CS 425 Midterm Prep

In a Chord ring using m = 9, nodes with the following peer ids (or node ids) join the system: 1, 12, 123, 234, 345, 456, 501. The successor of node 501 is:

1

A startup in your home garage is designing a new gossip-style failure detection similar to that discussed in lecture. In this new protocol, at a node (process or server) A, at local time = 140, its local entry for a node C is (address, counter, time) = (C, 340, 133). Tfail = 40. A receives a gossip message from node B containing one heartbeat as (sender-id, heartbeat counter), as shown below. Select the choice below where in all (four) cases, the left entry (heartbeat) leads to the right entry (updated heartbeat at A for C after receiving this heartbeat).

1 = (C, 349), A: (C, 349, 140) 2 = (C, 123), A: (C, 340,133) 3 = (C, 60), A: (C, 340, 133) 4 = (C, 355), A: (C, 355, 140)

A Pastry DHT has a peer P with the following neighbors. P currently has to route a query to key 101011001111. Which of the following neighbors is the best next-hop for this query?

101011001110 Pastry uses longest-prefix match to determine next hop for a Query.

A Gnutella topology looks like a balanced ternary tree with 4 levels of nodes, i.e., peers, as shown in the picture below. Thus, there is 1 root at Level 1, which has 3 children at Level 2, which each have 3 children at Level 3, which in turn each have 3 children at Level 4 - thus, there are a total of 40 nodes. If the root node (Level 1) sends a Query message with TTL=2, then what are the number of nodes receiving the Query message, not including the originating node? Enter your answer as a numeric value in the text box below. (1 point)

12

A Gnutella topology looks like a balanced ternary tree with 5 levels of nodes, i.e., peers. Thus, there is 1 root at Level 1, which has 3 children at Level 2, which each have 3 children at Level 3, which in turn each have 3 children at Level 4, which in turn each have 3 children at Level 5 - thus, there are a total of 121 nodes. If a child of the root (i.e., a Level 2 node) sends a Query message with TTL=5, then what are the number of nodes receiving the Query message, not including the originating node?

120 All nodes in the overlay receive this Query.

In a Chord ring using m = 9, nodes with the following peer ids (or node ids) join the system: 1, 12, 123, 234, 345, 456, 501. What node id is the file with id 120 stored at (assuming only one replica)?

123 The ith finger table entry (i = 0 and upwards) at node n is the first node at or clockwise to n+2i. A Query is forwarded, at each step, to the farthest clockwise neighbor which is still to the anticlockwise of the key, or failing that it is forwarded to the successor.

A datacenter run by an upstart company called Zahoo! consumed about 1000 kWh in 2014. If only 500 kWh of this total power was used in running IT equipment (servers, routers, etc.), then the PUE of Zahoo's datacenters is:

2.0

In a Chord ring using m = 8, nodes with the following peer ids (or node ids) join the system: 45, 32, 132, 234, 99, 199. What is the successor of node 199?

234

In a datacenter with 10,000 machines, the MTTF (mean time to failure) of a single server is 48 months. You can assume each month has 30 days. The MTTF (mean time to failure) until the next server fails in the data center is approximately:

3.4 hours

A Gnutella topology looks like a balanced ternary tree with 5 levels of nodes, i.e., peers. Thus, there is 1 root at Level 1, which has 3 children at Level 2, which each have 3 children at Level 3, which in turn each have 3 children at Level 4, which in turn each have 3 children at Level 5 - thus, there are a total of 121 nodes. If a leaf (Level 5) node sends a Query message with TTL=2, then what are the number of nodes receiving this message, not including the originating node? Enter your answer as a numeric value in the text box below.

4 The query is received by its 2 ancestors at Levels 4 and 3, and the other 2 children of the originator's parent.

In a Chord ring using m = 9, nodes with the following peer ids (or node ids) join the system: 1, 12, 123, 234, 345, 456, 501. If node 234 fails, which of the following nodes will not update any of their finger table entries or successors?

501 (1+128) mod 512 is 129, so 1 has 234 in its FTs (12+128) mod 512 is 140, so 12 has 234 in its FTs 123's successor is 234 (456+256) mod 512 is 200, so 456 has 234 in its FTs. (501+128) mod 512 = 117 and (501+256) mod 512 = 245. So 501 does not have 234 in its FTs.

A Gnutella topology looks like a balanced ternary tree with 5 levels of nodes, i.e., peers. Thus, there is one root at Level 1, which has 3 children at Level 2, which each have 3 children at Level 3, which in turn each have 3 children at Level 4, which in turn each have 3 children at Level 5 - thus, there are a total of 121 nodes. What is the minimum TTL required for any node's Query to reach every other node?

8 Feedback: In the worst case the Query takes 4 hops to get to the Level 1 node and 4 further hops to get down to another Level 4 node.

In a Chord ring using m = 8, nodes with the following peer ids (or node ids) join the system: 45, 32, 132, 234, 99, 199. What is the comma-separated list of 8 finger table entries at node 45?

99,99,99,99,99,99,132,199

In MapReduce, a Reduce function processes:

A set of all key-value pairs sharing the same key

Alice and Bob are employees in your company who have each been given the task of inventing a failure detection algorithm for an asynchronous system that tolerates 3 simultaneous process failures. You are called to judge whether their protocols satisfy the requirement. Alice's protocol has each process p randomly pick 3 other processes to send its heartbeats to, and thereafter p periodically sends its heartbeats to these processes. Bob's protocol has each process p randomly pick 3 other processes to receive heartbeats from, and thereafter each of these 3 picked processes periodically send its heartbeats to p. Note that in each of these protocols, the heartbeats are direct ones rather than gossip-style. Then, which of the following statements is correct?

Alice's protocol is complete but Bob's protocol is not.

In BitTorrent, a newly joined leecher X is trying to download a file with 6 blocks B1-B6. X has 3 neighbors: A, B, and C. These neighbors are storing the following blocks of the file: A: {B1, B2, B4, B5}; B:{B2, B3, B4, B5, B6}; C:{B1, B2, B6}. Then X will prefer downloading which block first?

B3

A Cassandra deployment uses the RackInferringSnitch. Two addresses 123.1.23.34 and 123.1.23.123:

Belong to the same rack but are different machines

In a heartbeat protocol for failure detection, increasing the timeout (used for declaring a member as failed), without changing any other protocol parameter, results in which of the following? (Select multiple correct answers.)

Decreases false positive rate Increases detection time

Napster servers, as discussed in lecture, do not store which of the following?

Files

In Cassandra, when a Memtable is full or old, it is

Flushed to disk along with an index table and a Bloom filter

In HBase, in order to guarantee that updates are not lost due to a crash, which of the following needs to be written before an update is added to MemStore?

HLog

Someone in your company decides to run Hadoop on a cluster of eight machines, out of which seven machines are each quad core 2.2 GHz, 16 GB RAM, 1 TB hard disk, and the eighth machine is single core 2.2 GHz, 2 GB RAM, 1 TB hard disk. Further, this person has initialized the cluster with the setting where each of the eight machines is configured to run at most four containers (Map or Reduce tasks) at any time - the containers at each machine equally split all the resources at the machine. For a job that contains 32 Map tasks, you claim that this will result in:

Long job completion time due to straggler tasks at the 2 GB node

A Cassandra deployment with 6 (N1 through N6) nodes across three racks: N1 and N2 are in rack 1; N3 and N4 in rack 2; N5 and N6 in rack 3. The Cassandra ring has the nodes in the following clockwise order: N1, N2, N3, N4, N5, N6. The NetworkTopologyStrategy is attempting to place 2 replicas of a given key. The first replica is placed by the partitioner at N3. The second replica will be placed at node:

N5 To find the second replica the NetworkTopologyStrategy goes clockwise around the ring starting from the first replica, until it hits a node in a different rack than the first replica.

Which of the following is faster?

Pull gossip

Which of the following Gnutella messages are flooded out and TTL restricted?

Query

The Hadoop Capacity Scheduler runs at:

RM

Someone gives you an arbitrary run (execution) trace from a system that implements eventual consistency. It is impossible that this run:

Returns an answer for a read from a write which was issued by a client after the read result was received at the client

Given that there are at most two failures at a time in the system, which of the following protocols does NOT satisfy completeness?

Ring-style heartbeating where each process heartbeats to only its clockwise neighbor

Which of these consistency models is a "strong" notion of consistency?

Sequential consistency

Which of the following is NOT a uniquely new trait of cloud computing problems compared to the previous generations of distributed systems problems?

Servers coordinating with each other

In a Pastry DHT that is locality-aware, the path of a query is very likely to:

Take short network jumps in its early hops and long network jumps in later hops

Eventual consistency does imply:

The set of replicas is always trying to catch up and converge to the latest writes

A Cassandra-like key-value store system uses write consistency level of size W and read level of size R. There are N replicas of each key, and N is an even integer that is large enough. You are told that to maintain the strong consistency needed by your application, all conflicting writes must be detected by at least one replica (i.e., any two sets of written replicas must overlap) and a read must return the value of the latest acknowledged write (i.e., a read replica set must overlap with every written replica set). Which of the following combinations does NOT maintain strong consistency?

W=N/2+1, R=N/2-1

A Cassandra-like key-value store system uses write consistency level of size W and read level of size R. There are N replicas of each key, and N is an even integer that is large enough. You are told that to maintain the strong consistency needed by your application, all conflicting writes must be detected by at least one replica (i.e., any two sets of written replicas must overlap) and a read must return the value of the latest acknowledged write (i.e., a read replica set must overlap with every written replica set). Which of the following combinations DOES maintain strong consistency?

W=N/2+3, R=N/2-1

Which of the following is a private cloud?

Yahoo's internal Grid clusters, accessible to only Yahoo employees

In MapReduce, one writes a program for Map that processes one input line at a time and outputs a (key, value) or nothing; and one writes a program for Reduce that processes an input of (key, all values for key). The iteration over input lines is done automatically by the MapReduce framework. You are given an input file containing information from an asymmetrical social network (e.g,. Twitter) about which users "follow" which other users. If user a follows b, the entry line is (a, b). In an input line, a and b are different from each other. You need to write a MapReduce program that outputs all pairs of users (a,b) who follow each other. Someone has written half of a MapReduce program where the shuffle traffic has key = (a,b), i.e., a pair of users. The Reduce function invoked on this key counts the number of occurrences of this key and if this count is 2, outputs the key as its final result. You're asked to write the Map function that takes as input (a,b) lines. The Map function should do the following:

if (a<b) then output <key,value>= <(a,b),1>, otherwise output <key,value>= <(b,a),1>