When using Aspect Oriented Programming with Policy injection, how do you deal with policy errors gracefully in your design?
In Aspect Oriented Programming the objects should be unconcerned with how a particular policy is configured, but a particular policy may throw particular errors. What's the most graceful design to deal with that?
If you consider the policy to be an "aspect" of the system then the specific aspect should handle everything that has to do with that policy. The objects should be unaware of the specific policy. This way, if you you need to change the policy in the future, you won't need to change anything in the objects.
Why not have your aspect that injects the policy validate it/handle errors that may occur?
Related
In the scala and scalajs library Diode, I have used but not entirely understood the PotAction class and only recently discovered the AsyncAction class, both of which seem to be favored in situations involving, well, asynchronous requests. While I understand that, I don't entirely understand the design decisions and the naming choices, which seem to suggest a more narrow use case.
Specifically, both AsyncAction and PotAction require an initialModel and a next, as though both are modeling an asynchronous request for some kind of refreshable, updateable content rather than a command in the sense of CQRS. I have a somewhat-related question open regarding synchronous actions on form inputs by the way.
I have a few specific use cases in mind. I'd like to know a sketch (not asking for implementation, just the concept) of how you use something like PotAction in conjunction with any of:
Username/password authentication in a conventional flow
OpenAuth-style authentication with a third-party involved and a redirect
Token or cookie authentication behind the scenes
Server-side validation of form inputs
Submission of a command for a remote shell
All of these seem to be a bit different in nature to what I've seen using PotAction but I really want to use it because it has already been helpful when I am, say, rendering something based on the current state of the Pot.
Historically speaking, PotAction came first and then at a later time AsyncAction was generalized out of it (to support PotMap and PotVector), which may explain their relationship a bit. Both provide abstraction and state handling for processing async actions that retrieve remote data. So they were created for a very specific (and common) use case.
I wouldn't, however, use them for authentication as that is typically something you do even before your application is loaded, or any data requested from the server.
Form validation is usually a synchronous thing, you don't do it in the background while user is doing something else, so again Async/PotAction are not a very good match nor provide much added value.
Finally for the remote command use case PotAction might be a good fit, assuming you want to show the results of the command to the user when they are ready. Perhaps PotStream would be even better, depending on whether the command is producing a steady stream of data or just a single message.
In most cases you should use the various Pot structures for what they were meant for, that is, fetching and updating remote data, and maybe apply some of the ideas or internal models (such as the retry mechanism) to other request types.
All the Pot stuff was separated from Diode core into its own module to emphasize that they are just convenient helpers for working with Diode. Developers should feel free to create their own helpers (and contribute back to Diode!) for new use cases.
I have been having this debate with a friend where i have a library (its python but I didn't include that as a tag as the question is applicable to any language) that has a few dependencies. The debate is whether to provide a default environment in the initialization or force the user of the code to explicitly set one.
My opinion is to force the user as its explicit and will avoid confusion and make it clear what they are pointing to.
My friend this is safer and more convenient to default to an environment and let the user override if he wants to.
Thoughts ? Are there any good references or examples / patterns in popular libraries that support either of our arguments? also, any popular blogs or articles that discuss this API design point?
I don't have any references, but here are my thoughts as a potential user of said library.
I think it's good to have a default configuration available to allow developers to quickly evaluate the library. I don't want to have to go through a bunch of configuration just to see if the library will do what I need. Once I'm happy that the library will do what i need it to do, then I'm happy to configure it the way I want.
A good example is Microsoft's ASP.Net MVC framework. When you create a new MVC project it hooks in a default authentication and membership provider, which allows the developer to very quickly get a functioning application up and running. It is also easy to configure different providers to be used if the default one's don't meet the requirements of the application in question.
As a slightly different example, Atlassian Confluence is wiki software which supports many different back-end databases. Atlassian could have chosen to have no default DB configuration, but instead Confluence ships with a default, simple, file-based database to allow users to evaluate the software. For production installations you can then hook up to Oracle, SQL Server, mySQL or whatever else you like.
There may be instances where a default configuratino for a library doesn't really make sense, but I think that would be a special case, rather than a general rule.
It depends. If you can provide sensible defaults, you might want to do that: it will make life easer on the occasional user of the library as they can set only the relevant settings, as opposed to the whole environment (with possibly settings the implications of which they don't fully understand (yet)). You are correct, that in situations it is possible this leads to frustration and confusion as the defaulted settings might cause unexpected behavior (unexpected by the (inexperienced) user) -- you have to weigh the reduced frustration of convenience against the price of not-understood defaults to make the choice for each of these possible-to-default settings, which choice might affect the choice for other, related settings as well
On the other hand, if there is no sensible default (e.g. DB credentials, remote address), you should require the user to provide those settings.
The key in both cases is to provide enough information in the documentation of the library and in the error messages (either for missing settings or conflicting ones) that the user can figure out what those settings actually mean/control without having to read through the source code of the library. This part is hard because 1) it is usally tedious from the point of view of the library developer (so it is often skimped) and 2) the documentation has to be written from the mindset of a newbie to the library, which is often different from the library developer's mindset -- the latter knows the implicit connections/implications, the former has to be told about those in an understandable way.
Although not exactly identical in terms of problem domain, this strikes me as the Convention over Configuration argument.
There has been quite a lot momentum behind CoC in recent years, and in my mind, it makes a whole lot of sense. As long as flexibility is not lost, you have everything to gain. Lower friction development is what we are all after, and if I've got to configure every aspect of your API in order to get it working, I'm less inclined to use it over another API of equal functionality.
I happen to like Hanselman's podcasts, so if you want a little light listening, check out this podcast.
I think your question needs some clarification. For starters, I don't think a library should have any runtime configuration. In terms of dependencies, library dependencies should be handled in a manner appropriate to the environment they are being written for. In python, those dependencies should be in the setup.py file (under requirements), and ultimately that file should meet the requirements of whatever service you plan on making it available on (i.e. pypi for python).
For applications, it is completely okay to require runtime configuration, but you should try to have sensible defaults. If your application depends on libraries, that dependency should be handled in the same way a library dependency would be handled, even though that information may be redundant in the context of an installer (if needed). For the most part first-run scripts and their ilk should be apart of the installer/rpm.
For Web Frameworks, it is typical that your app would carry configuration with it, and likely that it would need to be installed in a different way than traditional applications. Here, about the only thing you can do is try to follow the conventions of whatever framework you are writing in.
Aspect-oriented programming is a subject matter that has been very difficult for me to find any good information on. My old Software Engineering textbook only mentions it briefly (and vaguely), and the wikipedia and various other tutorials/articles I've been able to find on it give ultra-academic, highly-abstracted definitions of just what it is, how to use it, and when to use it. Definitions I just don't seem to understand.
My (very poor) understanding of AOP is that there are many aspects of producing a high-quality software system that don't fit neatly into a nice little cohesive package. Some classes, such as Loggers, Validators, DatabaseQueries, etc., will be used all over your codebase and thus will be highly-coupled. My (again, very poor) understanding of AOP is that it is concerned with the best practices of how to handle these types of "universally-coupled" packages.
Question : Is this true, or am I totally off? If I'm completely wrong, can someone please give a concise, laymen explanation for what AOP is, an example of a so-called aspect, and perhaps even provide a simple code example?
Separation of Concerns is a fundamental principle in software development, there is a classic paper by David Parnas On the Criteria To Be Used in Decomposing Systems into Modules that may introduce you to the subject and also read Uncle Bob's SOLID Principles.
But then there are Cross Cutting concerns that might be included in many use cases like authentication, authorization, validation, logging, transaction handling, exception handling, caching, etc that spawn all the layers in software. And if you want to tackle the problem without duplication and employing the DRY principle, you must handle it in a sophisticated way.
You must use declarative programming, that simply in .net could be annotating a method or a property by an attribute and what happened later is changing the behavior of code in runtime depending of those annotations.
You can find a nice chapter on this topic in Sommerville's Software engineering book
Useful links
C2 wiki CrossCuttingConcern, MSDN, How to Address Crosscutting Concerns in Aspect Oriented Software Development
AOP is a technique where we extract and remove the cross cutting concerns (logging, Exception handling, ....) from our code into it’s own aspect. leaving our original code focusing only on the business logic. not only this makes our code more readable, maintainable but also the code is DRY.
This can be better explained by an example:
Aspect Oriented Programming (AOP) in the .net world using Castle Windsor
or
Aspect Oriented Programming (AOP) in the .net world using Unity
AOP is about crosscutting concerns i.e. things that you need to do throughout the whole application. For instance logging. Suppose you want to trace when you enter and exit a method. This is very easy with aspects. You basically specify a "handler" for an event, such as entering a method. If necessary you can also specify with "wildcards" which methods you are interested in and then it is just a matter of writing the handler code, that could for instance log some info.
Aspect Oriented Programming is basically for separating the cross-cutting concerns (Non-functional) and develop it as aspects, like, security, logging, monitor etc., kept it aside whenever you need in your application, you can use it as plug & play. Only benefit we can achieve is clean code ,less code and programmers can focus on business logic(core concerns) , so that better modularity and quality system can be developed.
I have a strong preference for working in code, leverage IntelliSense and opening up all of the power of the C# language to work with WCF but I want to make sure that I'm not moving in a direction that ultimately will limit the WCF feature set I can access. My experience is so limited with WCF that I don't understand the benefits of using the configuration files, especially if you can do everything in code (?).
Note: I'm using .NET 3.5.
Can you do 'everything' with WCF programmatically or are configuration files required for the full WCF feature set?
You can do about 99.8% of things in code as well as config.
Some things can be done only in code - like setting user name and password on a call that requires those two for authentication.
And there appear to be a few things that can be done in config only - see this other recent SO question for one example.
But I think, if you prefer code, you should be fine for the vast majority of cases.
Marc
An overgrown comment...
Marc_s' answer and the question's perspective is good (two +1s from me).
I have no doubt that the following will not be news to either of you, but wanted to point it out in case someone encounters this and isn't aware of the cons of a purely programmatic approach.
Moving to programmatic configuration from config-file based setup means
you lose the ability to adjust (read: hack!) things in the field -- your only avenue of recourse will be to recompile and redeploy binaries. For many scenarios (including one of mine) this is not n.
you lose the ability to switch between multiple sets of configurations by juggling them in the config file.
I admit that both of the cited 'losses' are debatable - they can encourage bad habits and prevent you from reaching the most solid solution for your customers in the quickest manner possible.
UPDATE: I've implemented a mechanism where I use ChannelFactory<T> but pick up a customised config from the app.config if it's present, or provide a default if it isn't (my scenario is that I'm a guest in someone else's process and hence can't assume a config fuile is easy to update / has been updated, yet dont want to lose the option of tweaking settings after deployment)
This question already has answers here:
Closed 14 years ago.
Duplicate:
What is aspect-oriented programming?
Every time I here a podcast or read a blog entry about it, even here, they make it sound like string theory or something. Is the best way to describe it OOP with Dependency Injection on steroids?
Every time someone tries to explain it, it’s like, Aspects, [Adults from Peanuts Cartoon sound], Orthogonal, [more noise], cross cutting concerns, etc. Seriously, can anyone describe it in layman’s terms.
Laymans terms so let me give you an example. Let's say you have a web application, and you need to add error logging / auditing. One implementation would be to go into every public method and add your try catch blocks etc...
Well Aspect oriented says hogwash with that, let me inject my method around your method so for example instead of calling YourClass.UpdateModel(), the system might call,
LoggingHandler.CallMethod() this method might then redirect the call to UpdateModel but wraps it in a try catch block to handle logging errors.
Now the trick is that this redirection happens automagically, through configuration or by applying attributes to methods.
This works for as you said cross cutting things which are very common programing elements that exist in every domain, such as: Logging, Auditing, Transaction Mgmt, Authorization.
The idea behind it is to remove all this common plumbing code out of your business / app tier so you can focus on solving the problem not worrying about logging this method call or that method call.
Class and method attributes in .NET are a form of aspect-oriented programming. You decorate your classes/methods with attributes. Behind the scenes this adds code to your class/method that performs the particular functions of the attribute. For example, marking a class serializable allows it to be serialized automatically for storage or transmission to another system. Other attributes might mark certain properties as non-serializable and these would be automatically omitted from the serialized object. Serialization is an aspect, implemented by other code in the system, and applied to your class by the application of a "configuration" attribute (decoration) .
AOP is all about managing the common functionality (which span across the application, hence cross cutting) within the application such that it is not embedded within the business logic.
Examples to such cross cutting concerns are logging, managing security, transaction management etc.
Frameworks allows this to managed automatically with the help of some configuration files.
I currently use Post Sharp, i would read the info from their website. I use it to provide a security around method calls.
"PostSharp is an open platform for the analysis and transformation of .NET assemblies. It comes with PostSharp Laos, a powerful yet simple plug-in that let you develop custom attributes that actually adds behavior of your code. PostSharp Laos is the leading aspect-oriented programming (AOP) solution for the .NET Framework."
The classic examples are security and logging. Instead of writing code within your application to log occurance of x or check object z for security access control there is a language contraption "out of band" of normal code which can systematically inject security or logging into routines that don't nativly have them in such a way that even though your code doesn't supply it -- its taken care of.
A more concrete example is the operating system providing access controls to a file. A software program does not need to check for access restrictions because the underlying system does that work for it.
If you think you need AOP in my experience you actually really need to be investing more time and effort into appropriate meta-data management within your system with a focus on well thought structural / systems design.