What are different policies to prevent congestion at different layers?

- Many times it happens that the demand for the resource is more than what network can offer i.e., its capacity. 

- Too much queuing occurs in the networks leading to a great loss of packets. 

- When the network is in the state of congestive collapse, its throughput drops down to zero whereas the path delay increases by a great margin. 

- The network can recover from this state by following a congestion control scheme.

- A congestion avoidance scheme enables the network to operate in an environment where the throughput is high and the delay is low. 

- In other words, these schemes prevent a computer network from falling prey to the vicious clutches of the network congestion problem. 

- Recovery mechanism is implemented through congestion and the prevention mechanism is implemented through congestion avoidance. 

- The network and the user policies are modeled for the purpose of congestion avoidance. 

- These act like a feedback control system. 

The following are defined as the key components of a general congestion avoidance scheme:

Ø  Congestion detection

Ø  Congestion feedback

Ø  Feedback selector

Ø  Signal filter

Ø  Decision function

Ø  Increase and decrease algorithms

- The problem of congestion control gets more complex when the network is using a connection-less protocol. 

- Avoiding congestion rather than simply controlling it is the main focus. 

- A congestion avoidance scheme is designed after comparing it with a number of other alternative schemes. 

- During the comparison, the algorithm with the right parameter values is selected. 

For doing so few goals have been set with which there is an associated test for verifying whether it is being met by the scheme or not:

Ø  Efficient: If the network is operating at the “knee” point, then it is said to be working efficiently.

Ø  Responsiveness: There is a continuous variation in the configuration and the traffic of the network. Therefore the point for optimal operation also varies continuously.

Ø Minimum oscillation: Only those schemes are preferred that have smaller oscillation amplitude.

Ø Convergence: The scheme should be such that it should bring the network to a point of stable operation for keeping the workload as well as the network configuration stable. The schemes that are able to satisfy this goal are called convergent schemes and the divergent schemes are rejected.

Ø Fairness: This goal aims at providing a fair share of resources to each independent user.

Ø  Robustness: This goal defines the capability of the scheme to work in any random environment. Therefore the schemes that are capable of working only for the deterministic service times are rejected.

Ø  Simplicity: Schemes are accepted in their most simple version.

Ø Low parameter sensitivity: Sensitivity of a scheme is measured with respect to its various parameter values. The scheme which is found to be too much sensitive to a particular parameter, it is rejected.

Ø Information entropy: This goal is about how the feedback information is used. The goal is to get maximum info with the minimum possible feedback.

Ø Dimensionless parameters: A parameter having the dimensions such as the mass, time and the length is taken as a network configuration or speed function. A parameter that has no dimensions has got more applicability.

Ø Configuration independence: The scheme is accepted only if it has been tested for various different configurations.

Congestion avoidance scheme has two main components:

Ø  Network policies: It consists of the following algorithms: feedback filter, feedback selector and congestion detection.

Ø  User policies: It consists of the following algorithms: increase/ decrease algorithm, decision function and signal filter.

These algorithms decide whether the network feedback has to be implemented via packet header field or as source quench messages.

What is hard disk and what is its purpose?

- HDD or Hard Disk Drive is for data storage. 

- It is used for storage and retrieval of the digital information or data that is stored on it. 

- The data is stored or retrieved by means of its discs that rotate rapidly. 

Hard Disk and its Purpose

 

- These discs are known as the platters and are coated with some sort of magnetic material. 

- The major characteristic as well as benefit of hard disk drives is that they retain the data even when the power supply is switched off. 

- From hard disk, the data can be read in a manner of random access. 

- This means that the storing and retrieval of the individual blocks of the data can be done either sequentially or in any order that the user may like. 

- A hard disk may consist of one or more than one of those rigid platters. 

- These rotating discs have magnetic heads that are located on an actuator arm that is continuously moving and reads and writes data on their surfaces. 

- IBM was the first to introduce the hard disk in the year of 1956. 

- Hard disk drives are the most dominant and the prominent secondary storage device for the computers since 1960s. 

- Since then, it has been continuously improved. 

- The HDD units are produced by more than 200 companies; among them most prominent developers are Toshiba, Seagate, Western digital etc. 

HDD’s primary characteristics are:

Ø  Capacity and

Ø  Performance

- The former is specified in terms of the unit prefixes. 

- In some systems, the capacity of the hard disk drive might be unavailable to the user since being used by the operating system and the file system and may have a possibility of occurrence of redundancy.

- The latter is specified in terms of the movements of the heads for a file i.e., the average access time in addition to the time taken for moving the file under the head i.e., the average latency and data rate. 

HDDs are available in two most common factors namely:

Ø  3.5 inch for desktop computers

Ø  2.5 inch for laptops

HDDs might be connected to the system by any of the following standard interface cables:

Ø  Serial ATA or SATA cable

Ø  USB cable

Ø  Serial attached SCSI or SAS cable

- In the year of 2012, flash memory emerged as a tough primary competitor for the hard disk drives. 

- These flash memories are some sort of solid state drives or SSDs. 

- However, the HDDs will still continue to dominate the secondary storage for its advantages such as price per unit of storage and recording capacity. 

- But there is a different scenario is the case of portable electronics.

- Here, the flash drives are considered to be more useful then rotating HDDs because here the durability and physical size of the drive has also to be considered more when compared to price and capacity.

HDD uses the magnetic recording technology where the data is recorded by magnetizing a thin film of material that is typically ferromagnetic on a disk. 

- The binary data bits are represented by the sequential change in the direction of the magnetization.

- An encoding scheme is used for encoding the user data. 

- An example of such encoding scheme run – length limited encoding. 

- It is these schemes that determine how the magnetic transitions would represent the data.

The latest HDD technologies are:

Ø  Shingled write

Ø  CPP/ MGR heads

Ø  Heat assisted magnetic recording

Ø  Bit – patterned recording

What is virtual user script? Why do you need to parametrize fields in your virtual user script?

Nowadays, the concept of the virtual user is quite common in the field of load testing. 

What is a Virtual User?

- Virtual user is one of the quite useful tools have been ever invented in the field of software engineering. 

- Virtual users can be used in a number of ways for testing load, stress or capacity of any software system or application you want. 

- It will be the easiest to define a virtual user as a virtualized representation of a real world user which has been specifically designed for simulating the same interactions and behaviors with the software system or application or web site to be tested in a way that a real world user would do exactly. 

Let us take an example, suppose at a peak your web site got 100 users over a particular hour. It becomes quite easy for you to simulate the same behavior using 100 virtual users making use of the scripts that invoke quite similar interaction and the navigation exactly as your real users do. Today such virtual users are also available that can playback the recorded scripts. 

In this article we discuss about what a virtual user script is and what is the need for parameterizing the fields in a virtual user script.  

What is a Virtual User Script & Need to Parametrize fields?

The fact that almost all of the scripts of the virtual user are automated is quite common. 

- For each and every automated script a particular entry point is marked by the script statement in virtual user. 

- From this point, the actions of the automated scripts are much similar to that of the real world users. 

- At any point these virtual user scripts can be made to click on any of the buttons or the windows, type some words, and move the mouse around and so on.

- Not only scripts, there are certain functions that are supported well by the virtual user and these functions can act as an extension to the virtual user scripts. 

- These functions have been given a general name called “tasks”. 

- These tasks consist of a procedure along with a list of parameters and in some cases they may also include an optional return value. 

- The information regarding the environment of the target computer is also collected by the virtual user with help of a statement called the match statement. 

- A specific environment element is searched by this match statement with the help of some descriptor traits like the following:

1. Static text
2. Edit text
3. Pictures
4. Icons
5. User items and so on.

- There is one more kind of statement that is used and is called the collect statement.

- It is used to collect all elements of a certain type in to the list.

- Afterwards the virtual user interacts with the software system or application environment using certain keywords like:

1. Select
2. Drag
3. Type
4. Close
5. Click and so on.

- Common objects of the operating system like windows, buttons, bars, menus and scrolls are accessed by the virtual users. 

- The virtual user software package comes with a log file feature using which one can write out all the information from within a script. 

- This feature also provides the scripter with information regarding the current run time state of the virtual user scripts. 

- These virtual user scripts can also be debugged whenever required by logging with the printh statement. 

Another fact about virtual user is that there exists no type checking which serves as a very good idea to log the parameters that serve as input to every task.

What are different aspects of stress testing?

Stress testing can be defined as a form of testing that is carried out to determine the stability and the stress handling capacity of a software system or module. Stress testing is all about testing the software system or application beyond the normal operational capacities. It is the testing of software system or application to its breaking or fatal point. This is done in order to observe the results.

Stress testing has a much broader meaning. What is basically understood by a stress test?
- It is referred to a test that mainly focuses upon the availability, error handling and robustness of a software system or application.
- In stress testing the software system or application is subjected heavy loads of tasks.
- It is not about considering the proper behavior under the normal operational conditions or user environment.

Typically the goal of stress testing is to test whether or not the software system or application crashes or fails in the case of catastrophic problems like unavailability of sufficient computing resources. These computational resources may include disk space or memory. It is also done to determine if the system crashes or fails under the situation of denial of service attacks and unusually high concurrency.

Stress testing, load testing, volume testing all seem like much similar kinds of testing.

A look at the following examples of stress testing will clear up the confusions regarding stress testing:


- Stress testing for web server:
A web server can be subjected to stress testing using bots, scripts and several denial of service tools to determine its performance and behavior during the peak data and tasks load.

- Stress testing can be studied in contrast with load testing.
Load testing is basically carried out to examine the entire testing environment and the huge database. It is also carried out to determine the response time of the software system or application whereas the stress testing exclusively focuses upon identifying the transactions and pushing them to a level at which a break occurs in the execution of the transaction software system.

Another point is that during the stress testing if the transactions are duly stressed, then the chance is that the database may not experience a huge data load. However if the transactions are not stressed, the data base may experience a heavy work load.

SOME IMPORTANT POINTS:
- Stress testing is another word for system stress testing.
- It can be defined as the loading of the concurrent users beyond and over the level that can be handled by the system.
- This leads to the breakage of the weakest link in the whole software system or application.
- While carrying out the stress testing the software engineers, developers or testers need to test the whole software system or application under the desired expected stress as well as under the accelerated stress.
- The goal here is to determine the working life of the software system.
- It is also aimed at determining the modes of failure for the software system or application.
- For the hardware counterpart of a complete system, the stress testing can be defined as the subjecting of the concerned hardware to the exaggerated levels of stress.
- This is basically done to determine the stability of the hardware system when used in a normal environment rather than a testing environment.
- Before modifying the CPU parameters during the processes of over clocking, over volting, under volting and under clocking, it is necessary to verify whether or not the new CPU parameters like frequency and core voltage are suitable for taking the heavy CPU loads.
- Stress testing for such parameters is usually carried out by executing a CPU intensive program for a prolonged significant period of time. It is observed if the system crashes or hangs.

Volume tests - Volume Testing of Messaging systems

Volume tests are often most appropriate to messaging, batch and conversion processing type situations. In a volume test, there is often no such measure as response time. Instead, there is usually a concept of throughput. A key to effective volume testing is the identification of the relevant capacity drivers. A capacity driver is something that directly impacts on the total processing capacity. For a messaging system, a capacity driver may well be the size of messages being processed.

Most messaging systems do not interrogate the body of the messages they are processing, so varying the content of the test messages may not impact the total message throughput capacity, but significantly changing the size of the messages may have a significant effect. However, the message header may include indicators that have a very significant impact on processing efficiency. For example, a flag saying that the message need not be delivered under certain circumstances is much easier to deal with than a message with a flag saying that it must be held for delivery for as long as necessary to deliver the message, and the message must not be lost. In the former example, the message may be held in memory, but in the later example, the message must be physically written to disk multiple times.

Before conducting a meaningful test on a messaging system, the following must be known:

- the capacity drivers for the messages.
- the peak rate of messages that need to be processed, grouped by capacity driver.
- the duration of peak message activity that needs to be replicated.
- the required message processing rates.


A test can then be designed to measure the throughput of a messaging system as well as the internal messaging system metrics while that throughput rate is being processed. Such measures would typically include CPU utilization and disk activity. It is important that a test be run, at peak load, for a period of time equal to or greater than the expected production duration of peak load.