What is Paging? Why it is used?

- Paging is a very important concept for the computer operating systems required for managing the memory. 

- It is essentially a memory management scheme which is used for storing as well as retrieving data from the secondary memory devices.

- Under this scheme, the data is retrieved from the secondary storage devices and handed over to the operating systems. 

- The data is in the form of blocks all having the same size. 

- These data blocks are called as the pages. 

- In paging, for a process the physical address space can be kept as non–contiguous itself. 

- Paging is a very important concept for implementing the virtual memory in the operating systems designed for contemporary and general use. 

- This allows the disk storage to be used for the data that is not able to fit in to the RAM. 

- The main functions of the paging technique are carried out when a program attempts to access the pages that have no mapping to the physical RAM. 

- This situation is commonly known as the page fault. 

- In this situation, the OS comes to take control of the error. 

- This is done in a way that is invisible to the application. 

The operating system carries out the following tasks in paging:

Ø  Locates the data address in an auxiliary storage.

Ø Obtains a vacant page frame in the physical memory to be used for storing the data.

Ø  Loads the data requested by the application in to the page frame obtained in the previous step.

Ø  Make updates to the page table for showing the new data.

Ø Gives back the execution control to the program.This maintains a transparency. it again tries to execute the instruction because of which the fault occurred.

- If space is not available on RAM for storing all the requested data, then another page from RAM cannot be removed. 

- If all of the page frames are filled up, then a page frame can be obtained from the table which contains data that will be shortly emptied. 

- A page frame is said to become dirty if it is modified since its last read operation in to the RAM. 

- In such a case it has to be written back in to its original location in the drive before it is freed. 

- If this is not done, a fault will occur which will require obtaining an empty frame and reading the contents from drive in to this page. 

- The paging systems must be efficient so as to determine which frames are to be emptied. 

- Presently many page replacement algorithms have been designed for accomplishing this task. 

- Some of the mostly used for replacement are:

Ø  LRU or least recently used

Ø  FIFO or first in first out

Ø  LFU or least frequently used.

- To further increase responsiveness, paging systems may employ various strategies to predict which pages will be needed soon. 

- Such systems will attempt to load pages into main memory preemptively, before a program references them. 

- When demand paging is used, paging takes place only when some data request and not prior to it. 

- In a demand pager, execution of a program begins with none of the pages loaded in to the RAM. 

Explain the check points in QTP? In how many ways we can add check points to an application using QTP?

Check points in quick test professional have always proved to be an efficient tool. In this article, we discuss exclusively about the check points in quick test professional and the ways in which the check points can be added to an application with the help of quick test professional. 

What is Check Point? How it is added to an application?

- Check point can be thought of as a point to confirm or verify whether the application under test or AUT is working properly or not. 

- This is verified by making comparisons between the current value of some property and the expected value of the same property decided earlier. 

- Whenever a check point is added by the user, an equivalent check point is added by the quick test professional to the present row in the key word view as well as a “check check point” statement is added to the expert view. 

- The name of the test object also serves as a name for the check point that has been added to it. 

- The check points can be added via an active screen by placing the cursor on the desired location and clicking the insert standard check point option. 

Types of Check Points

It is on the basis of this expected value that the check points have been classified in to various types as described below:

1. Page check point: 

- This check point is the check point that is created exclusively for a web page. - This check point keeps a count of all the images and links present on that particular web page. 

- Another purpose for which page check point is used for checking the time taken by the web page to load fully i.e., the load time.

2. Bitmap check point: 

- This check point is the check point that helps in checking of the bitmap of either a full web page or an image. 

- Two images are provided to this check point, one the actual image and the other one expected image. 

- These two images are compared pixel by pixel by the bit map check point.

3. Image check point: 

- This check point is used for keeping a check on certain properties belonging to a web page such as the source file location of the image. 

- However, being a image check point, it cannot check pixels of an image like a bitmap check point does.

4. Text check point: 

- This check point is the check point that is employed in checking the expected text in a web application or web page. 

- This text may belong to a specific region of the text displayed or application under test.

5. Accessibility check point: 

- This check point verifies whether or not the accessibility standard of the application under test matches with the accessibility guidelines laid down by the W3C or world wide web consortium for information systems and technology which are web based.

6. Data base check point: 

- This check point performs the task of generating a query while the recording is in progress. 

- The values stored in the data base serve as expected values. 

- This query is executed during the run time to obtain the actual value which is then compared with the expected value.

7. Table check point: 

This check point is the one that is used for checking various table properties such as cell width, row width and so on.

8. XML check point: 

This check point is the one that is used to verify the following:

a)     XML schema

b)     XML data etc.

How will you check a web application for broken links using QTP?

Checking for the broken and ill links forms a very important aspect of any web testing profile. The below mentioned are few testings which take the priority of the web testing check list:

1. Usability testing
2. Functionality testing
3. Compatibility testing
4. Interface testing
5. Security testing
6. Performance testing

Out of all these the functionality testing tests the following basic aspects of a web page or site or application:

1. all the links in web pages
2. data base connection
3. forms in the web pages
4. cookies

When it comes to checking of the links the following links are tested:

1. out going links from a specific domain
2. internal links
3. links jumping on same page
4. links for sending mails
5. orphan pages and lastly
6. broken  links

Our focus is on last aspect i.e., testing of the broken links in a web page, site or web application with the help of the quick test professional. 

What are broken links?

- Broken links are also termed as dead links since they are permanently unavailable and hence cannot be accessed by the user. 

- If you would have observed you might have surely come across 404 error while browsing some site. 

- This signifies the presence of a broken link. 

- Using quick test professional, the broken links can be identified during the run time itself by following any one of the two methods mentioned below:

1. via manual creation of a check point
2. via automatic page check point

How a broken link be checked manually by creating a page check point?

- A direct menu option is not available in the quick test professional for incorporating a page check point. 

- Therefore, you need to take assistance of a standard check point. 

- What you actually need, is to click on the start recording session and then select insert option, then check point and finally standard check point option.

- You can use F12 if it works for you.

- Get the hand pointer on the web page and click OK. 

- You will get a page check point properties dialog box which has a broken link check box. 

- You need to check that box and click OK. 

- After this, you need to run the scripts and check the status of links in broken link result. 

- If the links pointing only to the current host have to be checked then you need to check the box titled check only links to current host by going to the tools menu and then selecting web options. 

- Similarly, if the links pointing to all the hosts have to be checked then this box has to be kept unchecked. 

For following the second method you need to go through the following:

Tools à options à web à advanced

- You will get two boxes labeled “broken links” and “create a check point for each page while recording”, check these both boxes. 

Now every time the quick test professional starts recording on a new page, a check point will be automatically included for all the broken links. 

One thing that you should know is that the quick test professional experiences some issues with the windows vista if the internet explorer version that you using is 7.0. 

Tabbed browsing is something that is not supported by the quick test professional especially in Microsoft internet explorer. 

An alternative to this can be to disable tabbed browsing first and then proceed with quick test professional. In internet explorer the quick test professional may not be able to recognize the web objects. 

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. 

What are different HTML errors?

HTML or hyper text mark up language is perhaps one of the most used mark up languages for the web pages, sites and applications with which most of us are familiar. This language is written using the HTML elements which mainly constitute of the tags enclosed in the angle brackets like:

These elements are housed in the web page or site. Most of the HTML tags are implemented in pairs of two.

HTML ELEMENTS AND CONTENT
- Some tags are even empty and are commonly known as empty elements.
- These are usually unpaired unlike the filled elements which are paired.
- In the paired tags, the first tag is called the start tag and the second one is then called the end tag.
- Between these two tags, any text, comments, tags etc can be added by the designer of the web sites.
- The content that is to be added should only be of textual type.
- Due to some wrong designing principles often some errors are introduced in to the html of the web site or the page.
- One of the most common errors is the insertion of the graphical content in to the html.
- Such content is neither displayed nor is it interpreted.
- It simply causes the malfunctioning of the web site.
- Other elements of the html allow the addition of the graphical content and only these should be used whenever some graphics are to be inserted in to the page.
- Html elements provide a means for the creation of the structured documents that denote the structural semantics for textual content like:

1. Lists
2. Links
3. Paragraphs
4. Headings
5. Quotes and so on.

DIFFERENT HTML ERRORS AND THEIR IMPACT

- Html can also be embedded in to the scripts like javascript which also some times leads to errors when inserted incorrectly.
- Such errors affect the behaviour of the web sites and cause them to behave abnormally.
- Whenever the site is affected by an error or a bug is encountered, a set of error messages is generated by the business logic component which is then stored as a string in any of the available scopes.
- For using such functions, you need to define an application scope with the name of default attribute, because if such a scope is not found, then nothing is to be rendered by the business logic.
- Some designers forget to define such scopes and therefore end up with unnecessary errors in their web sites.
- Many more errors occur whenever a requested is generated by the client to the web server.
- In such cases the server responds with some status messages, few of which have been mentioned below along with the possible errors:

1. 400:
Bad syntax of the request is preventing the server to process it.

2. 401:
The request though being cannot be processed because of unavailability of the authentication processes.

3. 404 not found:
The page requested by the client is found to be unavailable at that moment.

4. 405 method not allowed:
The client made request in such a way that is not supported by that site.

5. 407 proxy authentication required:
The client has not authenticated its proxy.

6. 408 request time out:
The server’s request time expired while waiting for the client to generate one.

7. 409 conflict:
The server is not able to process the request because of the presence of some conflict in it.

8. 410 gone:
The page that was requested is no longer available on the web.

What are the important elements in search engine optimization Part I?

SEO web design is one of the most appropriate strategies to keep the competition.
A successful search engine optimization has few important elements includes a well targeted keyword focus, strong page element optimization and good quality content.

Keywords are most important element for search engines. These keywords are what search strings are matched against.The most crucial step is to find the correct keyword that can be used to optimize.

- Choosing the right keyword to optimize. It is best to focus on less competitive and highly specific keywords. The keyword should have the property of describing what is the purpose of your website. Also, check whether these keywords are relevant to your website or not.

- The website should be rich in keywords and have good keyword density. It is a measure to judge how relevant is the page.Keeping 5 to 10 keywords is termed as OK keyword density. Keyword stuffing should not be practiced.

- Quality of keywords and their position on the seb page is also very important. Keywords that appear in the heading, title are more effective than the keywords that are placed at the bottom.

- A balance has to be there between url keywords and site usability.
- The contents of page title tag are displayed during search so this is the special place where keywords can be given.

- From an SEO point of view, it is good to have as many headings with good keywords in it. The headings should not be very long as readability becomes difficult.

Optimal Page Replacement Algorithm

Each page can be labeled with the number of instructions that will be executed before that page is first referenced. The optimal page algorithm simply says that the page with the highest label should be removed. If one page will not be used for 8 million instructions and another page will not be used for 6 million instructions, removing the former pushes the page fault that will fetch it back as far into the future as possible.
The only problem with this algorithm is that it is unrealizable. At the time of the page fault, the operating system has no way of knowing when each of the pages will be referenced next. Still, by running a program on a simulator and keeping track of all page references, it is possible to implement optimal page replacement on the second run by using the page reference information collected during the first run.

Despite this limitation, algorithms exist that can offer near-optimal performance — the operating system keeps track of all pages referenced by the program, and it uses those data to decide which pages to swap in and out on subsequent runs. This algorithm can offer near-optimal performance, but not on the first run of a program, and only if the program's memory reference pattern is relatively consistent each time it runs.

In short :
* Belady's optimal algorithm for the minimum number of page faults.
* Replace the page that will be referenced furthest in the future or not at all.
* Problem: we cannot implement it, because we cannot predict the future.
* This is the best case.
* Can use it to compare against other algorithms.

Performance of Demand Paging

Advantages of Demand Paging :
* Only loads pages that are demanded by the executing process.
* As there is more space in main memory, more processes can be loaded reducing context switching time which utilizes large amounts of resources.
* Less loading latency occurs at program start-up, as less information is accessed from secondary storage and less information is brought into main memory.
* Does not need extra hardware support than what paging needs, since protection fault can be used to get page fault.

Disadvantages of Demand Paging :
* Individual programs face extra latency when they access a page for the first time. So demand paging may have lower performance than anticipatory paging algorithms such as pre-paging.
* Programs running on low-cost, low-power embedded systems may not have a memory management unit that supports page replacement.
* Memory management with page replacement algorithms becomes slightly more complex.
* Possible security risks, including vulnerability to timing attacks.

Performance Of Demand Paging :
Let p be the probability of a page fault (0<=p<=1). We would expect p to be close to zero i.e. there will be only few page faults. The effective access time is then :
effective access time = (1-p) * ma + p * page fault time
To compute the effective access time, we must know how much time is needed to service a page fault. A page fault causes the following sequence to occur :
- Trap to the operating system.
- Save the user registers and process state.
- Determine that the interrupt was a page fault.
- Check that the page reference was legal and determine the location of the page on the disk.
- Issue a read from the disk to a free frame.
- While waiting, allocate the CPU to some other user.
- Interrupt from the disk.
- Save the registers and process state for the other user.
- Determine that the interrupt was from the disk.
- Correct the page table and other tables to show that the desired page is now in memory.
- Wait for the CPU to be allocated to this process again.
- Restore the user registers, process state, and new page table, then resume interrupted instruction.

Overview of Demand Paging

Demand paging follows that pages should only be brought into memory if the executing process demands them. This is often referred to as lazy evaluation as only those pages demanded by the process are swapped from secondary storage to main memory. Contrast this to pure swapping, where all memory for a process is swapped from secondary storage to main memory during the process start-up.

In pure demand paging a page is never moved from the backing store into main memory until that page is referenced. It is the responsibility of the operating system to check where the page is in main memory and OS uses an internal table for this. Operating system reads that page after finding it, and in order to reflect change, the page table is updated. So by using this process it is possible to run a process even its entire memory image is not taken from backing store into main memory.
In this way demand paging is a better approach then paging and it also increases the degree of multiprogramming and allows a process to run even it exceed the physical space allocated for it.

An invalid page is one that currently resides in secondary memory. When a process tries to access a page, the following steps are generally followed:
- Attempt to access page.
- If page is valid (in memory) then continue processing instruction as normal.
- If page is invalid then a page-fault trap occurs.
- Check if the memory reference is a valid reference to a location on secondary memory. If not, the process is terminated (illegal memory access). Otherwise, we have to page in the required page.
- Schedule disk operation to read the desired page into main memory.
- Restart the instruction that was interrupted by the operating system trap.

Overview of Shadow Paging

A computer system, like any other mechanical or electrical system is subject to failure. There are a variety of causes, including disk crash, power failure, software errors, a fire in the machine room, or even sabotage. Whatever the cause, information may be lost. The database must take actions in advance to ensure that the atomicity and durability properties of transactions are preserved. An integral part of a database system is a recovery scheme that is responsible for the restoration of the database to a consistent stage that existed prior to the occurrence of the failure.

Shadow paging is a technique used to achieve atomic and durable transactions, and provides the ability to manipulate pages in a database. During a transaction, the pages affected by the transaction are copied from the database file into a workspace such as volatile memory, and modified in that workspace. When a transaction is committed, all of the pages that were modified by the transaction are written from the workspace to unused pages in the database file. During execution of the transaction, the state of the database exposed to the user is that is which the database existed prior to the transaction, since the database file still contains the original versions of the modified pages, as they existed before being copied into the workspace if a user accesses the database before the transaction is complete, or upon recovery of failure, it will appear as though the transaction has not occurred.

- Shadow paging is an alternative to log-based recovery; this scheme is useful if transactions execute serially.
- Basic Idea: Maintain two page tables during the lifetime of a transaction – the current page table, and the shadow page table.
- Store the shadow page table in nonvolatile storage, such that state of the database prior to transaction execution may be recovered.
* Shadow page table is never modified during execution.
- Initially, both the page tables are identical. Only current page table is used for data item accesses during execution of the transaction.
- Whenever any page is about to be written for the first time
* A copy of this page is made onto an unused page.
* The current page table is then made to point to the copy.
* The update is performed on the copy.

Introduction: WWW - The Client Side

The Internet / World Wide Web consists of pages and each page contains links or pointers to other pages. Users follow the link by clicking on them. This process can be repeated indefinitely, possibly traversing hundreds of linked pages. Pages that point to other pages are said to use "hypertext".

Pages are viewed with a program called a browser. When a page is requested, the browser fetches, interprets the text and formats the commands that it contains and displays the page. Strings of text that are links to other pages are called hyperlinks.

Most browsers have numerous buttons and features to navigate the Web. In addition to having ordinary text and hypertext, web pages also contain icons, line drawings, maps and photographs. Some pages also consist of audio tracks, video clips, or both. When hypertext pages are combined with other media, the result is called hypermedia. Many Web pages consists of large images which take a long time to load.

Some browsers deal with slow loading of images by first fetching and displaying the text, then getting the images. Some Web pages contain forms that request the user to enter information. Some browsers use the local disk to cache pages that they have fetched. A check is made before a page is fetched to see if it is in the local cache. If so, check if it is up to date. If so,there is no need to load the page again.

To host a web browser, a machine must be directly connected to Internet or have a SLIP or PPP connection to a router or other machine that is directly on Internet.