程序代写代做代考 scheme distributed system database algorithm concurrency crawler Java cache compiler 1
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The Solutions to Tutorial Questions and Lab Projects of Week 1
Tutorial Questions
1. Give five types of hardware resource and five types of data or software resource that can
usefully be shared. Give examples of their sharing as it occurs in distributed systems.
Answer
Hardware:
CPU: compute server (executes processor-intensive applications for clients), remote
object server (executes methods on behalf of clients), worm program (shares cpu
capacity of desktop machine with the local user). Most other servers, such as file
servers, do some computation for their clients, hence their cpu is a shared resource.
memory: cache server (holds recently-accessed web pages in its RAM, for faster access
by other local computers).
disk: file server, virtual disk server (see Chapter 8), video on demand server (see
Chapter 15).
screen: network window systems, such as X-11, allow processes in remote computers to
update the content of windows.
printer: networked printers accept print jobs from many computers, managing them
with a queuing system.
network capacity: packet transmission enables many simultaneous communication
channels (streams of data) to be transmitted on the same circuits.
Data/software:
web page: web servers enable multiple clients to share read-only page content (usually
stored in a file, but sometimes generated on-the-fly).
file: file servers enable multiple clients to share read-write files. Conflicting updates may
result in inconsistent results. Most useful for files that change infrequently, such as
software binaries.
object: possibilities for software objects are limitless. E.g. shared whiteboard, shared
diary, room booking system, etc.
database: databases are intended to record the definitive state of some related sets of
data. They have been shared ever since multi-user computers appeared. They include
techniques to manage concurrent updates.
newsgroup content: the netnews system makes read-only copies of the recently-posted
news items available to clients throughout the Internet. A copy of newsgroup content is
maintained at each netnews server that is an approximate replica of those at other
servers. Each server makes its data available to multiple clients.
video/audio stream: servers can store entire videos on disk and deliver them at
playback speed to multiple clients simultaneously.
exclusive lock: a system-level object provided by a lock server, enabling several clients
to coordinate their use of a resource (such as printer that does not include a queuing
scheme).
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2. A user arrives at a railway station that she has never visited before, carrying a PDA that is
capable of wireless networking. Suggest how the user could be provided with information
about the local services and amenities at that station, without entering the station’s name or
attributes. What technical challenges must be overcome?
Answer
The user must be able to acquire the address of locally relevant information as automatically
as possible. One method is for the local wireless network to provide the URL of web pages
about the locality over a local wireless network.
For this to work: (1) the user must run a program on her device that listens for these URLs,
and which gives the user sufficient control that she is not swamped by unwanted URLs of the
places she passes through; and (2) the means of propagating the URL (e.g. infrared or an
802.11 wireless LAN) should have a reach that corresponds to the physical spread of the
place itself.
3. What are the advantages and disadvantages of HTML, URLs and HTTP as core technologies
for information browsing? Are any of these technologies suitable as a basis for client-server
computing in general?
Answer
HTML is a relatively straightforward language to parse and render but it confuses
presentation with the underlying data that is being presented.
URLs are efficient resource locators but they are not sufficiently rich as resource links. For
example, they may point at a resource that has been relocated or destroyed; their
granularity (a whole resource) is too coarse-grained for many purposes.
HTTP is a simple protocol that can be implemented with a small footprint, and which can be
put to use in many types of content transfer and other types of service. Its verbosity (HTML
messages tend to contain many strings) makes it inefficient for passing small amounts of
data.
HTTP and URLs are acceptable as a basis for client-server computing except that (a) there is
no strong type-checking (web services operate by-value type checking without compiler
support), (b) there is the inefficiency that we have mentioned.
4. A search engine is a web server that responds to client requests to search in its stored
indexes and (concurrently) runs several web crawler tasks to build and update the indexes.
What are the requirements for synchronisation between these concurrent activities?
Answer
The crawler tasks could build partial indexes to new pages incrementally, then merge them
with the active index (including deleting invalid references). This merging operation could be
done on an off-line copy. Finally, the environment for processing client requests is changed
to access the new index. The latter might need some concurrency control, but in principle it
is just a change to one reference to the index which should be atomic.
5. The host computers used in peer-to-peer systems are often simply desktop computers in
users’ offices or homes. What are the implications of this for the availability and security of
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any shared data objects that they hold and to what extent can any weaknesses be overcome
through the use of replication?
Answer
Problems:
• People often turn their desktop computers off when not using them. Even if on most
of the time, they will be off when user is away for an extended time or the computer
is being moved.
• The owners of participating computers are unlikely to be known to other
participants, so their trustworthiness is unknown. With current hardware and
operating systems the owner of a computer has total control over the data on it and
may change it or delete it at will.
• Network connections to the peer computers are exposed to attack (including denial
of service).
The importance of these problems depends on the application. For the music downloading
that was the original driving force for peer-to-peer it isn’t very important. Users can wait
until the relevant host is running to access a particular piece of music. There is little
motivation for users to tamper with the music. But for more conventional applications such
as file storage availability and integrity are all-important.
Solutions:
Replication:
• If data replicas are sufficiently widespread and numerous, the probability that all are
unavailable simultaneously can be reduced the a negligible level.
• One method for ensuring the integrity of data objects stored at multiple hosts
(against tampering or accidental error) is to perform an algorithm to establish a
consensus about the value of the data (e.g. by exchanging hashes of the object’s
value and comparing them). This is discussed in Chapter 15. But there is a simpler
solution for objects whose value doesn’t change (e.g. media files such as music,
photographs, radio broadcasts or films).
Secure hash identifiers:
• The object’s identifier is derived from its hash code. The identifier is used to address
the object. When the object is received by a client, the hash code can be checked for
correspondence with the identifier. The hash algorithms used must obey the
properties required of a secure hash algorithm as described in Chapter 7.
6. Distinguish between buffering and caching.
Answer
Buffering: a technique for storing data transmitted from a sending process to a receiving
process in local memory or secondary (disk) storage until the receiving process is ready to
consume it. For example, when reading data from a file or transmitting messages through a
network, it is beneficial to handle it in large blocks. The blocks are held in buffer storage in
the receiving process’ memory space. The buffer is released when the data has been
consumed by the process.
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Caching: a technique for optimizing access to remote data objects by holding a copy of them
in local memory or secondary (disk) storage. Accesses to parts of the remote object are
translated into accesses to the corresponding parts of the local copy. Unlike buffering, the
local copy may be retained as long as there is local memory available to hold it. A cache
management algorithm and a release strategy are needed to manage the use of the memory
allocated to the cache. (If we interpret the word ‘remote’ in the sense of ‘further from the
processor’, then this definition is valid not only for client caches in distributed systems but
also for disk block caches in operating systems and processor caches in cpu chips.)
7. Describe possible occurrences of each of the main types of security threat (threats to
processes, threats to communication channels, denial of service) that might occur in the
Internet.
Answer
Threats to processes: without authentication of principals and servers, many threats exist.
An enemy could access other user’s files or mailboxes, or set up ‘spoof’ servers. E.g. a server
could be set up to ‘spoof’ a bank’s service and receive details of user’s financial transactions.
Threats to communication channels: IP spoofing – sending requests to servers with a false
source address, man-in-the-middle attacks.
Denial of service: flooding a publicly-available service with irrelevant messages.
Lab Projects
Task 1
1. Download the WebClient1.java from Week 1 block of the course Moodle site.
2. Compile and run the Java program.
3. What protocols have been used for the communication between WebClient1 and the
server?
Answer
HTTPS
4. What has been downloaded by WebClient1? You should answer this question by reading
the source of WebClient1 and comparing its output (page1.txt) with the page source of
https://www.oracle.com/.
Answer
The home page of www.oracle.com.
By checking the contents of page1.txt and comparing with the contents of the page (IE:
ViewSource; Firefox: ViewPage Source), the program downloaded the page and wrote it
into file page1.txt.
https://www.oracle.com/
http://www.theaustralian.com.au/
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Task 2
1. Download the WebClient2.java from Week 1 block of the course Moodle site
2. Compile and run the Java program.
3. What protocols have been used for the communication between WebClient2 and the
server?
Answer
HTTPS
4. What have been output from WebClient2? Answer this question by reading the code of
WebClient2 and checking the properties of the web page.
Answer
The program output information of digital certificates from the server. The digital
certificates can be checked by IE: ‘right click’Propertiescertificate or Firefox: ‘right
click’View Page InfoSecurityView Certificate.
