Lab 3

Lab 3 Due Wednesday Nov 06, 11:59pm ET (3A) and Wednesday Nov 20, 11:59pm ET (3B)

Important Dates and Other Stuff

Part 3A Due ~~Monday, 11/04,~~ Wednesday, 11/06, 11:59pm.

Part 3B Due ~~Monday, 11/18,~~ Wednesday, 11/20, 11:59pm.

Resources

Lab 3 walk-through [video] [slides]

Introduction

In this lab you’ll implement Raft, a replicated state machine protocol. 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 state machine replication. It organizes client requests into a sequence, called the log, and ensures that all the replicas agree on 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.

NOTE: 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 do not need to implement: log persistence, 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 the Runway model for Raft or the original Raft Visualization useful to get a sense of the high-level workings of Raft.

Hint: Start early. Although the amount of code to implement isn’t large, getting it to work correctly will be very challenging. Both the algorithm and the code is tricky and there are many corner cases to consider. When one of the tests fails, it may take a bit of puzzling to understand in what scenario your solution isn’t correct, and how to fix your solution.
Hint: Read and understand the extended Raft paper and the Raft lecture notes before you start. Your implementation should follow the paper’s description closely, particularly Figure 2, since that’s what the tests expect.

This lab is due in two parts. You must submit each part on the corresponding due date. This lab does not involve a lot of code, but concurrency makes it very challenging to debug; START EACH PART EARLY!

Getting Started

We supply you with skeleton code and tests. You can download the software from here. The skeleton code and tests are in raft, and a simple RPC-like system is in labrpc.

To download the tar file from Zeus:

$ cd $HOME
$ wget https://tddg.github.io/cs4740-fall24/assets/lab_code/lab3.tar
$ tar -xvf lab3.tar
$ ls
labgob/  labrpc/  raft/

Copy these three directories into src/ under your CS4740 lab directory:

$ cp -r labgob labrpc raft cs4740-fall24-labs/src/
$ cd cs4740-fall24-labs/src
$ ls
kvsrv  labgob  labrpc  main  mapreduce  models  porcupine  raft

NOTE: You will have to overwrite the following two directories: labgob/ and labrpc/ from Lab 2, as Lab 3 requires a slightly different implementation of these two libraries.

You will be working on src/raft for this lab assignment.

To get up and running, execute the following commands:

...
$ cd cs4740-fall24-labs
$ export "GO111MODULE=off"
$ export GOPATH="$PWD"
$ cd src/raft
$ go test
Test (3A): initial election ...
--- FAIL: TestInitialElection3A (4.84s)
    config.go:326: expected one leader, got none
Test (3A): election after network failure ...
--- FAIL: TestReElection3A (4.90s)
    config.go:326: expected one leader, got none
Test (3B): basic agreement ...
--- FAIL: TestBasicAgree3B (10.03s)
    config.go:471: one(100) failed to reach agreement
...

When you’ve finished the two parts of the lab, your implementation should pass all the tests in the src/raft directory:

$ go test
Test (3A): initial election ...
  ... Passed --   3.0  3   46   13164    0
Test (3A): election after network failure ...
  ... Passed --   4.9  3  120   24309    0
Test (3B): basic agreement ...
  ... Passed --   1.1  3   16    4598    3
Test (3B): agreement despite follower disconnection ...
  ... Passed --   6.4  3  110   29737    8
Test (3B): no agreement if too many followers disconnect ...
  ... Passed --   3.9  5  184   38454    3
Test (3B): concurrent Start()s ...
  ... Passed --   0.8  3   10    2862    6
Test (3B): rejoin of partitioned leader ...
  ... Passed --   4.5  3  128   30888    4
Test (3B): leader backs up quickly over incorrect follower logs ...
  ... Passed --  31.2  5 2204 1779548  102
Test (3B): RPC counts arent too high ...
  ... Passed --   2.3  3   36   10862   12
PASS
ok  	raft	93.964s

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 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 network conditions under which your code should work correctly. Don’t rely on modifications to labrpc because we will test your code with the labrpc as handed out.

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 3A

Task: Implement leader election and heartbeats (AppendEntries RPCs with no log entries). The goal for Part 3A 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 3A to test your 3A code.

Hint: Add any state you need to the Raft struct in raft.go. You’ll also need to define a struct to hold information about each log entry. Your code should follow Figure 2 in the paper as closely as possible.
Hint: Fill in the RequestVoteArgs and RequestVoteReply structs. Modify Make() to create a background goroutine that will kick off leader election periodically by sending out RequestVote RPCs when it hasn’t heard from another peer for a while. This way a peer will learn who is the leader, if there is already a leader, or become the leader itself. Implement the RequestVote() RPC handler so that servers will vote for one another.
Hint: To implement heartbeats, define an AppendEntries RPC struct (though you may not need all the arguments yet), and have the leader send them out periodically. Write an AppendEntries RPC handler method that resets the election timeout so that other servers don’t step forward as leaders when one has already been elected.
Hint: Make sure the election timeouts in different peers don’t always fire at the same time, or else all peers will vote only for themselves and no one will become the leader.
Hint: The tester requires that the leader send heartbeat RPCs no more than ten times per second.
Hint: The tester requires your Raft to elect a new leader within five seconds of the failure of the old leader (if a majority of peers can still communicate). Remember, however, that leader election may require multiple rounds in case of a split vote (which can happen if packets are lost or if candidates unluckily choose the same random backoff times). You must pick election timeouts (and thus heartbeat intervals) that are short enough that it’s very likely that an election will complete in less than five seconds even if it requires multiple rounds.
Hint: The paper’s Section 5.2 mentions election timeouts in the range of 150 to 300 milliseconds. Such a range only makes sense if the leader sends heartbeats considerably more often than once per 150 milliseconds. Because the tester limits you to 10 heartbeats per second, you will have to use an election timeout larger than the paper’s 150 to 300 milliseconds, but not too large, because then you may fail to elect a leader within five seconds.
Hint: You may find Go’s rand useful.
Hint: You’ll need to write code that takes actions periodically or after delays in time. The easiest way to do this is to create a goroutine with a loop that calls time.Sleep(). The hard way is to use Go’s time.Timer or time.Ticker, which are difficult to use correctly.
Hint: If you are puzzled about locking, you may find this advice helpful.
Hint: If your code has trouble passing the tests, read the paper’s Figure 2 again; the full logic for leader election is spread over multiple parts of the figure.
Hint: A good way to debug your code is to insert print statements when a peer sends or receives a message, and collect the output in a file with go test -run 3A > out. Then, by studying the trace of messages in the out file, you can identify where your implementation deviates from the desired protocol. You might find DPrintf in util.go useful to turn printing on and off as you debug different problems.
Hint: Go RPC sends only struct fields whose names start with capital letters. Sub-structures must also have capitalized field names (e.g. fields of log records in an array). The labgob package will warn you about this; don’t ignore the warnings.
Hint: You should check your code with go test -race, and fix any races it reports.

Be sure you pass the 3A tests before submitting Part 3A, so that you see something like this:

$ go test -run 3A
Test (3A): initial election ...
  ... Passed --   2.5  3   30    0
Test (3A): election after network failure ...
  ... Passed --   4.5  3   78    0
PASS
ok      raft    7.016s

Each “Passed” line contains four 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, 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.

Your code will be tested on Autolab. No marks will be awarded if your code does not pass the test. You will receive full marks only if your code successfully passes the test.

Please post questions on Ed.

Point distribution

There are a total of 2 tests for Part 3A. Each individual test takes 10 points. That is, Part 3A carries 20 points.

Your code will be tested on Autolab. No marks will be awarded if your code does not pass the test. You will receive full marks only if your code successfully passes the test.

Submitting 3A on Autolab

You must turn in your lab assignment using Autolab. Read this document for instructions on how to sign-up for Autolab. You may skip the Autolab signup step if you have done this in Lab 0.

Create a tar file of only the following Go source file: raft.go. Please, .tar only, not .tgz, nor .7z/.zip. Name your tar file as lab3a-handin.tar.

$ tar -cvf lab3a-handin.tar raft.go

Please do not put any directory in your tar file as our autograder is scripted to directly fetch src files not directories. Use the following command to examine the content of your tar file:

$ tar -tvf lab3a-handin.tar
-rw-r--r--  0 yue    staff    8911 Oct 30 16:38 raft.go

When you upload your lab code, Autolab will automatically untar it and test it. You should verify that the result that Autolab generates is what you expect. Test your code on Zeus before submitting it to Autolab. Your code is tested in a cloud Linux VM. Assignments that do not compile or run will receive a maximum of 50%. Note that we have provided ample resources for you to verify that our view of your assignment is the same as your own: you will see the result of the test execution for your assignment when you submit it.

You can resubmit your lab an unlimited number of times before the deadline. Note the late submission policy: assignments will be accepted up until 3 days past the deadline at a penalty of 10% per late day; after 3 days, no late assignments will be accepted, no exceptions.

Please also submit your tar file on Canvas. The submission should include one Go source file, and this is just for the GTAs’ record.

NOTE: Canvas submission will remain open after the due date, but we will not use Canvas submission timestamp. Instead, we will use the timestamp of your last Autolab submission for late penalty.

Part 3B

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.

Task: 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 TestBasicAgree3B() test (in test_test.go). Once you have that working, you should get all the 3B tests to pass (go test -run 3B).

Hint: You will need to implement the election restriction (section 5.4.1 in the paper).
Hint: One way to fail the early Lab 3B tests is to hold un-needed elections, that is, an election even though the current leader is alive and can talk to all peers. This can prevent agreement in situations where the tester believes agreement is possible. Bugs in election timer management, or not sending out heartbeats immediately after winning an election, can cause un-needed elections.
Hint: You may need to write code that waits for certain events to occur. Do not write loops that execute continuously without pausing, since that will slow your implementation enough that it fails tests. You can wait efficiently with Go’s channels, or Go’s condition variables, or (if all else fails) by inserting a time.Sleep(10 * time.Millisecond) in each loop iteration.
Hint: Give yourself time to rewrite your implementation in light of lessons learned about structuring concurrent code. In later labs you’ll thank yourself for having Raft code that’s as clear and clean as possible. For ideas, you can re-visit the Raft structure, locking and guide, pages.

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 some typical output for Lab 3B:

$ time go test -run 3B
Test (3B): basic agreement ...
  ... Passed --   1.2  3   16    4598    3
Test (3B): agreement despite follower disconnection ...
  ... Passed --   6.5  3  112   30220    7
Test (3B): no agreement if too many followers disconnect ...
  ... Passed --   3.8  5  180   37559    3
Test (3B): concurrent Start()s ...
  ... Passed --   0.7  3    8    2274    6
Test (3B): rejoin of partitioned leader ...
  ... Passed --   4.6  3  128   31025    4
Test (3B): leader backs up quickly over incorrect follower logs ...
  ... Passed --  31.0  5 2200 1771420  103
Test (3B): RPC counts arent too high ...
  ... Passed --   2.3  3   36   10862   12
PASS
ok  	raft	53.088s

real	0m53.303s
user	0m1.600s
sys	0m0.881s

The “ok raft 53.088s” means that Go measured the time taken for the 3B tests to be 53.088 seconds of real (wall-clock) time. The “user 0m1.600s” means that the code consumed 1.600 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 3B 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.

Be sure you pass the 3A and 3B tests before submitting Part 3B.

Point distribution

There are a total of 7 tests for Part 3B. Each individual test takes 5 points. That is, Part 3B carries 35 points.

Your code will be tested on Autolab. No marks will be awarded if your code does not pass the test. You will receive full marks only if your code successfully passes the test.

Submitting 3B on Autolab

You must turn in your lab assignment using Autolab. Read this document for instructions on how to sign-up for Autolab.

Create a tar file of only the following Go source file: raft.go. Please, .tar only, not .tgz, nor .7z/.zip. Name your tar file as lab3b-handin.tar.

$ tar -cvf lab3b-handin.tar raft.go

Please do not put any directory in your tar file as our autograder is scripted to directly fetch src files not directories. Use the following command to examine the content of your tar file:

$ tar -tvf lab3b-handin.tar
-rw-r--r--  0 yue    staff    10583 Nov 07 09:40 raft.go

Please also submit your tar file on Canvas. The submission should include one Go source file, and this is just for the GTAs’ record.

NOTE: Canvas submission will remain open after the due date, but we will not use Canvas submission timestamp. Instead, we will use the timestamp of your last Autolab submission for late penalty.

Acknowledgment

The lab assignment is adapted from MIT’s 6.824 course. Thanks to Frans Kaashoek, Robert Morris, and Nickolai Zeldovich for their support.

Lab 3 Due Wednesday Nov 06, 11:59pm ET (3A) and Wednesday Nov 20, 11:59pm ET (3B)

Important Dates and Other Stuff

Resources

Introduction

Getting Started

The code

Part 3A

Point distribution

Submitting 3A on Autolab

Sharing your repo with GTA

Part 3B

Point distribution

Submitting 3B on Autolab

Sharing your repo with GTA

Acknowledgment