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 playerbase, 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.

Each player must initialize XO locally using the xo init command. This command generates a public and private key pair that is used to identify players on the network. It stores this information locally in a file. The xo init command has two options: --username and --url.

  • The --username option is used locally as a nickname for the key pair. If a --username is not specified, OS environment variables will be parsed to find a suitable value.
  • The --url option should be set to the URL of the REST API. The XO client will send requests to this URL in order to determine the validity of each move and update the game state on the blockchain. If no URL is specified, the URL will default to


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

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.

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.

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 during initialization. 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 --auth-user is not the same username that is entered with the --username option during initialization.

See also

xo Family CLIs

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 two players to play the game:

$ xo init --username jack
$ xo init --username jill

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

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

If you are using the XO CLI to play a game on one machine, xo init can also be used to set the active player. If the above commands are run in the order shown, both players will be created, but “jill” will be the active player. To switch the active player, simply run the xo init command again with the name of thedesired player.

Set “jack” as the active player now with:

$ xo init --username jack

Create A Game

Create a game with the following command:

$ xo create game1

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
game1                                           --------- P1-NEXT

Take A Space As Player One

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

$ xo take game1 4


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

To take a space on the board as player two, “jill” needs to be set as the active player. Run the following command:

$ xo init --username jill

Now take a space on the board as player two:

$ xo take game1 3

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 game1

You will see the current state of the board displayed:

GAME:     : game1
PLAYER 1  : 024c8f
PLAYER 2  : 03f8f2

    |   | O
  X |   |
    |   |

Continue Game

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


Be sure to switch users before taking each move to simulate two distinct users playing.

$ xo show game1
GAME:     : game1
PLAYER 1  : 024c8f
PLAYER 2  : 03f8f2
STATE     : P2-WIN

  X |   | O
  X | O |
  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 Specification