Wednesday, 13 April 2016

Dependency Injection for Common Global Dependencies

The use of singletons can often be replaced by simply adjusting scoping of objects. The vast majority of dependencies fit this pattern, with a few exceptions such as DateTime instances, or logging.

Sometimes you just need these dependencies everywhere. You can find yourself passing these dependencies down into the deep depths of your code base. Such changes are often dangerous, time consuming and undesirable.

DateTime

For a while the use of some date/time abstraction was my default approach to handling dates and times. This fake clock or calendar instance when combined with DI at the lowest level does actually work. However if we stop and think about the abstraction it is clearly unnecessary in many cases. Unless your domain is dealing with date and times explicitly, you don't really need an abstraction. In other words, other than the system where the code is running when or why would you provide a different implementation?

The approach taken as part of the example within the Dependency Elimination Principle is my current solution to date/times and DI. This is still dependency injection, except the value is provided, not the method of obtaining the value. This is essentially one of the benefits of functional programming.

Logging

All systems need some form of logging. Commonly either the standard library or a highly rated logging framework is used. The general advice has been to use the logging component directly, rather than providing your own abstraction. Most frameworks already provide interfaces or base classes that make this easy to achieve.

Even so logging suffers the same issue as date/times when it comes to DI. You often need the logging component everywhere, whether it is simply to pass on to other services.

Logging and DI generally do not go well together. Instead simple use the logging instance directly. A good logging framework would be fast, so any automated tests will not notice the difference. Likewise whether logging is configured or not, this should not cause tests to fail. In summary, not every object has to be provided via dependency injection. Loggers being a prime example.

Due to this directly using a logging instance is the preferred approach. Do not rely on DI. However semantic or structured logging does change this suggestion as the use of a domain explicit interface can provide benefits. Semantic logging will be expanded in a future post.

Others

Date/Time and Logging are the two most common global dependencies. The majority of all other dependencies can and probably should be satisfied by traditional DI where possible. As always each dependency should be validated prior to introduction. It may be possible to either eliminate or replace the component in question.

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