Archive for the ‘Design’ Category
How to get it
Source code: hg.tath.am/reliability-patterns
What it solves
In our homes, we use circuit breakers to quickly isolate an electrical circuit when there’s a known fault.
Michael T. Nygard introduces this concept as a programming pattern in his book Release It!: Design and Deploy Production-Ready Software.
The essence of the pattern is that when one of your dependencies stops responding, you need to stop calling it for a little while. A file system that has exhausted its operation queue is not going to recover while you keep hammering it with new requests. A remote web service is not going to come back any faster if you keep opening new TCP connections and mindlessly waiting for the 30 second timeout. Worse yet, if your application normally expects that web service to respond in 100ms, suddenly starting to block for 30s is likely to deteriorate the performance of your own application and trigger a cascading failure.
Electrical circuit breakers ‘trip’ when a high current condition occurs. They then need to be manually ‘reset’ to close the circuit again.
Our programmatic circuit breaker will trip after an operation has more consecutive failures than a predetermined threshold. While the circuit breaker is open, operations will fail immediately without even attempting to be executed. After a reset timeout has elapsed, the circuit breaker will enter a half-open state. In this state, only the next call will be allowed to execute. If it fails, the circuit breaker will go straight back to the open state and the reset timer will be restarted. Once the service has recovered, calls will start flowing normally again.
Writing all this extra management code would be painful. This library manages it for you instead.
How to use it
Taking advantage of the library is as simple as wrapping your outgoing service call with
// Note: you'll need to keep this instance around var breaker = new CircuitBreaker(); var client = new SmtpClient(); var message = new MailMessage(); breaker.Execute(() => client.SendEmail(message));
The only caveat is that you need to manage the lifetime of the circuit breaker(s). You should create one instance for each distinct dependency, then keep this instance around for the life of your application. Do not create different instances for different operations that occur on the same system.
(Managing multiple circuit breakers via a container can be a bit tricky. I’ve published a separate example for how to do it with Autofac.)
It’s generally safe to add this pattern to existing code because it will only throw an exception in a scenario where your existing code would anyway.
You can also take advantage of built-in retry logic:
breaker.ExecuteWithRetries(() => client.SendEmail(message), 10, TimeSpan.FromSeconds(20));
Why is the package named ReliabilityPatterns instead of CircuitBreaker?
Because I hope to add more useful patterns in the future.
This blog post in picture form
As this is a suggestion which comes up quite regularly, I felt it valuable to document some of the research I have collected about the readability of light text on dark backgrounds.
The science of readability is by no means new, and some of the best research comes from advertising works in the early 80s. This information is still relevant today.
First up is this quote from a paper titled “Improving the legibility of visual display units through contrast reversal”. In present time we think of contrast reversal meaning black-on-white, but remember this paper is from 1980 when VDUs (monitors) where green-on-black. This paper formed part of the research that drove the push for this to change to the screen formats we use today.
However, most studies have shown that dark characters on a light background are superior to light characters on a dark background (when the refresh rate is fairly high). For example, Bauer and Cavonius (1980) found that participants were 26% more accurate in reading text when they read it with dark characters on a light background.
Reference: Bauer, D., & Cavonius, C., R. (1980). Improving the legibility of visual display units through contrast reversal. In E. Grandjean, E. Vigliani (Eds.), Ergonomic Aspects of Visual Display Terminals (pp. 137-142). London: Taylor & Francis
Ok, 26% improvement – but why?
People with astigmatism (aproximately 50% of the population) find it harder to read white text on black than black text on white. Part of this has to do with light levels: with a bright display (white background) the iris closes a bit more, decreasing the effect of the "deformed" lens; with a dark display (black background) the iris opens to receive more light and the deformation of the lens creates a much fuzzier focus at the eye.
Jason Harrison – Post Doctoral Fellow, Imager Lab Manager – Sensory Perception and Interaction Research Group, University of British Columbia
The "fuzzing” effect that Jason refers to is known as halation.
It might feel strange pushing your primary design goals based on the vision impaired, but when 50% of the population of have this “impairment” it’s actually closer to being the norm than an impairment.
The web is rife with research on the topic, but I think these two quotes provide a succinct justification for why light text on a dark background is a bad idea.