Develop Reference

How does a geolocator work?

I need to develop an Android app which is basically a geolocator, allowing people to reach each other via Google maps. All being said, I don't know how do I accomplish such thing since I don't know how it does work and if I need a server or database on the way. I'd be grateful if anyone could shed some light on this topic.

As you said you will need Android application and Server through which Android clients will communicate.
Android Application
will be getting data of other Android devices from server and then showing those data on the screen
will be periodically sending own location to the server
Server
is responsible for storing geolocation of Android clients, data can be stored in some database or memory of the server, depends on the use case, how long you need to have those data, how important they are, etc.
is responsible for sending stored geolocation data to Android clients
you have various options, you can write your own server in whatever technology you wish - it can be PHP server, NodeJS server, Java server... or you will use some serverless solution. Check for example
https://firebase.google.com
has to be hosted somewhere, you have many options:
there are "traditional" hosting providers, where you are usually limited by technologies you can use, for example you can just use PHP or you will use products from Heroku or Amazon AWS.. there is more, Microsoft has Microsoft Azure or Redhat has Openshift.. There are differences in prices, services, etc., it is up to you to look and decide what is the best for you.
I for sure missed lot of things. It is not in my power to describe every possible corner case because the question is too broad but rather provide you with higher overview of the problem and possibly point you where to go next. I suggest to you to read more about problematic and then get back with more specific issues to solve.

How to use Android App as a "Producing client" for Kafka?

Is it possible / does it make sense to use an Android app as a "Producing client" for Apache Kafka?
Let's say my Android App need to capture and analyse reaction time data. Goal is to collect all data and show the average reaction time in real-time in the App.
The alternative is having an app server of some kind as an
intermediary that accepts messages from the android app and posts them to
Kafka, rather than having the app be a Kafka Producer on its own.

Even if it's possible, in my opinion it has some disadvantages.
In general I like clients to be as simple as possible to avoid maintenance issues. Instead I'd route all client requests through a REST API on my app server. The disadvantages are not related to Kafka, but are common problems of native clients.
Coupling
You're coupling the Android app closely to your messaging infrastructure. If you later decide that a Kafka solution is too much and Plain Old Java would be good enough, you'll first have to update the Android app and wait until enough users do an update.
Network issues + delivery guarantees
Kafka clients also require a direct connection to each of the brokers. Mobile clients can have very inconsistent/spotty network connectivity, making direct client access susceptible to dropped events and overall network connectivity issues.
Authentication
Probably you already have some kind of authentication in your app. You can also create authenticated connections to Kafka. So you'll have two authentication paths, whereas with an app server Kafka only needs to check if the requests are coming from the trusted app server, which means less implementation effort.
...

I think it would make lots of sense:
Kafka-clients.jar provide auto-reconnecting capability which is very useful when the phone is on a flaky internet connection
The Kafka-clients.jar is quite thin and does not include any of Kafka Server code (it doesn't even depend on Scala).
Unfortunately, it isn't compatible with Android just now: KAFKA-7025 . If you'd like to see this happen, please upvote the JIRA issue.

I believe that an Android Application can use a secured connection to a Kafka Broker cluster using SASL, for example. However, it must be done in coupling with any other communication, which can support synchronized keys rotation, which is initialized by any remote server with synchronization with the broker's cluster.
Any concrete instance of mobile application can listen to a concrete topic, and produce messages to a related topic, which is created when registering the instance using a REST server. Any deserializer verifies headers or Keys for a token, which are appointed using REST while registering on the same service. Custom encryption can be provided similarly.
Technically it is solvable. But what are the benefits, in front of using Firebase, for instance? Expenses I see from the start. Benefits???

How to develop and deploy a back-end server for my Android application?

I am developing a voting application based on Android. This app will be used to create user accounts, receive notifications and caste vote.
I want to develop a back-end server for this application which will also have an Administrator interface to create new voting events. I want to know what all options I have to implement this project.
The server must maintain user account records in a secure manner, send out one-time-passwords from email, have sessions implemented for users logging in from the Android App, send out notifications about upcoming events to the app.
What back-end technology, database, parsing(for communication between server and android application), encryption(for secure communication) should I use to make this possible?
I also want to know how would I be able to host this server, both on a local virtual machine as well as on an online hosting.

The scope of your project is far too large if you're wanting sessions, administration views, E-mail integration, encryption, etc., without even knowing how it would all interact, or even what language to use.
As for what languages / services you would need, it varies for each task:
For administration views, you're probably looking at either ASP.NET or PHP with a CMS.
In terms of security, you'd want either MySQLi, or preferably, PDO. Make sure to read up on how to prevent things like LFI, RFI, SQLi, and XSS if you want semi-decent security.
Both ASP.NET and PHP can handle sessions, and they're fairly easy to understand in either language, so that wouldn't be too much of a concern.
While you can send out E-mails directly using scripts like PHP's mail() function, you probably want a system that can easily monitor things like how often users click on certain links, where the people who click the links are located, what campaigns are most effective, etc. For that, you'd want a service like MailChimp, Marketo, or Pardot.
As for how to have the back-end interact with the front-end, you're working with Android, so you're probably working with Java. That means you would be limited to functions like HttpMethod. Having said that, you can create plain HTML applications, or use a service like PhoneGap. In either of those situations, you have AJAX at your disposal, which will make things a lot easier.
For hosting, simply do a Google search for 'web hosting service'. There's literally thousands of decent hosts. Some have better pricing than others, but some have better features. I can't really recommend any brilliant ones, as they all meet different needs. To host locally, you'll want either IIS, or WAMP / LAMP depending on whether you are running Windows or Linux.
If you're not familiar with almost every term that I've mentioned, then the scope of your project is too large, and you'll definitely want to scale back.
I don't mean to scare you off, just making sure you're aware of just how difficult it would be to put all of that together :)

Android two player game to be played in internet

I have developed a simple two player chess game in android to be played using Bluetooth. I want to extend it by making it possible to be played through internet. whenever a player makes a move, the move should be transferred to the other player via internet.
How to make this possible?
I have heard of C2DM mechanism.Does that suites the scenario i described and is it reliable?
Thanks:)

Yes, C2DM is ideally suited to this type of game. This is what I am using for my own game (http://www.chesspresso.net) which is a correspondence chess client for android.
Things to consider when using C2DM:
You don't send the info to the devices, you notify the devices that a move has been made. You don't use C2DM to transfer data, you use it to notify that something has changed.
Its available for 2.2+ Android, which is the majority of devices. But if you wanted to support older devices you'd have to consider an alternative. I am using polling for older devices.
You have to request for developer access, then once your app is ready you have to request production status. If you don't do this you'll hit the developer status quota very quickly once its released! They are very generous with production quota, but you have to explain what you're using it for and it also can take a few weeks to get accepted!
Your users will have to have a google account that is authorised, otherswise C2DM won't work. Most users will have an account associated with their device, but some don't so this means that you'll possibly want to validate for the presence of an account to notify the user.
Its reliable, but every now and again a device will have to wait for the message. Sometimes a few minutes. Usually its instant.
Hope that helps!
UPDATE:
C2DM has now been deprecated, and replaced by Google's GCM.
Also, I strongly suggest looking at other options as tying yourself down to a Google specific API means you won't be able to support external marketplaces. For alternatives, I am currently evaluating Amazon SNS and I will also be looking at Urban Airship. There are possibly other alternatives I have not considered evaluating yet.
UPDATE:
Evaluation update of non google based push notifications:
Amazon SNS is just not a project for this task and Urban Airship for the vast majority of apps is too expensive. Unfortunately all the other alternatives are all very expensive also, especially if your app (like mine) relies heavily on push.

A good way of doing that is using a simple direct TCP connection between the peers.
If you're new to socket programming on Java, try this:
All About Sockets

Another option is to use some sort of IM as a communication medium for app. For eg. Use Asmack to connect to XMPP Im like GTalk. Prompt user to create an account there, for your game.
And use it to send and receive commands via IM. This way you won't need to setup your mediating server.
This works if user knows who he is playing with. To collect the user data and let them search for available players, you still need to setup a server. IRC chat room may be an option to avoid this also.
GTalk was just an example. You can use any IM or IRC also.

C2DM it's not design to transfer informations, even if they are small like "horse in b4" or things like this. It's designed to inform the device of something, maybe a newer version of a document or more articles on a website.. Stuff like this.. It's not designed to communicate device to device. And also it may be not fast enough for a real time chess play.
You should look for a more traditional way of communicate via internet or to search for some libraries (I'm pretty sure that something exists..) that will help you.

IMHO, C2DM is exactly the kind of thing you would want for a chess game; to be notified when the oppo has made his turn (which may be minutes /hours / days later ?). I have discussed my game with a few google android devs and they've stated that C2DM is ideal for this. You'll need to go via a centralised server though (well, not essential but very advisable) as there may be issues with resync'ing game state etc. Worried about "hitting the limit" ? Well, for a start my c2dm acct is restricted to "just" 100,000 messages per day. I guess you're buying the drinks if you hit that !!

Chess is often played by email. You could do that.
Of course, any centralized/federated messaging system will work.
What might be better for your use is to add a jabber client to the application and have the program generate an account name that is used for automated messaging. You could host the jabber server or generate the accounts on a free provider.

Google App Engine if you know Python or Java.
Alternatively there are two web app API styles in wide use today: SOAP XML and RESTful web services.
If you know RoR I would recommend using JSON/REST, because you can just use Phusion Passenger with Apache to deploy your app. Free, extremely easy, and makes your server very reliable.
You could, and I only mention this because my friends do this all the time, use twitter as a server between the games.
I also found a lib called mages which looks quite promising.
Good luck.

I did this for my online 2D rpg: http://developingthedream.blogspot.com
Basically, use a middle-man server to co-ordinate data between all your clients.
You simply open a socket and communicate with the middle server and it takes care of passing on the information to any other connected clients.
I wouldn't recommend C2DM because of the message limit, and because the latency is still to big. Using your own server you can optimize it, plus you'll be the only one using the service so your data will be delivered faster.

I think that C2DM is not right way for playing chess because there is no warranty that messages will be delivered. You need more reliable way for data transfer

Android P2P Multiplayer game (with a) XMPP/Google talk b) JXTA peerdroid c) other way)

I am an android developer and I made some board games. Now i want to make some of my board games multiplayer. I don't want to create and host my own web service, so i thought about P2P.
The first thing i found was the XMPP protocol, however it's not real P2P, but if i can use the existing google talk service, i'm ready to go. Is this possible while using your existing google account without interfering with the normal working of your google talk client?
Then i heard about JXTA, a real P2P solution, and it's already ported from J2ME to Android (http://code.google.com/p/peerdroid/).
Maybe i am overcomplexing things here (as i do sometimes)
I just want to know the easiest way to do simple P2P for a boardgame.
All your opinions are welcome! Thanks in advance

Kristof, Did you get an answer to your question? I've been working on a multi-player application recently as well, though I've chosen to host the server (originally). I'm now reconsidering my choice, though, but the library I'm using fully supports peer to peer communications. The underlying protocol is built on top of Google Protobuf. It's essentially a full duplex RPC stack built on top of Netty, which can use Protobuf. Here's the URL to the RPC protobuf stack: http://code.google.com/p/protobuf-rpc-pro/
The author has been very helpful and I've found a couple of bugs, nothing major. I also had very little issues getting these libraries working on my Android phone, but they're not terribly "compact." Nothing extraordinarily large, just not small :). So far, I've had no issues getting the communications working both synchronously and asynchronously. As such, I may be moving my game over to a "peer to peer" style, and just provide the necessary location/registration server that would be used to find existing games/server.

Using XMMP should be possible for you case. Look at smack from igniterealtime. They have a nice and active java api that helps to build you own jabber extension packets that can be used to transport the changes in game state.
I think that using two google talk clients at the same time with the same account could be difficult. But it would be great because you could invite all the friends from you list to play with you directly from your game. This way you could easily get more people to play your game.

You can't use the existing Google Talk Service/Connection from what we've seen. You should take Janusz's advice and check out the smack library, that's your best bet. To allow multiple non-interfering connections with the same GTalk login take a look at the resource component of the XMPP address, it's what allows you to be logged into two clients at the same time to the same account. You can effectively make your game another client. More here: http://code.google.com/appengine/docs/java/xmpp/overview.html#JIDs_and_Resources
Finally, there appears to be some new functionality in 2.2 relating to device push communication, but I haven't looked into it yet. 2.2 is not widely deployed yet either, so probably of limited use.

Basicly, if you want to write a multi user game or a game built on top of XMPP (Jabber), you should have a serious look at pubsub extension of XMPP. It's designed for pushing data from a server to clients, in the opposite way of HTTP. In HTTP the client has to pull information from the server all the time to be able to know when some new data is pushed. While XMPP is designed to push data to clients, when something happens. Less resources used on server and clients.
You should not use the Chat part of XMPP, as that will interfere with the users presence.
In pubsub you can create a tree of nodes, where clients can listen to any new data published in any node or subnod in the tree that the client subscribes to. So if some client publish data on one node, all clients that has subscribed to that node, or any parent, will be notified about this data.
The good part with XMPP is that it's extensible so you can extend the protocol with your own extensions. I also give you user authorization, authentification and encryption, and you don't need to debug that yourself.
You could use any XMPP-server with good support for pubsub or you can host one server yourself. There are plenty of servers usable for this. GTalk doesn't have support for PubSub last time I looked.