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2.7 Objek dalam sistem file misalnya file, direktori dapat dibaca / ditulis / diciptakan / dihapus

dengan menggunakan hak-hak pengguna lokal yang menjalankan program. Jaringan

komunikasi program mungkin mencoba untuk membuat soket, terhubung ke mereka,

mengirim pesan dll Akses ke printer. Hal ini juga dapat menyamar sebagai pengguna

dengan berbagai cara, misalnya, mengirim / menerima email.

(Objects in the file system e.g. files, directories can be read/written/created/deleted using

the rights of the local user who runs the program. Network communication - the program

might attempt to create sockets, connect to them, send messages etc. Access to printers. It

may also impersonate the user in various ways, for example, sending/receiving email.)

2.11 We cannot send a limit for the time taken by a server to respond a client request because

the arrival of client requests is not predictible. For example we can have limits for

executing a computation corresponding to a request, but we can not predict how much a

request will have to wait until its execution time arrives. If the server uses threads

there is not possible for the server to allocate sufficient time to a particular request withinany given time limit. If the requests are carryed out one at a time, they may wait in the

queue for an unlimited amount of time. We can imagine the following solution

trying to set a time limit in responding to client requests:

we can limit the numb er of clients to suit the server capacity; to deal with many

clients, we should use a server with a higher capacity (many processors).

we can replicate the service.

setting a limit for the number of clients that are able to suit the service we offer

induces the lost of some client requests.

the introduction of replicas brings the problem of keeping them consistent. This

requires also an amount of time that could be used for responding client requests.

2.12 Time taken by OS communication services in the sending and receiving processes - these

tasks would need to be guaranteed sufficient processor cycles.

Time taken to access network. The pair of communicating processes would need to

be given guaranteed network capacity.

The time to transmit the data is a constant once the network has been accessed.

To provide the above guarantees we would need more resources and associated costs. The

guarantees associated with accessing the network can for example be provided with ATM

networks, but they are expensive for use as LANs.

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To give guarantees for the processes is more complex. For example, for a server to

guarantee to receive and send messages within a time limit would mean limiting the

number of clients.

2.14 Sevice A can have

arbitrary failures: checksums apply only to headers => message bodies may be

corrupted, duplicated messages.

omission failures: lost messages

The lost of messages implies that we have no validity property using the service A. The

corrupted and duplicated messages deny the integrity property of service A.

Service B can have

omission failures: lost messages, dropped messages.

The integrity prop erty holds but the validity prop erty is denied by the lost of messages.

So, the service B can NOT be called reliable.

2.15 An invocation may suffer from the following failures:

crash failures: X or Y may crash. Therefore an invocation may suffer from crash

failures.

omission failures: as SB suffers from omission failures the request or reply message

may be lost.