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What is the difference between a framework and a library? [closed]
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What is the difference between a Framework, a Toolkit and a Library?
The most important difference, and in fact the defining difference between a library and a framework is Inversion of Control.
What does this mean? Well, it means that when you call a library, you are in control. But with a framework, the control is inverted: the framework calls you. (This is called the Hollywood Principle: Don't call Us, We'll call You.) This is pretty much the definition of a framework. If it doesn't have Inversion of Control, it's not a framework. (I'm looking at you, .NET!)
Basically, all the control flow is already in the framework, and there's just a bunch of predefined white spots that you can fill out with your code.
A library on the other hand is a collection of functionality that you can call.
I don't know if the term toolkit is really well defined. Just the word "kit" seems to suggest some kind of modularity, i.e. a set of independent libraries that you can pick and choose from. What, then, makes a toolkit different from just a bunch of independent libraries? Integration: if you just have a bunch of independent libraries, there is no guarantee that they will work well together, whereas the libraries in a toolkit have been designed to work well together – you just don't have to use all of them.
But that's really just my interpretation of the term. Unlike library and framework, which are well-defined, I don't think that there is a widely accepted definition of toolkit.
Martin Fowler discusses the difference between a library and a framework in his article on Inversion of Control:
Inversion of Control is a key part of
what makes a framework different to a
library. A library is essentially a
set of functions that you can call,
these days usually organized into
classes. Each call does some work and
returns control to the client.
A framework embodies some abstract
design, with more behavior built in.
In order to use it you need to insert
your behavior into various places in
the framework either by subclassing or
by plugging in your own classes. The
framework's code then calls your code
at these points.
To summarize: your code calls a library but a framework calls your code.
Diagram
If you are a more visual learner, here is a diagram that makes it clearer:
(Credits: http://tom.lokhorst.eu/2010/09/why-libraries-are-better-than-frameworks)
The answer provided by Barrass is probably the most complete. However, the explanation could easily be stated more clearly. Most people miss the fact that these are all nested concepts. So let me lay it out for you.
When writing code:
eventually you discover sections of code that you're repeating in your program, so you refactor those into Functions/Methods.
eventually, after having written a few programs, you find yourself copying functions you already made into new programs. To save yourself time you bundle those functions into Libraries.
eventually you find yourself creating the same kind of user interfaces every time you make use of certain libraries. So you refactor your work and create a Toolkit that allows you to create your UIs more easily from generic method calls.
eventually, you've written so many apps that use the same toolkits and libraries that you create a Framework that has a generic version of this boilerplate code already provided so all you need to do is design the look of the UI and handle the events that result from user interaction.
Generally speaking, this completely explains the differences between the terms.
Introduction
There are various terms relating to collections of related code, which have both historical (pre-1994/5 for the purposes of this answer) and current implications, and the reader should be aware of both, particularly when reading classic texts on computing/programming from the historic era.
Library
Both historically, and currently, a library is a collection of code relating to a specific task, or set of closely related tasks which operate at roughly the same level of abstraction. It generally lacks any purpose or intent of its own, and is intended to be used by (consumed) and integrated with client code to assist client code in executing its tasks.
Toolkit
Historically, a toolkit is a more focused library, with a defined and specific purpose. Currently, this term has fallen out of favour, and is used almost exclusively (to this author's knowledge) for graphical widgets, and GUI components in the current era. A toolkit will most often operate at a higher layer of abstraction than a library, and will often consume and use libraries itself. Unlike libraries, toolkit code will often be used to execute the task of the client code, such as building a window, resizing a window, etc. The lower levels of abstraction within a toolkit are either fixed, or can themselves be operated on by client code in a proscribed manner. (Think Window style, which can either be fixed, or which could be altered in advance by client code.)
Framework
Historically, a framework was a suite of inter-related libraries and modules which were separated into either 'General' or 'Specific' categories. General frameworks were intended to offer a comprehensive and integrated platform for building applications by offering general functionality, such as cross platform memory management, multi-threading abstractions, dynamic structures (and generic structures in general). Historical general frameworks (Without dependency injection, see below) have almost universally been superseded by polymorphic templated (parameterised) packaged language offerings in OO languages, such as the STL for C++, or in packaged libraries for non-OO languages (guaranteed Solaris C headers). General frameworks operated at differing layers of abstraction, but universally low level, and like libraries relied on the client code carrying out it's specific tasks with their assistance.
'Specific' frameworks were historically developed for single (but often sprawling) tasks, such as "Command and Control" systems for industrial systems, and early networking stacks, and operated at a high level of abstraction and like toolkits were used to carry out execution of the client codes tasks.
Currently, the definition of a framework has become more focused and taken on the "Inversion of Control" principle as mentioned elsewhere as a guiding principle, so program flow, as well as execution is carried out by the framework. Frameworks are still however targeted either towards a specific output; an application for a specific OS for example (MFC for MS Windows for example), or for more general purpose work (Spring framework for example).
SDK: "Software Development Kit"
An SDK is a collection of tools to assist the programmer to create and deploy code/content which is very specifically targeted to either run on a very particular platform or in a very particular manner. An SDK can consist of simply a set of libraries which must be used in a specific way only by the client code and which can be compiled as normal, up to a set of binary tools which create or adapt binary assets to produce its (the SDK's) output.
Engine
An Engine (In code collection terms) is a binary which will run bespoke content or process input data in some way. Game and Graphics engines are perhaps the most prevalent users of this term, and are almost universally used with an SDK to target the engine itself, such as the UDK (Unreal Development Kit) but other engines also exist, such as Search engines and RDBMS engines.
An engine will often, but not always, allow only a few of its internals to be accessible to its clients. Most often to either target a different architecture, change the presentation of the output of the engine, or for tuning purposes. Open Source Engines are by definition open to clients to change and alter as required, and some propriety engines are fixed completely. The most often used engines in the world however, are almost certainly JavaScript Engines. Embedded into every browser everywhere, there are a whole host of JavaScript engines which will take JavaScript as an input, process it, and then output to render.
API: "Application Programming Interface"
The final term I am answering is a personal bugbear of mine: API, was historically used to describe the external interface of an application or environment which, itself was capable of running independently, or at least of carrying out its tasks without any necessary client intervention after initial execution. Applications such as Databases, Word Processors and Windows systems would expose a fixed set of internal hooks or objects to the external interface which a client could then call/modify/use, etc to carry out capabilities which the original application could carry out. API's varied between how much functionality was available through the API, and also, how much of the core application was (re)used by the client code. (For example, a word processing API may require the full application to be background loaded when each instance of the client code runs, or perhaps just one of its linked libraries; whereas a running windowing system would create internal objects to be managed by itself and pass back handles to the client code to be utilised instead.
Currently, the term API has a much broader range, and is often used to describe almost every other term within this answer. Indeed, the most common definition applied to this term is that an API offers up a contracted external interface to another piece of software (Client code to the API). In practice this means that an API is language dependent, and has a concrete implementation which is provided by one of the above code collections, such as a library, toolkit, or framework.
To look at a specific area, protocols, for example, an API is different to a protocol which is a more generic term representing a set of rules, however an individual implementation of a specific protocol/protocol suite that exposes an external interface to other software would most often be called an API.
Remark
As noted above, historic and current definitions of the above terms have shifted, and this can be seen to be down to advances in scientific understanding of the underlying computing principles and paradigms, and also down to the emergence of particular patterns of software. In particular, the GUI and Windowing systems of the early nineties helped to define many of these terms, but since the effective hybridisation of OS Kernel and Windowing system for mass consumer operating systems (bar perhaps Linux), and the mass adoption of dependency injection/inversion of control as a mechanism to consume libraries and frameworks, these terms have had to change their respective meanings.
P.S. (A year later)
After thinking carefully about this subject for over a year I reject the IoC principle as the defining difference between a framework and a library. There ARE a large number of popular authors who say that it is, but there are an almost equal number of people who say that it isn't. There are simply too many 'Frameworks' out there which DO NOT use IoC to say that it is the defining principle. A search for embedded or micro controller frameworks reveals a whole plethora which do NOT use IoC and I now believe that the .NET language and CLR is an acceptable descendant of the "general" framework. To say that IoC is the defining characteristic is simply too rigid for me to accept I'm afraid, and rejects out of hand anything putting itself forward as a framework which matches the historical representation as mentioned above.
For details of non-IoC frameworks, see, as mentioned above, many embedded and micro frameworks, as well as any historical framework in a language that does not provide callback through the language (OK. Callbacks can be hacked for any device with a modern register system, but not by the average programmer), and obviously, the .NET framework.
A library is simply a collection of methods/functions wrapped up into a package that can be imported into a code project and re-used.
A framework is a robust library or collection of libraries that provides a "foundation" for your code. A framework follows the Inversion of Control pattern. For example, the .NET framework is a large collection of cohesive libraries in which you build your application on top of. You can argue there isn't a big difference between a framework and a library, but when people say "framework" it typically implies a larger, more robust suite of libraries which will play an integral part of an application.
I think of a toolkit the same way I think of an SDK. It comes with documentation, examples, libraries, wrappers, etc. Again, you can say this is the same as a framework and you would probably be right to do so.
They can almost all be used interchangeably.
very, very similar, a framework is usually a bit more developed and complete then a library, and a toolkit can simply be a collection of similar librarys and frameworks.
a really good question that is maybe even the slightest bit subjective in nature, but I believe that is about the best answer I could give.
Library
I think it's unanimous that a library is code already coded that you can use so as not to have to code it again. The code must be organized in a way that allows you to look up the functionality you want and use it from your own code.
Most programming languages come with standard libraries, especially some code that implements some kind of collection. This is always for the convenience that you don't have to code these things yourself. Similarly, most programming languages have construct to allow you to look up functionality from libraries, with things like dynamic linking, namespaces, etc.
So code that finds itself often needed to be re-used is great code to be put inside a library.
Toolkit
A set of tools used for a particular purpose. This is unanimous. The question is, what is considered a tool and what isn't. I'd say there's no fixed definition, it depends on the context of the thing calling itself a toolkit. Example of tools could be libraries, widgets, scripts, programs, editors, documentation, servers, debuggers, etc.
Another thing to note is the "particular purpose". This is always true, but the scope of the purpose can easily change based on who made the toolkit. So it can easily be a programmer's toolkit, or it can be a string parsing toolkit. One is so broad, it could have tool touching everything programming related, while the other is more precise.
SDKs are generally toolkits, in that they try and bundle a set of tools (often of multiple kind) into a single package.
I think the common thread is that a tool does something for you, either completely, or it helps you do it. And a toolkit is simply a set of tools which all perform or help you perform a particular set of activities.
Framework
Frameworks aren't quite as unanimously defined. It seems to be a bit of a blanket term for anything that can frame your code. Which would mean: any structure that underlies or supports your code.
This implies that you build your code against a framework, whereas you build a library against your code.
But, it seems that sometimes the word framework is used in the same sense as toolkit or even library. The .Net Framework is mostly a toolkit, because it's composed of the FCL which is a library, and the CLR, which is a virtual machine. So you would consider it a toolkit to C# development on Windows. Mono being a toolkit for C# development on Linux. Yet they called it a framework. It makes sense to think of it this way too, since it kinds of frame your code, but a frame should more support and hold things together, then do any kind of work, so my opinion is this is not the way you should use the word.
And I think the industry is trying to move into having framework mean an already written program with missing pieces that you must provide or customize. Which I think is a good thing, since toolkit and library are great precise terms for other usages of "framework".
Framework: installed on you machine and allowing you to interact with it. without the framework you can't send programming commands to your machine
Library: aims to solve a certain problem (or several problems related to the same category)
Toolkit: a collection of many pieces of code that can solve multiple problems on multiple issues (just like a toolbox)
It's a little bit subjective I think. The toolkit is the easiest. It's just a bunch of methods, classes that can be use.
The library vs the framework question I make difference by the way to use them. I read somewhere the perfect answer a long time ago. The framework calls your code, but on the other hand your code calls the library.
In relation with the correct answer from Mittag:
a simple example. Let's say you implement the ISerializable interface (.Net) in one of your classes. You make use of the framework qualities of .Net then, rather than it's library qualities. You fill in the "white spots" (as mittag said) and you have the skeleton completed. You must know in advance how the framework is going to "react" with your code. Actually .net IS a framework, and here is where i disagree with the view of Mittag.
The full, complete answer to your question is given very lucidly in Chapter 19 (the whole chapter devoted to just this theme) of this book, which is a very good book by the way (not at all "just for Smalltalk").
Others have noted that .net may be both a framework and a library and a toolkit depending on which part you use but perhaps an example helps. Entity Framework for dealing with databases is a part of .net that does use the inversion of control pattern. You let it know your models it figures out what to do with them. As a programmer it requires you to understand "the mind of the framework", or more realistically the mind of the designer and what they are going to do with your inputs. datareader and related calls, on the other hand, are simply a tool to go get or put data to and from table/view and make it available to you. It would never understand how to take a parent child relationship and translate it from object to relational, you'd use multiple tools to do that. But you would have much more control on how that data was stored, when, transactions, etc.
There is so much written about unit testing but I have hardly found any books/blogs about integration testing? Could you please suggest me something to read on this topic?
What tests to write when doing integration testing?
what makes a good integration test?
etc etc
Thanks
Anything written by Kent Beck, father of both JUnit and SUnit, is a great place to start (for unit tests / test writing in general). I'm assuming that you don't mean "continuous integration," which is a process-based build approach (very cool, when you get it working).
In my own experience, integration tests look very similar to regular unit tests, simply at a higher level. More mock objects. More state initialization.
I believe that integration tests are like onions. They have layers.
Some people prefer to "integrate" all of their components and test the "whole" product as an the "integration" test. You can certainly do this, but I prefer a more incremental approach. If you start low-level and then keep testing at higher composition layers, then you will achieve integration testing.
Maybe it is generally harder to find information on integration testing because it is much more specific to the actual application and its business use. Nevertheless, here's my take on it.
What applies to unit-tests also applies to integration tests: modules should have an easy way to mock their externals inputs (files, DB, time...), so that they can be tested together with the other unit-tests.
But what I've found extremely useful, at least for data-oriented applications, is to be able to create a "console" version of the application that takes input files that fully determine its state (no dependencies on databases, network resources...), and outputs the result as another file. One can then maintain pairs of inputs / expected results files, and test for regressions as part of nightly builds, for example. Having this console version allows for easier scripting, and makes debugging incredibly easier as one can rely on a very stable environment, where it is easy to reproduce bugs and to run the debugger.
J.B. Rainsberger has written about them. Here's a link to an InfoQ article with more info.
http://www.infoq.com/news/2009/04/jbrains-integration-test-scam
I've heard this term used a lot in the same context as logging, but I can't seem to find a clear definition of what it actually is.
Is it simply a more general class of logging/monitoring tools and activities?
Please provide sample code/scenarios when/how instrumentation should be used.
I write tools that perform instrumentation. So here is what I think it is.
DLL rewriting. This is what tools like Purify and Quantify do. A previous reply to this question said that they instrument post-compile/link. That is not correct. Purify and Quantify instrument the DLL the first time it is executed after a compile/link cycle, then cache the result so that it can be used more quickly next time around. For large applications, profiling the DLLs can be very time consuming. It is also problematic - at a company I worked at between 1998-2000 we had a large 2 million line app that would take 4 hours to instrument, and 2 of the DLLs would randomly crash during instrumentation and if either failed you would have do delete both of them, then start over.
In place instrumentation. This is similar to DLL rewriting, except that the DLL is not modified and the image on the disk remains untouched. The DLL functions are hooked appropriately to the task required when the DLL is first loaded (either during startup or after a call to LoadLibrary(Ex). You can see techniques similar to this in the Microsoft Detours library.
On-the-fly instrumentation. Similar to in-place but only actually instruments a method the first time the method is executed. This is more complex than in-place and delays the instrumentation penalty until the first time the method is encountered. Depending on what you are doing, that could be a good thing or a bad thing.
Intermediate language instrumentation. This is what is often done with Java and .Net languages (C~, VB.Net, F#, etc). The language is compiled to an intermediate language which is then executed by a virtual machine. The virtual machine provides an interface (JVMTI for Java, ICorProfiler(2) for .Net) which allows you to monitor what the virtual machine is doing. Some of these options allow you to modify the intermediate language just before it gets compiled to executable instructions.
Intermediate language instrumentation via reflection. Java and .Net both provide reflection APIs that allow the discovery of metadata about methods. Using this data you can create new methods on the fly and instrument existing methods just as with the previously mentioned Intermediate language instrumentation.
Compile time instrumentation. This technique is used at compile time to insert appropriate instructions into the application during compilation. Not often used, a profiling feature of Visual Studio provides this feature. Requires a full rebuild and link.
Source code instrumentation. This technique is used to modify source code to insert appropriate code (usually conditionally compiled so you can turn it off).
Link time instrumentation. This technique is only really useful for replacing the default memory allocators with tracing allocators. An early example of this was the Sentinel memory leak detector on Solaris/HP in the early 1990s.
The various in-place and on-the-fly instrumentation methods are fraught with danger as it is very hard to stop all threads safely and modify the code without running the risk of requiring an API call that may want to access a lock which is held by a thread you've just paused - you don't want to do that, you'll get a deadlock. You also have to check if any of the other threads are executing that method, because if they are you can't modify it.
The virtual machine based instrumentation methods are much easier to use as the virtual machine guarantees that you can safely modify the code at that point.
(EDIT - this item added later) IAT hooking instrumentation. This involved modifying the import addess table for functions linked against in other DLLs/Shared Libraries. This type of instrumentation is probably the simplest method to get working, you do not need to know how to disassemble and modify existing binaries, or do the same with virtual machine opcodes. You just patch the import table with your own function address and call the real function from your hook. Used in many commercial and open source tools.
I think I've covered them all, hope that helps.
instrumentation is usually used in dynamic code analysis.
it differs from logging as instrumentation is usually done automatically by software, while logging needs human intelligence to insert the logging code.
It's a general term for doing something to your code necessary for some further analysis.
Especially for languages like C or C++, there are tools like Purify or Quantify that profile memory usage, performance statistics, and the like. To make those profiling programs work correctly, an "instrumenting" step is necessary to insert the counters, array-boundary checks, etc that is used by the profiling programs. Note that in the Purify/Quantify scenario, the instrumentation is done automatically as a post-compilation step (actually, it's an added step to the linking process) and you don't touch your source code.
Some of that is less necessary with dynamic or VM code (i.e. profiling tools like OptimizeIt are available for Java that does a lot of what Quantify does, but no special linking is required) but that doesn't negate the concept.
A excerpt from wikipedia article
In context of computer programming,instrumentation refers to an
ability to monitor or measure the level of a product's performance, to
diagnose errors and to write trace information. Programmers implement
instrumentation in the form of code instructions that monitor specific
components in a system (for example, instructions may output logging
information to appear on screen). When an application contains
instrumentation code, it can be managed using a management tool.
Instrumentation is necessary to review the performance of the
application. Instrumentation approaches can be of two types, source
instrumentation and binary instrumentation.
Whatever Wikipedia says, there is no standard / widely agreed definition for code instrumentation in IT industry.
Please consider, instrumentation is a noun derived from instrument which has very broad meaning.
"Code" is also everything in IT, I mean - data, services, everything.
Hence, code instrumentation is a set of applications that is so wide ... not worth giving it a separate name ;-).
That's probably why this Wikipedia article is only a stub.
I am in the process of building interactive front-ends to a
distributed application which to date has been used to run workloads
that had a batch-job like structures and needed no UI at all. The application is mostly written in Perl and C and runs on a mix of Unix and Windows machines, but I think this isn't relevant to the UI.
The first such frontend is going have a command-line user interface --
currently, I envision something similar to the CLIs of the Procurve
switches and Cisco routers that I have worked with.
Like modern network gear CLIs, commands are going to resemble
simple sentences, (i.e. show vlans ports 1-4) and the CLI will
have some implicit state, much in the way that Unix shells and
cmd.exe in Windows have environment variables and current working
directories. Moreover, I'd like to implement great tab completion that
is aware of the application's state as much as possible and I want to be able to do that with as
little application-specific code as possible.
The low-level functionality (terminal I/O) seems easy to implement on
top of GNU Readline or similar libraries, but that's only where the
real fun starts. So far I have looked at the Perl modules
Term::Shell
and
Term::ShellUI,
but I'm not convinced that I want to use either of them. I am still
considering rolling my own solution and at the moment I am primarily looking for
inspiration.
Can you recommend any application or library, regardless of
implementation language, that implements a good CLI from which I can
borrow ideas?
I suggest you take a look at the philosophy underlying Microsoft PowerShell. From the idea of piping typed objects between commands to the consistency of its commands and argument syntax, I think it can be a source of inspiration.
You could try having a look at libcli :
"Libcli provides a shared library for
including a Cisco-like command-line
interface into other software."
http://code.google.com/p/libcli/
BTW - I forgot to mention that it is GNU Lesser GPL and actually used by Cisco in some products.
As for your last sentence/question, I'm particularly fond of zsh completion and line editing (zle).