I'm starting to work with ROS and plugins and I would like to understand one thing: What the use of different namespaces for the base class and the plugin class in ROS ?
I can understand the utility of namespace to differentiate similar nodes or Topics used by different nodes but I quite don't understand its use when we talk about plugins.
To be clear, why in the tutorial the base class use the namespace polygon_base and the plugins use the namespace polygon_plugin
thank you for your help
Initial disclaimer: Plugins are chiefly used by libraries who need the extended features Plugins provide. For the average ROS developer, there are usually better design choices.
The use of namespaces in ROS is to reduce confusion, especially with similarly and simultaneously operating things. For example, topic namespacing is (almost exclusively) used for running the same launch file multiple times, most commonly to launch/simulate multiple robots, or multiple sensors.
In the tutorial you mentioned (with badly formatted URL) (https://wiki.ros.org/pluginlib/Tutorials/Writing and Using a Simple Plugin), the only real use of namespaces are C++ namespaces. I think they used two different namespaces to disambiguate two similar kinds of code. The polygon_base code is not intended to be used as a plugin, it simply is a base. If it could be used as a plugin, it would be included in the polygon_plugins namespace.
In short, pick what is most clear and communicative as your naming schemes.
Related
I have been using the Kubernetes and Helm for a while and have now come across Kustomize for the first time.
But what exactly is the difference between Kustomize and Helm?
Are both simply different solutions for bundling K8s elements such as services, deployments, ...? Or does it make sense to use both Helm and Kustomize together?
The best way to describe the differences is to refer to them as different types of deployment engines. One is a Templating Engine and one is an Overlay Engine.
So what are these? Well when you use a templating engine you create a boilerplate example of your file. From there you abstract away contents with known filters and within these abstractions you provide references to variables. These variables are normally abstracted to another file where you insert information specific to your environment Then, on runtime, when you execute the templating engine, the templates are loaded into memory and all of the variables are exchanged with their placeholders.
This is different from an overlay engine in a few nuanced ways. Normally about how information gets into configuration examples. Noticed how I used the word examples there instead of templates. This was intentional as Kustomize doesn't use templates. Instead, you create a Kustomization.yml file. This file then points to two different things. Your Base and your Overlays. At runtime your Base is loaded into memory and if any Overlays exist that match they are merged over top of your Base configuration.
The latter method allows you to scale your configurations to large numbers of variants more easily. Imagine maintaining 10,000 different sets of variables files for 10,000 different configurations. Now imagine maintaining a hierarchy of modular and small configurations that can be inherited in any combination or permutation? It would greatly reduce redundancy and greatly improve manageability.
Another nuance to make note of is ownership of the projects. Helm is operated by a third party. Kustomize is developed directly by the Kubernetes team. In fact, Kustomize functionality is directly supported in Kubectl. You can build and perform a Kustomize project like so: kubectl apply -k DIR. However, the version of kustomize embedded in the kubectl binary is out of date and missing some new features.
There are a few other improvements in Kustomize too that are somewhat more minor but still worth mentioning. It can reference bases from the internet or other non-standard paths. It supports generators to build configuration files for you automatically based on files and string literals. It supports robust & granular JSON patching. It supports injecting metadata across configuration files.
The following links were added in the comments below for more comparisons:
https://medium.com/#alexander.hungenberg/helm-vs-kustomize-how-to-deploy-your-applications-in-2020-67f4d104da69
https://codeengineered.com/blog/2018/helm-kustomize-complexity/
Almost everything. Like asking what's the difference between Apache and Nginx :) They do vaguely similar jobs but quantifying the differences is kind of impossible.
The short version is that Helm is a template-driven system based on decentralized model for chart sharing. Kustomize is based on deep merges and other structured transforms of YAML data.
There are cases where using both is reasonable, such as feeding the output from helm template into kustomize for overlays.
Both has its Pros and Cons. Lets look in this table
Helm is particularly useful for packaging, porting and installing apps that are well-defined, while Kustomize works best for modifying existing Kubernetes apps.
The fact that Kustomize and Helm offer unique specific benefits the best course of action would be to use the two tools side by side.
What mechanism for sharing can I put in place to allow two separate typescript projects to communicate along common interfaces?
The basic idea is that I have one main project that will implement the interfaces, and there will be another project that consumes them. However I don't want either of the projects to be dependent on the other.
Specifically: How can I can create and distribute a library of contracts (and perhaps any functionality that complements them if applicable).
Are there any established or emerging conventions in the TypeScript ecosystem to distribute libraries of interfaces via NPM?
Several years and many projects later, the simplest answer to this question today is to simply publish a .d.ts to an npm package and consume it! This project can be built from typescript sources and even include built code.
Of course, a lot of this answer now is long after TypeScript and the JS community have matured and come together to establish these conventions.
Just define the interface in a separate file, or collect all shared interfaces in a file interfaces.ts (or a interfaces.d.ts). Both projects can reference this file, you can have copies of that file in both projects.
TypeScript is not like i.e. C# where you have to reference an underlying assembly (although this will be fixed in C# with assembly neutral interfaces).
But beware: TypeScript has no dedicated runtime that assures your interfaces match. As everything is still JavaScript, at runtime you have to make sure the interfaces match. If one projects uses i.e. another version of the interfaces with properties that didn't exist in previous versions, you will still run into trouble.
Are there any established or emerging conventions in the TypeScript ecosystem to distribute libraries of interfaces via NPM
Use the typescript key in your package.json to point to the definition file :
https://github.com/DefinitelyTyped/tsd#link-to-bundled-definitions
I have been using CDH and HDP for a while (both in the pseudo-distributed mode) on a VM as well as installing natively on Ubuntu. Although my question is probably relevant to all Projects within the Apache Hadoop Ecosystem, let me ask this specifically in the context of Avro.
What is the best way to go about figuring out what the different packages and the classes within the packages do. I usually end up referring to the Javadoc for the project (Avro in this case) but the overviews for packages and classes end up being awfully inadequate.
For e.g. Take two of the Avro packages: org.apache.avro.specific and org.apache.avro.generic These are used for creating Specific and Generic Readers and Writers (respectively) but I'm not a 100% sure what these are for. I have used the Specific Package for in cases when I have used Avro Code Generation and the Generic ones when I don't want to use code generation. However, I am not sure if that is the only reason for using one vs. the other.
Another example: The Encoder\Decoder Classes are used for low-level SerDe, the DatumReader\DatumWrite for a "medium-level" Serde while most application layer interactions with Avro will probably use Generic\Specific Readers\Writers. Without having struggled through the pain of using these classes, how is a user to know what to use for what?
Is there a better way to get a good overview of each package (clearly the javadoc is not well documented) and the classes within the package?
PS: I have similar questions for essentially all other Hadoop Projects (Hive, HBASE etc.) - the Javadocs seem to be grossly inadequate overall. I just wonder what other developers end up doing to figure these out.
Any inputs would be great.
I download the source code and skim through it to get the idea what it does. If there is javadoc, I read that too. I tend to concentrate on the interfaces that I need and move on from there, that way I put everything into context and it makes it easier to figure out the usage. I use the call hierarchy and the type hierarchy views a lot.
These are very general guidelines, and ultimately it is the time you spend with the project that will make you understand it.
Hadoop ecosystem is quickly growing and changes are introduced on monthly bases. that's why javadoc is not so good. Another reason is that hadoop software tends to lean towards the infrastructure and not towards the end user. People developing tools will spend time learning the APIs and internals while everybody else is kinda supposed to be blissfully ignorant of all those, and just use some high level domain specific language for the tool.
A repeating theme in my development work has been the use of or creation of an in-house plug-in architecture. I've seen it approached many ways - configuration files (XML, .conf, and so on), inheritance frameworks, database information, libraries, and others. In my experience:
A database isn't a great place to store your configuration information, especially co-mingled with data
Attempting this with an inheritance hierarchy requires knowledge about the plug-ins to be coded in, meaning the plug-in architecture isn't all that dynamic
Configuration files work well for providing simple information, but can't handle more complex behaviors
Libraries seem to work well, but the one-way dependencies have to be carefully created.
As I seek to learn from the various architectures I've worked with, I'm also looking to the community for suggestions. How have you implemented a SOLID plug-in architecture? What was your worst failure (or the worst failure you've seen)? What would you do if you were going to implement a new plug-in architecture? What SDK or open source project that you've worked with has the best example of a good architecture?
A few examples I've been finding on my own:
Perl's Module::Plugable and IOC for dependency injection in Perl
The various Spring frameworks (Java, .NET, Python) for dependency injection.
An SO question with a list for Java (including Service Provider Interfaces)
An SO question for C++ pointing to a Dr. Dobbs article
An SO question regarding a specific plugin idea for ASP.NET MVC
These examples seem to play to various language strengths. Is a good plugin architecture necessarily tied to the language? Is it best to use tools to create a plugin architecture, or to do it on one's own following models?
This is not an answer as much as a bunch of potentially useful remarks/examples.
One effective way to make your application extensible is to expose its internals as a scripting language and write all the top level stuff in that language. This makes it quite modifiable and practically future proof (if your primitives are well chosen and implemented). A success story of this kind of thing is Emacs. I prefer this to the eclipse style plugin system because if I want to extend functionality, I don't have to learn the API and write/compile a separate plugin. I can write a 3 line snippet in the current buffer itself, evaluate it and use it. Very smooth learning curve and very pleasing results.
One application which I've extended a little is Trac. It has a component architecture which in this situation means that tasks are delegated to modules that advertise extension points. You can then implement other components which would fit into these points and change the flow. It's a little like Kalkie's suggestion above.
Another one that's good is py.test. It follows the "best API is no API" philosophy and relies purely on hooks being called at every level. You can override these hooks in files/functions named according to a convention and alter the behaviour. You can see the list of plugins on the site to see how quickly/easily they can be implemented.
A few general points.
Try to keep your non-extensible/non-user-modifiable core as small as possible. Delegate everything you can to a higher layer so that the extensibility increases. Less stuff to correct in the core then in case of bad choices.
Related to the above point is that you shouldn't make too many decisions about the direction of your project at the outset. Implement the smallest needed subset and then start writing plugins.
If you are embedding a scripting language, make sure it's a full one in which you can write general programs and not a toy language just for your application.
Reduce boilerplate as much as you can. Don't bother with subclassing, complex APIs, plugin registration and stuff like that. Try to keep it simple so that it's easy and not just possible to extend. This will let your plugin API be used more and will encourage end users to write plugins. Not just plugin developers. py.test does this well. Eclipse as far as I know, does not.
In my experience I've found there are really two types of plug-in Architectures.
One follows the Eclipse model which is meant to allow for freedom and is open-ended.
The other usually requires plugins to follow a narrow API because the plugin will fill a specific function.
To state this in a different way, one allows plugins to access your application while the other allows your application to access plugins.
The distinction is subtle, and sometimes there is no distiction... you want both for your application.
I do not have a ton of experience with Eclipse/Opening up your App to plugins model (the article in Kalkie's post is great). I've read a bit on the way eclipse does things, but nothing more than that.
Yegge's properties blog talks a bit about how the use of the properties pattern allows for plugins and extensibility.
Most of the work I've done has used a plugin architecture to allow my app to access plugins, things like time/display/map data, etc.
Years ago I would create factories, plugin managers and config files to manage all of it and let me determine which plugin to use at runtime.
Now I usually just have a DI framework do most of that work.
I still have to write adapters to use third party libraries, but they usually aren't that bad.
One of the best plug-in architectures that I have seen is implemented in Eclipse. Instead of having an application with a plug-in model, everything is a plug-in. The base application itself is the plug-in framework.
http://www.eclipse.org/articles/Article-Plug-in-architecture/plugin_architecture.html
I'll describe a fairly simple technique that I have use in the past. This approach uses C# reflection to help in the plugin loading process. This technique can be modified so it is applicable to C++ but you lose the convenience of being able to use reflection.
An IPlugin interface is used to identify classes that implement plugins. Methods are added to the interface to allow the application to communicate with the plugin. For example the Init method that the application will use to instruct the plugin to initialize.
To find plugins the application scans a plugin folder for .Net assemblies. Each assembly is loaded. Reflection is used to scan for classes that implement IPlugin. An instance of each plugin class is created.
(Alternatively, an Xml file might list the assemblies and classes to load. This might help performance but I never found an issue with performance).
The Init method is called for each plugin object. It is passed a reference to an object that implements the application interface: IApplication (or something else named specific to your app, eg ITextEditorApplication).
IApplication contains methods that allows the plugin to communicate with the application. For instance if you are writing a text editor this interface would have an OpenDocuments property that allows plugins to enumerate the collection of currently open documents.
This plugin system can be extended to scripting languages, eg Lua, by creating a derived plugin class, eg LuaPlugin that forwards IPlugin functions and the application interface to a Lua script.
This technique allows you to iteratively implement your IPlugin, IApplication and other application-specific interfaces during development. When the application is complete and nicely refactored you can document your exposed interfaces and you should have a nice system for which users can write their own plugins.
I once worked on a project that had to be so flexible in the way each customer could setup the system, which the only good design we found was to ship the customer a C# compiler!
If the spec is filled with words like:
Flexible
Plug-In
Customisable
Ask lots of questions about how you will support the system (and how support will be charged for, as each customer will think their case is the normal case and should not need any plug-ins.), as in my experience
The support of customers (or
fount-line support people) writing
Plug-Ins is a lot harder than the
Architecture
Usualy I use MEF. The Managed Extensibility Framework (or MEF for short) simplifies the creation of extensible applications. MEF offers discovery and composition capabilities that you can leverage to load application extensions.
If you are interested read more...
In my experience, the two best ways to create a flexible plugin architecture are scripting languages and libraries. These two concepts are in my mind orthogonal; the two can be mixed in any proportion, rather like functional and object-oriented programming, but find their greatest strengths when balanced. A library is typically responsible for fulfilling a specific interface with dynamic functionality, whereas scripts tend to emphasise functionality with a dynamic interface.
I have found that an architecture based on scripts managing libraries seems to work the best. The scripting language allows high-level manipulation of lower-level libraries, and the libraries are thus freed from any specific interface, leaving all of the application-level interaction in the more flexible hands of the scripting system.
For this to work, the scripting system must have a fairly robust API, with hooks to the application data, logic, and GUI, as well as the base functionality of importing and executing code from libraries. Further, scripts are usually required to be safe in the sense that the application can gracefully recover from a poorly-written script. Using a scripting system as a layer of indirection means that the application can more easily detach itself in case of Something Bad™.
The means of packaging plugins depends largely on personal preference, but you can never go wrong with a compressed archive with a simple interface, say PluginName.ext in the root directory.
I think you need to first answer the question: "What components are expected to be plugins?"
You want to keep this number to an absolute minimum or the number of combinations which you must test explodes. Try to separate your core product (which should not have too much flexibility) from plugin functionality.
I've found that the IOC (Inversion of Control) principal (read springframework) works well for providing a flexible base, which you can add specialization to to make plugin development simpler.
You can scan the container for the "interface as a plugin type advertisement" mechanism.
You can use the container to inject common dependencies which plugins may require (i.e. ResourceLoaderAware or MessageSourceAware).
The Plug-in Pattern is a software pattern for extending the behaviour of a class with a clean interface. Often behaviour of classes is extended by class inheritance, where the derived class overwrites some of the virtual methods of the class. A problem with this solution is that it conflicts with implementation hiding. It also leads to situations where derived class become a gathering places of unrelated behaviour extensions. Also, scripting is used to implement this pattern as mentioned above "Make internals as a scripting language and write all the top level stuff in that language. This makes it quite modifiable and practically future proof". Libraries use script managing libraries. The scripting language allows high-level manipulation of lower level libraries. (Also as mentioned above)
I've noticed in pretty much every company I've worked that they have a common library that is generally shared across a number of projects. More often than not this has been a single companyx-commons project that ends up as a dumping ground for common programs including:
Command Line Parsers
File Utilities
Framework Helpers
etc...
Some of these are well thought out and some duplicate functionality found in Apache commons-lang, commons-io etc..
What are the things you have in your common library and more importantly how do you structure the common libraries to make them easy to improve and incorporate across other projects?
In my experience, the single biggest factor in the success of a common library is user buy-in; users in this case being other developers; and culture of your workplace/team(s) will be a big factor.
Separate libraries (projects/assemblies if you're in .Net) for different application tiers is essential (e.g: there's obviously no point putting UI and data access code together).
Keep things as simple as possible; what you don't put in a common library is often at least as important as what you do. Users of the library won't want to have to think, so usage needs to be super easy.
The golden rule we stuck to was keeping individual functions focused on a single task - do one thing and do it well (or very very well); don't try and provide something that tries to take every possibility into account, the more reusable you think you're making it - the less likely it is to be used. Code Complete (the book) has some excellent content on common libraries.
A good approach to setting/improving a library up is to do regular code reviews and retrospectives; find good candidates that you've already come up with and consider re-factoring them into a library for future projects; a good candidate will be something that more than one developer has had to do on more that one project (for example).
Set-up some sort of simple and clear governance of the libraries - someone who can 'own' a specific library and ensure it's overal quality (such as a senior dev or team lead).
I have so far written most of the common libraries we use at our office.
We have certain button classes that are just slightly more useful to us than the standard buttons
A database management class that does some internal caching and can connect to ODBC, OLEDB, SQL, and Access databases without even the flip of a parameter
Some grid and list controls that are multi threaded so we can add large amounts of data to them without the program slowing and without having to write all the multithreading code every time there is a performance issue with a list box/combo box.
These classes make it easier for all of us to work on each other's code and know how exactly they work since we all use the exact same interfaces throughout our products.
As far as organization goes, all of the DLL's are stored along with their source code on a shared development drive in the office that we all have access to. (We're a pretty small shop)
We split our libraries by function.
Commmon.Ui.dll has base classes for ui elements.
Common.Data.Dll is sort of a wrapper around Enterprise library Data access classes.
Common.Business is a dumping ground for other common classes that don't fit into one of those.
We create other specialized dlls as needs arise.