Introduction to the XO Transaction Family

What is XO?

XO is an example transaction family contained within the Sawtooth SDK. It is an implementation of the popular game Tic-tac-toe (otherwise known as Noughts and Crosses or X’s and O’s).

X’s and O’s-style games have been played across the world for many centuries. The origin of this type of game is unclear; however, some historians believe that the first version of Tic-tac-toe may have originated in ancient Egypt. Others believe that the modern Tic-tac-toe is an evolution of a game known as Terni Lapilli, which was popular within the Roman Empire.

Tic-tac-toe also has historical significance within the field of Computer Science. In 1952, it became the basis of one of the first-ever video games when OXO was developed by Alexander S. Douglas at the University of Cambridge. More information regarding the origins and properties of Tic-tac-toe can be found here.

XO was chosen as an example transaction family for Sawtooth due to its simplicity, global player base, and straightforward implementation as a computer program. This example transaction family demonstrates the functionality of Sawtooth, and the code that implements it serves as a reference for building other transaction processors.

How to Play XO

Game Rules

Tic-tac-toe is a two-player game in which players take turns marking spaces on a 3x3 grid. Conventionally, player one marks spaces with an ‘X’ and player two marks spaces with an ‘O’; and player one always takes the first turn.

A player wins the game by marking 3 spaces in a horizontal, vertical, or diagonal row. If all 9 spaces on the grid have been marked but no player has achieved a winning board position, the game is considered a draw.

Playing XO Using the Command-line Interface

A command-line interface is provided for playing XO which handles the construction and submission of transactions. Instructions on how to use the CLI are detailed below:

Start up a validator, XO transaction processor, and the REST API. For more information on configuring and running Sawtooth components, see Installing and Running Sawtooth.

A user creates a new game using the xo create [name] command.

  • The name argument is the identifier for the new game to be created.

Players take turns using the xo take [name] [space] command to mark spaces on the grid.

  • The name argument is the identifier for an existing game.
  • The space argument is the space that is to be marked, which is an integer in the range of 1 through 9.


The first player to issue an xo take command to a newly created game is recorded by their username as player one. The second player to issue a take command is recorded by their username as player two.

xo create and xo take each take --username and --key-dir arguments. These are used to locate the user’s private key file: <key-dir>/<username>.priv. key-dir defaults to ~/.sawtooth/keys and username defaults to the name of the OS user.


All xo commands also support --auth-user and --auth-password, but these options are not used in this example.

Each time a user attempts to take a space, the transaction processor will verify that their username matches the name of the player whose turn it is. This ensures that no player is able to mark a space out of turn.

The XO transaction processor will scan for winning or draw board conditions after each turn. If either condition occurs, further take actions on the finished game will not be allowed.


Either user can use the xo reset command to delete their local XO data. This includes the saved URL and username.

Viewing the Game State

The XO CLI also supports commands to view the state of ongoing or finished games. These commands are xo show [game] and xo list

  • xo show [game] shows the board of a specified game, as well as data about the players and game state.
  • xo list shows a list of all ongoing and finished games.


The XO client optionally supports authentication. If the REST API is connected to an authentication proxy, you can point the XO client at it with the --url argument. You must enter your authentication information using the --auth-user [user] and --auth-password [password] options for each XO command.

Note that the value of the --auth-user argument is not the same username that is entered with the --username argument.


The xo commands all take a --url argument that defaults to This should be set to the URL of the REST API. The XO client will send requests to update and query the blockchain to this URL.

See also


Playing XO with the XO Client (Example)

Now that we have gone through the basics of the XO transaction processor, we are ready to play a game. The steps below show you how to set up and play a game using the XO CLI.

Start the Necessary Components

To play XO, ensure that the following components are running and connected:

  1. At least one validator
  2. An XO family transaction processor
  3. The REST API

Create Players

Create keys for two players to play the game:

$ sawtooth keygen jack
$ sawtooth keygen jill

The command produces output similar to the following for both players:

writing file: /home/ubuntu/.sawtooth/keys/jack.priv
writing file: /home/ubuntu/.sawtooth/keys/jack.addr
writing file: /home/ubuntu/.sawtooth/keys/jill.priv
writing file: /home/ubuntu/.sawtooth/keys/jill.addr

Create a Game

Create a game with the following command:

$ xo create game --username jack

To see list of the created games, enter the following command:

$ xo list

The command outputs a list of the games that have been created:

GAME            PLAYER 1        PLAYER 2        BOARD     STATE
game                                            --------- P1-NEXT

Take a Space as Player One

Start playing by taking a space as the first player, “jack”:

$ xo take game 5 --username jack


The board spaces are numbered from 1 to 9. The upper-left corner is number 1, and the lower right corner is number 9.

Take a Space as Player Two

Now take a space on the board as player two:

$ xo take game 1 --username jill

Show the Current State of the Game Board

Whenever you want to see the current state of the game board, enter the following command:

$ xo show game

You will see the current state of the board displayed:

GAME:     : game
PLAYER 1  : 02403a
PLAYER 2  : 03729b

  O |   |
    | X |
    |   |

Continue the Game

You can continue the game until one of the players wins, or the game ends in a draw:

$ xo show game
GAME:     : game
PLAYER 1  : 02403a
PLAYER 2  : 03729b

  O | X | O
  X | X | O
  X | O | X

XO Transaction Family Specification

The XO Transaction Family Specification contains technical information about the XO transaction family. This specification can be found here: XO Transaction Family