Introduction To Micro Controller

The World Wide Web can be considered as a large distributed information system that provides access to shared data objects. As one of the most popular applications currently running on the Internet, The World Wide Web is of an exponential growth in size, which results in network congestion and server overloading. Also, the WWW has documents that are of diverse nature and so everyone can find information according to their liking. But, this scorching rate of growth has put a heavy load on the Internet communication channels. This situation is likely to continue in the foreseeable future, as more and more information services move onto web. The result of all this is increased access latency for the users. Access latency, which is the time interval between the user giving out a request and its actual completion, could result from many reasons. Servers can get flooded with more requests than they can optimally handle. The network path between the user and the server could become congested due to increased traffic on any or some of the constituent links.

Advantages of Caching

Documents can be cached on the clients, the proxies, and the servers. The effects of Web caching are two-fold. First, it has been shown that caching documents can improve Web performance significantly. There are several advantages of using Web caching

1. Web caching reduces bandwidth consumption, thereby decreases network traffic and lessens network congestion.

2. Web caching reduces access latency due to two reasons:

a) Frequently accessed documents are fetched from nearby proxy caches instead of remote data servers, the transmission delay is minimized.
b) Because of the reduction in network traffic, those documents not cached can also be retrieved relatively faster than without caching due to less congestion along the path and less workload at the server.

3. Web caching reduces the workload of the remote Web server by disseminating data among the proxy caches over the wide area network.

4. If the remote server is not available due to the remote server's crash or network partitioning, the client can obtain a cached copy at the proxy. Thus, the robustness of the Web service is enhanced.

5. A side effect of Web caching is that it provides us a chance to analyze an organization's usage patterns.

Disadvantages of using a caching:

1. The main disadvantage is that a client might be looking at stale data due to the lack of proper proxy updating.

2. The access latency may increase in the case of a cache miss due to the extra proxy processing. Hence, cache hit rate should be maximized and the cost of a cache miss should be minimized when designing a caching system.

3. A single proxy cache is always a bottleneck. A limit has to be set for the number of clients a proxy can serve. An efficiency lower bound (i.e. the proxy system is ought to be at least as efficient as using direct contact with the remote servers) should also be enforced.

4. A single proxy is a single point of failure.

5. Using a proxy cache will reduce the hits on the original remote server which might disappoint a lot of information providers, since they cannot maintain a true log of the hits to their pages. Hence, they might decide not to allow their documents to be cacheable.

Proxy Caching

Proxy servers are generally used to allow access to the Internet from users within a firewall. For security reasons, companies run a special type of HTTP servers called "proxy" on their firewall machines. A Proxy server typically processes requests from within a firewall by forwarding them to the remote servers, intercepting the responses, and sending the replies back to the clients. Since the same proxy servers are typically shared by all clients inside of the firewall, naturally this leads to the question of the effectiveness of using these proxies to cache documents. Clients within the same firewall usually belong to the same organization and likely share common interests. They would probably access the same set of documents and each client tends to browse back and forth within a short period of time. Therefore on the proxy server a previously requested and cached document would likely result in future hits. Web caching at proxy server can not only save network bandwidth but also lower access latency for the clients.

Transparent Caching:

Transparent proxy caching eliminates one of the big drawbacks of the proxy server approach: the requirement to configure Web browsers. Transparent caches work by intercepting HTTP requests and redirecting them to Web cache servers or cache clusters. This style of caching establishes a point at which different kinds of administrative control are possible; for example, deciding how to load balance requests across multiple caches. There are two ways to deploy transparent proxy caching: at the switch level and at the router level.

Router-based transparent proxy caching uses policy-based routing to direct requests to the appropriate cache(s). For example, requests from certain clients can be associated with a particular cache. In switch-based transparent proxy caching, the switch acts as a dedicated load balancer. This approach is attractive because it reduces the overhead normally incurred by policy-based routing. Although it adds extra cost to the deployment, switches are generally less expensive than routers

Cache coherency

Caches provide lower access latency with a side effect: every cache sometimes provide users with stale pages - pages which are out of date with respect to their master copies on the Web servers where the pages originated. Every Web cache must update pages in its cache so that it can give users pages which are as fresh as possible

Conclusion

As Web services becomes more and more popular, users will suffer from network congestion and server overloading. Web caching is recognized to be one of the effective techniques to alleviate server bottleneck and reduce network traffic, thereby minimize the user access latency. The main challenges in Web caching are proxy placement, dynamic data caching, cache routing. The research frontier in Web performance improvement lies in developing effcient, scalable, robust, adaptive and stable Web caching scheme that can be easily deployed in current and future network