# CSE 452 / M 552: Distributed Systems

## Course Overview

Distributed systems are central to modern computing, from web applications to e-commerce to content distribution. This senior-level course will cover abstractions and implementation techniques for building distributed systems, including client-server computing, the web, cloud computing, peer-to-peer systems, and distributed storage systems. Topics will include remote procedure call, preventing and finding errors in distributed programs, maintaining consistency of distributed state, fault tolerance, high availability, and scaling.

Alumni of this course say that it is among the most intellectually challenging courses that they have taken at UW, and among the most relevant to their future careers. We will attempt to live up to that recommendation. We believe the best way to learn the material is to implement the ideas presented in the course, and so there is a substantial programming project.

The calendar page provides a detailed topic list for the course, including readings, problem sets, and labs. In addition, we are using Gitlab, Ed, and Gradescope for various parts of the course.

## Lecture and Section

Lectures are MWF at 3:30 in Gowen 301. We will record lectures via Panopto and make them available to all students in the course.

Section times and locations are listed on the UW time schedule. Section will largely focus on the labs and you will need to attend to be able to complete the assignments. Sections are not recorded.

## Course Staff

You can email the entire staff (including James, despite the fact that the email address says "TAs") at cse452-tas@cs.washington.edu, but we usually prefer you make a private post on Ed if possible.

## Staff Contact

The best way to contact the staff is to make a post on Ed. If your question is likely to be useful to other students, please consider making it public. (You can make the post "anonymous" to hide your identity from your classmates, but note that course staff can still see your identity on anonymous posts. See anonymous feedback at the very bottom of this page for submitting feedback without revealing your identity to course staff.) If your message is not relevant to other students, make it private. We prefer you send messages via Ed if at all possible, because it allows any staff member to assist you. If you need to contact an individual staff member directly, you can also use email.

## Assignments

There are four kinds of assignments in this class.

• Labs: These are significant programming projects. Four total, due on Friday every two or three weeks.
• Design documents: One per lab, usually due a week before the lab (except for lab 1).
• Problem sets: Due on Fridays weekly in weeks when labs are not due. Gives you practice with lecture material.
• Blog posts: During the last two weeks of the quarter, we will assign and discuss five research papers. Prior to class you will write a post on the discussion board about some aspect of the paper. Due the night before the lecture in which the paper is discussed.

See the sections below for more information on each kind of assignment, as well as the sections on grading, the late policy, and academic honesty.

### Labs and design documents

The core of the course is to build a highly available, scalable, fault tolerant, and transactional key-value store. Key-value stores are widely used in cloud computing. The project is written in Java, derived from a similar one designed for the MIT graduate distributed systems course. A hallmark of our project infrastructure is extensive support for thorough testing and debugging. Each lab has a model-checking-based test suite that you can use to validate your implementation.

Lab 0 and Lab 1 of the project are to be done individually. (Note that Lab 0 does not have a turnin.) For the remaining labs (2-4), students in 452 may optionally work with a partner. (M 552 students must work alone.)

The labs are autograded by a model checker, which is a program that exhaustively tests your solution in all cases up to a certain depth bound. This means that you will need to come up with solutions that work in all cases. Except for the design document, the labs are self-grading—we give you all of the test cases we run.

The best way to write code that works in all cases is to carefully think through your design before writing any code. Design before code is especially important for distributed systems where the number of possible code paths is exponential in the number of messages, the cost of uncaught errors in production code is enormous, and it is often infeasible to catch every possible error through testing.

This is very different from other CSE classes you may have taken. If you think we are just saying that and we don't really mean it, please understand that it may take you as much as ten times as long to complete the assignments if you start writing code before you carefully think through the design with your partner. Debugging typos can be laborious but is a reality we all face. Debugging distributed system design errors by testing is extremely time consuming.

To encourage you to design then code, we ask for a design document for each lab, worth a sizable portion of the total value of the lab, due a week in advance of the lab's due date (except for lab 1) and then optionally revised and resubmitted with the lab. You (and your partner, if you are working with one) should complete the design document before writing any code. Of course, you may find from time to time that you need to update the design as you later write code, but you want to try to minimize that.

Finally, we also ask you to provide a short post-mortem with each lab: what worked for you, and what didn't.

We strongly recommend you get an early start on the labs. Many students underestimate the difficulty of the assignments, and leave themselves too little time to finish before the deadline. The most common comment about the labs is that students wished they had gotten an earlier start.

### Problem sets

For most lecture topics (which may span one or more class meetings), we will assign a problem set specific to that topic. The problem sets give you a chance to practice working with the material presented in lecture. These are to be completed individually.

There is no textbook for this course. Instead, in the first 8 weeks, we will assign various tutorial and research papers as optional background reading.

In the final two weeks of the class, we will shift gears and read and discuss five research papers describing various practical distributed systems. For these papers, we ask students to submit a blog post on each paper. The blog discussions are submitted on Ed.

Blog posts must have two clearly labeled parts.

• First, semi-objectively reconstruct some aspect of the paper in your own words. For example, if the paper presents a system design, you might summarize the design. If the paper presents empirical evidence for a conclusion, summarize the statement of the conclusion, the evidence for it, and why the conclusion actually follows from the evidence. Usually this part will not contain first-person pronouns like "I" and "me".
• Second, extend your reconstruction in one of the following ways. Typically this part will contain first-person pronouns like "I" and "me".
• Add missing details or fill in gaps in the paper's presentation. For example, if the paper does not describe some aspect of how their system works, you could figure out how it does (e.g. by reading the source code or follow up work, or by (educated) conjecture). Another example would be if the paper makes a claim whose justification is missing, you could provide that justification. Whatever you do, you should clearly distinguish things you know (cite your sources) versus things you think might be true (also fine, but make sure it's clear to your reader that your are making a conjecture).
• Present background knowledge from previous work. If you found the paper difficult to read because it was not self contained, find another resource (paper, textbook, Wikipedia, etc.) that helps. Cite this resource and briefly summarize what additional information you learned.
• Present related follow-up work. Find a later paper that builds on the original paper and describe what it adds.
• Present a modern-day interpretation of the paper. Many of the papers we read are several years old. What has changed about distributed systems since they were written? How would you change their solution to adapt to modern practice.
• Critique the paper's design. Do you buy the argument, or is it flawed or just confusing? Is the empirical evidence convincing or is it missing something? An important kind of critique is asking questions about the design. You do not necessarily need to answer questions that you pose. In any case, your critique must be specific and relevant. Do not just say "I thought it was cool to see how they used Paxos to do X." Go deeper than that.

Because the two parts of the blog post are related (the second part extends the same aspect that the first part reconstructed), you will usually have to have the second part in mind when choosing which aspect to reconstruct. Typically, each part of your blog will be at least one paragraph long. (So your entire blog will be at least two such paragraphs.) We prefer quality over quantity. Try to focus deeply on just one aspect of the paper about which you have something interesting to say. If you have several ideas, pick just one. If you find that you don't end up having anything interesting to say, change your topic.

Your blog post may also extend a discussion started by someone else. When writing this type of post, you still need to reconstruct and extend. For example, you might add or better explain details that the original post missed in your reconstruction, and then use these additional details to respond to the original extension (either by strengthening the original argument by presenting new evidence, or by arguing for a different conclusion by presenting other evidence).

Blog posts are graded on a completion basis with 12.5 points for each part. Occasionally we will award an additional 10 points or so for especially insightful posts.

Blog posts are due the night before the lecture in which the paper is discussed.

• Lab 1: 160 points (note the lab 1 tests sum to 320, so for this lab points are divided by 2) + 25 design document
• Lab 2: 330 points + 50 design document
• Lab 3: 355 points + 65 design document
• Lab 4: 495 points
• Problem sets: 250 points total
• Blog posts: 25 points each for 5 papers, totaling 125 points
• Bonus points: Due to a canceled blog post, problem set, and design doc, free 145 points

Total points possible: 2000

There is no midterm or final exam.

The course is not curved. Your grade on a 4.0 scale is computed by the following formula.

$\min\left( \left.\left\lfloor\frac{\mathrm{points}}{45}\right\rfloor \middle/ 10 - 0.1 \right.,\ 4.0 \right)$

In other words, your grade will be computed by the following table:

≥1845 4.0
≥1800 3.9
≥1755 3.8
≥1710 3.7
≥1665 3.6
≥1620 3.5
≥1575 3.4
≥1530 3.3
≥1485 3.2
≥1440 3.1
≥1395 3.0
... ...
... ...
... ...

This grading scheme may be somewhat unfamiliar to you. We will discuss it on the first day of lecture. Be sure you understand it, and feel free to ask any questions about it.

An important consequence the additive points-based grading scheme is that there is a sense in which every assignment is optional. If you are unable to complete some assignment, simply make sure to complete the remaining ones—we give you room to miss points and still get a good grade. For example, if you miss 15% of the available points, that is still some form of an A grade.

Since you will receive the same number of points as your (optional) partner for your combined work on labs 2-4, it is essential during partner-matching that you communicate expectations about your grade target. If you find yourself stuck in a situation where your partner wants to do significantly more or less work than you do, please contact the staff.

Most students find that they are able to complete all of the assignments to a high degree of quality, but that the assignments require a decent amount of effort. Students find the class time-consuming but rewarding, and so grades in this class are generally high.

If you are taking the class S/NS, note that per university policy, undergraduates receive S credit for any grade 2.0 or higher, while graduate students (including BS/MS students) receive S credit for any grade 2.7 or higher. Thus, an undergraduate would need to earn at least 945 points to receive S credit, while a graduate student would need to earn at least 1260 points to receive S credit.

## Late Policy

Our late policy is designed to give students maximal flexibility without having to ask us for permission in most cases, while still allowing us to grade assignments and get them back to students in a timely fashion. If you have an extenuating circumstance that causes you not to be able to complete the work on time, especially if due to something outside of your control, please contact the staff. Often, we are able to work something out that is agreeable to everyone involved.

Each kind of assignment has a separate late policy. Be sure you understand the differences.

• All assignments, regardless of late policy or extensions, must be turned in by the end of the day Tuesday, March 14, 2023, unless the instructor has given specific and individual permission.

• Each lab comes with a 48-hour grace period, during which work is accepted without penalty. We then deduct 1% off your score for that lab for each additional day that it is late. In other words, if you are making progress, even slowly, you should keep working on the lab, but if you are stuck you should go ahead and turn it in. Since turning assignments in late cuts into the available time you have for the next lab, only use this flexibility if necessary. To turn in your lab after the 48-hour grace period, contact the staff.

• For design documents, there is no grace period. This is because these are graded manually and quick feedback is essential to the course calendar.

• If you turn in a lab and later on discover that you have a solution that would receive a higher grade from the automated grader, you may resubmit it for a regrade for up to 50% of the missing points. You need to contact the staff to exercise this option. Only the autograder is subject to regrading—any manual grading (e.g., the design document) is not subject to regrading.

• Problem sets have a 48-hour grace period, during which work is accepted without penalty. No credit is granted after the grace period expires.

• Blog posts are due the night before the lecture in which the paper is discussed. Or, you can turn in a blog post within 24 hours after the original deadline for half credit. After 24 hours, no credit will be given.

Note that, unlike some other course policies you might be familiar with, in this class there is no cap on how much total grace time you can use over the quarter. You can use the grace period on every single problem set and lab and still get full credit. Note that blog posts are different, since the 24 hour grace period on a blog post is for half credit.

You are encouraged to discuss all aspects of the course with and ask for help from the instructor, the TAs, and other students. However, do not cross this line:

Do not share code or written text. Do not look at lab or problem set solutions that might be on the Internet. Do not use someone else's code or text in your solutions or responses.

It is ok to share ideas, explain your code at a high level to someone to see if they know why it doesn't work, or to help someone else debug if they've run into a wall.

Some work in this class will be completed with a partner. The rest is to be done individually. We will clearly mark each piece of work as to whether it is to be done with your partner or individually. Please contact the staff if you are unsure about any part of how this policy applies to an assignment.

• Lab 1 and its design document are individual.
• Labs 2-4 and their design documents are optionally with a partner (for 452 students only, not M 552 students). You must arrange your partner in advance in consultation with the course staff.
• Problem sets are individual.
• Blog posts are individual.

## Partner work

452 students can optionally work with a partner on labs 2 through 4. (M 552 students will work alone.)

The labs are difficult, and working with a partner can make things easier because you can discuss the details of your design together. Working with a partner is also a serious responsibility: your partner is relying on you to communicate and collaborate effectively. Do not agree to work with a partner if you are not willing to commit to these responsibilities.

A common misconception about working with a partner is that you should "split up the work". This is usually a terrible idea, especially in this class, because the hard part of the lab is the design process. It might take you 10 or more iterations to design your protocol correctly, and all parts of a distributed protocol can depend heavily on all other parts of the protocol, so there is no way to "split up" this design work. Instead, you should plan to pair program (i.e., work together synchronously) during the design process until you are confident that you have a correct design and have written the design document together. After that, you can either continue to pair program your implementation (we recommend this approach) or you can try to split up the implementation work. When in doubt, work together synchronously. When you do work separately, establish a clear communication channel and keep each other posted about where you are stuck. Again, do not commit to working with a partner if you are not interested in pair programming.

To summarize, by entering a partnership, you are agreeing, at minimum, to:

• Communicate frequently with your partner during the weeks where design documents and labs are due.
• Collaborate synchronously on the design document and any challenging implementation tasks.
• Inform your partner in advance if you will be unavailable for communication or collaboration.

We take the partner agreement extremely seriously and will enforce it. If you flagrantly and repeatedly abandon your partner, you can expect to take a zero on the lab regardless of how much work you put into it.

If your partner breaks this agreement, first try to communicate with them about it. If that doesn't work, contact the course staff.

If you do decide to work with a partner, you can either find a partner on your own (and let us know) or you can ask us to match you with someone. We will send more information about the partnering process later in the quarter closer to lab 2.

When looking for a partner, be sure to communicate about grade expectations. Also, if you plan to opt in to W credit, you should find a partner who wants W credit as well.

## 452 vs M 552

The only difference between 452 and M 552 is that M 552 students must work on the labs alone.

## W credit

This quarter we are expanding a pilot program to offer opt-in W credit for CSE 452.

The difference between the W credit and non-W credit versions of the course is that students who want W credit will submit revised versions of their design documents after each lab. The revisions will take into account staff feedback on the design, and include any updates to the design that you encountered while implementing the lab.

We will create a form to opt in to W credit during week 2.

## Anonymous feedback

Anonymous feedback can be sent to the instructor or TAs via feedback.cs.washington.edu.