In OSGI bundle we can have a lot of components having Activate method.What will be sequence in which they will be called after the bundle is activated?
Components will be activated once all their mandatory references are present. If more than one component can be activated at a certain time then you have no guarantee for the sequence.
If the bundles have different Start Levels then all the activation methods of bundles in lower start levels will be executed before the activation methods of bundles on higher start level.
With this logic, you can control the activation method sequence or you can determine what happened based on the logs.
In practice, your bundles must be ordered by dependencies rather than start levels.
As Christian said this is not defined. Once dependencies are satisfied OSGI will activate that service.
As a second point - your services should not depend on a startup sequence. If they do that's bad practice. Why is this relevant to your project? Perhaps we can help you architect it.
As others have pointed out, this is not defined.
That said, sometimes there are good reasons to call several methods in given order during activation. If that is your case, you could always have a single "Activate" method that calls all the others. This way you can apply whatever order logic fits best for your usecase. It could even take into account configuration values if needed.
Related
The Problem:
I'm trying to have Puppet manage some of the details of several scheduled tasks without managing whether those tasks are enabled. To that end, I declare scheduled_task resources without any explicit enabled attributes, with the intention of communicating that whether the tasks are enabled is not to be altered by Puppet. Puppet, however, is persistent in re-enabling the tasks on every run, just as if I had specified enabled => true for each of them. How can I make it stop doing that?
Already Tried:
I've considered setting the attributes for each datacenter via hiera, but the reality is that this makes failovers and switches more complicated than necessary. I don't want to change my puppet code every time that needs to happen. I also can't shut-off the puppet-agent runs. I need to maintain the integrity of our rolling deployment system.
Providers?
I've read a little about providers, seems like I can handle the behavior there. However, I'm having a hard time figuring out where there is (if any) documentation that explains how to use them to override specific resource properties.
Notify/Subscribe:
I've thought about using notify/subscribe to only set the triggers to enabled on creation. I'm not thinking this is the right solution, because it's not one resource subscribing to/notifying another, it's properties being set on a single resource. If there's some magical way of doing this or something similar, I'd love to know.
Bottom-line: I just need puppet to create disabled tasks, and let me turn them on/off without changing the state in subsequent runs.
Is there a way to ignore resource attribute defaults in Puppet entirely?
Only by explicitly declaring a value for every attribute of every resource.
But that doesn't seem to be the question you really wanted to ask. You seem to be exploring ways to assign attribute values without specifying them as literals in your resource declarations, and I guess you're looking for some kind of layered approach, with the bottom layer replacing or overriding types' and providers' built-in defaults.
As you thought, you could conceivably do this at least in part via providers. You would need to write a custom provider for your resource type and specify that it be used by each resource instance. But don't. This way is complicated to implement and confusing to anyone who later has to read your manifests (maybe including future you).
Notify / subscribe, on the other hand, are simply the wrong tools for the job. They do not do what you seem to think they do, or perhaps you just haven't thought that idea through.
I think you're selling Hiera short and / or inappropriately minimizing the complexity of the task. Probably a mixture of both -- I'm inclined to guess that Hiera can do more for you than you appreciate, but also that the complications you envision will manifest to some extent, in some form, no matter what you do.
Nevertheless, there is an approach that seems to match pretty closely what I think you want: resource default declarations (I link to the latest docs, but this feature is present, in substantially the same form, in every Puppet version released in at least the last nine years). Thus, you might cause all scheduled_task resources to be disabled unless you explicitly say otherwise by putting this resource default declaration at an appropriate place:
Scheduled_task {
enabled => false
}
When choosing where to put such a statement, do note that, unlike anything else in modern Puppet, resource default statements have dynamic scope. The manual discusses that in somewhat more detail.
Revisited:
In light of the clarification of the question, I'll clarify that resource types' providers are where resource management behavior lives. Therefore, if Puppet's behavior on target machines is not what you want in the event of an altogether missing property declaration, then modifying the provider or writing your own are pretty much your only alternatives. Of course, if you have a support contract then perhaps you don't have to do that in-house.
If you don't want to hack on providers -- an altogether reasonable position -- then you're left with managing the properties to their desired values. Supposing that you employ Hiera effectively, however, you should not need to modify any manifests to control which servers have their tasks enabled.
Recently, I found my way to The Clean Architecture post by Uncle Bob. But when I tried to apply it to a current project, I got stuck when a usecase needed to depend on another usecase.
For example, my Domain Model is Goal and Task. One Goal can have many Tasks. When I update a Task, it needs to update the information of its parent Goal. In other words, UpdateTask usecase will have UpdateGoal usecase as a dependecy. I am not sure if this is acceptable, or, if we should avoid usecase level dependencies.
A use case is related to a functionality of your application. Generally when we need to invoke from one use case to another there is something that does not work.
When you update a goal in isolation, it is not the same scenario as when you update it by a change in a task, in fact, it is sure that not all data is updated, but a part.
Surely you will have to use the goal repository and the goal entity but it is a completely different scenario. In your case you are not duplicating logic, only calls to the repository or the entity, saving code lines can be expensive in the future.
In short, it is not a good idea to have dependence between use cases.
I'm developing an application with MVC3 and Entity Framework. I have service layers for two entities that have one similarity.
The two services are DurationService and FieldService. The former handles a list of Days and daysettings. These settings contain information about the timeslots per day (start time, end time, a list of possible break times). The latter service handles a list of Fields and fieldsettings. These fieldsettings are used to determine the field availability.
Both services need to check whether the break times overlap. I coded this for the DurationService but have now noticed that the FieldService requires exactly the same method. I don't want to violate the DRY principle so my question is, how do I best handle this?
Do I make a static class which both services can call? Do I use some kind of inheritance (even though this method is the only method they will share).
Looks like architecture is preventing you from doing the obviously right things. Don't let that happen.
Inheritance is probably not the right solution. A static helper class will do. Simple problems require simple solutions.
A simple billing system (on top of ColdBox MVC) is ballooning into a semi-enterprisey inventory + provisioning + issue-tracking + profit tracking app. They seem to be doing their own thing yet they share many things including a common pool of Clients and Staff (login's), and other intermingled data & business logic.
How do you keep such system modular? from a maintenance, testability & re-usability stand point?
single monolithic app? (i.e. new package for the base app)
ColdBox module? not sure how to make it 'installable' and what benefits does it bring yet.
Java Portlet? no idea, just thinking outside the box
SOA architecture? through webservice API calls?
Any idea and/or experience you'd like to share?
I would recommend you break the app into modular pieces using ColdBox Modules. You can also investigate on separate business logic into a RESTful ColdBox layer also and joining the system that way also. Again, it all depends on your requirements and needs at the moment.
Modules are designed to break monolithic applications into more manageable parts that can be standalone or coupled together.
Stop thinking about technology (e.g. Java Portals, ColdBox modules, etc...) and focus on architecture. By this I mean imagining how you can explain your system to an observer. Start by drawing a set of boxes on a whiteboard that represent each piece - inventory, clients, issue tracking, etc... - and then use lines to show interactions between those systems. This focuses you on a separation of concerns, that is grouping together like functionality. To start don't worry about the UI, instead focus on algorithms and data.
If you we're talking about MVC, that step is focusing on the model. With that activity complete comes the hard part, modifying code to conform to that diagram (i.e the model). To really understand what this model should look like I suggest reading Domain Driven Design by Eric Evans. The goal is arriving at a model whose relationships are manageable via dependency injection. Presumably this leaves you with a set of high level CFCs - services if you will - with underlying business entities and persistence management. Their relationships are best managed by some sort of bean container / service locator, of which I believe ColdBox has its own, another example is ColdSpring.
The upshot of this effort is a model that's unit testable. Independent of of the user interface. If all of this is confusing I'd suggest taking a look at Working Effectively with Legacy Code for some ideas on how to make this transition.
Once you have this in place it's now possible to think about a controller (e.g. ColdBox) and linking the model to views through it. However, study whatever controller carefully and choose it because of some capability it brings to the table that your application needs (caching is an example that comes to mind). Your views will likely need to be reimagined as well to interact with this new design, but what you should have is a system where the algorithms are now divorced from the UI, making the views' job easy.
Realistically, the way you tackle this problem is iteratively. Find one system that can easily be teased out in the fashion I describe, get it under unit tests, validate with people as well, and continue to the next system. While a tedious process, I can assure it's much less work than trying to rewrite everything, which invites disaster unless you have a very good set of automated validation ahead of time.
Update
To reiterate, the tech is not going to solve your problem. Continued iteration toward more cohesive objects will.
Now as far as coupled data, with an ORM you've made a tradeoff, and monolithic systems do have their benefits. Another approach would be giving one stateful entity a reference to another's service object via DI, such that you retrieve it through that. This would enable you to mock it for the purpose of unit testing and replace it with a similar service object and corresponding entity to facilitate reuse in other contexts.
In terms of solving business problems (e.g. accounting) reuse is an emergent property where you write multiple systems that do roughly the same thing and then figure out how to generalize. Rarely if ever in my experience do you start out writing something to solve some business problem that becomes a reusable component.
I'd suggest you invest some time in looking at Modules. It will help with partitioning your code into logical features whilst retaining the integration with the Model.
Being ColdBox there is loads of doc's and examples...
http://wiki.coldbox.org/wiki/Modules.cfm
http://experts.adobeconnect.com/p21086674/
You need to get rid of the MVC and replace it with an SOA architecture that way the only thing joining the two halves are the service requests.
So on the server side you have the DAO and FACADE layers. And the client side can be an MVC or what ever architecture you want to use sitting somewhere else. You can even have an individual client for each distinct business.
Even for the server side you can break the project down into multiple servers: what's common between all businesses and then what's distinct between all of them.
The problem we're facing here luckily isn't unique.
The issue here seems not to be the code itself, or how to break it apart, but rather to understand that you're now into ERP design and development.
Knowing how best to develop and grow an ERP which manages the details of this organization in a logical manner is the deeper question I think you're trying to get at. The design and architecture itself of how to code from this flows from an understanding of the core functional areas you need.
Luckily we can study some existing ERP systems you can get a hold of to see how they tackled some of the problems. There's a few good open source ERP's, and what brought this tip to my mind is a full cycle install of SAP Business One I oversaw (a small-mid size ERP that bypasses the challenges of the big SAP).
What you're looking for is seeing how others are solving the same ERP architecture you're facing. At the very least you'll get an idea of the tradeoffs between modularization, where to draw the line between modules and why.
Typically an ERP system handles everything from the quote, to production (if required), to billing, shipping, and the resulting accounting work all the way through out.
ERPS handle two main worlds:
Production of goods
Delivery of service
Some businesses are widget factories, others are service businesses. A full featured out of the box ERP will have one continuous chain/lifecycle of an "order" which gets serviced by a number of steps.
If we read a rough list of the steps an ERP can cover, you'll see the ones that apply to you. Those are probably the modules you have or should be breaking your app into. Imagine the following steps where each is a different document, all connected to the previous one in the chain.
Lead Generation --> Sales Opportunities
Sales Opportunities --> Quote/Estimate
Quote Estimate --> Sales Order
Sales Order --> Production Order (Build it, or schedule someone to do the work)
Production order --> Purchase orders (Order required materials or specialists to arrive when needed)
Production Order --> Production Scheduling (What will be built, when, or Who will get this done, when?)
Production Schedule --> Produce! (Do the work)
Produced Service/Good --> Inventory Adjustments - Convert any raw inventory to finished goods if needed, or get it ready to ship
Finished Good/Service --> Packing Slip
Packing Slip items --> Invoice
Where system integrators come in is using the steps required, and skipping over the ones that aren't used. This leads to one thing for your growing app:
Get a solid data security strategy in place. Make sure you're confortable that everyone can only see what they should. Assuming that is in place, it's a good idea to break apart the app into it's major sections. Modules are our friends. The order to break them up in, however, will likely have a larger effect on what you do than anything.
See which sections are general, (reporting, etc) that could be re-used between multiple apps, and which are more specialized to the application itself. The features that are tied to the application itself will likely be more tightly coupled already and you may have to work around that.
For an ERP, I have always preferred a transactional "core" module, which all the other transaction providers (billing pushing the process along once it is defined).
When I converted a Lotus Notes ERP from the 90's to the SAP ERP, the Lotus Notes app was excellent, it handled everything as it should. THere were some mini-apps built on the side that weren't integrated as modules which was the main reason to get rid of it.
If you re-wrote the app today, with today's requirements, how would you have done it differently? See if there's any major differences from what you have. Let the app fight for your attention to decide what needs overhauling / modularization first. ColdBox is wonderful for modularization, whether you're using plugin type modules or just using well separated code you won't go wrong with it, it's just a function of developer time and money available to get it done.
The first modules I'd build / automate unit testing on are the most complex programatically. Chances are if you're a decent dev, you don't need end to end unit testing as of yesterday. Start with the most complex, move onto the core parts of the app, and then spread into any other areas that may keep you up at night.
Hope that helped! Share what you end up doing if you don't mind, if anything I mentioned needs further explanation hit me up on here or twitter :)
#JasPanesar
I'm drinking the coolade and loving it - interfaces, IoC, DI, TDD, etc. etc. Working out pretty well. But I'm finding I have to fight a tendency to make everything an interface! I have a factory which is an interface. Its methods return objects which could be interfaces (might make testing easier). Those objects are DI'ed interfaces to the services they need. What I'm finding is that keeping the interfaces in sync with the implementations is adding to the work - adding a method to a class means adding it to the class + the interface, mocks, etc.
Am I factoring the interfaces out too early? Are there best practices to know when something should return an interface vs. an object?
Interfaces are useful when you want to mock an interaction between an object and one of its collaborators. However there is less value in an interface for an object which has internal state.
For example, say I have a service which talks to a repository in order to extract some domain object in order to manipulate it in some way.
There is definite design value in extracting an interface from the repository. My concrete implementation of the repository may well be strongly linked to NHibernate or ActiveRecord. By linking my service to the interface I get a clean separation from this implementation detail. It just so happens that I can also write super fast standalone unit tests for my service now that I can hand it a mock IRepository.
Considering the domain object which came back from the repository and which my service acts upon, there is less value. When I write test for my service, I will want to use a real domain object and check its state. E.g. after the call to service.AddSomething() I want to check that something was added to the domain object. I can test this by simple inspection of the state of the domain object. When I test my domain object in isolation, I don't need interfaces as I am only going to perform operations on the object and quiz it on its internal state. e.g. is it valid for my sheep to eat grass if it is sleeping?
In the first case, we are interested in interaction based testing. Interfaces help because we want to intercept the calls passing between the object under test and its collaborators with mocks. In the second case we are interested in state based testing. Interfaces don't help here. Try to be conscious of whether you are testing state or interactions and let that influence your interface or no interface decision.
Remember that (providing you have a copy of Resharper installed) it is extremely cheap to extract an interface later. It is also cheap to delete the interface and revert to a simpler class hierarchy if you decide that you didn't need that interface after all. My advice would be to start without interfaces and extract them on demand when you find that you want to mock the interaction.
When you bring IoC into the picture, then I would tend to extract more interfaces - but try to keep a lid on how many classes you shove into your IoC container. In general, you want to keep these restricted to largely stateless service objects.
Sounds like you're suffering a little from BDUF.
Take it easy with the coolade and let it flow naturally.
Remember that while flexibility is a worthy objective, added flexibility with IoC and DI (which to some extent are requirements for TDD) also increases complexity. The only point of flexibility is to make changes downstream quicker, cheaper or better. Each IoC/DI point increases complexity, and thus contributes to making changes elsewhere more difficult.
This is actually where you need a Big Design Up Front to some extent: identify what areas are most likely to change (and/or need extensive unit testing), and plan for flexibility there. Refactor to eliminate flexibility where changes are unlikely.
Now, I'm not saying that you can guess where flexibility will be needed with any kind of accuracy. You'll be wrong. But it's likely that you'll get something right. Where you later find you don't need flexibility, it can be factored out in maintenance. Where you need it, it can be factored in when adding features.
Now, areas which may or may not change depends on your business problem and IT environment. Here are some recurring areas.
I'd always consider external
interfaces where you integrate to
other systems to be highly mutable.
Whatever code provides a back end to
the user interface will need to support change in the UI. However, plan for changes in functionality primarily: don't go overboard and plan for different UI technologies (such as supporting both a smart client and a web application – usage patterns will differ too much).
On the other hand, coding for
portability to different databases
and platforms is usually a waste
of time at least in corporate
environments. Ask around and check
what plans may exist to replace or
upgrade technologies within the
likely lifespan of your software.
Changes to data content and formats are a tricky
business: while data will
occasionally change, most designs
I've seen handle such changes
poorly, and thus you get concrete
entity classes used directly.
But only you can make the judgement of what might or should not change.
I usually find that I want interfaces for "services" - whereas types which are primarily about "data" can be concrete classes. For instance, I'd have an Authenticator interface, but a Contact class. Of course, it's not always that clear-cut, but it's an initial rule of thumb.
I do feel your pain though - it's a little bit like going back to the dark days of .h and .c files...
I think the most important "agile" principle is YAGNI ("You Ain't Gonna Need It"). In other words, don't write extra code until it's actually needed, because if you write it in advance the requirements and constraints may well have changed when(if!) you finally do need it.
Interfaces, dependency injections, etc. - all this stuff adds complexity to your code, making it harder to understand and change. My rule of thumb is to keep things as simple as possible (but no simpler) and to not add complexity unless it gains me more than enough to offset the burden it imposes.
So if you are actually testing and having a mock object would be very useful then by all means define an interface that both your mock and real classes implement. But don't create a bunch of interfaces on the purely hypothetical grounds that it might be useful at some point, or that it is "correct" OO design.
Interfaces have the purpose of establishing a contract and are particularly useful when you want to change the class performing a task on the fly. If there is no need to change the classes an interface just might get in the way.
There are automatic tools to extract the interface, so maybe you'd better delay the interface extraction later in the process.
It depends very much on what you are providing... if you are working on internal things then the advice of "don't do them until needed" is reasonable. If, however, you are making an API that is to be consumed by other developers then changing things around to interfaces at a later date can be annoying.
A good rule of thumb is to make interfaces out of anything that needs to be subclasses. This is not an "always make an interface in that case" sort of thing, you still need to think about it.
So, the short answer is (and this works with both internal things and providing an API) is that if you anticipate more than one implementation is going to be needed then make it an interface.
Somethings that generally would not be interfaces would be classes that only hold data, like say a Location class that deals with x and y. THe odds of there being another implementation of that is slim.
Do not create the interfaces first - you ain't gonna need them. You cannot guess for which classes you'll need an interface of for which classes you don't. Therefore do not spend any time to burden the code with useless interface now.
But extract them when you feel the urge to do so - when you see the need of an interface - at the refactoring step.
Those answers can help too.