Zend Framework 3 singletons

I'm creating a new application in Zend Framework 3 and I have a question about a design pattern

Without entering in much details this application will have several Services, as in, will be connecting to external APIs and even in multiple databases, the workflow is also very complex, a single will action can have multiple flows depending on several external information (which user logged in, configs, etc.).

I know about dependency injections and Zend Framework 3 Service Manager, however I am worried about instantiating several services when the flow will actually use only a few of them in certain cases, also we will have services depending on other services as well, for this, I was thinking about using singletons.

Is singleton really a solution here? I was looking a way to user singletons in Zend Framework 3 and haven't figured out a easy way since I can't find a way to user the Service Manager inside a service, as I can't retrieve the instance of the Service Manager outside of the Factory system.

What is an easy way to implement singletons in Zend Framework 3?

Solution

Why use singletons?

You don't need to worry about too many services in your service manager since they are started only when you get them from the service manager.

Also don't use the service manager inside another class except a factory. In ZF3 it's removed from the controllers for a reason. One of them is testability. If all services are inject with a factory, you can easily write tests. Also if you read your code next year, you can easily see what dependencies are needed inside a class.

If you find there are too many services being injected inside a class which are not always needed you can:

Use the ProxyManager. This lazy loads a service but doesn't start it until a method is called.
Split the service: Move some parts from a service into a new service. e.g. You don't need to place everything in an UserService. You can also have an UserRegisterService, UserEmailService, UserAuthService and UserNotificationsService.

In stead of ZF3, you can also think about zend-expressive. Without getting into too much detail, it is a lightweight middleware framework. You can use middleware to detect what is needed for a request and route to the required action to process the request. Something like this can probably also done in ZF3 but maybe someone else can explain how to do it there.