Remote Invocation

Question 1

Which systems usally use Request-Reply protocols?

Answer 1

Synchronous systems

Question 2

What are the advatanges of UDP over TCP for R-R protocols? (3)

Answer 2

No connection establishing overhead.
No acknowledgements are required since the reply acts as the ack.
Flow control is redundant

Question 3

What are the parameters of doOperation()? (3)

Answer 3

Remote reference r
operationID
arguments

Question 4

What is the protocol for R-R?

Answer 4

Client: doOperation(r,opID,args); (wait)
Server: getRequest();
Server: selectOp();
Server executeOp();
Server: sendReply(); - using ip address and port
Client: (continue)

Question 5

What are the failure models of R-R protocols? (3)

Answer 5

Process Crashes
Channel Ommision Failures
Messages not guarenteed to arrive in order.

Question 6

What happens in the event of a timeout in Request-Reply protocols? (2)

Answer 6

Exception/error code returned.

Retransmission

Question 7

Why are duplicate requests removed?

Answer 7

To prevent multiple executions of the same request. (Only remove if reply hasn’t been sent)

Question 8

Why is a history of replies kept?

Answer 8

Avoids reexecution

Question 9

What are the 3 styles of exchange protocol?

Answer 9

Request (R)
Request-Reply (RR)
Requst-Reply-Acknowledgement (RRA)

Question 10

What does RPC do?

Answer 10

Allow procdedures to be called on remote machines as if they were local.

Question 11

What does RPC hide?

Answer 11

Encoding/deconding of parameters and results.
Passing of messages
Preserving of required semantics for PC.

Question 12

What is the interface design issue of RPC?

Answer 12

Specification of procedures offered by a server.
Separation of interface and implementation.
Clients cannot directly access variables of server models.

Question 13

What are the call semantics of RPC? Define each (3)

Answer 13

Maybe: No fault tolerance
At-least once: Retransmits request messages after failure.
At-most one: Retransmits requests after failures, duplicate filtering and retrans replies included.

Question 14

What are the transparency problems of RPC? (3)

Answer 14

More vulnerable to failure than local procedures.
Greater latency than local.
RPC doesn’t off call-by-reference (can’t use addresses)

Question 15

What is RMI?

Answer 15

Remote Method Invocation - Calling object can invokd a method of a remote object.

Question 16

What are the similarities and differences between RMI and RPC? (3/2)

Answer 16

Similarities - Uses R-R protocol, uses interfaces, transparency
Differences - RMI uses full OOP, all objects have unique object references and can be passed as parameters.

Question 17

What is the distributed object model? (3)

Answer 17

Physical distribution of objects into different processes/computers.
Client-Server architecture
Synchronisation needed for concurrent access.

Question 18

Why are remote interfaces used?

Answer 18

To specify which methods can be called remotely.

Question 19

What are the actions upom method invocation? (3)

Answer 19

State change
Further invocation
Initialisation of new objects.

Question 20

How is garbage collection handled in RMI?

Answer 20

Cooperatively with local and remote GC.
Usually using a dedicated module to carry out distributed garbage collection.

Question 21

What failures cause exceptions in RMI?

Answer 21

Crash failures
Ommision failures: Losing of request/reply
Arbitrary Failure: Due to remoteness of object