Sabtu, 17 November 2012

Fundamental HTTP Request dan Response dalam Ajax

Sebuah aplikasi web sebenarnya hanyalah media perantara antara user dengan webserver yang melayani permintaan dan menerima respon dari server web kemudian menyampaikannya ke user melalui. Saat ini standar yang digunakan browser untuk bekomunikasi dengan server web adalah HTTP (HyperText Transfer Protocol) versi HTTP/1.1.

Ajax, tentunya juga menggunakan protokol HTTP ini untuk melakukan komunikasi dengan server. Maka, dalam rangka untuk memahami bagaimana konsep Ajax, penting untuk mengetahui bagaimana sebuah situs web memproses permintaan dan menerima respon dari server web.
Bisa dikatakan HTTP bekerja layaknya e-mail. Permintaan dikirim, header tertentu diteruskan yang memungkinkan server web untuk tahu persis apa yang harus ditangani dan bagaimana untuk menangani permintaan. Kebanyakan header dalam HTTP bersifat opsional, namun ada satu header yang harus ada yaitu host header. Header ini sangat penting karena header ini yang memungkinkan server tahu dari mana request berasal dan kemana harus mengembalikan response proses.

Setelah permintaan telah diterima, server kemudian mengirimkan respon kembali. Ada banyak kode respon dalam HTTP yang dinyatakan dalam angka yang berbeda:

Error code Error text Description HTTP version
Informational responses
100 Continue This interim response indicates that everything so far is OK and that the client should continue with the request or ignore it if it is already finished. HTTP/1.1 only
101 Switching Protocol This code is sent in response to an Update: request header by the client, and indicates that the protocol the server is switching too. It was introduced to allow migration to an incompatible protocol version, and is not in common use. HTTP/1.1 only
Successful responses
200 OK The request has succeeded. The meaning of a success varies depending on the HTTP method:
  • GET: The resource has been fetched and is transmitted in the message body.
  • HEAD: The entity headers are in the message body.
  • POST: The resource describing the result of the action is transmitted in the message body.
  • TRACE: The message body contains the request message as received by the server.
The methods PUT, DELETE, and OPTIONS can never result in a 200 OK response.
HTTP/0.9 and later
201 Created The request has succeeded and a new resource has been created as a result of it. This is typically the response sent after a PUT request. HTTP/0.9 and later
202 Accepted The request has been received but not yet acted upon. It is non-committal, meaning that there is no way in HTTP to later send an asynchronous response indicating the outcome of processing the request. It is intended for cases where another process or server handles the request, or for batch processing. HTTP/0.9 and later
203 Non-Authoritative Information This response code means returned meta-information set is not exact set as available from the origin server, but collected from a local or a third party copy. Except this condition, 200 OK response should be preferred instead of this response. HTTP/0.9 and 1.1
204 No Content There is no content to send for this request, but the headers may be useful. The user-agent may update its cached headers for this resource with the new ones. HTTP/0.9 and later
205 Reset Content This response code is sent after accomplishing request to tell user agent reset document view which sent this request. HTTP/1.1 only
206 Partial Content This response code is used because of range header sent by the client to separate download into multiple streams. HTTP/1.1 only
Redirection messages
300 Multiple Choice The request has more than one possible responses. User-agent or user should choice one of them. There is no standardized way to choice one of the responses. HTTP/1.0 and later
301 Moved Permanently This response code means that URI of requested resource has been changed. Probably, new URI would be given in the response. HTTP/0.9 and later
302 Found This response code means that URI of requested resource has been changed temporarily. New changes in the URI might be made in the future. Therefore, this same URI should be used by the client in future requests. HTTP/0.9 and later
303 See Other Server sent this response to directing client to get requested resource to another URI with an GET request. HTTP/0.9 and 1.1
304 Not Modified This is used for caching purposes. It is telling to client that response has not been modified. So, client can continue to use same cached version of response. HTTP/0.9 and later
305 Use Proxy This means requested response must be accessed by a proxy. This response code is not largely supported because security reasons. HTTP/1.1 only
306 unused This response code is no longer used, it is just reserved currently. It was used in a previous version of the HTTP 1.1 specification. HTTP/1.1 only
307 Temporary Redirect Server sent this response to directing client to get requested resource to another URI with same method that used prior request. This has the same semantic than the 302 Found HTTP response code, with the exception that the user agent must not change the HTTP method used: if a POST was used in the first request, a POST must be used in the second request. HTTP/1.1 only
308 Permanent Redirect This means that the resource is now permanently located at another URI, specified by the Location: HTTP Response header. This has the same semantics as the 301 Moved Permanently HTTP response code, with the exception that the user agent must not change the HTTP method used: if a POST was used in the first request, a POST must be used in the second request.
Note: This is an experimental response code whose specification is currently in draft form.
Client error responses
400 Bad Request This response means that server could not understand the request due to invalid syntax. HTTP/0.9 and later
401 Unauthorized Authentication is needed to get requested response. This is similar to 403, but in this case, authentication is possible. HTTP/0.9 and later
402 Payment Required This response code is reserved for future use. Initial aim for creating this code was using it for digital payment systems however this is not used currently. HTTP/0.9 and 1.1
403 Forbidden Client does not have access rights to the content so server is rejecting to give proper response. HTTP/0.9 and later
404 Not Found Server can not find requested resource. This response code probably is most famous one due to its frequency to occur in web. HTTP/0.9 and later
405 Method Not Allowed The request method is known by the server but has been disabled and cannot be used. The two mandatory methods, GET and HEAD, must never be disabled and should not return this error code. HTTP/1.1 only
406 Not Acceptable This response is sent when the web server, after performing server-driven content negotiation, doesn't find any content following the criteria given by the user agent. HTTP/1.1 only
407 Proxy Authentication Required This is similar to 401 but authentication is needed to be done by a proxy. HTTP/1.1 only
408 Request Timeout This response is sent on an idle connection by some servers, even without any previous request by the client. It means that the server would like to shut down this unused connection. This response is used much more since some browsers, like Chrome or IE9, use HTTP preconnection mechanisms to speed up surfing (see bug 634278, which tracks the future implementation of such a mechanism in Firefox). Also note that some servers merely shut down the connection without sending this message. HTTP/1.1 only
409 Conflict This response would be sent when a request conflict with current state of server. HTTP/1.1 only
410 Gone This response would be sent when requested content has been deleted from server. HTTP/1.1 only
411 Length Required Server rejected the request because the Content-Length header field is not defined and the server requires it. HTTP/1.1 only
412 Precondition Failed The client has indicated preconditions in its headers which the server does not meet. HTTP/1.1 only
413 Request Entity Too Large Request entity is larger than limits defined by server; the server might close the connection or return an Retry-After header field. HTTP/1.1 only
414 Request-URI Too Long The URI requested by the client is too long for the server to handle. HTTP/1.1 only
415 Unsupported Media Type The media format of the requested data is not supported by the server, so the server is rejecting the request. HTTP/1.1 only
416 Requested Range Not Satisfiable The range specified by the Range header field in the request can't be fulfilled; it's possible that the range is outside the size of the target URI's data. HTTP/1.1 only
417 Expectation Failed This response code means the expectation indicated by the Expect request header field can't be met by the server. HTTP/1.1 only
Server error responses
500 Internal Server Error The server has encountered a situation it doesn't know how to handle. HTTP/0.9 and later
501 Not Implemented The request method is not supported by the server and cannot be handled. The only methods that servers are required to support (and therefore that must not return this code) are GET and HEAD. HTTP/0.9 and later
502 Bad Gateway This error response means that the server, while working as a gateway to get a response needed to handle the request, got an invalid response. HTTP/0.9 and later
503 Service Unavailable The server is not ready to handle the request. Common causes are a server that is down for maintenance or that is overloaded. Note that together with this response, a user-friendly page explaining the problem should be sent. This responses should be used for temporary conditions and the Retry-After: HTTP header should, if possible, contain the estimated time before the recovery of the service. The webmaster must also take care about the caching-related headers that are sent along with this response, as these temporary condition responses should usually not be cached. HTTP/0.9 and later
504 Gateway Timeout This error response is given when the server is acting as a gateway and cannot get a response in time. HTTP/1.1 only
505 HTTP Version Not Supported The HTTP version used in the request is not supported by the server. HTTP/1.1 only
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Minggu, 04 November 2012

AJAX: Melangkah lebih jauh dengan XMLHttpRequest

AJAX didasarkan pada penggunaan engine javascript yang tertanam hampir di semua browser modern. Dengan kontra bahwa perilaku kode javascript yang kita ciptakan akan bervariasi sesuai dengan engine javascript yang digunakan oleh masing-masing browser. Untuk mengetahui lebih lanjut engine javascript browser, Anda dapat membacanya lebih lanjut di wikipedia.


Seperti yang sudah dibahasa pada posting saya sebelumnya, AJAX menggunakan object javascript XMLHTTPRequest untuk berkomunikasi dengan web server.
Objek javascript XMLHttpRequest atau yang biasa disingkat xhr,seperti namanya,akan memungkinkan kita untuk membuat request melalui protokol http ke web server dengan return XML.

Membuat objek XMLHttpRequest

var xhr = null;
if (window.XMLHttpRequest) //Firefox dan lainnya
    xhr = new XMLHttpRequest();
else if (window.ActiveXObject) {//Internet Explorer
    try {
        xhr = new ActiveXObject ("MSXML2.XMLHTTP");
    } catch(e) {
        xhr = new ActiveXObject ("Microsoft.XMLHTTP");
} else {
    //XMLHttpRequest tidak didukung oleh browser Anda
    alert ("Browser Anda tidak mendukung objek XMLHttpRequest ...");
    xhr = false;

Penggunaan Objek XMLHttpRequest

Property / Method Deskripsi
open("method","url",flag) Membuka koneksi ke server.
method: metode koneksi "GET" atau "POST"
url: url tujuan koneksi
flag: tipe boolean, true jika asynchronous.
setRequestHeader("nama","nilai") Mengeset HTTP header dalam request.
send("params") mengirim parameter request.
Jika menggunakan metode GET, parameter akan diset null namun parameter dikirim melalui url query string.
Jika menggunakan metode POST, parameter akan dikirim melalui form.
abort() Membatalkan request
onreadystatechange fungsi yang akan dieksekusi ketika terjadi perubahan pada objek.
readyState fungsi yang menguji fungsi onreadystatechange.
0: Not Initialized (belum / tidak di inisiasi)
1: Open (telah dibuka)
2: Sent (telah dikirim)
3: In Progress (dalam proses)
4: Ready (semua data telah dimuat)
status respon status dalam kode
statusText respon status dalam format teks
responseText respon pesan yang dikembalikan server dalam format text
responseXML respon pesan yang dikembalikan server dalam format XML
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