Transaction Processor Tutorial Python¶
This tutorial covers the creation of a new Sawtooth transaction family in Python, based on the Sawtooth SDK. We will construct a transaction handler which implements a distributed version of the multi-player game tic- tac-toe.
The SDK contains a fully-implemented version of tic-tac-toe. This tutorial is meant to demonstrate the relevant concepts, rather than to create a complete implementation. See the SDK for full implemenations in multiple languages.
A general description of tic-tac-toe, including the rules, can be found on Wikipedia at:
A full implementation of the tic-tac-toe transaction family can be found in
This tutorial assumes that you have gone through Getting Started and are familiar with the concepts introduced there.
You should be familiar with the concepts introduced in the Getting Started guide and have a working Sawtooth environment prior to completing this tutorial.
The Transaction Processor¶
There are two top-level components of a transaction processor: a processor class and a handler class. The SDK provides a general-purpose processor class. The handler class is application-dependent and contains the business logic for a particular family of transactions. Multiple handlers can be connected to an instance of the processor class.
Handlers get called in two ways:
- Various “metadata” methods
The metadata is used to connect the handler to the processor, and
we’ll discuss it at the end of this tutorial. The bulk of the handler, however,
is made up of
apply and its helper functions, so that’s where we’ll start.
apply gets called with two arguments,
state_store. The argument
transaction is an instance of the class
Transaction that is created from the protobuf definition. Also,
state_store is an instance of the class State from the python SDK.
transaction holds the command that is to be executed (e.g. taking a space or
creating a game), while
state_store stores information about the current
state of the game (e.g. the board layout and whose turn it is).
The transaction contains payload bytes that are opaque to the validator core, and transaction family specific. When implementing a transaction handler the binary serialization protocol is up to the implementer.
Without yet getting into the details of how this information is encoded, we can
start to think about what
apply needs to do.
apply needs to
- unpack the command data from the transaction,
- retrieve the game data from the state store,
- play the game, and
- save the updated game data.
Accordingly, a top-down approach to
apply might look like this:
def apply(self, transaction, state_store): signer, game_name, action, space = \ self._unpack_transaction(transaction) board, state, player1, player2 = \ self._get_state_data(game_name, state_store) updated_game_data = self._play_xo( board, state, player1, player2, signer, action, space ) self._store_game_data(game_name, updated_game_data, state_store)
Note that the third step is the only one that actually concerns tic-tac-toe; the other three steps all concern the coordination of data.
Transactions and Batches! contains a detailed description of how transactions are structured and used. Please read this document before proceeding, if you have not reviewed it.
So how do we get data out of the transaction? The transaction consists of a
header and a payload. The header contains the “signer”, which is used to
identify the current player. The payload will contain an encoding of the game
name, the action (‘create’ a game, ‘take’ a space), and the space (which will be
an empty string if the action isn’t ‘take’). So our
_unpack_transaction function will look like this:
def _unpack_transaction(self, transaction): header = TransactionHeader() header.ParseFromString(transaction.header) signer = header.signer try: game_name, action, space = self._decode_data(transaction.payload) except: raise InvalidTransaction("Invalid payload serialization") return signer, game_name, action, space
Before we say how exactly the transaction payload will be decoded, let’s look at
_get_state_data. Now, as far as the handler is concerned, it
doesn’t matter how the game data is stored. The only thing that matters is that
given a game name, the state store is able to give back the correct game data.
(In our full XO implementation, the game data is stored in a Merkle-radix tree.)
def _get_state_data(self, game_name, state_store): game_address = self._make_game_address(game_name) state_entries = state_store.get([game_address]) try: return self._decode_data(state_entries.data) except IndexError: return None, None, None, None except: raise InternalError("Failed to deserialize game data.")
By convention, we’ll store game data at an address obtained from hashing the game name prepended with some constant:
def _make_game_address(self, game_name): prefix = self._namespace_prefix game_name_utf8 = game_name.encode('utf-8') return prefix + hashlib.sha512(game_name_utf8).hexdigest()
Finally, we’ll store the game data. To do this, we simply need to encode the updated state of the game and store it back at the address from which it came.
def _store_game_data(self, game_name, game_data, state_store): game_address = self._make_game_address(game_name) encoded_game_data = self._encode_data(game_data) addresses = state_store.set([ StateEntry( address=game_address, data=encoded_game_data ) ]) if len(addresses) < 1: raise InternalError("State Error")
So, how should we encode and decode the data? We have many options in binary encoding schemes; the binary data stored in the validator state is up to the implementer of the handler. In this case, we’ll encode the data as a simple UTF-8 comma-separated value string, but we could use something more sophisticated, BSON.
def _decode_data(self, data): return data.decode().split(',') def _encode_data(self, data): return ','.join(data).encode()
Playing the Game¶
All that’s left to do is describe how to play tic-tac-toe. The details here are
fairly straighforward, and the
_play_xo function could certainly be implemented in
different ways. To see our implementation, go to
core/sdk/examples/xo_python. We choose to represent the board as
a string of length 9, with each character in the string representing a space
taken by X, a space taken by O, or a free space. Updating the board
configuration and the current state of the game proceeds straightforwardly.
And that’s all there is to
apply! All that’s left to do is set up the
XoTransactionHandler class and its metadata. The metadata is used to
register the transaction processor with a validator by sending it information
about what kinds of transactions it can handle.
class XoTransactionHandler: def __init__(self, namespace_prefix): self._namespace_prefix = namespace_prefix @property def family_name(self): return 'xo' @property def family_versions(self): return ['1.0'] @property def encodings(self): return ['csv-utf8'] @property def namespaces(self): return [self._namespace_prefix] def apply(self, transaction, state_store): # ...