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Is it good to keep updating room database with every action or to do it at the end?

I am working on a note taking app and i am storing various images and texts using room persistence. I was wondering if it is better to keep updating the database every time user performs an action (like update the existing note) or is it better to do it at the end when the activity terminates or moves on to another activity.
What are the pros and cons of both the options.
Is there any better way than that ?

In my opinion you should update it on every action. Looks like this way google keep works to provide syncronization with server. And you should consider that your app can be closed without calling even onPause (e.g. due to some system crash), so this approach is more safe. So I don't really see any cons in updating database on every action.

IMHO one important thing is to never assume the users just stay within same app for more than 5 minute. They will switch to another app or do lock screen then leave your app for 30 minute and demand same exact screen when they come back to your app. If the activity already cleared by OS then... you know the rest.
So I'll definitely go with the first approach(update db for every action).

The best practice is to update your database on every change. If you worry about performance it doesn't have a sensible impact on your app performance. In applications with hundreds of database transactions per second like multipart download managers maybe it impacts the CPU and disk usage but your task is to slow for performance issues. Although you could use onStop() method to save data on the database before the user exit based on Android document but this work has no efficiency in your case and Android doesn't guarantee to call onStop in every situation.

Keep some variables initialized on destroy()

I have an app that requires fairly substantial initialization (CPU-wise, not memory). When the user hits the back button, I'd prefer to leave the hard computations and allocated structures intact so there is no 3-4 second delay every time they hit the icon.
What is the proper way to do this? Should part of the app be a service?

Decouple your UI and core logic. Since you want your application keep running in the background implement a service component and maintain the state of your native library initialisation and other stuff which you wanted to be available for the UI. Service plus a state machine approach will give you better benefit in such cases.

When will one need to create a separate process in a Application?

I was reading a article in Android developer blog Process and Threads which talks about creating new process for specific component of Application. But I failed to understand when will creating a new process in my application becomes a absolute need. Could you please help me understand following doubts I have in this regard.
When as a developer I should feel I need to have a separate process for a Android component/s?
Does introducing a new process has any side effect on application's overall performance?
Any other info is greatly appreciated.
Thanks,
SKU

Having a separate process can be useful if there are components of your application that do not necessarily need to both be running to be useful to the user, and the background task is critical to application "correctness" (either now or in the future). The classic example of this is an app that has a service where the service saves or uploads some data that is critical to your application (critical meaning the only way to get the data back is to have the user re-enter it!). The service might be responsible for doing something like uploading or saving data, while the activity is just the interface for the user. So developers should decouple these two components to prevent problems that may arise from my next point..
Android was designed to run in a resource (especially memory) constrained environment, so processes deemed unimportant are killed periodically to open up memory for important ones by the "low memory killer" (LMK) (if you Google this you'll get tons of information on the topic). Things like foreground processes are understandably given a higher priority since they're currently in use, but they're sometimes killed off as well for reasons like consuming too much memory. Now, imagine you need to save off some data to a database after the user does something in the app and you use a service to do so to ensure that it is done even if the user navigates away from the app. Unless you create the service in its own process the process containing both the activity and the service is likely to be killed since the process belongs to a non-foreground activity.
However it is not always necessary to place the service in its own process, oftentimes simply giving the service its own thread will suffice; it's very application specific. I would only place a service in its own process if it took longer than maybe a few seconds (long enough for the user to navigate away from my application and for the LMK to step in) to perform some task in the background and that task related to the "correctness" of my application (I.E. saving data for later). For something like caching, stick to threads, since if the process gets prematurely killed you can just recreate that data later.
Another reason to have a separate process is if you're running a global service (a service that can be used by applications other than your own) that maybe you provide an interface with via an Activity for configuration.
As for the performance question, there will definitely be a performance hit for something like this. Interprocess communication is not cheap, so you should really only use a separate process if you fit into a specific use case, like the ones mentioned above. Also, there's a certain amount of memory overhead for maintaining a process, so that's another performance hit.

1.)You need to do something on seperate process or thread when you don't want your app to behave slowly. As by introducing threads you force your app not to run on UI thread. Thus making your app responsive to other events. For Example : you can use threads when you have to fetch some data from web service so that it happens in background and doesn't effect your app.
2.)Threads should not be used..We should use AsyncTask or loaders rather in android.

1.) In android 4.0 (and possibly 3.0, not sure though) The device does not let you use the HTTP Agent in the main thread, for this slows the UI..
This is when threads come in handy.
Also with the use of functions that need alot of cpu, if these are run in the UI thread, the UI will lag and not respond until the function finishes.
2.) as stated at 1, it will actually improve the visual performance of your app ;)

Is an always-on Android service ever not an anti-pattern?

I understand why an always-on service is normally an anti-pattern in Android, but my app really seems to be begging for one:
On first load, the app has to go through potentially thousands of small entities from the database to construct the initial state. There's not much data brought into memory (most is lazy loaded later), but that first scan is unavoidable by the nature of the app. This scan can take at worst 6-7 seconds with slow hardware and a big dataset, average is probably around 3. The app is a "impulse use in short bursts" type of thing, so those repeated loads are really not desirable.
I think this begs for a background service to be perpetually alive and holding that state, thus avoiding that load time. It will always be ready to be killed, and not in the foreground, so should the system or user decide that they have it out for the service, no harm is done. But if the service is left in peace, the app will start instantly, and in my case that does a lot for the user experience.
Am I still wrong?

I think this begs for a background service to be perpetually alive and holding that state, thus avoiding that load time.
As the British say, bollocks.
On first load, the app has to go through potentially thousands of small entities from the database to construct the initial state.
Then fix that. Either simplify this work, or persist the initial state in a simpler form for later reuse (e.g., JSON).
If it is OK for you to use a cached result of this work that is held in RAM, it is OK for you to use a cached result of this work that is held in an easier-to-read-in persistent data structure.

An "always-on" service would essentially act like a daemon process and there are plenty of services on Android phones that never turn off.
In this case, though, it seems a better solution would be to simply have a splash screen and/or wait dialog that sits there until the data is loaded. It seems like a bad idea to me to take up resources when the app isn't running just so the app will load faster when the user finally opens it. If the average use of the app is much smaller than the load time, then it would probably be even better to speed up the scan in some way.

People use taskkillers to kill such kind of services. My view is, that when you make the user aware of why your service is running (say, this will load the app quicker), he will understand it and not kill it. You could also ofcourse add an option to use the service or not.

android design considerations: AsyncTask vs Service (IntentService?)

I'm designing an android app which will need to do the following steps:
user pushes a button or otherwise indicates to "sync data".
sync process will use REST web services to move data to and from the server.
the data will be stored locally in a sqlite database.
the sync process should provide status updates/messages to the UI
the user should not be allowed to wander off to other parts of the application and do more work during the sync process.
The first time the sync process runs, it may take 10-20 minutes.
After the initial sync, less data will be transferred and stored and
I expect the process to take 1-2 minutes or less.
I've been doing a lot of reading about android's AsyncTask and various examples of using a Service ... But I don't fully understand the design considerations and trade-offs of choosing one design over the other. I currently have my demo project stubbed out using an AsyncTask. After watching (most of) Developing Android REST client applications: http://code.google.com/events/io/2010/sessions/developing-RESTful-android-apps.html# I'm left confused the design patterns described here feel overly
complex, perhaps because I just "don't get it" yet.
I come from a java, spring, web and desktop application background. Thinking and designing in terms of a handheld device is quite new to me. (What happens when the screen layout is changed? What happens when the phone rings while I'm running a sync?) Taking 2 steps back, if the initial sync IS going to be such a long running process, is there a better way for me to think about the problem->solution, the user experience, the user expectations of an application running on a phone?
Would love to hear from some more experienced android developers out there who have already wrestled with these questions.

In my opinion this is the most tricky/hard part of a mainstream/average Android development. For instance on BlackBerry this is IN TIMES easier.
Definitely you need to use a Service.
AsyncTask does not suit, because it is tightly "bound" to your Activity via a Context handle (otherwise you would not be able to update UI of the Activity from your AsyncTask). However an Activity can be killed by OS once the Activity went in background. An example reason of going to background can be an incoming call - user switches to Phone application so your Activity becomes invisible. In this case (depending on the current RAM state) OS may decide to kill one of the background (invisible to the user) activities.
Some devs workaround this by arranging a static stuff for having a long-running actions inside of. Some recommend to use Application instance. This is because static stuff and Application exist while the whole app process exists. However those are incorrect workarounds. Processes in Android are also may be killed when OS decides it is time to. Android OS have its own considerations about what it can kill and in what order. All processes are devided to 5 levels of "killability". Here is the doc where those levels are specified. It is interesting to read there:
Because a process running a service is
ranked higher than one with background
activities, an activity that initiates
a long-running operation might do well
to start a service for that operation,
rather than simply spawn a thread —
particularly if the operation will
likely outlast the activity. Examples
of this are playing music in the
background and uploading a picture
taken by the camera to a web site.
Using a service guarantees that the
operation will have at least "service
process" priority, regardless of what
happens to the activity.
Your Activity where users initiate a long-running action should show a ProgressDialog to make sure user does not do anything else while the action is running. The guide is here.
Also, you'd most likely want to use the NotificationManager for notifying the user about your long-running action completion (or failure) if your Activity is currently invisible. Here is the NotificationManager info to start from.

There are multiple considerations that you must weigh in order to best decide how to approach your situation. It sounds like you need a good comparison between the two approaches... So here is a list of similarities, and differences and additional considerations that must be taken into account when working on a handheld device.
A Service is a part of your Application that has no UI. It may be called by a UI(Activity) to be started, or may be started by any other component of your Application. When developing, you have the freedom to place it on a different thread, or even run it in a different Task or Process. This allows you to ultimately separate it from your UI. Additionally, you may start the Service to run independently (startService) or bind your activity to it (bindService) depending upon your needs. By using custom Handlers, you can set callbacks to update the UI with your progress. A Service does not necessarily end if a User changes Activities, but may be ended at ANY time by the OS.
A AsyncTask is always instantiated from the UI thread. It only allows specific callbacks, but simplifies the process of multi-threading for the purposes of relatively short transactions (as compared to dedicated separate threaded services) that are inherently tied to actions performed by an Activity. Whenever a User changes Activities, the AsyncTask is put on "pause" and may even die because there is no UI thread for your Activity any longer.
The thing that I would be most concerned about is if the app is going to take 10-20 minutes the first time, I would ASSUME that the User will either change tasks temporarily or set the phone down until it completes (which can cause all of the same complications if the phone sleeps). Given this consideration, a threaded service bound to your activity may be your best choice. To protect your UI, I would make a Progress Dialog for your Activity that receives your progress callbacks. This limits user input in YOUR app and allows your service to continue the way that it needs to. Then override the Activity onResume to check the status of your Service and if it is running. Then you can reset the Dialog immediately.
Given that this is my preferred method, I would also take into account that the OS may kill the App at any time anyway. So make sure to have some way to detect an incomplete or partial sync. Then you may resume automatically when your Activity or Service restarts.

With AsyncTask if the user goes to another Activity you can't transfer that object to the other Activity so it dies. There are tricks you can play when say the user rotates the screen or something like that, but that doesn't extend to general purpose destruction. AsyncTask can randomly die.
Google Sync is run as a Service in the background because syncing can take a while to complete. You might need to follow their path and create your own sync service that you can communicate with. Here is some thoughts how to accomplish that:
http://mylifewithandroid.blogspot.com/2008/01/about-binders.html
You can definitely communicate between Service and Activity, but it's tricky to do it right.

The choice is mainly dependent on the app design. Since both AsyncTask and IntentService stands their ground, what you may want from the app(user experience) is more important and then choose either or both. Some scenarios are mentioned below (mostly what I experienced while developing apps)
Assume apps that have feeds pages - where more than one api calls are made to make the page presentable ( /getFriends, /getDates, /getPictures etc.) you can warp all such api calls to a single AsyncTask with executor which is multithreaded and the sequence of execution doesn't matter. In contrast to IntentService which runs all calls in sequence in a single worker thread. For a high end device with multi-core the call from AsyncTask is more effective. And if you start the AsyncTask on UI thread then updating IU is a piece of cakes(read less boiler plate code). And even if an user leaves the page, with intelligent use of not holding on to the context the app doesn't crash.
Assuming you are trying to write an app which doesn't need the user to be on view/activity/fragment and the total execution time to show something is not mission critical (assume sync service or user notification/alarm) then IntentService is a better choice. (no hassle to start Asynctask on UI thread so that you don't need to write a Handler to force changes on UI etc. etc. and less boiler plate code)
From my experience - write small app for both and compare the pros and cons to get a better idea. (p.s I'd suggest take a look at the iosched app from google to get a better idea - they use both Asynctask and IntentService)

I tend to prefer the IntentService + BroadcastReceiver combo because they give you a really strong degree of control
You definitely have to make sure the UI is running if you are updating something on the screen. ASyncTask crashes were at once reported to be one of the top causes of Android crashes. This can be avoided by keeping some sort of "activityIsAlive" variable and skipping or delaying a UI update if the activity is dead.
The IntentService + BroadcastReceiver combo is a little more resistant to the crash because most tutorials tell you to shut off the BroadcastReceiver onPause or onStop. If you do not do this, again you'll have to turn off the UI update. There's a runOnUiThread command somewhere that will help you do UI updates.
The IntentService + BroadcastReceiver combo is also more extensible. You can create functions and extend BroadcastReceiver to make a more elegant REST processing solution. However, it does require more plumbing vs an ASyncTask
If you do delay the UI update, you may be able to rig it on OnWindowFocusChangedListener. When that function receives true, it means that the UI is alive.
tldr; Make sure the Activity and/or Fragment is alive before updating the UI if you are running something in the background
2015 Edit: check out Loaders as well. A little harder to grasp because there's a lot going on behind the scenes