Socket.io

What is the benefit of transforming data into packets?

• Communication – it is easy (and often free) to communicate using email, text messages, voice calls and video calls.
• Roaming – if information is stored on a network, it means users are not fixed to one place. They can use computers anywhere in the world to access their information.
• Sharing information – it is easy to share files and information over a network. Music and video files, for instance, can be stored on one device and shared across many computers, so every computer does not need to fill the hard drive with files.
• Sharing resources – it is easy to share resources such as printers. Twenty computers in a room could share one printer over a network.
• Sharing software – it is possible to stream software using web applications. This avoids needing to download and store the whole software file.

UDP is often refereed to as a connectionless protocol. Why is this?

UDP is a simple, connectionless protocol that is the layer 4 protocol of choice for such applications as streaming audio and video and VoIP (voice on Internet Protocol — see Chapter 6) packet delivery. Applications using UDP are frequently characterized by the need for rapid (low-latency) delivery, where it is not feasible to correct for transmission errors. UDP is also used where a transmission is bound for more than one receiver, making acknowledgment moot.

Connectionless protocols operate in this manner. One casts a datagram onto the network with the understanding that it will be delivered on a best-effort basis to whomever it is addressed to. In addition, we accept that there is no notification of a failure, nor can we make assumptions about the sequence of delivery. UDP is a great example of this sort of communication.

Can a socket server application have multiple socket connections?

The answer differs depending on what OS is being considered. In general though:

For TCP, no. You can only have one application listening on the same port at one time. Now if you had 2 network cards, you could have one application listen on the first IP and the second one on the second IP using the same port number.

For UDP (Multicasts), multiple applications can subscribe to the same port.

Can a socket connection application be connected to multiple socket servers?

Yes ,Multiple connections on the same server can share the same server-side IP/Port pair as long as they are associated with different client-side IP/Port pairs, and the server would be able to handle as many clients as available system resources allow it to.

Can an application be both a socket server and a socket connection?

Yes and no. It isn’t very clear what you’re trying, though. So the answer isn’t very clear either. However, if you want to use a client socket and a server socket within a single application, then yes! You can do that if you’re willing to use two different ports. Or if the sockets won’t be using the same port at the same time.

Terms:

Term	Def
Observer Pattern	The observer pattern is a software design pattern in which an object, named the subject, maintains a list of its dependents, called observers, and notifies them automatically of any state changes, usually by calling one of their methods.
Listener	An event is an important part of JavaScript.A web page respond according to an event occurred. Some events are user generated and some are generated by API’s. An event listener is a procedure in JavaScript that waits for an event to occur. The simple example of an event is a user clicking the mouse or pressing a key on the keyboard.
Event Handler	Events are actions that occur, usually as a result of something the user does.
Event Driven Programming	In computer programming, event-driven programming is a programming paradigm in which the flow of the program is determined by events such as user actions (mouse clicks, key presses), sensor outputs, or message passing from other programs or threads. Event-driven programming is the dominant paradigm used in graphical user interfaces and other applications (e.g., JavaScript web applications) that are centered on performing certain actions in response to user input. This is also true of programming for device drivers (e.g., P in USB device driver stacks[1]).
Event Loop	In computer science, the event loop is a programming construct or design pattern that waits for and dispatches events or messages in a program. The event loop works by making a request to some internal or external “event provider” (that generally blocks the request until an event has arrived), then calls the relevant event handler (“dispatches the event”). The event loop is also sometimes referred to as the message dispatcher, message loop, message pump, or run loop.
Event Queue	An event queue is a repository where events from an application are held prior to being processed by a receiving program or system. Event queues are often used in the context of an enterprise messaging system.
Call Stack	In computer science, a call stack is a stack data structure that stores information about the active subroutines of a computer program. This kind of stack is also known as an execution stack, program stack, control stack, run-time stack, or machine stack, and is often shortened to just “the stack”. Although maintenance of the call stack is important for the proper functioning of most software, the details are normally hidden and automatic in high-level programming languages. Many computer instruction sets provide special instructions for manipulating stacks.
Emit/Raise/Trigger	In node.js an event can be described simply as a string with a corresponding callback. An event can be “emitted” (or in other words, the corresponding callback be called) multiple times or you can choose to only listen for the first time it is emitted. The on or addListener method (basically the subscription method) allows you to choose the event to watch for and the callback to be called. The emit method (the publish method), on the other hand, allows you to “emit” an event, which causes all callbacks registered to the event to ‘fire’, (get called).The emit() function raises the specified event. First parameter is name of the event as a string and then arguments. An event can be emitted with zero or more arguments. You can specify any name for a custom event in the emit() function.The trigger() method triggers the specified event and the default behavior of an event (like form submission) for the selected elements.
Subscribe	The .subscribe() function is similar to the Promise.then(), .catch() and .finally() methods in jQuery, but instead of dealing with promises it deals with Observables.That means it will subscribe itself to the observable of interest and wait until it is successful and then execute the first passed callback function
database	In computing, a database is an organized collection of data stored and accessed electronically from a computer system. Where databases are more complex they are often developed using formal design and modeling techniques.