Memcached vs APC vs Varnish

Alternative PHP Cache (APC)
APC provides two caching mechanisms.

APC caches the opcode generated during the PHP execution cycle and then later will be able to avoid loading and parsing PHP files repeatedly.
APC also provides a key-value, object cache for PHP applications in shared memory.

Memcached
Memcached is a distributed, key-value, object cache in memory. This is similar to the object cache provided by APC but there are some important differences. It’s in-memory, while APC’s object cache is in shared memory. This will make Memcached faster, but will also require the memory allocation for it’s storage. The other major difference is that Memcached is distributed. This means that it runs across multiple servers. For example, if you’re load-balancing your application. Generally, the need for a distributed object cache is why you would use Memcached.

Varnish
Varnish is a caching HTTP reverse proxy. The reverse proxy part means that it sits between your application and the outside world. Visiting your domain will actually connect to Varnish. Varnish will then make the corresponding request to your application and then deliver it to the client. It will cache the results of these requests based on a configuration file you can write in the Varnish Configuration Language.

Varnish is neither an opcode cache like APC’s opcode cache component, nor an object cache like those in APC or Memcached. Varnish operates outside of your application and caches the entire HTTP response such as the whole HTML document returned by your application.

Varnish is extremely effective because it will cache the final resulting HTML document after all PHP or other server side processing is done. This means that when varnish delivers a page from cache, it avoids running the PHP code at all and consequently any database queries involved in generating the document. Delivering a cached page from Varnish is like delivering a static HTML file.

https://www.neonrain.com/web-cache-apc-vs-memcached-vs-varnish/

What is Clickjacking?

A clickjacked page tricks a user into performing undesired actions by clicking on a concealed link. On a clickjacked page, the attackers load another page over it in a transparent layer. The users think that they are clicking visible buttons, while they are actually performing actions on the hidden/invisible page. The hidden page may be an authentic page; therefore, the attackers can trick users into performing actions which the users never intended. There is no way of tracing such actions to the attackers later, as the users would have been genuinely authenticated on the hidden page.

https://en.wikipedia.org/wiki/Clickjacking

What information does the apache access log contain?

127.0.0.1 - - [05/Feb/2012:17:11:55 +0000] "GET / HTTP/1.1" 200 140 "-" "Mozilla/5.0 (Windows NT 6.1; WOW64) AppleWebKit/535.19 (KHTML, like Gecko) Chrome/18.0.1025.5 Safari/535.19"

%h is the remote host (ie the client IP)
%l is the identity of the user determined by identd (not usually used since not reliable)
%u is the user name determined by HTTP authentication
%t is the time the request was received.
%r is the request line from the client. (“GET / HTTP/1.0”)
%>s is the status code sent from the server to the client (200, 404 etc.)
%b is the size of the response to the client (in bytes)
Referer is the Referer header of the HTTP request (containing the URL of the page from which this request was initiated) if any is present, and “-” otherwise.
User-agent is the browser identification string.

http://stackoverflow.com/questions/9234699/understanding-apaches-access-log

Sort an array of 0s, 1s and 2s

Given an array A[] consisting 0s, 1s and 2s, write a function that sorts A[]. The functions should put all 0s first, then all 1s and all 2s in last.

Example
Input = {0, 1, 1, 0, 1, 2, 1, 2, 0, 0, 0, 1};
Output = {0, 0, 0, 0, 0, 1, 1, 1, 1, 1, 2, 2}

This is a variation of famous Dutch national flag problem
The problem was posed with three colours, here `0?, `1? and `2?. The array is divided into four sections:

a[1..Lo-1] zeroes (red)
a[Lo..Mid-] ones (white)
a[Mid..Hi] unknown
a[Hi+1..N] twos (blue)
The unknown region is shrunk while maintaining these conditions

Lo := 1; Mid := 1; Hi := N;
while Mid <= Hi do
Invariant: a[1..Lo-1]=0 and a[Lo..Mid-1]=1 and 
a[Hi+1..N]=2; a[Mid..Hi] are unknown.

case a[Mid] in
0: swap a[Lo] and a[Mid]; Lo++; Mid++
1: Mid++
2: swap a[Mid] and a[Hi]; Hi–

Sort an array of 0s, 1s and 2s

Key Terms and Concepts in Scalability

https://www.webforefront.com/performance/scaling101.html

Performance tuning.- This step would consist of refactoring a web application’s source code, analyzing a web application’s configuration settings, attempting to further parallelize a web application’s logic, implementing caching strategies, detecting hot spots and another series of often invasive — code wise that is — procedures throughout a web application’s tiers. These topics will be detailed as the book progresses.

Vertical Scaling: In scaling terminology, this implies that a box or node on which a web application is running can be upwardly equipped with more CPU, Memory, Bandwidth or I/O capacity. Thus if a web application encounters a greater demand for any of these resources, and you are able to move a web application or one of its tiers to a box or node with greater capacity, you will have vertically scaled an application.

Horizontal Scaling: Horizontal scaling refers to assigning resources on a lateral basis. In scaling terminology, this implies that the node on which a web application is running cannot be equipped with more CPU, Memory, Bandwidth, or I/O capacity and thus a web application is split to run on multiple boxes or nodes.
The process of horizontally scaling a web application is more elaborate than that of vertically scaling. The reason is that by relying on horizontal scaling, you need to devise a way to decouple a web application. The simplest way to decouple a web application is by means of its tiers. Recapping from the previous paragraphs, these tiers would be:

Static content tier.- Consists of static images or other resources that make up an application’s interface and don’t need processing. In most web applications this would be JPEG/GIF images, Javascript libraries, Cascading style sheets or pre-built HTML pages.
Business logic tier.- Consists of a web framework for processing data provided by users or stored in the permanent storage tier. Depending on your preferences, this could be Ruby on Rails, PHP Cake, Django, Grails(Java) or MonoRail(.NET) among many others.
Permanent storage tier.- Consists of a permanent storage location for data. In most web applications this is a relational database system(RDBMS).

Javascript Questions Set3

1) What is event delegation?

Event delegation is when you bind an event listener to a parent (or ancestor) element rather than the element(s) you are particularly interested in. When the event is triggered you can check the event target to make sure it was actually the triggered on the element of interest.

Event bubbling provides the foundation for event delegation in browsers. Now you can bind an event handler to a single parent element, and that handler will get executed whenever the event occurs on any of its child nodes (and any of their children in turn). This is event delegation. Here’s an example of it in practice:

Let’s say that we have a parent UL element with several child elements:

  • Item 1
  • Item 2
  • Item 3

Let’s also say that something needs to happen when each child element is clicked. You could add a separate event listener to each individual LI element, but what if LI elements are frequently added and removed from the list? Adding and removing event listeners would be a nightmare, especially if addition and removal code is in different places within your app. The better solution is to add an event listener to the parent UL element. But if you add the event listener to the parent, how will you know which element was clicked?

Simple: when the event bubbles up to the UL element, you check the event object’s target property to gain a reference to the actual clicked node. Here’s a very basic JavaScript snippet which illustrates event delegation:

// Get the element, add a click listener...
document.getElementById("parent-list").addEventListener("click", function(e) {
// e.target is the clicked element!
// If it was a list item
if(e.target && e.target.nodeName == "LI") {
    // List item found!  Output the ID!
    console.log("List item ", e.target.id.replace("post-"), " was clicked!");
       }
 });

2) What are the 3 ways to bind an event to an element ?
1) HTML Event Handlers
< input type="text" id="username" onblur="checkUserName()">
Not recommended any more. Better to separate JS and HTML

2) DOM Event Handlers
function checkUserName() { blah } ;
var el = document.getElementById(‘username’);
el.onblur = checkUserName;

Disadvantage is that you can only attach one function to each event Handler.

3) DOM Event Listeners
Advantage : They can deal with more than one function at a time.
var el = document.getElementById(‘username’);
el.addEventListener(‘blur’, checkUserName, false);

3) How do you convert arguments into an actual array. arguments are pseudo array in js.

var args = Array.prototype.slice.call(arguments);

4) How would you simulate Function.prototype.bind in older browser that don’t support it

Function.prototype.bind = Function.prototype.bind || function(context){
  var self = this;

  return function(){
    return self.apply(context, arguments);
  };
}

4) How to pass parameters to callback functions?

By default you cannot pass arguments to a callback function. For example:

function callback() {
  console.log('Hi human');
}

document.getElementById('someelem').addEventListener('click', callback);

But using closure concept you can do so :

function callback(a, b) {
  return function() {
    console.log('sum = ', (a+b));
  }
}

var x = 1, y = 2;
document.getElementById('someelem').addEventListener('click', callback(x, y));

What are B-Trees

https://www.youtube.com/watch?v=k5J9M5_IMzg

a B-tree is a self-balancing tree data structure that keeps data sorted and allows searches, sequential access, insertions, and deletions in logarithmic time. The B-tree is a generalization of a binary search tree in that a node can have more than two children (Comer 1979, p. 123). Unlike self-balancing binary search trees, the B-tree is optimized for systems that read and write large blocks of data. B-trees are a good example of a data structure for external memory. It is commonly used in databases and filesystems.

All terminal nodes are the same distance from the base