Mod qos

Key takeaways

Mod qos belongs to computer science; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Mod qos to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Mod qos from memory before moving on to harder problems.

Reference excerpt

mod_qos is a quality of service (QoS) module for the Apache HTTP server implementing control mechanisms that can provide different priority to different requests.

Description A web server can only serve a limited number of concurrent requests. QoS is used to ensure that important resources stay available under high server load. mod_qos is used to reject requests to unimportant resources while granting access to more important applications. It is also possible to disable access restrictions, for example, for requests to very important resources or for very important users. Control mechanisms are available at the following levels:

Request level control: mod_qos controls the number of concurrent requests to a name space (URL). It is used to define different priorities to different pages or applications within a web server. Connection level control: mod_qos controls the number of TCP connections to the web server. This helps limit the connections coming from a single client or from unknown networks, in order to reduce the maximum number of concurrent connections to a virtual server or to implement dynamic HTTP keep-alive settings. Bandwidth level control: throttles requests/responses to certain URL on the web server. Generic request line and header filter dropping suspicious request URLs or HTTP headers. The module can be useful when used in a reverse proxy in order to divide up resources to different webserver.

Use Cases

Slow Application

The first use case shows how mod_qos can avoid service outage of a web server due to slow responses of a single application. In case an application (here /ccc) is very slow, requests wait until a timeout occurs. Due to many waiting requests, the web server runs out of free TCP connections and is not able to process other requests to application /aaa or /bbb. mod_qos limits the concurrent requests to an application in order to assure the availability of other resources.

HTTP keep-alive The keep-alive extension to HTTP 1.1 allows persistent TCP connections for multiple request/responses. This accelerates access to the web server due to less and optimised network traffic. The disadvantage of these persistent connections is that server resources are blocked even though no data is exchanged between client and server. mod_qos allows a server to support keep-alive as long as sufficient connections are free, stopping the keep-alive support when a defined connection threshold is reached.

Client opens many concurrent connections A single client may open many simultaneous TCP connections in order to download different content from the web server. While the client gets many connections other users may not be able to access the server since no free connections remain for them. mod_qos can limit the number of concurrent connections for a single IP source address.

Many requests to a single URL If you have to limit the number of requests to a URL, mod_qos can help with that too. mod_qos limits the maximum number of requests per second to this URL. The module may also control bandwidth. Simply specify the maximum allowed bandwidth and moq_qos starts throttling when it becomes necessary.

Mitigating low-bandwidth denial of service attacks mod_qos may help to protect an Apache web server against low-bandwidth DoS attacks by enforcing a minimum upload/download throughput a client must generate.

History The initial release of mod_qos was created in May 2007 and published on SourceForge.net as an open source software project. It was able to limit the number of concurrent HTTP requests for specified resources (path portion of request URLs) on the web server. More features were added and some of them were useful to protect Apache servers against DoS attacks. In 2012, mod_qos was included to the Ubuntu Linux distribution. Major releases:

May 2007, Version 1: Limits concurrent requests on a per URL path basis. July 2007, Version 2.2: Introduction of support utilities. August 2007, Version 3: Introduces connection level controls as well as a status viewer. September 2007, Version 4: Request/response throttling and generic request filtering. December 2007, Version 5: Limiting by user defined events. March 2008, Version 6: Per client (IP) control mechanisms. May 2008, Version 7: Enforces minimum upload/download throughput a client must generate. September 2009, Version 9: Anomaly detection using client characteristic measurement. February 2012, Version 10: Adds geolocation features (ISO 3166 country codes). May 2014, Version 11: Highly improves response throttling (bytes/sec limitation). July 2015, Version 11.15: Serialization not only on a per server but also on a per client (IP) level. May 2016, Version 11.28: Supports differentiated services by controlling the DSCP IP field.

References

External links Official website

Worked examples

Example 1 — a first encounter with Mod qos

Start with the simplest possible case. Write down what Mod qos claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In computer science, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Mod qos before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Mod qos ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Mod qos

In research: Mod qos appears in computer science research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Mod qos in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Mod qos is common in secondary-school and first-year university syllabi. It links to neighbouring topics Apache httpd modules, Free web server software, Network performance, so understanding it makes those chapters shorter.
In everyday life: Look for Mod qos outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

How to study Mod qos in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Mod qos means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Mod qos out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Mod qos in simple terms?

mod_qos is a quality of service (QoS) module for the Apache HTTP server implementing control mechanisms that can provide different priority to different requests. Description A web server can only serve a limited number of concurrent requests.

Why does Mod qos matter?

Because it connects several computer science ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Mod qos?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Mod qos.