AJAX Concept Advantage Development prospect working principle underlying technical state disadvantage framework

1. Concept

Ajax asynchronous JavaScript and XML, asynchronous JS and XML.

This explanation is obsolete, and now Ajax is the technology that allows browsers and servers to communicate without having to refresh the current page.

It was Microsoft's technology, Google in Google Earth, Google suggest and Gmail, and other Ajax technology widely used, the emergence of the popularity of Ajax. In fact, Google's current lead in Ajax technology is beyond Microsoft's reach.

2. Development prospects

Gmail has made almost no difference to the functionality of Outlook Express when sending and receiving mail, and it doesn't need to install a client program. There is no existing browser that can handle complex images like Photoshop and other desktop programs. But we cannot ignore the impact of it.

3. Advantages (Enhanced user experience)

User experience For example, if your home is now in the community because of some situation and water, now the relevant departments announced two programs, one is completely water off 8 hours, in this 8 hours completely water, 8 hours after the return to normal. Two is not completely water off 10 hours, in this 10 hours of water is not completely broken, but the flow is much smaller than the original, after 10 hours to return to normal flow, then, if it is you you will choose which way? Obviously the latter.

Asynchronous transmission is a character-oriented transmission, its unit is a character, and synchronous transmission is a bit-oriented transmission, its unit is frame, it is transmitted when the receiver and the sender's clock is consistent.

Async generally each group is a 8-bit character, in the head and tail of each group has a start bit and a stop bit, it in the transmission process of the receiver and the sender of the clock does not require consistency, that is, the asynchronous sender can send these groups at any time, and the receiver does not know when it arrives.

Each asynchronous transfer of information begins with a starting bit, which notifies the receiver that the data has arrived, giving the receiver the time to respond, receive, and cache the data bits, and at the end of the transfer, a stop bit indicates the termination of the transmission information. The stop bit causes the signal to change back to 1, and the signal remains at the next start bit.

The bit groupings for synchronous transmissions are much larger. Instead of sending each character independently, each character has its own start and stop bits, but it combines them together to send. We call these combinations a data frame, or simply a frame. Ensure that the receiver's sampling speed and the arrival speed of the bit are consistent, so that the receiving and receiving parties into synchronization.

Synchronization also has benefits: synchronous transmissions are often much faster than asynchronous transmissions. The receiver does not have to start and stop the operation for each character. Once a frame synchronization character is detected, it receives them when the next data arrives. In addition, the overhead of synchronous transmission is relatively low.

Disadvantage: The longer the data bit, the larger the buffer required to cache the data, which limits the size of one frame. In addition, the larger the frame, the longer it occupies the continuous time of the transmission media. In extreme cases, this will cause other users to wait too long.

4. The principle of work and the underlying technology on which to work

The XMLHttpRequest object is used to send an asynchronous request to the server, get the data from the server, and then use JavaScript to manipulate the DOM and update the page.

XMLHttpRequest the properties of this object.

Its properties are:

onReadyStateChange event handlers for events that are triggered by each state change.

ResponseText the string form of the data returned from the server process.

Responsexml a DOM-compatible document data object that is returned from the server process.

Status number codes returned from the server, such as common 404 (not Found) and 200 (ready)

Status Text string information accompanying the state code

ReadyState Object State Value

0 (uninitialized) object has been established but not initialized (the open method has not been called)

1 (Initialize) object has been established, the Send method has not been called

2 (send data) The Send method has been called, but the current state and HTTP headers are unknown

3 (data transfer) has received some of the data, because the response and HTTP header is not complete, then through Responsebody and ResponseText to obtain some of the data will be error,

4 (complete) The data is received, at which time the complete response data can be obtained by Responsexml and ResponseText

5. Disadvantages

Disadvantages of Ajax

Let me focus on the drawbacks of Ajax, because most of the things we pay attention to are the benefits of Ajax, such as the improvement of the user experience. And the shortcomings of Ajax have been neglected.

The drawbacks of Ajax described below are inherently generated.

1, Ajax killed the back button, that is, the browser fallback mechanism of destruction. The Back button is an important feature of a standard web site, but it does not work well with JS. This is a serious problem with Ajax, because users often want to be able to undo the previous operation by going backwards. So is there any way to do this question? The answer is yes, using Gmail know, Gmail under the Ajax technology to solve the problem, in Gmail can be back, but it does not change the mechanism of Ajax, it is only a relatively stupid but effective way, that is, the user click the Back button to access the history To reproduce the changes on the page by creating or using a hidden iframe. (for example, when a user clicks back in Google Maps, it searches in a hidden iframe and then reflects the search results on an AJAX element to restore the application state to its current state.) ）

However, although this problem can be solved, but the development cost of it is very high, and the AJAX framework required by the rapid development is a divergence. This is a very serious problem caused by Ajax.

2, security issues

Technology also poses new security threats to IT companies, and Ajax technology is just like building a direct channel to enterprise data. This allows developers to inadvertently expose more data and server logic than ever before. The logic of Ajax can be hidden from the client's security scanning technology, allowing hackers to create new attacks from remote servers. And Ajax also makes it difficult to avoid some known security weaknesses, such as cross-site footstep attacks, SQL injection attacks, and credentials-based security vulnerabilities.

3, the support of the search engine is weaker.

4. The abnormal mechanism of the program is destroyed. At least for now, these Ajax frameworks, like Ajax.dll,ajaxpro.dll, can disrupt the program's exception mechanism. On this issue, I have encountered in the development process, but looked at the Internet almost no relevant introduction. Then I did a test by myself, using Ajax and the traditional form submission mode to delete a piece of data ... It has brought us a lot of difficulties in debugging.

5, in addition, like some other aspects of the problem, such as violating the URL and resource positioning of the original intention. For example, I give you a URL address, if you use Ajax technology, perhaps you see below the URL address and what I see under this URL is different. This is contrary to the original intention of resource positioning.

6, some handheld devices (such as mobile phones, PDAs, etc.) are not very good to support Ajax, for example, we open the browser on the mobile phone using AJAX technology site, it is currently not supported, of course, this problem and we do not have much to do with.

6.several frameworks for Ajax

The more Ajax frameworks we use today are mainly Ajax.dll,ajaxpro.dll,magicajax.dll and Microsoft's Atlas framework. Ajax.dll and Ajaxpro.dll These two frameworks are not very different, and Magicajax.dll just package more, for example, it can directly return the dataset dataset, as we have said before, Ajax is returning a string, Magicajax just encapsulated it. But this feature can bring us a lot of convenience, for example, our page has a list, and the list of data is constantly changing, then we can use Magicajax to deal with, the operation is very simple, after adding Magicajax, The list control to be updated is placed within the Magicajax control, and the time to define the update interval within the pageload is OK, and the principle of Atlas is similar to Magicajax. However, one of the problems to be aware of is that these frameworks only support IE, do not do browser-compatible aspects of the processing, with the anti-compilation tools to see their code can be known.

In addition to these frameworks, the more common way we use them is to create XMLHttpRequest objects ourselves, which is more flexible than the previous frameworks. In addition, here also mention the aspnet2.0 of the asynchronous callback interface, it can also be implemented as Ajax partial non-flush, but its implementation is actually based on the XMLHttpRequest object, but also only support IE, of course, this is Microsoft's competitive strategy.

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AJAX Concept Advantage Development prospect working principle underlying technical state disadvantage framework