A software framework is code that calls your (application) code. That's how we distinguish a framework from a library. Libraries have aspects of frameworks so there is a gray area.

My friend Christian Theune puts it like this: a framework is a text where you fill in the blanks. The framework defines the grammar, you bring some of the words. The words are the code you bring into it.

If you as a developer use a framework, you need to tell it about your code. You need to tell the framework what to call, when. Let's call this configuring the framework.

There are many ways to configure a framework. Each approach has its own trade-offs. I will describe some of these framework configuration patterns here, with brief examples and mention of some of the trade-offs. Many frameworks use more than a single pattern. I don't claim this list is exhaustive -- there are more patterns.

The patterns I describe are generally language agnostic, though some depend on specific language features. Some of these patterns make more sense in object oriented languages. Some are easier to accomplish in one language compared to another. Some languages have rich run-time introspection abilities, and that make certain patterns a lot easier to implement. A language with a powerful macro facility will make other patterns easier to implement.

Where I give example code, I will use Python. I give some abstract code examples, and try to supply a few real-world examples as well. The examples show the framework from the perspective of the application developer.

Pattern: Callback function The framework lets you pass in a callback function to configure its behavior. Fictional example This is a Form class where you can pass in a function that implements what should happen when you save the form. from framework import Form def my_save ( data ): ... application code to save the data somewhere ... my_form = Form ( save = my_save ) Real-world example: Python map A real-world example: map is a (nano)framework that takes a (pure) function: >>> list ( map ( lambda x : x * x , [ 1 , 2 , 3 ])) [ 1 , 4 , 9 ] You can go very far with this approach. Functional languages do. If you glance at React in a certain way, it's configured with a whole bunch of callback functions called React components, along with more callback functions called event handlers. Trade-offs I am a big fan of this approach as the trade-offs are favorable in many circumstances. In object-oriented languages this pattern is sometimes ignored because people feel they need something more complicated like pass in some fancy object or do inheritance, but I think callback functions should in fact be your first consideration. Functions are simple to understand and implement. The contract is about as simple as it can be for code. Anything you may need to implement your function is passed in as arguments by the framework, which limits how much knowledge you need to use the framework. Configuration of a callback function can be very dynamic in run-time -- you can dynamically assemble or create functions and pass them into the framework, based on some configuration stored in a database, for instance. Configuration with callback functions doesn't really stand out, which can be a disadvantage -- it's easier to see someone subclasses a base class or implements an interface, and language-integrated methods of configuration can stand out even more. Sometimes you want to configure multiple related functions at once, in which case an object that implements an interface can make more sense -- I describe that pattern below. It helps if your language has support for function closures. And of course your language needs to actually support first class functions that you can pass around -- Java for a long time did not.

Pattern: Subclassing The framework provides a base-class which you as the application developer can subclass. You implement one or more methods that the framework will call. Fictional example from framework import FormBase class MyForm ( FormBase ): def save ( self , data ): ... application code save the data somewhere ... Real-world example: Django REST Framework Many frameworks offer base classes - Django offers them, and Django REST Framework even more. Here's an example from Django REST Framework: class AccountViewSet ( viewsets . ModelViewSet ): """ A simple ViewSet for viewing and editing accounts. """ queryset = Account . objects . all () serializer_class = AccountSerializer permission_classes = [ IsAccountAdminOrReadOnly ] A ModelViewSet does a lot: it implements a lot of URLs and request methods to interact with them. It integrates with Django's ORM so that you get a REST API that you can use to create and update database objects. Subclassing questions When you subclass a class, this is what you might need to know: What base classes are there?

What methods can you override?

When you override a method, can you call other methods on self ( this ) or not? Is there is a particular order in which you are allowed to call these methods?

( ) or not? Is there is a particular order in which you are allowed to call these methods? Does the base class provide an implementation of this method, or is it really empty?

If the base class provides an implementation already, you need to know whether it's intended to be supplemented, or overridden, or both.

If it's intended to be supplemented, you need to make sure to call this method on the superclass in your implementation.

If you can override a method entirely, you may need to know what methods to use to to play a part in the framework -- perhaps other methods that can be overridden.

Does the base class inherit from other classes that also let you override methods? when you implement a method, can it interact with other methods on these other classes? Trade-offs Many object-oriented languages support inheritance as a language feature. You can make the subclasser implement multiple related methods. It seems obvious to use inheritance as a way to let applications use and configure the framework. It's not surprising then that this design is very common for frameworks. But I try to avoid it in my own frameworks, and I often am frustrated when a framework forces me to subclass. The reason for this is that you as the application developer have to start worrying about many of the questions above. If you're lucky they are answered by documentation, though it can still take a bit of effort to understand it. But all too often you have to guess or read the code yourself. And then even with a well designed base class with plausible overridable methods, it can still be surprisingly hard for you to do what you actually need because the contract of the base class is just not right for your use case. Languages like Java and TypeScript offer the framework implementer a way to give you guidance ( private/protected/public , final ). The framework designer can put hard limits on which methods you are allowed to override. This takes away some of these concerns, as with sufficient effort on the part of the framework designer, the language tooling can enforce the contract. Even so such an API can be complex for you to understand and difficult for the framework designer to maintain. Many languages, such as Python, Ruby and JavaScript, don't have the tools to offer such guidance. You can subclass any base class. You can override any method. The only guidance is documentation. You may feel a bit lost as a result. A framework tends to evolve over time to let you override more methods in more classes, and thus grows in complexity. This complexity doesn't grow just linearly as methods get added, as you have to worry about their interaction as well. A framework that has to deal with a variety of subclasses that override a wide range of methods can expect less from them. Too much flexibility can make it harder for the framework to offer useful features. Base classes also don't lend themselves very well to run-time dynamism - some languages (like Python) do let you generate a subclass dynamically with custom methods, but that kind of code is difficult to understand. I think the disadvantages of subclassing outweigh the advantages for a framework's external API. I still sometimes use base classes internally in a library or framework -- base classes are a lightweight way to do reuse there. In this context many of the disadvantages go away: you are in control of the base class contract yourself and you presumably understand it. I also sometimes use an otherwise empty base class to define an interface, but that's really another pattern which I discuss next.

Pattern: interfaces The framework provides an interface that you as the application developer can implement. You implement one or more methods that the framework calls. Fictional example from framework import Form , IFormBackend class MyFormBackend ( IFormBackend ): def load ( self ): ... application code to load the data here ... def save ( self , data ): ... application code save the data somewhere ... my_form = Form ( MyFormBackend ()) Real-world example: Python iterable/iterator The iterable/iterator protocol in Python is an example of an interface. If you implement it, the framework (in this case the Python language) will be able to do all sorts of things with it -- print out its contents, turn it into a list, reverse it, etc. class RandomIterable : def __iter__ ( self ): return self def next ( self ): if random . choice ([ "go" , "stop" ]) == "stop" : raise StopIteration return 1 Faking interfaces Many typed languages offer native support for interfaces. But what if your language doesn't do that? In a dynamically typed language you don't really need to do anything: any object can implement any interface. It's just you don't really get a lot of guidance from the language. What if you want a bit more? In Python you can use the standard library abc module, or zope.interface . You can also use the typing module and implement base classes and in Python 3.8, PEP-544 protocols. But let's say you don't have all of that or don't want to bother yet as you're just prototyping. You can use a simple Python base class to describe an interface: class IFormBackend : def load ( self ): "Load the data from the backend. Should return a dict with the data." raise NotImplementedError () def save ( self , data ): "Save the data dict to the backend." raise NotImplementedError () It doesn't do anything, which is the point - it just describes the methods that the application developer should implement. You could supply one or two with a simple default implementation, but that's it. You may be tempted to implement framework behavior on it, but that brings you into base class land. Trade-offs The trade-offs are quite similar to those of callback functions. This is a useful pattern to use if you want to define related functionality in a single bundle. I go for interfaces if my framework offers a more extensive contract that an application needs to implement, especially if the application needs to maintain its own internal state. The use of interfaces can lead to clean composition-oriented designs, where you adapt one object into another. You can use run-time dynamism like with functions where you assemble an object that implements an interface dynamically. Many languages offer interfaces as a language feature, and any object-oriented language can fake them. Or have too many ways to do it, like Python.

Pattern: imperative registration API You register your code with the framework in a registry object. When you have a framework that dispatches on a wide range of inputs, and you need to plug in application specific code that handles it, you are going to need some type of registry. What gets registered can be a callback or an object that implements an interface -- it therefore builds on those patterns. The application developer needs to call a registration method explicitly. Frameworks can have specific ways to configure their registries that build on top of this basic pattern -- I will elaborate on that later. Fictional Example from framework import form_save_registry def save ( data ): ... application code to save the data somewhere ... # we configure what save function to use for the form named 'my_form' form_save_registry . register ( 'my_form' , save ) Real-world example: Falcon web framework A URL router such as in a web framework uses some type of registry. Here is an example from the Falcon web framework: class QuoteResource : def on_get ( self , req , resp ): ... user code ... api = falcon . API () api . add_route ( '/quote' , QuoteResource ()) In this example you can see two patterns go together: QuoteResource implements an (implicit) interface, and you register it with a particular route. Application code can register handlers for a variety of routes, and the framework then uses the registry to match a request's URL with a route, and then can all into user code to generate a response. Trade-offs I use this pattern a lot, as it's easy to implement and good enough for many use cases. It has a minor drawback: you can't easily see that configuration is taking place when you read code. Sometimes I expose a more sophisticated configuration API on top of it: a DSL or language integrated registration or declaration, which I discuss later. But this is foundational. Calling a method on a registry is the most simple and direct form to register things. It's easy to implement, typically based on a hash map, though you can also use other data structures, such as trees. The registration order can matter. What happens if you make the same registration twice? Perhaps the registry rejects the second registration. Perhaps it allows it, silently overriding the previous one. There is no general system to handle this, unlike patterns which I describe later. Registration can be done anywhere in the application which makes it possible to configure the framework dynamically. But this can also lead to complexity and the framework can offer fewer guarantees if its configuration can be updated at any moment. In a language that supports import-time side effects, you can do your registrations during import time. That makes the declarations stand out more. This is simple to implement, but it's also difficult to control and understand the order of imports. This makes it difficult for the application developer to do overrides. Doing a lot of work during import time in general can lead to hard to predict behavior.

Pattern: convention over configuration The framework configures itself automatically based on your use of conventions in application code. Configuration is typically driven by particular names, prefixes, and postfixes, but a framework can also inspect other aspects of the code, such as function signatures. This is typically layered over the procedural registration pattern. Ruby on Rails made this famous. Rails will automatically configure the database models, views and controllers by matching up names. Fictional example # the framework looks for things prefixed form_save_. It hooks this # up with `myform` which is defined elsewhere in a module named `forms` def form_save_myform ( data ): ... application code to save the data somewhere ... Real-world example: pytest pytest uses convention over configuration to find tests. It looks for modules and functions prefixed by test_ . pytest also goes further and inspects the arguments to functions to figure out more things. def test_ehlo ( smtp_connection ): response , msg = smtp_connection . ehlo () assert response == 250 assert 0 # for demo purposes In this example, pytest knows that test_ehlo is a test, because it is prefixed with test_ . It also knows that the argument smtp_connection is a fixture and looks for one in the same module (or in its package). Django uses convention over configuration in places, for instance when it looks for the variable urlpatterns in a specially named module to figure out what URL routes an application provides. Trade-offs Convention over configuration can be great. It allows the user to type code and have it work without any ceremony. It can enforce useful norms that makes code easier to read -- it makes sense to prefix tests with test_ anyway, as that allows the human reader to recognize them. I like convention over configuration in moderation, for some use cases. For more complex use cases I prefer other patterns that allow registration with minimal ceremony by using features integrated into the language, such as annotation or decorator syntax. The more conventions a framework has, the more disadvantages show up. You have to learn the rules, their interactions, and remember them. You may sometimes accidentally invoke them even though you don't want to, just by using the wrong name. You may want to structure your application's code in a way that would be very useful, but doesn't really work with the conventions. And what if you wanted your registrations to be dynamic, based on database state, for instance? Convention over configuration is a hindrance here, not a help. The developer may need to fall back to a different, imperative registration API, and this may be ill-defined and difficult to use. It's harder for the framework to implement some patterns -- what if registrations need to be parameterized, for instance? That's easy with functions and objects, but here the framework may need more special naming conventions to let you influence that. That may lead the framework designer to use classes over functions, as in many languages these can have attributes with particular names. Static type checks are of little use with convention over configuration -- I don't know of a type system that can enforce you implement various methods if you postfix your class with the name View , for instance. If you have a language with enough run-time introspection capabilities such as Ruby, Python or JavaScript, it's pretty easy to implement convention over configuration. It's a lot harder for languages that don't offer those features, but it may still be possible with sufficient compiler magic. But those same languages are often big on being explicit, and convention over configuration's magic doesn't really fit well with that.

Pattern: metaclass based registration When you subclass a framework-provided baseclass, it gets registered with the framework. Some languages such as Python and Ruby offer meta-classes. These let you do two things: change the behavior of classes in fundamental ways, and do side-effects when the class is imported. You can do things during class declaration that you normally only can do during instantiation. A framework can exploit these side-effects to do some registration. Fictional example from framework import FormBase class MyForm ( FormBase ): def save ( self , data ): ... application code save the data somewhere ... # the framework now knows about MyForm without further action from you Real-world example: Django When you declare a Django model by subclassing from its Model base class, Django automatically creates a new relational database table for it. from django.db import models class Person ( models . Model ): first_name = models . CharField ( max_length = 30 ) last_name = models . CharField ( max_length = 30 ) Trade-offs I rarely use these because they are so hard to reason about and because it's so easy to break assumptions for the person who subclasses them. Meta-classes are notoriously hard to implement. If they're not implemented correctly, they can also lead to surprising behavior that you may need to deal with when you use the framework. Basic assumptions that you may have about the way a class behaves can go out of the door. Import-time side-effects are difficult to control -- in what order does this happen? Python has a simpler way to do side-effects for class declarations using decorators. A base-class driven design for configuration may lead the framework designer towards meta-classes, further complicating the way the framework uses. Many languages don't support this pattern. It can be seen as a special case of language integrated registration, discussed next.

Pattern: language integrated registration You configure the application by using framework-provided annotations for code. Registrations happen immediately. Many programming languages offer some syntax aid for annotating functions, classes and more with metadata. Java has annotations. Rust has attributes. Python has decorators which can be used for this purpose as well. These annotations can be used as a way to drive configuration in a registry. Fictional example from framework import form_save_registry # we define and configure the function at the same time @form_save_registry.register ( 'my_form' ) def save ( data ): ... application code to save the data somewhere ... Real-world example: Flask web framework A real-world example is the @app.route decorator of the Flask web framework. from flask import Flask app = Flask ( __name__ ) @app.route ( '/' ) def hello_world (): return 'Hello, World!' Trade-offs I use this method of configuring software sometimes, but I'm also aware of its limitations -- I tend to go for language integrated declaration, discussed below, which looks identical to the end user but is more predictable. I'm warier than most about exposing this as an API to application developers, but am happy to use it inside a library or codebase, much like base classes. The ad-hoc nature of import-time side effects make me reach for more sophisticated patterns of configuration when I have to build a solid API. This pattern is lightweight to implement at least in Python -- it's not much harder than a registry. Your mileage will vary dependent on language. Unlike convention over configuration, configuration is explicit and stands out in code, but the amount of ceremony is kept to a minimum. The configuration information is co-located with the code that is being registered. Unlike convention over configuration, there is a natural way to parameterize registration with metadata. In languages like Python this is implemented as a possibly significant import-time side-effect, and may have surprising import order dependencies. In a language like Rust this is done by compiler macro magic -- I think the Rocket web framework is an example, but I'm still trying to understand how it works.

Pattern: DSL-based declaration You use a DSL (domain specific language) to configure the framework. This DSL offers some way to hook in custom code. The DSL can be an entirely custom language, but you can also leverage JSON, YAML or (shudder) XML. You can also combine these: I've helped implement a workflow engine that's configured with JSON, and expressions in it are a subset of Python expressions with a custom parser and interpreter. It is typically layered over some kind of imperative registration system. Fictional example { "form" : { "name" : "my_form" , "save" : "my_module.save" } } We have a custom language (in this case done with JSON) that lets us configure the way our system works. Here we plug in the save behavior for my_form by referring to the function save in some Python module my_module . Real-world example: Plone CMS framework Pyramid and Plone both are descendants of Zope, and you can use ZCML, a XML-derived configuration language with them both. Here is some ZCML from Plone: <configure xmlns= "http://namespaces.zope.org/zope" xmlns:browser= "http://namespaces.zope.org/browser" i18n_domain= "my.package" > <!-- override folder_contents --> <configure package= "plone.app.content.browser" > <browser:page for= "Products.CMFCore.interfaces._content.IFolderish" class= "my.package.browser.foldercontents.MyFolderContentsView" name= "folder_contents" template= "folder_contents.pt" layer= "my.package.interfaces.IMyPackageLayer" permission= "cmf.ListFolderContents" /> </configure> </configure> This demonstrates a feature offered by a well-designed DSL: a way to do a structured override of behavior in the framework. Trade-offs Custom DSLs are a very powerful tool if you actually need them, and you do need them at times. But they are also a lot more heavyweight than the other methods discussed, and that's a drawback. A custom DSL is thorough: a framework designer can build it with very clean boundaries, with a clear grammar and hard checks to see whether code conforms to this grammar. If you build your DSL on JSON or XML, you can implement such checks pretty easily using one of the various schema implementations. A custom DSL gives the potential for non-developers to configure application behavior. At some point in a DSL there is a need to interface with user code, but this may be abstracted away quite far. It lets non-developers reuse code implemented by developers. A DSL can be extended with a GUI to make it even easier for non-developers to configure it. Since code written in a DSL can be stored in a database, you can store complex configuration in a database. A DSL can offer certain security guarantees -- you can ensure that DSL code can only reach into a limited part of your application. A DSL can implement a declaration engine with sophisticated behavior -- for instance the general detection of configuration conflicts (you try to configure the same thing in conflicting ways in multiple places), and structured, safe overrides that are independent of code and import order. A DSL doesn't have to use such sophistication, but a framework designer that designs a DSL is naturally lead in such a direction. A drawback of DSL-based configuration is that it is quite distant from the code that it configures. That is fine for some use cases, but overkill for others. A DSL can cause mental overhead -- the applciation developer not only needs to read the application's code but also its configuration files in order to understand the behavior of an application. For many frameworks it can be much nicer to co-locate configuration with code. A DSL also provides little flexibility during run-time. While you could generate configuration code dynamically, that's a level of meta that's quite expensive (lots of generate/parse cycles) and it can lead to headaches for the developers trying to understand what's going on. DSL-based configuration is also quite heavy to implement compared to many other more lightweight configuration options described.

Pattern: imperative declaration You use a declaration engine like in a DSL, but you drive it from programming language code in an imperative way, like imperative registration. In fact, an imperative declaration system can be layered over a imperative registration system. The difference from imperative registration is that the framework implements a deferred configuration engine, instead of making registrations immediately. Configuration commands are first collected in a separate configuration phase, and only after collection is complete are they executed, resulting in actual registrations. Fictional example from framework import Config def save ( data ): ... application code to save the data somewhere ... config = Config () config . form_save ( 'my_form' , save ) config . commit () The idea here is that configuration registries are only modified when config.commit() happens, and only after the configuration has been validated. Real-world example: Pyramid web framework From the Pyramid web framework: def hello_world ( request ): return Response ( 'Hello World!' ) with Configurator () as config : config . add_route ( 'hello' , '/' ) config . add_view ( hello_world , route_name = 'hello' ) This looks very similar to a plain registry, but inside something else is going on: it first collects all registrations, and then generically detects whether there are conflicts, and generically applies overrides. Once the code exits the with statement, config is complete and committed. Trade-offs This brings some of the benefits of a configuration DSL to code. Like a DSL, the configuration system can detect conflicts (the route name 'hello' is registered twice), and it allows sophisticated override patterns that are not dependent on the vagaries of registration order or import order. Another benefit is that configuration can be generated programmatically, so this allows for a certain amount of run-time dynamism without some the costs that a DSL would have. It is still good to avoid such dynamism as much as possible though, as it can make for very difficult to comprehend code. The code that is configured may still not be not co-located with the configuration, but at least it's all code, instead of a whole new language.

Pattern: language integrated declaration You configure the application by using framework-provided annotations for code. This configuration is declarative and does not immediately take place. Language integration declaration looks like language integrated registration, but uses a configuration engine like with imperative declaration. Fictional example from framework import Config config = Config () # we define and configure the function at the same time @config.form_save ( 'my_form' ) def save ( data ): ... application code to save the data somewhere ... # elsewhere before application starts config . commit () Real-world example: Morepath web framework My own Morepath web framework is configured this way. import morepath class App ( morepath . App ): pass @App.path ( path = '/hello' ) class Hello ( object ): pass @App.view ( model = Hello ) def view_get ( self , request ): return "Hello world!" Here two things happen: an instance of Hello is registered for the route /hello , and a GET view is registered for such instances. You can supply these decorators in any order in any module -- the framework will figure it out. If you subclass App , and re-register the /hello path, you have a new application with new behavior for that path, but the same view. Trade-offs I like this way of configuring code very much, so I built a framework for it. This looks very similar to language-integrated registration but the behavior is declarative. It's more explicit than convention over configuration, but still low on ceremony, like language-integrated registration. It co-locates configuration with code. It eliminates many of the issues with the more lightweight language-integrated registration while retaining many of its benefits. It imposes a lot of structure on how configuration works, and this can lead to useful properties: conflict detection and overrides, for instance. It's a lot more heavy-weight than just passing in a callback or object with an interface -- for many frameworks this is more than enough ceremony, and nothing beats how easy that is to implement and test. You can't store it in a database or give it to a non-programmer: for that, use a DSL. But if want a configuration language that's powerful and friendly, this is a good way to go. It's a lot more difficult to implement though, which is a drawback. If you use Python, you're in luck: I've implemented a framework to help you build this, called Dectate. My Morepath web framework is built on it. In Dectate, import-time side-effects are minimized: when the decorator is executed the parameters are stored, but registration only happens when commit() is executed. This means there is no dependence on run-time import order, and conflict detection and overrides are supported in a general way.