How can data caching have a negative effect on load testing results?

It is quite a heavy task to retrieve data from a certain repository if we see it through a performance point of view. It becomes much more difficult when the data repository lies too far from the application server.Retrieving data becomes difficult also when a specific type of data is accessed over and over again. 

Caching is a technique that has been developed as a measure for reducing the work load and the time consumed for retrieval of the data. 

In this article, we have discussed about the negative effects that simple data caching can have up on the load testing. 

Rules for Caching Concepts

Some rules have been laid down for the caching concepts which have been mentioned below:

1. The data caching is useful if used only for a short period of time and does not works when used through the life cycle of the software system or application.

2. Only that data which is not likely to be changed quite often should be cached.

3. There are certain data repositories that have the capability of supporting the notification events in case if the modification of the data takes place outside the application.

If these above stated rules are not followed properly, the data caching is sure to have a negative impact up on the load testing. 

How data caching produces a negative impact on load testing?

- This is so because the data caching has got some pitfalls which come in our observation only when there are potential situations when there is a possibility of data expiry and software system or application using inconsistent data. 

- Using caching technique is quite simple but any fault can cause an impact on load testing.

- Load testing involves putting demands on the software system or application in order measure its response. 

- The outcomes of the load testing helps in measuring the difference between the responses of the software system or application under normal as well as peak load conditions. 

- Load testing is usually used as a means to have a measure of the maximum capacity at which the software system or application can operate easily. 

- Data caching initiates quick response from the software system or application for obtaining cookies etc. 

- Though data caching responds faster than the usual memory transactions, it has a negative impact on the result of the load testing i.e. you will not get the original results rather the results you will get will be the altered ones. 

What you will get to see is the wrong performance of the software system or application. 

What is the purpose of caching?

- Caching is done with the purpose of storing certain data so that that data in the subsequent stages can be served faster. 

- Data caching affects load testing results in a way until and unless the cache is cleared up by the testing tool after every iteration of the virtual user, an artificial faster page load time is started to be given by the caching mechanism. 

- Such artificial timings will alter your load testing results and invalidate them. - In caching, all the recently visited web pages are stored. 

- When we carry out load testing, our aim is always to check the software system or application under load. 

- So if by chance the caching option is left enabled, what will happen is that the software system or application will try retrieving the data from the data that is locally saved giving false measure of the performance determination. 

- So, the caching option should always be disabled while you carry out load testing. 

Describe some Caching Issues?

We all are familiar with what a cache is? 

"A cache can be defined as a memory component that is held for storing the data transparently in order to speed up the future serving of the data requests."

The data stored in a cache might be the data that has been required earlier for some operations. 

There are two events related to cache as mentioned below:

1. Cache hit: When the requested data is available in cache and

2. Cache miss: When the requested data is not available in cache and has to be looked up in to the RAM.

- The speed of processing is directly proportional to the number of requests that can be served via the cache. 

- Cache memories are quite costly and hence to make it cost efficient and keep the data usage as efficient as possible, smaller caches are used. 

- But since the time of its advent, cache has proven itself in the field of computing. 

"Caching can be thought of as a technique that is aimed at increasing the computing performance by keeping in itself the frequently accessed data."

There are basically three kinds of caching as we have stated below:

1. Caching output caching: In this type of caching the dynamic output that had been generated up on a request.

2. Fragment caching: In this type of caching the portion of the page that is generated by the request is cached since in many situations it is not practical to cache the whole page at once.

3. Data caching: In this type of caching the objects are cached pro grammatically.

What are different caching issues?

In this article we have taken in to discussion some very prominent caching issues. Most of the people experience problem in server caching of certain files. 

There are four major caching issues have been recognized which have been mentioned below:

1. Designing of a custom cache.

2. Securing of a custom cache.

3. Monitoring of a custom cache and lastly.

4. Synchronization of the caches in a server farm.

Besides these four major caching issues, there are many other minor caching issues.

- Some times it happens that the package delivery fails or an object or element appears like it has been corrupted and seems like such a failure has not got anything to do with the connection! 

- In such cases you can go on with a cache clear up. 

- If the situation is worse, you may also require clearing up the proxy cache!

- Web sites and browsers are looked up as a means of optimizing the resources which is done by them so well that they end up breaking down your dynamic web site content. 

- Your web site is not updated as you thought it will be done. 

Let us see an example, suppose you own a music web site which you frequently update with new music. Your clients come to your site every day so what happens is that the cache forces the web site to list the cached version of the play list and so the clients would never be able to listen to new and the latest music.

Such situation though enhances your internet experience, can also cause many other problems! Some times the cache will stash up an old page of the web site instead of showing up the latest one. You should make it a point to empty your browser’s cache from time to time. There are many internet service providers that cache pages to speed up the internet access like AOL. All the web pages that you visit are stashed up in the cache. 

Implementation techniques and Remote File Access for Distributed File Systems

Implementation of transparent naming requires a provision for the mapping of a file name to the associated location. Keeping this mapping manageable calls for aggregating sets of files into component units, and providing the mapping on a component unit basis rather than on a single file basis. To enhance the availability of the crucial mapping information, methods like replication, local caching, or both can be used.
A non-transparent mapping technique:
name ----> < system, disk, cylinder, sector >
A transparent mapping technique:
name ----> file_identifier ----> < system, disk, cylinder, sector >
So, when changing the physical location of a file, only the file identifier need be modified. This identifier must be "unique" in the universe.

Remote File Access
In remote-service mechanism, requests for accesses are delivered to the server, the server machine performs the accesses, and their results are forwarded back to the user. The remote-service method is analogous to performing a disk access for each access request. To ensure reasonable performance of a remote-service mechanism, caching is used. In DFSs, the goal is to reduce both network traffic and disk I/O.

Page/Disk Cache and Web Cache

PAGE CACHE :
Page Cache or disk cache is transparent buffer of disk-backed pages kept in main memory (RAM) by the operating system for quicker access. All memory that is not directly allocated to applications is usually utilized for page cache. Since non-dirty pages in the page cache have identical copies in secondary storage(hard disk), discarding and re-using their space is much quicker than paging out application memory, and is often preferred.The page cache also aids in writing to a disk. Pages that have been modified in memory for writing to disk, are marked "dirty" and have to be flushed to disk before they can be freed. When a file write occurs, the page backing the particular block is looked up. If it is already found in cache, the write is done to that page in memory. Otherwise, when the write perfectly falls on page size boundaries, the page is not even read from disk, but allocated and immediately marked dirty. Otherwise,the page(s)are fetched from disk and requested modifications are done.

WEB CACHE :
Web caching is the caching of web documents(e.g.,HTML pages, images) to reduce bandwidth usage, server load, and perceived lag. A web cache stores copies of documents passing through it; subsequent requests may be satisfied from the cache if certain conditions are met.
With a local cache in operation, user web object requests go via the local cache which then retains a copy of the said web object. This results in all subsequent requests for the same object being fulfilled from the local cache instead of from the site of origin. This process of web caching minimizes the amount of times identical web objects are transferred from remote websites by retaining copies of requested URLs in a cache. A web cache can be installed utilizing both software and hardware, and can run on various different platforms.
With a local cache in operation, subsequent requests for previously cached URLs result in the cached copy of the object being returned to the user; creating little or no extra network traffic, improving efficiency and reducing waiting time.