What is Web Server? | How Does a Web Server Work? - Server Watch - Server Watch

What is Web Server? | How Does a Web Server Work? - Server Watch - Server Watch

What is Web Server? | How Does a Web Server Work? - Server Watch - Server Watch

Posted: 15 Mar 2021 02:54 PM PDT

Why is it important to understand the answer to the question, how does a web server work? The success of a website doesn't just depend on its content and functionality but also the efficiency of the web server used to power it. This requires an understanding of a web server's capabilities and limitations. When discussing how a Web server works, it is not enough to simply outline a diagram of how low-level network packets go in and out of a Web server.

Years ago, when Web servers were first prototyped, they served simple HTML documents and images. Today, as we shall go into later in this tutorial, they are frequently used for much more.

The first step is to view the Web server as a black box and ask the questions: How does it work; what can it achieve? It's a safe assumption that most Internet users believe a Web site's success or failure is due to its content and functionality rather than the server used to power it. However, the choice of the correct server, and understanding its capabilities and limitations is an important step on the road to success.

What Does a Web Server Do?

So what does a web server do? It's software or hardware (or both together) that stores and delivers content to a web browser at a basic level. The servers communicate with browsers using Hypertext Transfer Protocol (HTTP). Web servers can also support SMTP (Simple Mail Transfer Protocol) and FTP (File Transfer Protocol). 

Web servers are also used for hosting websites and data for web applications. They can host single websites and multiple websites using virtualization.

How Does a Web Server Work Internally?

When the web server receives a request for content for a web page, such as


and maps that Uniform Resource Locator (URL)to a local file on the host server.

In this case, the file


is somewhere on the host file system. The server then loads this file from disk and serves it out across the network to the user's Web browser. This entire exchange is mediated by the browser and server talking to each other using HTTP. This workflow is shown in the figure below.

That's all there is to it. But if it's that simple, then why such an in-depth tutorial?

Because this simple arrangement, which allows the serving of static content such as HyperText Markup Language (HTML) and image files to a Web browser was the initial concept behind what we now call the World Wide Web. The beauty of its simplicity is that it has led to much more complex information exchanges being possible between browsers and Web servers.

Perhaps the most important expansion on this was the concept of dynamic content (i.e., Web pages created in response to a user's input, whether directly or indirectly). The oldest and most used standard for doing this is Common Gateway Interface (CGI). This is a pretty meaningless name, but it basically defines how a Web server should run programs locally and transmit their output through the Web server to the user's Web browser that is requesting the dynamic content.

For all intents and purposes the user's Web browser never really has to know that the content is dynamic because CGI is basically a Web server extension protocol. The figure below shows what happens when a browser requests a page dynamically generated from a CGI program.

The second important advance, and the one that makes e-commerce possible, was the introduction of HyperText Transmission Protocol, Secure (HTTPS). This protocol allows secure communication to go on between the browser and Web server.

In a nutshell, this means that it is safe for user and server to transmit sensitive data to each another across what might be considered an insecure network. What happens when the data arrives at either end is another matter, however, and should not be ignored. We will discuss this a bit later.

The simplicity of the above arrangements is deceptive, and underestimating its complexities often leads to bad decisions being made about the design of a Web-hosting infrastructure. It is too easy to focus on the design of the Web pages themselves and the technologies used to create dynamic content, such as Java, Javascript, Perl, C/C++, and ASP, and to subsequently miss the fact that each of these technologies can be aided, or hindered, by the platform on which they are to be run — the Web server itself.

In other words, explaining how a Web server works involve discussing more than just how a Web server serves documents. We will go over the following topics in our quest to finding out what and how a modern Web server goes about doing its activities.

We will start by explaining the low-level details underlying what a Web server must do. Then, we will discuss the issues surrounding the use of a Web server and how it fits within the scope of other infrastructural elements of the Internet. We will then close with a discussion of the relationship that applications have with Web servers.

Long covid: How to define it and how to manage it - The BMJ

Posted: 07 Sep 2020 12:00 AM PDT

  1. Nikki Nabavi, editorial scholar
  1. The BMJ

On 3 September The BMJ hosted an online webinar on the diagnosis, management, and prognosis of "long covid." An expert panel discussed its symptoms, course, and character and suggested strategies for managing it. Nikki Nabavi reports


Panel member Nisreen Alwan (box 1) began the discussion by defining long covid as "not recovering [for] several weeks or months following the start of symptoms that were suggestive of covid, whether you were tested or not."

Box 1

The webinar's expert panel

  • Nisreen Alwan—associate professor in public health, University of Southampton, and honorary consultant in public health, University Hospital Southampton NHS Foundation Trust. Has personal experience of long covid

  • Paul Garner—professor, Centre for Evidence Synthesis for Global Health, Liverpool School of Tropical Medicine, and coordinating editor, Cochrane Infectious Disease Group. He has personal experience of long covid

  • Fiona Godlee—editor in chief, The BMJ (chair)

  • Trisha Greenhalgh—professor of primary healthcare, Nuffield Department of Primary Care Health Sciences, University of Oxford, and practising GP

  • Nick Peters—professor of cardiology, Imperial College London, and consultant cardiologist

  • Valentina Puntmann—senior clinical investigator in cardiovascular imaging, University Hospital, Frankfurt

  • Tim Spector—professor of genetic epidemiology, King's College London, and leader of the Covid Symptoms Study


"Profound fatigue" was a common symptom in most people with long covid, she said, but added that a wide range of other symptoms included cough, breathlessness, muscle and body aches, and chest heaviness or pressure, but also skin rashes, palpitations, fever, headache, diarrhoea, and pins and needles. "A very common feature is the relapsing, remitting nature of the illness, where you feel as though you've recovered, then it hits you back," she said.

Nick Peters added to this definition by highlighting a "distinction between very sick people who have recovered to an extent and [and have been] left with some impact of their severe sickness, versus those who had a relatively mild sickness from the start, in whom it is ongoing."

Alwan described the fluctuations of her own illness: "It's a constant cycle of disappointment, not just to you but people around you, who really want you to recover."

Paul Garner, who also has long covid, described it as a "very bizarre disease" that had left him feeling "repeatedly battered the first two months" and then experiencing lesser episodes in the subsequent four months with continual fatigue. "Navigating help is really difficult," he said.

Tim Spector said that his team at the Covid Symptom Study had identified six clusters of symptoms for covid-19,1 a couple of which were associated with longer term symptoms, indicating a possible way of predicting early on what might occur. "If you've got a persistent cough, hoarse voice, headache, diarrhoea, skipping meals, and shortness of breath in the first week, you are two to three times more likely to get longer term symptoms," he said.

He said that patterns in the team's data suggested that long covid was about twice as common in women as in men and that the average age of someone presenting with it was about four years older than people who had what might be termed as "short covid."

But Spector added, "We do seem to be getting different symptom clusters in different ages, so it could be that there is a different type in younger people compared with the over 65s. As we get more data we should be able to break it into these groups and work out what is going on … which could be very interesting and help us to get early interventions for those at-risk groups."

Peters said that the data showed fatigue was the most common trait in people who had symptoms beyond three weeks. He also said that around 80% of people who had symptoms lasting more than three weeks reported "having had clear good days and bad days."

Cardiovascular effects

Valentina Puntmann, who coauthored a research paper in JAMA Cardiology examining the cardiovascular effects in unselected patients with covid-19,2 discussed one of the study's findings: the relatively high frequency of postviral or inflammatory myocarditis in people who have had covid-19.

She said, "We were all focusing on the lungs and respiratory symptoms to begin with, but the virus attacks the same receptor that is in the heart too, not just the lungs. Many patients who have had heart failure in the past who have now been very unwell with covid symptoms have sustained arrhythmias, meaning their hearts were beating very fast whilst they were ill, and they then went on to develop heart failure.

"If you have pre-existing cardiac conditions, and cardiomyocytes express the ACE2 receptor that the covid virus likes, you are more likely to develop the cardiac symptoms. Taking ACE inhibitors can be used as a preventive measure."

Puntmann said that the illness was "obviously a huge stress on the heart, particularly for those with pre-existing conditions, which can lead to hypoxia and ischaemia of the heart. The clinical course of covid-19 is also much worse in individuals with a troponin leak."


Alwan explained that as someone with long covid "you learn your patterns, learn what brings on utter exhaustion or the other symptoms, and try to avoid those things."

She added, "I'm fairly driven, and I thought I could beat this virus. A friend told me to stop dominating the virus and start accommodating it. Once you start accepting that, it becomes a bit easier. You have to drop your baseline by 90%; you are a different person."

Trisha Greenhalgh said that although many patients referred to long covid clinics were slowly getting better, some were not and urgently needed referring from primary care.

"It should be a story of gradual improvement, and if it's not, consider referring," she advised. She added, "The interface between the GP and the specialist investigations and monitoring is something we have to work on."

Greenhalgh and Matthew Knight, a respiratory physician who ran a clinic for people recovering from long covid, coauthored a recent BMJ article on the management of post-acute covid-19 in primary care.3 A major purpose of the paper, she said, was to reassure GPs that they have the clinical skills to manage these people, such as "listening to the patient, documenting what the symptoms are, how they change and how they fluctuate, and being alert to symptoms that might suggest they need referring."

More broadly, Alwan said that long covid cases should be incorporated into covid-19 statistics. "We are missing a huge opportunity to quantify and measure long covid in the same way that we're doing with positive test results and deaths," she said.

Garner called for research specialists to work together to create better guidance on how to improve investigation in patients with enduring symptoms. "We need rapid interdisciplinary communication," he said.

This article is made freely available for use in accordance with BMJ's website terms and conditions for the duration of the covid-19 pandemic or until otherwise determined by BMJ. You may use, download and print the article for any lawful, non-commercial purpose (including text and data mining) provided that all copyright notices and trade marks are retained.



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