How are web3 contract function calls and transactions carried out?

Question

Sorry if this sounds like a beginner question but after reading through a lot of material I have some questions.

Firstly, if a contract has already been created can I call its functions if I know its ABI & address? I think contract(ABI).at(contractAddress) does this but I found the description rather ambiguous.

Secondly, if I am running web3 by direct library import without MetaMask or Mist when I initiate a function call who is the msg.sender? If MetaMask or Mist are running is the msg.sender the currently signed in wallet?

Finally, if I want to create a browser Dapp, how do I ensure some functions can only be invoked by the Dapp and no one else (Not even the owner)?

Thanks in advance!

Answer 1

1- if you know the address and the ABI you can call the contract's function as described in the official doc :

// creation of contract object
var MyContract = web3.eth.contract(abi);

// initiate contract for an address
var myContractInstance = MyContract.at('0x78e97bcc5b5dd9ed228fed7a4887c0d7287344a9');

var result = myContractInstance.myConstantMethod('myParam');
console.log(result) // '0x25434534534'

myContractInstance.myStateChangingMethod('someParam1', 23, {value: 200, gas: 2000}, function(err, result){ ... });

2-msg sender is the address of the message//call sender's address so if you use any ethereum's wallet, the account's address is sent with your message (like an ip).

3-for this use a modifier :

modifier isAllowed() {
   if (msg.sender != allowed_address) {
        throw;
    }

    _;
}