6.824 Spring 2022 Paper Questions

For each paper, your assignment is two-fold. Before the start of lecture discussing the paper:

You can also upload your questions and answers using curl:

## Answer goes into lecN.txt
$ curl -F file=@lec2.txt \
       -F key=XXXXXXXX \
## Question goes into sqN.txt
$ curl -F file=@sq2.txt \
       -F key=XXXXXXXX \

The assigned reading for today is not a paper, but the Online Go tutorial. The assigned "question" is the Crawler exercise in the tutorial. Also, take a look at Go's RPC package, which you will use in lab 1.

Describe a sequence of events that would result in a client reading stale data from the Google File System.

How does VM FT handle network partitions? That is, is it possible that if the primary and the backup end up in different network partitions that the backup will become a primary too and the system will run with two primaries?

Consider the following code from the "incorrect synchronization" examples:

var a string
var done bool

func setup() {
	a = "hello, world"
	done = true

func main() {
	go setup()
	for !done {

Using the synchronization mechanisms of your choice, fix this code so it is guaranteed to have the intended behavior according to the Go language specification. Explain why your modification works in terms of the happens-before relation.

Suppose we have the scenario shown in the Raft paper's Figure 7: a cluster of seven servers, with the log contents shown. The first server crashes (the one at the top of the figure), and cannot be contacted. A leader election ensues. For each of the servers marked (a), (d), and (f), could that server be elected? If yes, which servers would vote for it? If no, what specific Raft mechanism(s) would prevent it from being elected?

The lecture today covers effective approaches to debugging the 6.824 labs. Submit a question about the testing and debugging process of your lab 1 solution: for example, something you struggled with, something you aren't confident about, or something you're curious about.

Could a received InstallSnapshot RPC cause the state machine to go backwards in time? That is, could step 8 in Figure 13 cause the state machine to be reset so that it reflects fewer executed operations? If yes, explain how this could happen. If no, explain why it can't happen.

Please read the paper's Appendices. In Spinnaker a leader to responds to a client request after the leader and one follower have written a log record for the request on persistent storage. Why is this sufficient to guarantee strong consistency even after the leader or the one follower fail?

(This paper relies on Zookeeper, which we will read later.)

The lecture today is a Q&A session about Lab 2A/2B. Submit a question about the lab: for example, something you wondered about while doing the lab, something you didn't understand, or just anything.

One use of Zookeeper is as a fault-tolerant lock service (see the section "Simple locks" on page 6). Why isn't possible for two clients to acquire the same lock? In particular, how does Zookeeper decide if a client has failed and it can give the client's locks to other clients?

Item 4 in Section 2.3 says that, if a client read request arrives and the latest version is dirty, the node should ask the tail for the latest committed version. Suppose, instead, that the node replied with its most recent clean version (ignoring any dirty version and not sending a version query to the tail). This change would cause reads to reflect the most recent committed write that the node is aware of. Explain how this could lead to violations of linearizability -- or violations of the paper's goal of strong consistency.

Suppose Chain Replication replied to update requests from the head, as soon as the next chain server said it received the forwarded update, instead of responding from the tail. Explain how that could cause Chain Replication to produce results that are not linearizable.

The second paragraph of Section 4.1 says "The runtime state maintained by the database lets us use single segment reads rather than quorum reads..." What runtime state does the database need to maintain in order to avoid having to read from a quorum?

Russ Cox is one of the leads on the Go project. What do you like best about Go? Why? Would you want to change anything in the language? If so, what and why?

6.033 Book. Read just these parts of Chapter 9: 9.1.5, 9.1.6, 9.5.2, 9.5.3, 9.6.3. The last two sections (on two-phase locking and distributed two-phase commit) are the most important. The Question: describe a situation where Two-Phase Locking yields higher performance than Simple Locking.

Spanner Suppose a Spanner server's TT.now() returns correct information, but the uncertainty is large. For example, suppose the absolute time is 10:15:30, and TT.now() returns the interval [10:15:20,10:15:40]. That interval is correct in that it contains the absolute time, but the error bound is 10 seconds. See Section 3 for an explanation TT.now(). What bad effect will a large error bound have on Spanner's operation? Give a specific example.

No compromises: distributed transactions with consistency, availability, and performance: Suppose there are two FaRM transactions that both increment the same object. They start at the same time and see the same initial value for the object. One transaction completely finishes committing (see Section 4 and Figure 4). Then the second transaction starts to commit. There are no failures. What is the evidence that FaRM will use to realize that it must abort the second transaction? At what point in the Section 4 / Figure 4 protocol will FaRM realize that it must abort?

MDCC Come back later for this year's question.

Resilient Distributed Datasets: A Fault-Tolerant Abstraction for In-Memory Cluster Computing What applications can Spark support well that MapReduce/Hadoop cannot support?

Naiad: A Timely Dataflow System: Consider the data-flow graph in Figure 3, and assume that the vertices are instantiated as follows:

Now assume that we introduce two records in epoch e = 1 at the In vertex: (a, 2) and (b, 6); and one record in e = 2: (a, 5).

Write down the timestamp changes that the records experience as they flow through the graph, e.g., (a, 2): at A, t = (1, []); at B, t = .... You may omit vertices that do not modify the timestamp.

Informally explain when E.OnNotify((1, [])) will be called. You do not need to step through the details of the pointstamp protocol.

Parameter Server: The parameter server model was developed for running machine-learning algorithms like sparse logistic regression (§ 5.1).

What do you expect the bottleneck – that is, the most loaded – resource (e.g., CPU, memory, network) to be at the workers and at the servers?

What resource utilization would you likely observe if using Spark's logistic regression (see §3.2.1 in the Spark paper) instead of the parameter server for the experiment in §5.1?

Frangipani: A Scalable Distributed File System: Suppose a server modifies an i-node, appends the modification to its log, then another server modifies the same i-node, and then the first server crashes. The recovery system will see the i-node modification in the crashed server's log, but should not apply that log entry to the i-node, because that would un-do the second server's change. How does Frangipani avoid or cope with this situation?

Memcache at Facebook. Section 3.3 implies that a client that writes data does not delete the corresponding key from the Gutter servers, even though the client does try to delete the key from the ordinary Memcached servers (Figure 1). Explain why it would be a bad idea for writing clients to delete keys from Gutter servers.

COPS. The last sentence in Section 4.3 says a client clears its context after a put, replacing the context with just the put. The text observes "This put depends on all previous key-version pairs and thus is nearer than them." Why does clearing the context and replacing it with just the put make sense? You might think that the client's subsequent puts would need to carry along the dependency information about previous gets. What entity ultimately uses the context information, and why does it not need the information about gets before the last put?

Explain why it's important that all viewers of a Certificate Transparency log see the same content. What attacks would be possible if a malicious log server was able to cause some readers of its log to see one sequence of records, and other readers to see a different set of records? What mechanisms ensure that all viewers see the same log content?

Why is it important that Blockstack names be unique, human-readable, and decentralized? Why is providing all three properties hard?

Managing Update Conflicts in Bayou Suppose we build a distributed filesystem using Bayou, and the system has a copy operation. Initially, file A contains "foo" and file B contains "bar". On one node, a user copies file A to file B, overwriting the old contents of B. On another node, a user copies file B to file A. After both operations are committed, we want both files to contain "foo" or for both files to contain "bar". Sketch a dependency check and merge procedure for the copy operation that makes this work. How does Bayou ensure that all the nodes agree about whether A and B contain "foo" or "bar"?

For the design described in Chord: a scalable peer-to-peer lookup service for Internet applications, if two Chord nodes x and y are located nearby each other in the Internet (e.g., they are both in the same datacenter), is the number of Chord hops small (say 1 or 2 instead of log N) when x is looking up a key that is stored on node y?

Dynamo Suppose Dynamo server S1 is perfectly healthy with a working network connection. By mistake, an administrator instructs server S2 to remove S1 using the mechanisms described in 4.8.1 and 4.9. It takes a while for the membership change to propagate from S2 to the rest of the system (including S1), so for a while some clients and servers will think that S1 is still part of the system. Will Dynamo operate correctly in this situation? Why, or why not?

Secure Untrusted Data Repository (SUNDR) In the simple straw-man, both fetch and modify operations are placed in the log and signed. Suppose an alternate design that only signs and logs modify operations. Does this allow a malicious server to break fetch-modify consistency or fork consistency? Why or why not?

Bitcoin Try to buy something with Bitcoin. It may help to cooperate with some 6.824 class-mates, and it may help to start a few days early. If you decide to give up, that's OK. Briefly describe your experience.

Ethereum Whitepaper. Use an Ethereum decentralized application (dapp) and send a transaction to a smart contract.

If you decide to give up, that's OK. Briefly describe your experience.

Experiences with a Distributed, Scalable, Methodological File System: AnalogicFS. In many ways, this experiences paper raises more questions than it answers. Please answer one of the following questions, taking into consideration the rich history of AnalogicFS and the spirit in which the paper was written:

a) The analysis of A* search shown in Figure 1 claims to be an introspective visualization of the AnalogicFS methodology; however, not all decisions are depicted in the figure. In particular, if I <= P, what should be the next node explored such that all assumptions in Section 2 still hold? Show your work.

b) Despite the authors' claims in the introduction that AnalogicFS was developed to study SCSI disks (and their interaction with lambda calculus), the experimental setup detailed in Section 4.1 involves decommissioned Gameboys instead, which use cartridge-based, Flash-like memory. If the authors had used actual SCSI disks during the experiments, how exactly might have their results changed quantitatively?

c) AnalogicFS shows rather unstable multicast algorithm popularity (Figure 5), especially compared with some of the previous systems we've read about in 6.824. Give an example of another system that would have a more steady measurement of popularity pages, especially in the range of 0.1-0.4 decibels of bandwidth.

d) For his 6.824 project, Ben Bitdiddle chose to build a variant of Lab 4 that faithfully emulates the constant expected seek time across LISP machines, as AnalogicFS does. Upon implementation, however, he immediately ran into the need to cap the value size to 400 nm, rather than 676 nm. Explain what assumptions made for the AnalogicFS implementation do not hold true for Lab 4, and why that changes the maximum value size.

Questions or comments regarding 6.824? Send e-mail to 6824-staff@lists.csail.mit.edu.

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