Skip to main content

Caching

The naive approach to implement caching is to just store everything in an in memory collection such as a hashtable. After all it works on my machine.

I've worked on systems in the past that used this technique but:

  • Bring in two processes and this falls apart
  • No Time to Live (TTL)
  • No cache eviction, memory will grow until it crashes the process

This sort of caching meant the system needed daily restarts due to each worker process starting to eat up more and more RAM. At the time I didn't realise this was the problem as to why daily restarts were required. These were automated so the team just sort of forgot about the problem after a while. This never felt right.

"Improper use of caching is the major cause of memory leaks, which turn into horrors like daily server restarts" - @mtnygard in Release It!.

Scale this system up, and daily becomes twice daily and so on. In a global market where software shouldn't be constrained by time zones or "working hours" this is wrong.

Solutions

There are numerous easy ways to solve these problems depending on the application in question.

Don't Roll your Own, Try a Third Party

Easy. Just use an off the shelf solution that solves the problems above plus includes a whole host of additional features.

Use your Standard Library

For example .NET includes caching functionality within the System.Runtime.Caching namespace. While there are limitations to this, it will work for some scenarios and solves some of the problems above.

Soft References

I've overlooked soft references in the past but for caching they can be incredibly useful. Use soft references for anything that isn't important or that can be recalculated. An example would be content displayed within an MVC view using the web servers session. Here if each item stored is a weak reference we introduce some benefits.

  • Stops your web server running of of memory - references will be reclaimed if memory starts to become a bottleneck.
  • Greater scalability with the same amount of memory - great for a sudden spike in traffic.

A web server's session being full of references that won't expire for a set period is a common cause of downtime. If soft references are used all we need to do is perform a simple conditional check prior to retrieval from the session. Different languages have similar features, e.g. Weak References in .NET.

Pre-Computation

Caching isn't always the best solution, in some cases pre-computation can be much easier and offer better performance. In other words at least some users will experience a slow response until the cache is warm, other techniques can be used to avoid this completely. I will expand on pre-computation in a future post.

Reference

More information can be found in the excellent book Release It!

Comments

Popular posts from this blog

Three Steps to Code Quality via TDD

Common complaints and problems that I've both encountered and hear other developers raise when it comes to the practice of Test Driven Development are: Impossible to refactor without all the tests breakingMinor changes require hours of changes to test codeTest setup is huge, slow to write and difficult to understandThe use of test doubles (mocks, stubs and fakes is confusing)Over the next three posts I will demonstrate three easy steps that can resolve the problems above. In turn this will allow developers to gain one of the benefits that TDD promises - the ability to refactor your code mercifully in order to improve code quality.StepsStop Making Everything PublicLimit the Amount of Dependencies you Use A Unit is Not Always a Method or ClassCode quality is a tricky subject and highly subjective, however if you follow the three guidelines above you should have the ability to radically change implementation details and therefore improve code quality when needed.

DRY vs DAMP in Tests

In the previous post I mentioned that duplication in tests is not always bad. Sometimes duplication becomes a problem. Tests can become large or virtually identically excluding a few lines. Changes to these tests can take a while and increase the maintenance overhead. At this point, DRY violations need to be resolved.SolutionsTest HelpersA common solution is to extract common functionality into setup methods or other helper utilities. While this will remove and reduce duplication this can make tests a bit harder to read as the test is now split amongst unrelated components. There is a limit to how useful such extractions can help as each test may need to do something slightly differently.DAMP - Descriptive and Meaningful PhrasesDescriptive and Meaningful Phrases is the alter ego of DRY. DAMP tests often use the builder pattern to construct the System Under Test. This allows calls to be chained in a fluent API style, similar to the Page Object Pattern. Internally the implementation wil…

Coding In the Real World

As a student when confronted with a problem, I would end up coding it and thinking - how do the professionals do this?For some reason I had the impression that once I entered the industry I would find enlightenment. Discovering the one true way to write high quality, professional code.It turns out that code in industry is not too far removed from the code I was writing back when I knew very little.Code in the real world can be:messy or cleanhard or easy to understandsimple or complexeasy or hard to changeor any combination of the aboveVery rarely will you be confronted with a problem that is difficult. Most challenges typically are formed around individuals and processes, rather than day to day coding. Years later I finally have the answer. Code in the real world is not that much different to code we were all writing when we first started out.If I could offer myself some advice back in those early days it would be to follow KISS, YAGNI and DRY religiously. The rest will fall into plac…

Feature Toggles

I'm a fan of regular releasing. My background and experience leads me to release as regularly as possible. There are numerous benefits to regular releases; limited risk, slicker release processes and the ability to change as requirements evolve.The problem with this concept is how can you release when features are not functionally complete?SolutionIf there is still work in progress, one solution to allow frequent releases is to use feature toggles. Feature toggles are simple conditional statements that are either enabled or disabled based on some condition.This simple example shows a feature toggle for an "Edit User" feature. If the boolean condition is false, then we only show the "New User" feature and the "Admin" feature. This boolean value will be provided by various means, usually a configuration file. This means at certain points we can change this value in order to demonstrate the "Edit User" functionality. Our demo environment could …

Reused Abstraction Principle

This is the second part of my series on abstractions.Part 1 - AbstractionsPart 3 - Dependency Elimination PrincipleThe Reused Abstraction Principle is a simple in concept in practice, but oddly rarely followed in typical enterprise development. I myself have been incredibly guilty of this in the past.Most code bases have a 1:1 mapping of interfaces to implementations. Usually this is the sign of TDD or automated testing being applied badly. The majority of these interfaces are wrong. 1:1 mappings between interfaces and implementations is a code smell.Such situations are usually the result of extracting an interface from an implementation, rather than having the client drive behaviour.These interfaces are also often bad abstractions, known as "leaky abstractions". As I've discussed previously, these abstractions tend to offer nothing more than simple indirection.ExampleApply the "rule of three". If there is only ever one implementation, then you don't need …