Light Clients and Android – What Can Be Used to Interact with the Ethereum Network from an Android Device?

1. Is the above architecture safe? (What happens if server is compromised of the server is biased)

Provided the users keys are stored only locally on the users device, this is a pretty safe architecture. However, I can imagine a possible attack vector where traffic between your users and your node is compromised, and incorrect information about the statement of the network is provided, causing your users to make ill-advised transactions. I'm not sure how likely that actually is for your application, but if it seems like something you should defend against, then using a secondary source of truth would help.

2. Do I need to create a Full-node Ethereum client corresponding to every Android app user.

No

3. What type of user's data will the Full-node server need to process a transaction on its behalf?

None, provided transactions are all properly signed before sending

To interact with the remote node, you can just provide the ip and port to web3.js like this:

web3.setProvider(new web3.providers.HttpProvider('http://ipaddress:port'));

I managed to do this today after some painful debugging. The result is here: https://github.com/zupzup/react-native-ethereum-wallet

It's not a full example yet, but it shows how to interact with the running geth node.

Here's what I did:

First step, add the geth dependency to build.gradle:

compile 'org.ethereum:geth:1.6.7'

Then, in MainActivity.java, try to get the node running. You'll see if it's running correctly in the Android Monitor. This looks something like this:

NodeConfig nc = new NodeConfig();
Node node = Geth.newNode(getFilesDir() + "/.eth1", nc);

Then, you need some way to expose this node to your Wrapping module. I just used a Singleton for this, but I guess there are cleaner ways to share state in Android.

Regarding the wrapping, I just used the official docs at https://facebook.github.io/react-native/docs/native-modules-android.html.

Basically, I created a method test, which fetches the node from the singleton and executes something on it (in my case a simple getBalanceAt query).

And this test method can be called from React-Native with a callback, if you register the wrapping module correctly. Like this:

NativeModules.TestNative.test("tojavandback", (str) => {
    Alert.alert(str);
})

This outputs, in my case, the amount of ether at the given address. It's just a simple example, but the whole end2end interaction works.

Hope this helps.