Distributed Systems: Concepts and Design by George Coulouris. Question 2.5

2.5) Suggest some applications for the peer process model, distinguishing between cases when the state of all peers needs to be identical and cases that demand less consistency.

Answer:

Cooperative work (groupware) applications that provide a peer process near to each user. 

Applications that need to present all users with identical state - shared whiteboard, shared view of a textual discussion 

Less consistency: where a group of users are working on a shared document, but different users access different parts or perhaps one user locks part of the document and the others are shown the new version when it is ready. 

Some services are effectively groups of peer processes to provide availability or fault tolerance. If they partition data then they don’t need to keep consistent at all. If they replicate then they do.

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Distributed Systems: Concepts and Design. by George Coulouris. Exercise Solutions 3. Question 2.4

2.4) 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 synchronization 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.

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Distributed Systems: Concepts and Design. by George Coulouris. Exercise Solutions 3

2.1 Describe and illustrate the client-server architecture of one or more major Internet applications (for example the Web, email or netnews).

2.1 - Answer:


Web:

Browsers are clients of Domain Name Servers (DNS) and web servers (HTTP). Some intranets are
configured to interpose a Proxy server. Proxy servers fulfil several purposes – when they are located at the same site as the client, they reduce network delays and network traffic. When they are at the same site as the server, they form a security checkpoint (see pp. 107 and 271) and they can reduce load on the server. N.B. DNS servers are also involved in all of the application architectures described below, but they ore omitted from the discussion for clarity.

Email:

Sending messages: User Agent (the user’s mail composing program) is a client of a local SMTP server and passes each outgoing message to the SMTP server for delivery. The local SMTP server uses mail routing tables to determine a route for each message and then forwards the message to the next SMTP server on the chosen route. Each SMTP server similarly processes and forwards each incoming message unless the domain name in the message address matches the local domain. In the latter case, it attempts to deliver the message to local recipient by storing it in a mailbox file on a local disk or file server. Reading messages: User Agent (the user’s mail reading program) is either a client of the local file server or a client of a mail delivery server such as a POP or IMAP server. In the former case, the User Agent reads messages directly form the mailbox file in which they were placed during the message delivery. (Exampes of such user agents are the UNIX mail and pine commands.) In the latter case, the User Agent requests information about the contents of the user’s mailbox file from a POP or IMAP server and receives messages from those servers for presentation to the user. POP and IMAP are protocols specifically designed to support mail access over wide areas and slow network connections, so a user can continue to access her home mailbox while travelling.

Netnews:

Posting news articles: User Agent (the user’s news composing program) is a client of a local NNTP server and passes each outgoing article to the NNTP server for delivery. Each article is assigned a unique identifier. Each NNTP server holds a list of other NNTP servers for which it is a newsfeed – they are registered to receive articles from it. It periodically contacts each of the registered servers, delivers any new articles to them and requests any that they have which it has not (using the articles’ unique id’s to determine which they are). To ensure delivery of every article to every Netnews destination, there must be a path of newsfeed connections from that reaches every NNTP server.
Browsing/reading articles: User Agent (the user’s news reading program) is a client of a local NNTP server.
The User Agent requests updates for all of the newsgroups to which the user subscribes and presents them to the user.

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