I need to build a rudimentary RESTful session management service in Scala. A user will login and receive a session id in return. This session id will be validated on each service call. Users will be logged out after a period of inactivity.
The session management service will (could) be a simple in-memory singleton, with a map of session ids to expiry times. Where a user's session has expired it should be removed from the map. The map can be read and written by multiple threads simultaneously.
Idiomatic Scala would suggest this map be immutable but how would I handle updates? As I see the options:
Synchronize access to a mutable map
Make the map immutable but synchronize access to its reference
What is the idiomatic way of handling this kind of problem?
Note: Akka is not an option, but other libraries are.
As a developer, you have a set of techniques to deal with concurrency that you can pick up. If you decide to go for synchronisation, you should be aware of the price:
Performance decreases as lock contention increase
Lock contention is a function of how long locks are held
You need to fine tune locking (or you'll end up in the degenerate case where the lock is held forever -> single threading)
Using a singleton with synchronized access makes latency increase very quickly. Assuming each request keeps the lock for 30ms, and requests arrive every 25ms, you will have a growing latency and your users will be really upset.
If your application is a trivial exercise, go for locking. If your application has speed/latency requirement, the sooner you abandon synchronization techniques the better. And by the way, storing session in memory won't work if you need to deploy your application in a highly available cluster.
I'm building a simple REST API for generating some objects that must be created and sent periodically out of the API. The nature of the objects doesn't matter, neither the framework supporting the REST interface (Spray, Play Framework, whatever else). My question is, what would be a good scalable actor design for this system using Akka? Suppose the service crashes or it's migrated or whatever that causes to stop it. In order to recover the description of the tasks about what objects must be sent and when, is akka-persistence a good way to go here? or it's better to persist such things in a traditional DB?
Thanks.
NOTE: also I would like to know, supposing there's some actor which is not stateful himself, but creates many children actors, if it's a good practice to use akka-persistence in order to replay the messages which causes this actor to create his children again (the children being also non-stateful).
In a traditional DB you would most likely end up modeling this with timestamps and events, and with event sourcing this is already the native model.
Akka-persistence would be a natural fit for this scenario since it will persist every event about what objects must be created and sent periodically out. The snapshot support will also help with speed of recovery when the number of events gets very large.
In the case of crashes or migration, the recovery process will handle this just fine.
Regarding your note, if the actor is truly stateless then there is no need to persist the events that cause the children to be created since they can be recreated on demand. If the existence of the children does need to be recovered, then the actor is not stateless. In that case then it may indeed make sense to persist those events.
After using EFProfiler (absolutely fantastic tool BTW!) for profiling a few of our Entity Framework applications, it seems that, in most cases, all of the Object Contexts are not closed.
For example, after running it locally, EF Profiler told me that there were 326 Object Context's opened, yet only 1 was closed?
So my question is, should I worry about this? Or is it self-contained within Entity Framework?
If you're not using an IoC container is there anyway you can close the ObjectContexts manually after each request, for example in the End Request of your Global.asax, thereby simulating a "per request" lifestyle for your contexts?
ObjectContexts will be disposed eventually if your application is not holding onto them explicitly, but in general, you should try to dispose them deterministically as soon as possible after you are done with them. In most cases, they will hold onto database connections until they are disposed. In my current web application, we use an IoC container (Autofac) to ensure that any ObjectContext opened during a request is disposed at the end of the request, and does not have to wait for garbage collection.
I suggest you do worry about it and try to fix the issue as Object Contexts are pretty "bulky". If you have too many of them your application may eventually end up using more memory than it needs to and IIS will be restarting your application more frequently then...
I'm reading this article on how to : correctly retain variable state in Android and I'm reminded that I've never gotten a good answer (and can't find one here) for why it's better to tussle with the Bundle (which isn't a HUGE hassle, but definitely has its limitations) rather than just always have an Application overridden in your App, and just store all your persistent data members there. Is there some leakage risk? Is there a way that the memory can be released unexpectedly? I'm just not clear on this... it SEEMS like it's a totally reliable "attic" to all the Activities, and is the perfect place to store anything that you're worried might be reset when the user turns the device or suspends the app.
Am I wrong on this? Would love to get some clarity on what the true life cycle of the memory is in the Application.
Based on the answers below, let me extend my question.
Suppose I have an app that behaves differently based on an XML file that it loads at startup.
Specifically, the app is a user-info gathering app, and depending on the XML settings it will follow an open ended variety of paths (collecting info A, but not J, and offering Survey P, followed by an optional PhotoTaking opportunity etc.)
Ideally I don't have to store the details of this behavior path in a Bundle (god forbid) or a database (also ugly, but less so). I would load the XML, process it, and have the Application hold onto that structure, so I can refer to it for what to do next and how. If the app is paused and the Application is released, it's not *THAT big a hassle to check for null in my CustomFlow object (that is generated as per the XML) and re-instantiate it. It doesn't sound like this would happen all that often, anyway. Would this be a good example of where Application is the *best tool?
The question as to which method is better largely depends upon what information you are storing and need access to and who (which components, packages, etc.) needs access to that information. Additionally, settings like launchMode and configChanges which alter the lifecycle can help you to determine which method is best for you.
First, let me note, that I am a huge advocate for extending the Application object and often extend the Application class, but take everything stated here in its context as it is important to understand that there are circumstances where it simply is not beneficial.
On the Lifecycle of an Application: Chubbard mostly correctly stated that the Application has the same life as a Singleton component. While they are very close, there are some minute differences. The Application itself is TREATED as a Singleton by the OS and is alive for as long as ANY component is alive, including an AppWidget (which may exist in another app) or ContentResolver.
All of your components ultimately access the same object even if they are in multiple Tasks or Processes. However, this is not guaranteed to remain this way forever (as the Application is not ACTUALLY a Singleton), and is only guaranteed in the Google Android, rather than the manufacturer overridden releases. This means that certain things should be handled with care within the Application Object.
Your Application object will not die unless all of your components are killed as well. However, Android has the option to kill any number of components. What this means is that you are never guaranteed to have an Application object, but if any of your components are alive, there IS an Application to associate it to.
Another nice thing about Application is that it is not extricably bound to the components that are running. Your components are bound to it, though, making it extremely useful.
Things to Avoid in Application Object:
As per ususal, avoid static Contexts. In fact, often, you shouldn't store a Context in here at all, because the Application is a Context itself.
Most methods in here should be static, because you are not guaranteed to get the same Application object, even though its extremely likely.
If you override Application, the type of you data and methods store here will help you further determine whether you need to make a Singleton component or not.
Drawables and its derivatives are the most likely to "leak" if not taken care of, so it is also recommended that you avoid references to Drawables here as well.
Runtime State of any single component. This is because, again, you are not guaranteed to get back the same Application object. Additionally, none of the lifecycle events that occur in an Activity are available here.
Things to store in the Application (over Bundle)
The Application is an awesome place to store data and methods that must be shared between components, especially if you have multiple entry points (multiple components that can be started and run aside from a launch activity). In all of my Applications, for instance, I place my DEBUG tags and Log code.
If you have a ContentProvider or BroadcastReceiver, this makes Application even more ideal because these have small lifecycles that are not "renewable" like the Activity or AppWidgetProvider and can now access those data or methods.
Preferences are used to determine, typically, run options over multiple runs, so this can be a great place to handle your SharedPreferences, for instance, with one access rather than one per component. In fact, anything that "persists" across multiple runs is great to access here.
Finally, one major overlooked advantage is that you can store and organize your Constants here without having to load another class or object, because your Application is always running if one of your components is. This is especially useful for Intent Actions and Exception Messages and other similar types of constants.
Things to store in Bundle rather than Application
Run-time state that is dependent upon the presence or state of a single component or single component run. Additionally, anything that is dependant upon the display state, orientation, or similar Android Services is not preferrable here. This is because Application is never notified of these changes. Finally, anything that depends upon notification from that Android System should not be placed here, such as reaction to Lifecycle events.
And.... Elsewhere
In regard to other data that needs to be persisted, you always have databases, network servers, and the File System. Use them as you always would have.
As useful and overlooked as the Application is, a good understanding is important as it is not ideal. Hopefully, these clarifications will give you a little understanding as to why gurus encourage one way over the other. Understand that many developers have similar needs and most instruction is based on what techniques and knowledge a majority of the community has. Nothing that Google says applies to all programmer's needs and there is a reason that the Application was not declared Final.
Remember, there is a reason Android needs to be able to kill your components. And the primary reason is memory, not processing. By utilizing the Application as described above and developing the appropriate methods to persist the appropriate information, you can build stronger apps that are considerate to the System, the User, its sibling components AND other developers. Utilizing the information that everyone here has provided should give you some great guidance as to how and when to extend your Application.
Hope this helps,
FuzzicalLogic
I prefer to subclass Application and point my manifest to that. I think that's the sane way of coding android although the Android architects from Google think you should use Singletons (eek) to do that. Singletons have the same lifetime as Application so everything that applies to them applies to Application except much less dependency mess Singletons create. Essentially they don't even use bundles. I think using subclass Application has dramatically made programming in Android much faster with far less hassle.
Now for the downside. Your application can be shutdown should the phone need more memory or your Application goes into the background. That could mean the user answered the phone or checked their email. So for example, say you have an Activity that forces the user to login to get a token that other Activities will use to make server calls. That's something you might store in your service object (not android service just a class that sends network calls to your server) that you store in your subclass of Application. Well if your Application gets shutdown you'll loose that token, and when the user clicks the back button your user might return to an Activity that assumes you are already authenticated and boom your service class fails to work.
So what can you do? Continue to use Bundle awfulness? Well no you could easily store security tokens into the bundle (although there might be some security issues with that depending on how this works for your app), or you have to code your Activities to not assume a specific state the Application is in. I had to check for a loss of the token and redirect the user back to the login screen when that happens. But, depending on how much state your Application object holds this could be tricky. But keep in mind your Application can know when it's being shutdown and persist it's internal state to a bundle. That at least allows you to keep your Objects in memory for 99% of the time your Application, and only save/restore when it gets shutdown rather than constantly serializing and deserializing with boiler plate code whenever you move between Activities. Using Application lets you centralize how your program can be brought up and shutdown, and since it normally lives longer than any one activity it can reduce the need for the program to reconstitute the guts of your App as the user moves between Activities. That makes your code cleaner by keeping out details of the app from every Activity, reduces overhead if your Application is already built, shares common instances/code, and allows Activities to be reclaimed without loosing your program all together. All good programs need a centralized hub that is the core, and subclassing Application gives you that while allowing you to participate in the Android lifecycle.
My personal favorite is to use http://flexjson.sourceforge.net/ to serialize my Java objects into bundles as JSON if I need to send objects around or save them. Far easier than writing to sqlite DB when all you need to do is persist data. And nice when sending data between two Activities using objects instead of broken apart primitives.
Remember by centralizing your model in the Application you create a place to share code between multiple Activities so you can always delegate an Activities persistence to an object in the Application by hooking the onPause() as well allowing persistence to be centrally located.
The short answer is: use bundles as it makes saving your state out when you're backgrounded easier. Also, it's complicated.
The long answer:
My understanding is, as soon as you Activity's onPause method is called (and onSaveInstanceState which gives you a bundle into which you should store your Activity's data) your process can be terminated without further warning. Later, when the user comes back to your application, your activity is given an onCreate call with that original bundle from which to restore its state. This will happen to all your activitys in what was your original stack.
Being able to restore your state from the bundle (which Android will save for you as your process goes away) is how Android maintain's the myth of multi-tasking. If you don't dump your activity's state out to a bundle each time onSaveInstanceState is called, your app will look like it's been restarted when the user may have just switched out for a second. This can be especially troubling when the system is resource constrained as the system would need to kill off processes more often in order to keep the device running quickly
Why the Application can be Bad
The Application does not actually get a chance to save any of its data if the process is shut down. It does have an onDestroy method but the docs will tell you that this actually never gets called by the system on an actual device. This means that, in the constrained case I mentioned above, any incidental information about what's going on within an Activity (if you've saved it in the Application) will be lost if the process is ended.
Developer's often miss this case (and it can be really annoying for users) because they're either running on a dev phone which never gets hit with using many applications at the same time. We're also never using the app for a while, then switching to another application and, after a while, switching back again.
You have to have one per thread, but beyond that ... should you have more? Is it harmful to have more?
For instance, I'm tempted to build my app around having one NSManagedObjectContext per tab, handling the subset of the overall persistent store that appears on that particular tab.
That way, I can have the tab GUI listen to notificataions on "its" context, and ignore others. Asynch network calls will change just one tab's context at a time.
(NB: not using NSFetchController because of the major bugs in that class pre iOS 3.2, and this is a 3.0+ app)
Have I misunderstood how to use NSManagedObjectContext? If so, can anyone point out "good"/"bad" reasons for using additional NSManagedObjectContext instances?
UPDATE: this worked fine for discrete contexts, but when I tried to extrapolate to using multiple contexts on one set of data, to handle temporary changes, it all goes horribly wrong: c.f. question on that topic: How to make/use temporary NSManagedObjects?
It is possible, yes, but be aware that you have to deal with merging them properly and making sure that you don't have stale data between contexts.