27 Oct 2010
Grep là công cụ cơ bản trong linux/unix dùng để tìm kiếm từ khoá trong file hoặc từ input. Ngoài grep ra còn có egrep giống như grep -E, rgrep giống như grep -r và fgrep giống như grep -F.
Dưới đây là vài ví dụ đơn giản có thể dùng hàng ngày
1. Tìm từ khoá keyword trong tất cả các file trong thư mục hiện thời và hiện đường dẫn của file đó
grep -H -r "keyword" *
2. Tìm từ khoá kết hợp output từ lệnh khác
cat filename | grep 'something'
cat /proc/cpuinfo | grep -i 'Model'
3. Đếm số kết quả tìm được
$ grep -c false /etc/passwd
grep là công cụ rất nhanh và hữu ích. Hãy tìm hiểu thêm ở đây hoặc gõ "man grep" trong terminal. Chúc bạn thành công.
29 Sep 2010
I have a chance to work with the system, consists of two separate servers , both using Django. The server can communicate with each other via socket. However, the problem with socket is that it is not easy to send the big data and retain different resources. Commonly, the two servers need to share a common resource, usually the database. It is not effective to maintain the local database in each server and replicate the data between them whenever there is certain change to the server.
Everything seems to work, server A and B communicate via server, and both access to the same database (in my case it is using MySQL database) ... but then a bug emerges ...
The server A creates some data and save it to the database, using Django ORM, and send a request to the server B telling B to process the data server A just saves to database. Server B gets the request and needed information to process the data the server A saved to the database. The problem is server B cannot find the data server A just saved. What is going on???
My first suspect is that when server A saves the data, it might take a little time to MySQL to save it, so before data is actually stored to db, server B gets the request and cannot find it. But this is not true, when the error also come after 30 seconds ... It cannot be that saving data to database takes more than 30 seconds.
Then I suspect the cache of Django ORM, it is possible that the Django ORM retains its own cache, it retrieves data without really hit the database, and only refresh after a certain time. But then I find out that Django ORM does not really have any caching mechanism at all. But when I tried to use direct access to MySQL, the data can be found. So what is going on?
The problem is due to a thing I never thought of. The cache of MySQL database. The scenario is as followed, to save the time, usually a server retain a certain connection (session) to the database server when requesting multiple queries in its life span. It is not efficient to create and close the connection when making connection for each query. Server A retain a connection (session) to MySQL server, writing or reading data has no problem at all. In the same meaning, the server B also retains the same connection with MySQL server. Even after the server A saves the data, MySQL server still not updates or reflects the new data to the connection made and retained by B. This is due to the cache set up in MySQL database. That's why server B cannot find the database made by A. So we know the reason, how can we solve the problem? Two options
1. Disable the cache in MySQL, usually this is not the good option since the cache will improve significantly the performance of MySQL and usually we don't have root access to MySQL db at all
2. Easier one and feasible one, restart the connection to database before processing new request sent from server A. It is a bit less efficient, but it's working properly. The problem is how to reset the connection to db in Django ORM. Django ORM in fact dos not support any method to reset the db connection. Finally I found the trick to do that.
from django.db import connection
This is not actually reseting the connection to db, but in fact close it. However, when you make a new query by Django ORM, the connection will be created again when it is seeing that the connection is closed before, it turns out a way to reset the db connection.
Even though this happens in the case the server is using Django framework in Python. But the problem might happen to other platforms as well. There are many chances you see the system with different servers access or share the same resource. Pay attention and make sure that the resource reflect the change effectively and the server can always obtain the latest state, without cache or delay in storing data. Otherwise, it can happens in a way that the server B lost the data or mis-used the data saved by other servers.
22 Sep 2010
The post assumes that you have a basic knowledge in Chrome extension development, if not check it out here http://code.google.com/chrome/extensions/getstarted.html
In the manifest json file, there might be two properties definitions for background-page and content-script. These properties are the most common used in the manifest file of Chrome extensions. They are also the most important entities to understand the architecture model of Chrome extension. Remind a bit about Chrome browser, in Chrome we can open many tabs as we want, but the extension is always stayed on top of those. The extension is like a small program sitting on top of the tabs and no matter how many tabs we open, there is only one instance of extension running and executing.
The architecture model of Chrome extension
Now back to the background page and content script. The background can access to mostly many properties of the browser such as history, bookmark ..etc... but cannot access the user page nor the content script. However, the background page can make a call to execute the methods inside the content script and if the content script want to access the data in the background page, it has to make the request to the background page via method
chrome.extension.sendRequest. The background page on the other hand can make any cross site requests but the content script cannot due to the security of cross site scripting. The only options for content script is to make the request with JSONP to a service or make the request to the background page to make the request to the required service.
To summarise things, if your extension is communicating with a certain Web services or APIs. That has to be done in the background page. If the extension wants to modify the view of user page, it must send request or execute the method defined in content script then the content script will access the DOM elements and modify the view accordingly. If the extension wishes to receive some data from the user page, then it has to be done in the content script and the content script will send the data to background page via the Chrome extension request. Finally note that the background has only one instances over many tabs or page, but the content script is created new on new page reloaded or open.
22 Sep 2010
I used to have many errors in Unicode and encoding in Python due to that I underestimated it. Unicode and encoding are very basic concepts to understand, but handling without care might give unexpected errors. There are chances that converting from a byte stream object to Unicode object will give error. For example
s = "Thank you pälä"
u = unicode(s)
The first line assigns the byte stream containing the character 'T', 'h' ..etc.. to the variable s. The next line will convert it to the Unicode object. However, the second will give you an exception due to that the default encoding in Python is ascii. Python will try to convert the byte stream data to Unicode string using ascii encoding, but the character ä which is encoded as 2 bytes 00 and E4 which is out of the range of first 128 characters (ascii codec can only process the first 128 characters in the ascii character map)
You might notice that the second line give no error with some other strings. In fact, that is when the byte stream data does not contain any Unicode character out of the range of the first 128 characters and because ascii characters set is a subset of Unicode, they are same for the first 128 characters.
To overcome the problem in converting the byte stream data to Unicode string. We must know the encoding of the string. There are certain cases that we cannot know the encoding in advance, the resolution is to guess it by trying various known and popular encodings such as ascii, UTF-8 and UTF-16 .etc… Assume we know the string is encoded in UTF-8, the second line can change to
u = unicode(s, 'utf-8')
That is the case of converting byte stream to Unicode, how about the opposite case? Look following example
u = "Hello pälä"
f = open(“file.txt”, “w”)
This will give an exception in the third line
UnicodeEncodeError: ‘ascii’ codec can’t encode character u’\xe4′ in position 7: ordinal not in range(128)
The reason is the same, when writing to file, sockets or some other media. It requires the byte stream object, the object contains byte by byte, not the Unicode string. Unicode string is a special object handled internally by Python, other media cannot understand it. Hence, before writing to file or sending data to the socket, Python will convert the Unicode string to the byte stream and guess what, it will use default encoding ascii to convert it. And since character ä is not in the range of 128 first characters, the result is that it will throw an exception error.
To overcome this problem, we must know the required encoding of the media we are going to deal with. Does the file require utf-8 encoded byte stream? There are many other encodings around contain the ä but the utf-8 is the most popular one. So the correct third line is
Unicode is a industry standard of characters set used in various applications worldwide. Encoding on the other hand is how data is stored in the disk file. When data is moved around different environment or media, such as from the client browser, web server, socket and database. The data is encoded to byte stream and decoded to Unicode string to process many times. Each media might requires different representation of the data to process. So pay attention to the encoding of the data, process it properly. The cost is much more when encoding is handled improperly, data lost, bug introduced, crashed .etc… The choice is yours :D
14 Aug 2010
[sourcecode lang="php" htmlscript="true"]
<link href="/css/stylesheet.css?<?php echo time(); ?>" rel="stylesheet" type="text/css" />
[sourcecode lang="php" htmlscript="true"]
<link href="/css/stylesheet.css?1281732818" rel="stylesheet" type="text/css" />
Trong ví dụ trên, hàm time() tạo ra 1 timestamp phía sau mỗi đường dẫn và làm cho browser load lại file đó mỗi lần user refresh lại trang. Thủ thuật này có thể áp dụng cho tất cả mọi loại file từ CSS, JS và file ảnh...
Tuy nhiên bạn nên cẩn thận khi sử dụng thì caching là điều tốt giúp giảm tải server và băng thông, vì vậy cách này nên dùng chỉ khi bạn đang develop hay test trang web. Trong môi trường production, các file CSS và JS nên được cache và thậm chí nén vì 2 mục đích trên.
Chúc bạn thành công
09 Aug 2010
Trong Symfony 1.4, mỗi action đều yêu cầu có 1 template hiển thị nội dung tương ứng, nhưng trong trường hợp bạn muốn đưa 1 file cho user download, bạn có thể dùng đoạn code dưới đây:
public function executeDownload(sfWebRequest $request)
$file = $this->getRoute()->getObject();
$this->forward404Unless(file_exists($file->getPath()), 'File not found');
$this->getResponse()->setHttpHeader('Content-Disposition', 'attachment; filename="' . basename($file->getPath()).'"');
Trong đoạn code trên, $file là một object lấy từ model object, bạn có thể tùy ứng thay đổi biến này để lấy các thông số như size, path...
Sau khi lấy được file cần thiết, chúng ta thông báo cho browser chuẩn bị download thông qua method setHttpHeader(), đây là method tương đương với hàm header() của PHP.
Sau đó đọc nội dung của file ra output buffer và các bạn đừng quên là phải có
để Symfony không cần template để render HTML nữa.
Chúc các bạn thành công
26 Jul 2010
Trên Debian/Ubuntu bạn có thể cài đặt Apache, MySQL và PHP chỉ với 1 dòng lệnh "apt-get" nhưng có một vấn đề là không ai có thể nhớ được hết tên các package. Cho dù bạn nhớ hết thì đây có thể là dòng lệnh mà bạn phải gõ:
apt-get install mysql-client mysql-common mysql-server mysql-server php5-mysql php-apc php-db php-pear php5 php5-cli php5-common php5-curl php5-gd php5-imagick php5-mcrypt php5-memcache php5-memcache php5-mysql php5-sqlite php5-suhosin apache2 apache2-doc apache2-mpm-prefork apache2-suexec apache2-utils apache2.2-common libapache2-mod-php5
Nhưng thật ra có 1 câu lệnh ngắn hơn giúp việc cài đặt LAMP server rất dễ dàng:
apt-get install phpmyadmin
Gói phpmyadmin yêu cầu bạn phải có những gói cơ bản như apache, php và mysql để có thể cài đặt. Nếu bạn cài phpmyadmin coi như bạn đã có một LAMP server hoàn chỉnh.
Chúc bạn cài đặt thành công :D
09 Jun 2010
Add this method to your ProjectConfiguration.class.php
public function configureDoctrine(Doctrine_Manager $manager)
09 May 2010
Today I stumbled upon this slide from BarCampNashville. It is a interesting feud for beginner to get a overall comparison between Symfony and Rails, Ruby and PHP. Enjoy
24 Apr 2010
Found today interesting talking about Doctrine 2 with Jonathan Wage. Thanks for the original post from SymfonyLab. Enjoy.
[vimeo http://vimeo.com/11146571 w=500&h=400]