# Getting Started

With BigchainDB and rethinkDB [installed and running](intalling.html) we can start creating and
transferring digital assets.

##### Importing BigchainDB

First, lets open a python shell
```shell
$ python
```

Then we can import and instantiate BigchainDB
```python
from bigchaindb import Bigchain
b = Bigchain()
```

When instantiating `Bigchain` witouth arguments it reads the configurations
stored in `$HOME/.bigchaindb`


##### Creating digital assets
In BigchainDB only the federation nodes are allowed to create digital assets
using the `CREATE` operation on the transaction.
Digital assets are usually created and assigned to a user, which in BigchainDB
is represented by a public key.

BigchainDB allows you to define your digital asset as a generic python dict that
will be used has a payload of the transaction.

```python
# create a test user
testuser1_priv, testuser1_pub = b.generate_keys()

# define a digital asset
digital_asset = {'msg': 'Hello BigchainDB!'}

# a create transaction uses the operation `CREATE` has no inputs
tx = b.create_transaction(b.me, testuser1_pub, None, 'CREATE', payload=digital_asset)

# all transactions need to be signed by the user creating the transaction
tx_signed = b.sign_transaction(tx, b.me_private)

# write the transaction to the bigchain
# the transaction will be store in a backlog where it will be validated,
# included in a block and written to the bigchain 
b.write_transaction(tx_signed)
```

##### Reading transactions from the bigchain

After a couple of seconds we can check if the transactions was included in the
bigchain.

```python
# retrieve a transaction from the bigchain
tx_retrieved = b.get_transaction(tx_signed['id'])

   'id': '6539dded9479c47b3c83385ae569ecaa90bcf387240d1ee2ea3ae0f7986aeddd',
   'transaction': {   'current_owner': 'pvGtcm5dvwWMzCqagki1N6CDKYs2J1cCwTNw8CqJic3Q',
                      'data': {   'hash': '872fa6e6f46246cd44afdb2ee9cfae0e72885fb0910e2bcf9a5a2a4eadb417b8',
                                   'payload': {'msg': 'Hello BigchainDB!'}},
                      'input': None,
                      'new_owner': 'ssQnnjketNYmbU3hwgFMEQsc4JVYAmZyWHnHCtFS8aeA',
                      'operation': 'CREATE',
                      'timestamp': '1455108421.753908'}}
```

The new owner of the digital asset is now
`ssQnnjketNYmbU3hwgFMEQsc4JVYAmZyWHnHCtFS8aeA` which is the public key of
`testuser1`


##### Transferring digital assets

Now that `testuser1` has a digital asset assigned to him he can now transfered it
to another user. Transfer transactions now require an input. The input will a
transaction id of a digital asset that was assigned to `testuser1` which in our
case is `6539dded9479c47b3c83385ae569ecaa90bcf387240d1ee2ea3ae0f7986aeddd`

```python
# create a second testuser
testuser2_priv, testuser2_pub = b.generate_keys()

# create a transfer transaction
tx_transfer = b.create_transaction(testuser1_pub, testuser2_pub, tx_retrieved['id'], 'TRANSFER')

# sign the transaction
tx_transfer_signed = b.sign_transaction(tx_transfer, testuser1_priv)

# write the transaction
b.write_transaction(tx_transfer_signed)

# check if the transaction is already in the bigchain
tx_transfer_retrieved = b.get_transaction(tx_transfer_signed['id'])

{   'id': '1b78c313257540189f27da480152ed8c0b758569cdadd123d9810c057da408c3',
    'signature': '3045022056166de447001db8ef024cfa1eecdba4306f92688920ac24325729d5a5068d47022100fbd495077cb1040c48bd7dc050b2515b296ca215cb5ce3369f094928e31955f6',
    'transaction': {   'current_owner': 'ssQnnjketNYmbU3hwgFMEQsc4JVYAmZyWHnHCtFS8aeA',
                       'data': None,
                       'input': '6539dded9479c47b3c83385ae569ecaa90bcf387240d1ee2ea3ae0f7986aeddd',
                       'new_owner': 'zVzophT73m4Wvf3f8gFYokddkYe3b9PbaMzobiUK7fmP',
                       'operation': 'TRANSFER',
                       'timestamp': '1455109497.480323'}}
```

##### Double Spends

BigchainDB makes sure that a digital asset assigned to a user cannot be
transfered multiple times.

If we try to create another transaction with the same input as before the
transaction will be marked invalid and the validation will trow a double spend
exception

```python
# create another transfer transaction with the same input
tx_transfer2 = b.create_transaction(testuser1_pub, testuser2_pub, tx_retrieved['id'], 'TRANSFER')

# sign the transaction
tx_transfer_signed2 = b.sign_transaction(tx_transfer2, testuser1_priv)

# check if the transaction is valid
b.validate_transaction(tx_transfer_signed2)
Exception: input `6539dded9479c47b3c83385ae569ecaa90bcf387240d1ee2ea3ae0f7986aeddd` was already spent

```