Web Services 2 Flashcards

Question

* Switching products * Cluster servers by one IP * Distribute workload (load balancing) * typically round-robin * Failure detection * Cisco, Foundry Networks, and F5Labs * Problem * Not sufficient for dynamic content

Answer 1

Flat Architecture - Switch Based ## Footnote setup helps manage internet traffic by spreading it out among multiple servers, but it might not be the best for handling content that changes a lot or is personalized for each user

Answer 2

* **Not sufficient for dynamic content** * **Adding/Removing nodes is difficult** Manual configuration required * **limited load balancing** in switch

Answer 3

Hierarchical Architecture ## Footnote ensures that responsibilities are clearly divided and that tasks are handled efficiently at different levels.

Answer 4

* **Better failover support** (Master restarts job if a slave fails) * **Separate dynamic and static** content (resource intensive jobs (CGI scripts) runs by slave) ## Footnote helps ensure smooth operation and efficiency by distributing tasks among different levels of servers. If one server fails, the others can step in to keep things running, and tasks are divided based on their nature to make the best use of resources.

Answer 5

* network congestion * server overloading

Answer 6

World Wide Web

Answer 7

web architecture is about **how your browser asks for web pages** from a server and how the server sends those pages back for you to see.

Answer 8

browser helps you access websites, a proxy **can hide your identity or enhance security**, a web server stores and serves websites, and a firewall protects your computer or network from threats. Each plays a role in your online experience and keeping you safe while browsing the internet

Answer 9

Web Caching System

Answer 10

1. **Reduces bandwidth consumption** (decrease network traffic) 2. **Reduces access latency** in the case of cache hit 3. **Reduces the workload** of the Web server 4. **Enhances the robustness of the Web service** 5. **Usage history collected by Proxy** cache can be used to determine the usage patterns and allow the use of different cache replacement and prefetching policies.

Answer 11

1.**Stale data can be serviced** due to the lack of proper updating 2. **Latency may increase** in the case of a cache miss 3. A single proxy cache is always a **bottleneck**. 4. A single proxy is a **single point of failure** 5. Client-side and proxy cache **reduces the hits on the original server.**

Answer 12

* Cache replacement * Prefetching * Cache coherency * Dynamic data caching

Answer 13

different size, accessing cost, access pattern.

Answer 14

* key-based * cost-based

Answer 15

1. LRU (Least Recently Used), 2. LFU (Least Frequently Used), 3. FIFO (First In First Out),

Answer 16

1. Key-based replacement policies 1. Cost-based replacement policies ## Footnote key-based replacement policies help decide which items to remove from storage when you need to make room for new ones. They each have their own way of choosing which items to ***remove based on factors like size, usage history, or download speed.***

Answer 17

* **Cost function** of factors such as last access time, cache entry time, transfer time cost, and so on * **Least Normalized Cost Replacement:** based on the access frequency, the transfer time cost and the size. * **Server-assisted scheme:** based on fetching cost, size, next request time, and cache prices during request intervals. ## Footnote help determine which items to keep or remove from storage based on factors like ***how often they're used, how long it takes to access them, and any associated costs***. Each policy uses different criteria to make these decisions, ultimately aiming to optimize storage efficiency and performance.

Answer 18

* ***The benefit from caching is limited.*** * Maximum cache hit rate - no more than 40-50% * to increase hit rate, anticipate future document requests and prefetch the documents in caches * ***documents to prefetch*** * considered as popular at servers * predicted to be accessed by user soon, based on the access pattern * ***It can reduce client latency at the expense of increasing the network traffic.*** ## Footnote * prefetching is like getting ready for a party by preparing snacks before guests arrive. * By predicting which documents users will want to access and fetching them in advance, prefetching can help reduce wait times for users, but it also means more data traveling over the network.

Answer 19

Cache Coherence ## Footnote * ensures everyone accessing the same document gets the most recent version. * HTTP commands like GET, Conditional GET, and Pragma no-cache help achieve this by fetching updated versions of documents as needed, and Last-Modified helps track when a document was last updated. * There are two approaches to cache consistency: strong, where everyone gets the same version immediately, and weak, where updates might take a bit longer to propagate to everyone

Answer 20

1. **GET**: retrieves a document given its URL 2. **Conditional GET**: GET combined with the header IF-Modified-Since. 3. **Pragma no-cache**: this header indicate that the object be reloaded from the server. 4. **Last-Modified**: returned with every GET message and indicate the last modification time of the document.

Answer 21

* ***Client validation (polling-every-time)*** * sends an IF-Modified-Since header with each access of the resources * server responses with a Not Modified message if the resource does not change * ***Server invalidation*** * whenever a resource changes, the server sends invalidation to all clients that potentially cached the resource. * Server should keep track of clients to use. * Server may send invalidation to clients who are no longer caching the resource. ## Footnote * ensures that clients always have the latest version of a webpage. * Clients ask the server if a webpage has changed since the last visit, and if it has, the server sends the updated version. If the webpage hasn't changed, the server tells the client it's still the same, saving unnecessary data transfer. * If the webpage does change, the server notifies all clients, even those who aren't currently caching the webpage, to ensure everyone stays up-to-date.

Answer 22

* ***Adaptive TTL (time-to-live)*** * adjust a TTL based on a lifetime (age) - if a file has not been modified for a long time, it tends to stay unchanged. * This approach can be shown to keep the probability of stale documents within reasonable bounds ( < 5%). * Most proxy servers use this mechanism. * No strong guarantee as to document staleness * Piggyback Invalidation * ***Piggyback Cache Validation (PCV)*** - whenever a client communicates with a server, it piggybacks a list of cached, but potentially stale, resources from that server for validation. * Piggyback Server Invalidation (PSI) - a server piggybacks on a reply to a client, the list of resources that have changed since the last access by the client. * If access intervals are small, then the PSI is good. But, if the gaps are long, then the PCV is good. * ## Footnote * adaptive TTL adjusts how long files stay in the cache based on how often they change. * Piggybacking adds extra information to messages sent between clients and servers to help them keep their caches up-to-date. * This ensures that even if some files in the cache are outdated, the probability of having stale documents is kept low, and the cache stays relatively fresh.

Answer 23

* ***Non-cacheable data*** * authenticated data, server dynamically generated data, etc. * how to make more data cacheable * how to reduce the latency to access non-cacheable data * ***Active Cache*** * allows servers to supply cache applets to be attached with documents. * the cache applets are invoked upon cache hits to finish necessary processing without contacting the server. * bandwidth savings at the expense of CPU costs * due to significant CPU overhead, user access latencies are much larger than without caching dynamic objects. * **Web server accelerator** resides in front of one or more Web servers provides an API which allows applications to explicitly add, delete, and update cached data. The API allows static/dynamic data to be cached. An example - the official Web site for the 1998 Olympic Winter Games whenever new content became available, updated Web reflecting these changes were made available within seconds. * Data Update Propagation (DUP, IBM Watson) is used for improving performance. * **Data Update Propagation (DUP)** * maintains data dependence information between cached objects and the underlying data which affect their values * upon any change to underlying data, determines which cached objects are affected by the change. * Such affected cached objects are then either invalidated or updated. * With DUP, about 100% cache hit rate at the 1998 Olympic Winter Games official Web site. * Without DUP, 80% cache hit rate at the 1996 Olympic Games official Web site. * ## Footnote * dynamic data caching is about finding ways to speed up access to data that changes frequently, like personalized information. * Active cache adds little helpers to cached documents to do some processing without contacting the server each time. While this saves on bandwidth, it can also increase user access times due to extra processing overhead. * DUP helps spread these updates across the caching system quickly and efficiently. It ensures that everyone who visits your website gets the most up-to-date information without any lag.

Answer 24

* ***Performance*** * Minimize document retrieval latency * ***Scalability*** * Reduce the amount of data transferred * Balance bandwidth use * ***Manageability***: easy to deploy and support * Maintain backward compatibility with existing client software * **Availability** * Maximize document availability (fault tolerant) * **Flexibility**: run on multiple hardware and OS platforms, support multiple protocols, HTTP, FTP, etc. * Retain transparency to user

Web Services 2 Flashcards

(50 cards)