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Distributed Sustems > Exams multiple > Flashcards

Exams multiple Flashcards

1
Q

What is meant by access transparency?

A

Local and remote resources are accessed using the same operations

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2
Q

what is meant by location transparency?

A

Remote resources are accessed using location independent names

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3
Q

What is meant by concurrency transparency?

A

Processes can access resources without interfering with each other

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4
Q

What is meant by failure transparency?

A

Failures are concealed for the users of a resource

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5
Q

What is meant by replication transparency?

A

Users of a resource access it as if it was not replicated

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6
Q

What is significant for a client server architecture?

A

The client is the active part

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7
Q

What would we call a system where one node is always reacting on requests and other nodes only communicate with this node?

A

A client server system

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8
Q

What is significant for a peer-to-peer architecture?

A

All nodes are active and can be the initiator of operations

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9
Q

What do we know in a synchronous system?

A

The upper bound of the time to perform an operation

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10
Q

What is significant for an asynchronous system?

A

That the time to perform an operation does not have an upper bound

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11
Q

What do we call a system where the maximum times for operations and messages delivery are known?

A

A synchronous system

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12
Q

What is provided by UDP?

A

A best effort delivery of messages to a process

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13
Q

What is provided by TCP?

A

A full-duplex stream between two processes

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14
Q

Which address can be found in the TCP header?

A

The port numbers

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15
Q

What is significant for synchronous communication?

A

The send operation blocks and waits for the receive operation

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16
Q

What is significant asynchronous communication?

A

A sender can continue without waiting for the receive operation

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17
Q

Can a synchronous communication interface be used in an asynchronous way?

A

Yes, by performing the send operation in a separate thread

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18
Q

What is the difference between the two nodes that are communicating over a stream socket?

A

Nothing, they have equal rights and responsibilities

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19
Q

The erlang call gen_tcp:recv(Socket,0) will return

A

The first part(possibly the whole) message sent to Socket

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20
Q

What is the purpose of the marshaling procedure?

A

To encode application layer structures in an external form

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21
Q

How are arguments passed in Java RMI?

A

At most once

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22
Q

What level of transparency is provided by method invocation in Java RMI?

A

Location transparency only since we need to capture remote exceptions

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23
Q

What level of transparency is provided by method invocation in Java RMI?

A

Location transparency only since we need to capture remote exceptions

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24
Q

How do Erlang processes communicate?

A

Asynchronous message passing

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25
Q

Does Erlang provide a form of location transparency?

A

Yes a process can use a process identifier without having to know the address of the node where the process lives

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26
Q

How is the destination defined in an Erlang send operation?

A

As process identifier or a local or remote registered name

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27
Q

What is a soft link in a file system?

A

A path that is resolved to another path

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28
Q

What is a hard link in a file system?

A

A mapping of a name to a file identifier

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29
Q

What is the purpose of the Unix Iseek operation?

A

Set the write/read pointer of an opened file

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30
Q

How is a NFS client-side cashe entry validated?

A

If the validity was checked less than t second ago or if the server notification time is equal to the client notification time

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31
Q

How is authentication control handled in Sun NFS?

A

Authentication is provided by RPC in each operation

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32
Q

How does a NFS server know at what position to read and write an opened file?

A

Each read and write operation holds the position

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33
Q

How is AFS client side cashing implemented?

A

The server promise to notify the client if a file is modified by another client

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34
Q

Can two client have an inconsistent view of a file using AFS?

A

Yes, if a call-back message is lost a cashed copy can be used although the original has been modified

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35
Q

What is true if A happened real time before B?

A

A could have happened before B

36
Q

What is true for events A and B?

A

If A caused B then A happened before B

37
Q

What can we know if we use Lamport clocks?

A

if L(a)<L(b) then could have caused b

38
Q

What can we know if we use Lamport clocks?

A

if but not only, if a happened before b then L(a)<L(b)

39
Q

What can we conclude looking at the Lamport clock timestamps of two events?

A

if L(a) !<L(b) then a did not happened before b

40
Q

What is the most that we know if we use vector clocks

A

V(a)<V(b) if and only if a happened before b

41
Q

What is the definition of a consist cut?

A

If e is in the cut and f happened before e then f is in the cut

42
Q

What is the definition of a stable global state predicate?

A

If a system enters a state where the predicate holds true it will remain true in all future states

43
Q

What is the definition of a unstable global state predicate?

A

The predicate could hold true in a state but then be false in future states.

44
Q

What is the difference between a URL and a URN?

A

URN resolves into any replica of a resource matching the URI.

45
Q

How are inconsistencies of the resolver cache handled in the DNS architecture?

A

Each entry has a time to live

46
Q

What is the advantage of using recursive navigation for DNS queries?

A

Servers can hide internal DNS hierachy

47
Q

How are inconsistent cashed entries removed from a DNS resolver?

A

All entries have en expiration time set when cashed

48
Q

How can we make two computer clocks perfectly synchronized?

A

we cannot

49
Q

What accuracy can be provided using Christian’s algorithm?

A

remember the equation!

50
Q

What is the purpose of Berkley algorithm?

A

to perform internal synchronization

51
Q

What does the reply from a NTP server contain?

A

send and receive time of request and send time of reply

52
Q

What is true if A happened before B?

A

A must have occurred in real time before B

53
Q

Give an example of a stable global state predicate

A

Deadlock

54
Q

What are the requirements for using the Bully algorithm ?

A

We must have reliable failure detectors

55
Q

What is the benefit of a reliable multicast?

A

messages are guaranteed to be delivered to all correct processes

56
Q

Can we implement a reliable multicast using only basic multicast?

A

yes, by re sending each received message to all the other nodes

57
Q

What is the definition of total order multicast?

A

Messages are delivered in the same sequence

58
Q

What is a dirty read during the transaction?

A

reading a value that has not been committed

59
Q

What is two-phase locking in a transaction?

A

not taking any locks once a lock has been released

60
Q

What does it mean that a transaction meets the atomicity property?

A

either all or no operations in the transaction are performed

61
Q

What does it mean that a transaction meets the isolation property?

A

intermediate results must no be visible to other transactions

62
Q

What does it mean that a transaction meets the durability property?

A

The effects of the transaction will remain even if we have a server crash

63
Q

What is two phase commit?

A

A protocol that ensures atomicity in a distributed transaction

64
Q

What is a phantom deadlock?

A

One that is detected but not stable

65
Q

What is a view, created by a group membership service?

A

The set of processes belonging to a group

66
Q

In view-synchronous group membership protocol, a process that enters the group and is included in the delivered vie, will be guaranteed to be delivered:

A

All messages starting from the view when it enters

67
Q

Why is view synchronous communication different from reliable multicast?

A

A message is sent and delivered only by processes belonging to the same view

68
Q

Distributed hash tables are designed to limit the number of hops to reach any key to

A

at most log(N)

69
Q

When is it better to use the Berkley algorithm rather than NTP for clock synchronization?

A

When implementing internal synchronization

70
Q

Assume that a NTP server cannot respond to received request in less than 40 ms, how does this influence the accuracy of synchronizing clients?

A

Not at all

71
Q

At time 117 you received a NTP reply with the following information: requst sent at 82, received at 111, reply sent at 120. How should you adjust your time?

A

Advanced 16 steps

72
Q

What is the difference between a Lamport clock and a Vector clock?

A

Only the vector clock gives a complete description of the happened before order

73
Q

When is it problematic to use vectors clocks?

A

When we have a dynamic set of process

74
Q

What do we know if we record a snapshot using the algorithm by Chandy and Lamport?

A

There is a linearization from the original state to the final state that passes through the snapshot

75
Q

Assuming that we collect all state transitions of nodes and have them tagged with vector clocks what can we then do?

A

For any unstable predicates determine if it possibly was true during the execution

76
Q

How can we detect that non-stable predicate definitely was true during ab execution?

A

Generate all consistent runs and show that the predicate is true at one point in all

77
Q

What is the advantage of a bully algorithm compared to a ring-based algorithm?

A

Better turnaround time

78
Q

What does Lamport clocks give us when implementing distributed mutual-exclusion?

A

The system is prevented to dead-lock

79
Q

What does Ricart and Agrawalas mutual exclusion algorithm perform better than a central solution?

A

Under high congestion since only one message is needed to release and obtain the lock

80
Q

How are FIFO, causal and total order multicast related?

A

A causal order is also a FIFO order

81
Q

What is a good reason for choosing UDP rather than TCP?

A

You have a small message that should be sent with littke delay

82
Q

What is a good reason for choosing TCP rather than UDP?

A

You have large messages or a sequence of messages

83
Q

What is the maximum TCP capacity in a 100MBPS link with 250ms round trip latency using a 64 Kbyte window size?

A

2Mbps

84
Q

What is the maximum TCP capasity in a 100 Mbps link with 25ms round trip latency using 64Kbyte window size?

A

20Mbps

85
Q

What is a remote object passed as a reference and not as a copy in Java RMI?

A

The object contains a mutable state that should not be duplicated

86
Q

Can we have a circular construction in Erlang?

A

Yes, processes can refer to each other in a circular way

Distributed Sustems (3 decks)

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