make sure your Raft works with the original labrpc, since that's what we'll use to test and grade your lab. Your Raft instances must interact only with RPC; for example, they are not allowed to communicate using shared Go variables or files.

Subsequent labs build on this lab, so it is important to give yourself enough time to write solid code.

Part 2A

Implement Raft 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, so that you see something like this:

$ go test -run 2A
Test (2A): initial election ...
  ... Passed --   4.0  3   32    9170    0
Test (2A): election after network failure ...
  ... Passed --   6.1  3   70   13895    0
ok      raft    10.187s

Each "Passed" line contains five numbers; these are the time that the test took in seconds, the number of Raft peers (usually 3 or 5), the number of RPCs sent during the test, the total number of bytes in the RPC messages, and the number of log entries that Raft reports were committed. Your numbers will differ from those shown here. You can ignore the numbers if you like, but they may help you sanity-check the number of RPCs that your implementation sends. For all of labs 2, 3, and 4, the grading script will fail your solution if it takes more than 600 seconds for all of the tests (go test), or if any individual test takes more than 120 seconds.

Handin procedure for lab 2A

First, please run the 2A tests one last time. Then, run make lab2a to upload your code to the submission site.

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 are logged in, and can be used to upload the lab from the console as follows.

$ cd ~/6.824
$ echo "XXX" > api.key
$ make lab2a

Check the submission website to make sure it sees your submission.

You may submit multiple times. We will use your last submission to calculate late days. Your grade is determined by the score your solution reliably achieves when we run the tester.

Part 2B

Implement the leader and follower code to append new log entries, so that the go test -run 2B tests pass.

The tests for upcoming labs may fail your code if it runs too slowly. You can check how much real time and CPU time your solution uses with the time command. Here's typical output:

$ time go test -run 2B
Test (2B): basic agreement ...
  ... Passed --   1.6  3   18    5158    3
Test (2B): RPC byte count ...
  ... Passed --   3.3  3   50  115122   11
Test (2B): agreement despite follower disconnection ...
  ... Passed --   6.3  3   64   17489    7
Test (2B): no agreement if too many followers disconnect ...
  ... Passed --   4.9  5  116   27838    3
Test (2B): concurrent Start()s ...
  ... Passed --   2.1  3   16    4648    6
Test (2B): rejoin of partitioned leader ...
  ... Passed --   8.1  3  111   26996    4
Test (2B): leader backs up quickly over incorrect follower logs ...
  ... Passed --  28.6  5 1342  953354  102
Test (2B): RPC counts aren't too high ...
  ... Passed --   3.4  3   30    9050   12
ok      raft    58.142s

real    0m58.475s
user    0m2.477s
sys     0m1.406s
The "ok raft 58.142s" means that Go measured the time taken for the 2B tests to be 58.142 seconds of real (wall-clock) time. The "user 0m2.477s" means that the code consumed 2.477 seconds of CPU time, or time spent actually executing instructions (rather than waiting or sleeping). If your solution uses much more than a minute of real time for the 2B tests, or much more than 5 seconds of CPU time, you may run into trouble later on. Look for time spent sleeping or waiting for RPC timeouts, loops that run without sleeping or waiting for conditions or channel messages, or large numbers of RPCs sent.

Handin procedure for lab 2B

First, double-check that your code passes the 2B tests, and still passes the 2A tests. Then, run make lab2b to upload your code to the submission site.

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 are logged in, which can be used to upload the lab from the console as follows.

$ cd ~/6.824
$ echo "XXX" > api.key
$ make lab2b

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.

A real implementation would write Raft's persistent state to disk each time it changed, and would read the 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.

Complete the functions persist() and readPersist() in raft.go by adding code to save and restore persistent state. You will need to encode (or "serialize") the state as an array of bytes in order to pass it to the Persister. Use the labgob encoder; see the comments in persist() and readPersist(). labgob is like Go's gob encoder but prints error messages if you try to encode structures with lower-case field names.

Insert calls to persist() at the points where your implementation changes persistent state. Once you've done this, you should pass the remaining tests.

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.

Your code should pass all the 2C tests (as shown below), as well as the 2A and 2B tests.

$ go test -run 2C
Test (2C): basic persistence ...
  ... Passed --   7.2  3  206   42208    6
Test (2C): more persistence ...
  ... Passed --  23.2  5 1194  198270   16
Test (2C): partitioned leader and one follower crash, leader restarts ...
  ... Passed --   3.2  3   46   10638    4
Test (2C): Figure 8 ...
  ... Passed --  35.1  5 9395 1939183   25
Test (2C): unreliable agreement ...
  ... Passed --   4.2  5  244   85259  246
Test (2C): Figure 8 (unreliable) ...
  ... Passed --  36.3  5 1948 4175577  216
Test (2C): churn ...
  ... Passed --  16.6  5 4402 2220926 1766
Test (2C): unreliable churn ...
  ... Passed --  16.5  5  781  539084  221
ok      raft    142.357s

Handin procedure for lab 2C

First, double-check that your code passes all the 2A, 2B, and 2C tests. Then, run make lab2c to upload your code to the submission site.

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 are logged in, which can be used to upload the lab from the console as follows.

$ cd ~/6.824
$ echo "XXX" > api.key
$ make lab2c

Please post questions on Piazza.