# Transaction

Bitcore provides a very simple API for creating transactions. We expect this API to be accessible for developers without knowing the working internals of bitcoin in deep. What follows is a small introduction to transactions with some basic knowledge required to use this API.

A Transaction contains a set of inputs and a set of outputs. Each input contains a reference to another transaction's output, and a signature that allows the value referenced in that ouput to be used in this transaction.

Note also that an output can be used only once. That's why there's a concept of "change address" in the bitcoin ecosystem: if an output of 10 BTC is available for me to spend, but I only need to transmit 1 BTC, I'll create a transaction with two outputs, one with 1 BTC that I want to spend, and the other with 9 BTC to a change address, so I can spend this 9 BTC with another private key that I own.

So, in order to transmit a valid transaction, you must know what other transactions on the network store outputs that have not been spent and that are available for you to spend (meaning that you have the set of keys that can validate you own those funds). The unspent outputs are usually referred to as "utxo"s.

Let's take a look at some very simple transactions:

```javascript
var transaction = new Transaction()
    .from(utxos)          // Feed information about what unspend outputs one can use
    .to(address, amount)  // Add an output with the given amount of satoshis
    .change(address)      // Sets up a change address where the rest of the funds will go
    .sign(privkeySet)     // Signs all the inputs it can
```

Now, this could just be serialized to hexadecimal ASCII values (`transaction.serialize()`) and sent over to the bitcoind reference client.

```bash
bitcoin-cli sendrawtransaction <serialized transaction>
```

## Transaction API

You can take a look at the javadocs for the [Transaction class here](link missing).

## Input

Transaction inputs are instances of either [Input](https://github.com/bitpay/bitcore/tree/master/lib/transaction/input) or its subclasses. The internal workings of it can be understood from the [API reference](link missing).

## Output

Transaction outputs are a very thin wrap around the information provided by a transaction output: its script and its output amount.

## Multisig Transactions

To send a transaction to a multisig address, the API is the same as in the above example. To spend outputs that require multiple signatures, the process needs extra information: the public keys of the signers that can unlock that output.

```javascript
  var multiSigTx = new Transaction()
      .from(utxo, publicKeys, threshold)
      .change(address)
      .sign(myKeys);

  var serialized = multiSigTx.serialize();
```

This can be serialized and sent to another party, to complete with the needed
signatures:

```javascript
  var multiSigTx = new Transaction(serialized)
      .from(utxo, publicKeys, threshold)       // provide info about the multisig output
                                               // (lost on serialization) 
      .sign(anotherSetOfKeys);

  assert(multiSigTx.isFullySigned());
```

## Advanced topics

### Internal Workings

There are a number of data structures being stored internally in a `Transaction` instance. These are kept up to date and change through successive calls to its methods.

* `inputs`: The ordered set of inputs for this transaction
* `outputs`: This is the ordered set of output scripts
* `_inputAmount`: sum of the amount for all the inputs
* `_outputAmount`: sum of the amount for all the outputs

TO BE IMPLEMENTED YET:
* `_fee`: if user specified a non-standard fee, the amount (in satoshis) will be stored in this variable so the change amount can be calculated.
* `_change`: stores the value provided by calling the `change` method.

### Unspent Output Selection

If you have a larger set of unspent outputs, only some of them will be selected to fulfill the amount. This is done by storing a cache of unspent outputs in a protected member called `_utxos`. When the `to()` method is called, some of these outputs will be selected to pay the requested amount to the appropriate address.

A nit that you should have in mind is that when the transaction is serialized, this cache can't be included in the serialized form. 

## Upcoming changes

We're debating an API for Merge Avoidance, CoinJoin, Smart contracts, CoinSwap, and Stealth Addresses. We're expecting to have all of them by some time in early 2015. First draft implementations of Payment Channel smart contracts extensions to this library are already being implemented independently.