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91 Cards in this Set
- Front
- Back
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Why is memory exchange difficult to use?
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every data transcer requires a send and receive, so receptive process must expect the send()
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Define shared memory.
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two or more processes share a part of their address space.
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What is the primary advantage of shared memory?
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fast and easy to use
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What are the two problems intorudced by shared memory's primary advantage?
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1) concurrent access can cause disasters, 2) synchronization access is needed
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What are the two disadvantages fo shared memory?
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1) not general, since sender and reciever must be on the same machine, 2) less secure since processes can directly access a part of the other process's shared address space
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Name the four defining issues of message passing.
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1) direct/indicrect communication, 2) blocking/nonblocking primitives, 3) exception handling, 4) quality of service
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T/F - In direct communication, send and receive system calls do not always need to specify processes as desitnation/source.
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F
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Give examples of send and recieve specifications that would take place in direct communication.
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send(process, msg,length); receive(process, msg, &length);
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Why is direct communication the most simple means of IPC?
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there is no intermediary between sender and receiver
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Make an analogy with which you could describe direct communication to a layman.
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ex: phones without switchboard, MAKE YOUR OWN FOR PRACTICE!
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T/F A process executing a recieve call only needs to know the identity of all processes likely to send messages WHEN the IPC is RGD
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F. RGD doesnt even mean anything. receiver ALWAYS needs to know!!
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Why does a receiving process need to know the identity of all processes likely to send it messages?
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duh
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Why is a receiving processes neccesity to know all processes likely to send messages bad for servers?
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because servers must answer requests from arbityary processes.
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How is indirect communication different from direct communication?
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indirect communication has an intermediary entity as a destination or source
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What is the intermediary of indirect communication called?
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the mailbox
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Given an example of a send or a recieve primative that is involved in indirect communicaiton.
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send(mailbox,msg,size);
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T/F a mailbox is created by the client.
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F, it is created by the kernel
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How does a kernel know that it is to create a mailbox?
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a request has been sent by a user process.
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Make an analogy with which you could describe indirect direct communication to a layman.
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ex: phones with a switchboard, MAKE YOUR OWN FOR PRACTICE!
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T/F Differnt processes can send messages to a single mailbox.
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T |
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T/F In indirect communication, a process cannot recieve messages from a processes it does not know about.
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F, different processes can send messages to a single mailbox, and the receptive process is informed by that mailbox
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T/F processes cannot wait for messages coming from different senders. they will only open the first message they receive
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F, but they do open messages as they arrive
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What are the two types of mailboxes?
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1) private, 2) public
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Is a BSD socket a private or public mailbox?
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private, the word "socket" reveals this |
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What is the difference between private and public mailboxes?
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1) private are attached to specific processes while 2) public are system objects
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T/F private mailboxes cease to exist when the process that requested creation (and its children) terminates
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T
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What is another name for a private mailbox?
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port
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What can receive messages trhough a private mailbox?
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the process and the children thereof that requestsed said mailbox's creation
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What are public mailboxes "owned" by?
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the system, man
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What are public mailboxes shared by?
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all processes having the right to receiver messages through it
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T/F public mailboxes can survive the termination of their creator process.
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T
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T/F public processes are best suited for passing messages between different machines.
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F
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Are System V UNIX message queues public mailboxes? What word(s) gives the answer away?
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Yes, "message queues"
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Is buffering needed for a blocking primative?
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Nope
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When does a blocking send return?
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when the receipient process has received the message.
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When does a non-blocking send return?
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when the messages has been accepted for delivery by the OS
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What does a non-blocking send assume about the receipient OS?
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it assumes that the OS can store the message in a buffer
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Make an analogy with which you could describe a non-blocking send to a layman.
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ex: letter is dropped in mailbox so you are DONE, you assume post office will handle the rest MAKE YOUR OWN FOR PRACTICE!
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When does a non-block receive return?
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as soon as it has either 1) retreived a message or 2) learned that the mailbox is empty
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Make an analogy with which you could describe a non-blocking receive to a layman.
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ex: checking the mail.
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What is another name for blocking receives?
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busy wait
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In three or more steps, explain the logic of a blocking send simulation.
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1) sender sends message and reqeusts an acknowledgement (ACK), 2) sender waits for ACK from receiver using a blocking receieve, 3) receiver sends ACK
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In three or more steps, explain the logic of a blocking send simulation.
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1) receipient receives message 2) receipient is delayed, 3) this process repeats until message space receives contents
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What are the three elements that define the "standard choice?"
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1) indirect naming, 2) non-blocking sends (fire and forget), 3) blocking receives (receiver needs data to continue)
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T/F Non-blocking primitives require buffering. Justify your answer.
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T, OS needs to store messages that have been sent but not yet received.
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What does it mean if a buffer has bounded capacity?
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the buffer can refuse to receive messages when it is full
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What is the limit on capacity of a buffer receiving non-blocking primitives?
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theoretically there is none, but typically the capacity is bounded.
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Are blocking receives and direct communication the best pairing of any pairing ever of all time forver?
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NO. They are not!! when paired, processes cannot wait for messages from several sources without using special parallel programming constructs, in fact, the receiving process can only receive messages from a specified process
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How can the sending and recieving of messages effect the quality of service?
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loss, corruption, duplication, arrival out of sequence
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What are positive acknowledgements?
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basic techniques for providing reliable delivery of messages
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Describe the scenarios of postive acknowledgements
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1) destination process sends an acknowledgment message (ACK) for every message that was correctly delivered, 2)sender resends any mdessage that was not ACK within a fixed time frame
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Name the three classes of service and how they handle messages
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1) datagrams -messages are sent one at a time, 2) virtual circuits - a connection oriented service that uses an ordered sequence fo messages, 3) streams - message boundaries are ignored and are sent as an ordered sequence of bytes
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How are messages sent when using datagrams? what potential consequences result from this?
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individually, some messages can be lost, duplicated, or might arrive out of sequence.
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what are the two kinds of datagrams?
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1) reliable datagrams 2) unreliable datagrams
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Why are reliable datagrams labeled so?
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because they resend messages until acknowledgement
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T/F unreliable datagrams work well when a message requests a reply
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T since a reply is an implicit ACK of a message
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Make an analogy for unreliable datagrams
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ex. you don't ACK emails, you just reply to them
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Why would an ACK be sent if a reply is an implicit response?
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The ACK is useful when a reply will not be timly
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What does UDP stand for?
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User Data Protocol
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T/F UDP is one of the lesser known protocols
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F
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T/F UDP provides a reliable datagram service
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F
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T/F UDP is best for short interactions
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T
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How many requests and replies is a UDP suited for?
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one of each
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When using virtual circuits, a ___ is established between the send and receiver.
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logical connection
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T/F Virtual circuits guarantee arrival of a sequence without loss or duplication.
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T
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Make an analogy for virtual circuits
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ex. words of a phone conversation
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Why are virtual circuits costlier than datagrams?
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the connection must be set up beore sending any data
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HTTP is an example of __?
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a virtual circuit
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T/F virtual circuits are best for transmitting large amounts of data.
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T
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T/F virtual circuits are best for transmitting data that requires two or less large messages.
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F, several messages are best suited
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T/F Streams are very similar to virtual cirtuits.
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T
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Do streams preserve message boundaries? How tdoes the receiver see the resultant message?
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streams do not preserve message boundaries. the receiver sees a seemless stream of bytes.
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What is an alternate name for TCP?
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stream protocol
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Why is TCP said to be heavyweight?
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it requires three messages (packets) to establish a virtual connection
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T/F TCP provides a reliable stream service
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T
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What are the three major advantages of using remote procedure calls?
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1) they hide all details of message passiong, 2) they provide a higher level of abstraction, 3) they extend a well know model of programming
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What is the disadvantage of using a RPC?
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RPCs do not always behave like regular procedure calls since the client and server do not share the same address space
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List the four steps of a user stub.
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1) argument marshaling, 2) send request message, 3) wait for server reply message, 4) argument unmarshaling
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List the five steps of the server stub.
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1) wait for client requests, 2) argument unmarshaling, 3) call server procedure, 4) argument marshaling, 5) send reply
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Which does the actual processing of user requests, the server procedure or the user stub?
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the server procedure
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T/F In RPC, the client and server processes share the same address space
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F
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T/F An RPC needs to handle partial failures
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T
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What are the ramifactions (2) of an RPC not having shared address space?
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1) no global variables, 2) cannot pass addresses (arguments by reference or dynamic data structures through pointers)
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If a remote procedure call cannot pass addresses, what are two ways arugments can be passed?
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1) pass current value of argument to remote procedure, 2) copy returned value in user program
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How can partial failures be detected?
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the client can send "are you alive?" mesages to the server at fixed intervals
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What is an idempotent request?
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a request that can be executed several times and have the same effect as having been executed once
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Given an example of an idempotent request.
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reading n bytes from a fixed location, not reading the next n bytes from a current location
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What does attaching a serial number to requests allow a server to do?
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detect replays of requests it has previously received and refuse to execute
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T/F Attaching serial numbers is a cheap method of handling partial executions.
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T
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T/F Using transaciton mechanisms are the most costly method of handling partial executions.
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T
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What does a transaction mechanism do?
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it guarantees that each request with EITHER be full executed OR have no effect.
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