To-Do: Run a marathon

Just like every other Sunday morning, I started the hike in Mission peak. The trial never gets easy even if you hike them every week. The climate was awesome as usual. While we were climbing down the trail we saw the runners of 23rd Annual Ohlone 50K Wilderness Run

Its a 31 mile run for some good cause. I saw people from all age groups run for the cause. When I saw them, I added a new item to my bucket list[list of things I plan to do before life ends] and that is to run a marathon for a cause. And I really do hope I am able to do it not just once but almost every year. I will make it happen soon!! For now I will just add my small achievement of touching the mission peak here.

View from Mission peak

Looking back at the trail

PHP MySql POST GET Cookies Session - Bits and information

I always feel its easier to remember details if we learn it as differences between two objects. So I want to blog the basic differences between cookies and session, POST and GET method. And bits and pieces of information about UNION,UNION ALL and some more. This post has so many random information. Let me know if you find something wrong or different.

Three ways to move information across web pages:

1. links: a link with href set to destination page.
   
2. Forms: set action to point to target, method of how server should process information sent, encoding type optional. Both 1 and 2 requires user action like a click.
   
3. Without any user action, user header in PHP to move to another page. header("Location: page.php");
   

Cookies:

• Cookies are stored at the client's computer.
• Once stored at client comp by an application, it can be accessed by any web page of that application.
• It contains variable = value pairs of information.
• Cookies are more useful for applications that has no database.
• Cookies will be available from the next page and not in the current page where it was set.
• In PHP, setcookie('variable','value'); sets the cookie and if you don't mention time, this cookie will expire when user exits the application.
• On setting the cookie, it gets stored in one of many built in arrays of PHP called $_COOKIE. For accessing your cookie variable just refer the array with the variable name like $_COOKIE['variable']
• If we want our cookie to be present even after user exits application then we have to mention the expiration time while we set the cookie. We can use two functions along with setcookie() - time and mktime. time() - returns the current time, we have to add the seconds for which the cookie needs to be alive. eg. setcookie('var','val',time()+3600) - to set cookie alive for 1 hour(60*60 seconds). mktime() - returns date and time and the order of arguments passed is hr,min,sec,mon,day, year. eg. setcookie('var','val',mktime(3,0,0,5,6,2010)); - to make it expire by 3am of May 6th 2010.
  
• To remove cookies, use the same setcookie without any values passed. eg. setcookie('var'); setcookie('var','');
  
• setcookie limitation: It has to be before any output is sent to the client browser. This limitation is with both session and cookie.
  

Session:

• Session details are stored in a file at the server side. In Unix and Linux systems, in \tmp folder and in Windows folder called sessiondata. We can change the location of where the file is stored by chaning session.save_path value in php.ini.
  
• Session ID - long nonsense number for every client which cannot be guessed or forged. In PHP, system variable is used for session ID - PHPSESSID.
• Session ID is passed to every page to access the session details. There are three ways to pass them. 1. If cookies turned on, use the cookies. 2. For links, use the URL. 3. Use hidden variables for form with POST method.
• Session variable is got and stored in $_SESSION built in array(similar to $_COOKIE array in cookies). They are accessed same way as in cookies.
• For session to work we need to enable, track_vars while installing PHP.(default its turned on from PHP 4.0).
• If cookie is turned off at the client side, trans_sid should be enabled to transfer session id. To enable that use session.use_trans_sid = 1 in php.ini.
  
• Start a session: session_start() - If sessionID is found, then load $_SESSION with variables and their values. If no ID is found, then it is first time so create a new session and set PHPSESSID.
• Save a session variable: $_SESSION['var'] = 'val';
• Close a session: session_destroy() - destroys the session details.
• session_id() - returns the PHPSESSID value - current session id.
• unset($_SESSION) - unset the session details in the current page.
• Limitation: session has to be set before any output is sent to the client browser.
  

We know that POST and GET are ways of indicating how the server should process the information sent by the form.

POST:

• Sends information in 2 steps. 1. browser contacts form processing server specified in action. 2. Once contact has been established, send data to server in separate transmission.
• On server side: 1. read parameters from a standard location. 2. After read, decode parameters before application use form variables.
• To get post variables from earlier form use built in array $_POST.
  

GET:

• In a single transmission, data is sent to the server. Data is appended by the browser to action URL.
• This is the default method if not specified otherwise.
  
• On server side, gets information passed at the end of the URL.

POST or GET what to use?

• GET: best transmission performance(single transmission) and apt for small forms with short/few fields.
• POST: Apt for forms with many/long text fields.
• If inexperienced with server programming use GET to avoid extra steps of processing- read/decode as in POST.
• For security purpose, use POST to avoid the information you transmit to be available in open for hackers to track. POST has security holes too but atleast it has encoding when transmitting.
• To invoke server processing outside form tag, eg in a tag, use GET because it lets us use form-like parameters as part of URL.
  

ENCTYPE field in FORM tag:

• Two types of enctype options available. 1. Multipart and 2. Text/plain
• Multipart: forms with file selection fields for upload by user.
• text/plain: used along with mailto in action attr of form tag. While sending forms to email server rather than a server.
• The default encoding type: Internet media type Application/x-www-form-urlencode.

UNION and UNION ALL:

• Union all - combines rows from multiple row sources into one result set. It includes duplicates.
• Union - does the same thing as union all but excludes duplicates, result would be sorted here without duplicates.

Polymorphism in OO langauges:

• Ability of 2 or more objects belonging to different classes to respond to exactly same message in different class specific ways.
  

Thread programming in Java

I spent a day in library refreshing stuffs I learned long back(not so long!) while I was in undergrad. Since my memory is fresh with these topics, I thought I should blog them so next time I can find it all in one place. On a different note, a day spent studying/reading in library is well spent! Okay back to Threads in Java.

Thread - class, Runnable - interface which has the thread implementations. For our application class to use thread we need to either implement or subclass or make the class a member class of our application. Thus three ways to create threads:

1. Extend Thread class: For standalone applications that don't need to extend another class.
2. Implement Runnable interface: For classes that need to extend other classes. Due to single inheritance, we can achieve threads by implementing the interface. Eg: Applets that extend JApplet and can't extend Thread too.
   
3. Construct a thread passing an inner class that is Runnable: For threads with tiny run methods and little outside interaction.

Thread life cycle functions:

• Thread() - When subclassing Thread class
• Thread(name) - when subclassing + for debugging
• start() - Begins process of assigning CPU time to a thread. Finally it results in calling run() method of the thread object.
• suspend(), resume(),stop() - deprecated. Because these suspend and resume functions can cause deadlocks.
• interrupt() - interrupt whatever the thread is doing
• boolean isAlive() - returns true if thread is neither finished or terminated by call to its stop()
• join() - to wait for a thread to finish
• get/setPriority() - higher priority threads gets first chance of CPU
• wait(), notify(), notifyAll() - semaphore and for thread synchronization.
• synchronised - keyword - can be used for any object in Java. When used over an object or a method, threads will wait if one thread has already started running.

Thread synchronization:
The wait, notify and notifyAll methods allow any java.lang.Object to be used as a synchronization target.

wait():
This causes the current thread to block in given object until awakened by notify or notifyAll.

notify():
Randomly selected thread waiting on this object is awakened. Then that thread tries to regain the monitor lock. If wrong thread is awakened, will result in deadlock.

notifyAll():
All threads waiting on this object is awakened. Then all try for the monitor lock. Hopefully one of them succeed.

Best practices:

• To avoid deadlock, use notifyAll to wake up waiting threads.
• To avoid using deprecated method - stop, use the variable as a flag and use it to find if the thread is done executing.