Homework 2

Total points 15

Question 1 In an object-based system, object C1 has a method C1.M1 that calls object C2’s method C2.M2. In which of the following cases is this call NOT a Remote Method Invocation? There may be MULTIPLE ANSWERS. 1 point

• C1 and C2 reside in different processes, and the two processes are running on different hosts.

• C1 and C2 are on different hosts.

• C1 and C2 reside in the same process.

• C1 and C2 reside in different processes, but the two processes are running on the same host.

SOLUTION-C1 and C2 reside in the same process.

Question 2 A particular RPC system chooses big-endian representation for its integers. Now consider a little-endian x86 machine (using this RPC system) that needs to send an RPC call with an integer parameter represented in memory as four bytes, with each byte represented in a hexadecimal format. The integer is stored as follows in the x86 machine: (lowest address byte) 12-34-56-78 (highest address byte).

In the marshalled RPC call message, this integer will be stored with: 1 point

• 12 in its lowest addressed byte

• 78 in its lowest addressed byte

• 56 in its lowest addressed byte

• None of these options

SOLUTION-78 in its lowest addressed byte

Question 3 Some operations are called idempotent operations because they can be repeated multiple times without any side effects. RPCs that implement idempotent operations are a good fit for at-least-once call semantics. Which of the following functions is NOT idempotent? In the following, y is a local variable held at the server executing the called function, while x is an argument passed by the calling function. (1 point) 1 point

• void f(int x) { y = 3; }

• void f(int x) { y = x+1; }

• int f(int x) { y = x; return y+1; }

• int f(int x) { y = y+x; return 1;}

SOLUTION-int f(int x) { y = x; return y+1; }

Question 4 The C in ACID stands for: (1 point) 1 point

• Commission

• Causality

• Consistency

• Coldness

SOLUTION-Consistency

Question 5 Which of the following ways will NOT prevent deadlocks from occurring (and persisting)? 1 point

• Transactions that are unable to successfully lock objects within a timeout must abort and release all held locks

• Transactions are multi-threaded

• Objects are acquired only in increasing order of object id, where object ids are integers

• Making all objects read-only and allowing only read locks on objects (no write locks).

SOLUTION-Transactions that are unable to successfully lock objects within a timeout must abort and release all held locks

Question 6 Which of the following increases concurrency (TPS) the most, while preserving correctness? 1 point

• Optimistic Concurrency Control

• None of these options

• Pessimistic Concurrency Control with one lock per object

• Pessimistic Concurrency Control with read-write locks per object

SOLUTION-Optimistic Concurrency Control

Question 7 In a system supporting read-write locks, which of the following is FALSE? 1 point

• A read lock on object O held by transaction T can be promoted only if no other transaction is currently holding a read lock on object O.

• A write lock on object O held by transaction T can be shared concurrently with a read lock held by another transaction U on a different object O2.

• A transaction T can acquire a read lock on object O only if no other transaction is currently holding a write lock on object O.

• A read lock on object O held by transaction T cannot be promoted if another transaction U holds a write lock on any other object O2.

SOLUTION-A read lock on object O held by transaction T cannot be promoted if another transaction U holds a write lock on any other object O2.

Question 8 Which of the following is NOT an optimistic concurrency control technique?

• Two-phase locking with read-write locks

• Eventual consistency using Las Writes Wins

• Never check for consistency violations until commit time; if consistency is detected abort transaction.

• Timestamp Ordering Technique

SOLUTION-Timestamp Ordering Technique

Question 8 With a per-replica availability of 1 Nine, what is the minimum number of replicas you need in order to get at least 5 Nines availability for an object? Please enter your answer in numeric value. 1 point

SOLUTION-7

Question 9 In a two-phase commit for transaction T, what is the earliest point of time that a responding can count portion of the transaction T's updates?

SOLUTION-Right after receiving a commit message from the coordinator

Question 9 In a two-phase commit run for transaction T involving 13 servers S1 through S13, all servers other than S8 respond back with a Yes message in the first phase (after receiving the Prepare message from the transaction coordinator), but the coordinator has not yet timed out for the replies. In which of the following cases will the two-phase commit run decide to commit the transaction? There may be MULTIPLE answers. 1 point

• S8 crashes after it sends back a Yes message in response to the coordinator’s Prepare message, and the coordinator receives the Yes message before the timeout. S8 does not recover for a long time.

• S8 receives the Prepare message but realizes that updates done by transaction T in S8’s memory are corrupted, so it responds back with a “No” to the coordinator.

• S8 crashes during the first phase just before it receives the Prepare message, and it does not recover for a long time.

• S8 crashes before the first phase and does not recover for a long time.

SOLUTION-

Question 10 Someone builds an active replication system where there is only one front-end, and the front-end for each operation (read or write) forwards only one operation at a time to all the replicas and waits for all of the replicas to respond to the coordinator before sending back an answer to the client. For a read operation, the front-end sends back the highest timestamped value. Which consistency model does this system implement? (Select the most appropriate answer.) 1 point

• Eventual consistency

• Causal consistency

• Monotonic consistency

• Linearizability (or a notion of strict consistency)

SOLUTION-Eventual consistency

Question 11 Below you are shown a set of transactions and their operations in the order they are executed in real time.