# Handle Wager Payments
Make sure you have everything you need before proceeding:
- You understand the concepts of modules and keepers.
- Go is installed.
- You have the checkers blockchain codebase up to the game wager. If not, follow the previous steps or check out the relevant version (opens new window).
In this section, you will:
- Work with the bank module.
- Handle money.
- Use mocks.
In the previous section, you introduced a wager. On its own, having a Wager field is just a piece of information, it does not transfer tokens just by existing.
Transferring tokens is what this section is about.
# Some initial thoughts
When thinking about implementing a wager on games, ask:
- Is there any desirable atomicity of actions?
- At what junctures do you need to handle payments, refunds, and wins?
- Are there errors to report back?
- What event should you emit?
In the case of this example, you can consider that:
- Although a game creator can decide on a wager, it should only be the holder of the tokens that can decide when they are being taken from their balance.
- You might think of adding a new message type, one that indicates that a player puts its wager in escrow. On the other hand, you can leverage the existing messages and consider that when a player makes their first move, this expresses a willingness to participate, and therefore the tokens can be transferred at this juncture.
- For wins and losses, it is easy to imagine that the code handles the payout at the time a game is resolved.
# Code needs
When it comes to your code:
- What Ignite CLI commands, if any, will assist you?
- How do you adjust what Ignite CLI created for you?
- Where do you make your changes?
- How would you unit-test these new elements?
- How would you use Ignite CLI to locally run a one-node blockchain and interact with it via the CLI to see what you get?
Here are some elements of response:
- Your module needs to call the bank to tell it to move tokens.
- Your module needs to be allowed by the bank to keep tokens in escrow.
- How would you test your module when it has such dependencies on the bank?
# What is to be done
A lot is to be done. In order you will:
- Make it possible for your checkers module to call certain functions of the bank to transfer tokens.
- Tell the bank to allow your checkers module to hold tokens in escrow.
- Create helper functions that encapsulate some knowledge about when and how to transfer tokens.
- Use these helper functions at the right places in your code.
- Update your unit tests and make use of mocks for that. You will create the mocks, create helper functions and use all that.
# Declaring expectations
On its own the Wager field does not make players pay the wager or receive rewards. You need to add handling actions that ask the bank module to perform the required token transfers. For that, your keeper needs to ask for a bank instance during setup.
The only way to have access to a capability with the object-capability model of the Cosmos SDK is to be given the reference to an instance which already has this capability.
Payment handling is implemented by having your keeper hold wagers in escrow while the game is being played. The bank module has functions to transfer tokens from any account to your module and vice-versa.
Alternatively, your keeper could burn tokens instead of keeping them in escrow and mint them again when paying out. However, this makes your blockchain's total supply falsely fluctuate. Additionally, this burning and minting may prove questionable when you later introduce IBC tokens.
Declare an interface that narrowly declares the functions from other modules that you expect for your module. The conventional file for these declarations is x/checkers/types/expected_keepers.go.
The bank module has many capabilities, but all you need here are two functions. Your module already expects one function of the bank keeper: SpendableCoins (opens new window). Instead of expanding this interface, you add a new one and redeclare the extra functions you need like so:
These two functions must exactly match the functions declared in the bank's keeper.go file (opens new window). Copy the declarations directly from the bank's file. In Go, any object with these two functions is a BankEscrowKeeper.
# Obtaining the capability
With your requirements declared, it is time to make sure your keeper receives a reference to a bank keeper. First add a BankEscrowKeeper to your keeper in x/checkers/keeper/keeper.go:
This BankEscrowKeeper is your newly declared narrow interface. Do not forget to adjust the constructor accordingly:
Next, update where the constructor is called and pass a proper instance of BankKeeper. This happens in app/app.go:
This app.BankKeeper is a full bank keeper that also conforms to your BankEscrowKeeper interface.
Finally, inform the app that your checkers module is going to hold balances in escrow by adding it to the whitelist of permitted modules:
If you compare it to the other maccperms lines, the new line does not mention any authtypes.Minter or authtypes.Burner. Indeed nil is what you need to keep in escrow. For your information, the bank creates an address for your module's escrow account. When you have the full app, you can access it with:
# Preparing expected errors
There are several new error situations that you can enumerate with new variables:
# Money handling steps
With the bank now in your keeper, it is time to have your keeper handle the money. Keep this concern in its own file, as the functions are reused on play and forfeit.
Create the new file x/checkers/keeper/wager_handler.go and add three functions to collect a wager, refund a wager, and pay winnings:
The Must prefix in the function expresses the fact that the transaction either takes place or a panic is issued. Indeed, if a player cannot pay the wager, it is a user-side error and the user must be informed of a failed transaction. If the module cannot pay, it means the escrow account has failed. This latter error is much more serious: an invariant may have been violated and the whole application must be terminated.
Now set up collecting a wager, paying winnings, and refunding a wager:
Collecting wagers happens on a player's first move. Therefore, differentiate between players:
When there are no moves, get the address for the black player:
Try to transfer into the escrow:
Then do the same for the red player (opens new window) when there is a single move.
Paying winnings takes place when the game has a declared winner. First get the winner. "No winner" is not an acceptable situation in this
MustPayWinnings. The caller of the function must ensure there is a winner:Then calculate the winnings to pay:
You double the wager only if the red player has also played and therefore both players have paid their wagers.
If you did this wrongly, you could end up in a situation where a game with a single move pays out as if both players had played. This would be a serious bug that an attacker could exploit to drain your module's escrow fund.
Then pay the winner:
Finally, refunding wagers takes place when the game has partially started, i.e. only one party has paid, or when the game ends in a draw. In this narrow case of
MustRefundWager:Refund the black player when there has been a single move:
If the module cannot pay, then there is a panic as the escrow has failed.
You will notice that no special case is made when the wager is zero. This is a design choice here, and which way you choose to go is up to you. Not contacting the bank unnecessarily is cheaper in gas. On the other hand, why not outsource the zero check to the bank?
# Insert wager handling
With the desired steps defined in the wager handling functions, it is time to invoke them at the right places in the message handlers.
When a player plays for the first time:
When a player wins as a result of a move:
When a game expires and there is a forfeit, make sure to only refund or pay full winnings when applicable. The logic needs to be adjusted:
# Unit tests
If you try running your existing tests you get a compilation error on the test keeper builder (opens new window). Passing nil would not get you far with the tests and creating a full-fledged bank keeper would be a lot of work and not a unit test. See the next section on integration tests for that.
Instead, you create mocks and use them in unit tests, not only to get the existing tests to pass but also to verify that the bank is called as expected.
# Prepare mocks
It is better to create some mocks (opens new window). The Cosmos SDK does not offer mocks of its objects so you have to create your own. For that, the gomock (opens new window) library is a good resource. Install it:
With the library installed, you still need to do a one time creation of the mocks. Run:
If your expected keepers change, you will have to run this command again. It can be a good idea to save the command for future reference. You may use a Makefile for that. Ensure you install the make tool for your computer. If you use Docker, add it to the packages and rebuild the image:
Create the Makefile:
At any time, you can rebuild the mocks with:
You are going to set the expectations on this BankEscrowKeeper mock many times, including when you do not care about the result. So instead of setting the verbose expectations in every test, it is in your interest to create helper functions that will make setting up the expectations more succinct and therefore readable, which is always a benefit when unit testing. Create a new bank_escrow_helpers.go file with:
A function to unset expectations (ie where anything can go). This is useful when your test does not check things around the escrow:
A function to quickly create the right type and value of coins to use for expectations:
A function to define an expectation of payment from a player:
A function to define an expectation of payment from the module:
Note that the two main expectation functions return *gomock.Call. This is so that you can continue adding expectations such as:
- Instructing the number of times the call is expected to be made: for instance
.Times(1). - Instructing the order of the expected calls: for instance
.After(<the previous call>).
# Make use of mocks
With the helpers in place, you can add a new function similar to CheckersKeeper(t testing.TB) but which uses mocks. Keep the original function, which passes a nil for bank:
The CheckersKeeperWithMocks function takes the mock in its arguments for more versatility. The CheckersKeeper remains for the tests that never call the escrow.
Now add a small function to those that set up the keeper before each test. It will be used for play and forfeit:
This function creates the mock and returns two new objects:
- The mock controller, so that the
.Finish()method can be called within the test itself. This is the function that will verify the call expectations placed on the mocks. - The mocked bank escrow. This is the instance on which you place the call expectations.
Both objects will be used from the tests proper.
Do the same with the create tests (opens new window) as you also use its function in move tests.
# Wager handler unit tests
After these adjustments, it is a good idea to add unit tests directly on the wager handling functions of the keeper. Create a new wager_handler_test.go file. In it:
Add a setup helper function that does not create any message server:
Add tests on the
CollectWagerfunction. For instance, when the game is malformed:Or when the black player failed to escrow the wager:
Or when the collection of a wager works:
Add similar tests to the payment of winnings from the escrow. When it fails:
Or when it works:
You will also need a test for refund (opens new window) situations.
# Add bank escrow unit tests
Now that the wager handling has been convincingly tested, you want to confirm that its functions are called at the right junctures. Add dedicated tests with message servers that confirm how the bank is called. Add them in existing files. They can duplicate existing tests but explicitly test the (mocked) bank calls, for instance:
When playing once:
Or twice (opens new window), or three times (opens new window).
When playing the game to its resolution by a win:
Or when a game is forfeited after having been played once:
Or after having been played twice (opens new window).
After doing all that, confirm that your tests run.
You have confirmed that your code behaves correctly given the expectations you have placed on the bank keeper.
# Interact via the CLI
With the tests done, see what happens at the command-line and see if the bank keeper behaves as you expected.
Keep the game expiry at 5 minutes to be able to test a forfeit, as done in a previous section. Now, you need to check balances after relevant steps to test that wagers are being withheld and paid.
How much do Alice and Bob have to start with?
This prints:
# A game that expires
Create a game on which the wager will be refunded because the player playing red did not join:
Confirm that the balances of both Alice and Bob are unchanged - as they have not played yet.
In this example, Alice paid no gas fees, other than the transaction costs, to create a game. The gas price is likely 0 here anyway. This is fixed in the next section.
Have Alice play:
Confirm that Alice has paid her wager:
This prints:
Wait 5 minutes for the game to expire and check again:
This prints:
# A game played twice
Now create a game in which both players only play once each, i.e. where the player playing black forfeits:
Confirm that both Alice and Bob paid their wagers. Wait 5 minutes for the game to expire and check again:
This shows:
This is correct: Bob was the winner by forfeit.
Similarly, you can test that Alice gets her wager back when Alice creates a game, Alice plays, and then Bob lets it expire.
It would be difficult to test by CLI when there is a winner after a full game. That would be better tested with a GUI, or by using integration tests as is done in the next section.
To summarize, this section has explored:
- How to work with the Bank module and handle players making wagers on games, now that the application supports live games playing to completion (with the winner claiming both wagers) or expiring through inactivity (with the inactive player forfeiting their wager as if losing), and no possibility of withheld value being stranded in inactive games.
- How to add handling actions that ask the
bankmodule to perform the token transfers required by the wager, and where to invoke them in the message handlers. - How to create a new wager-handling file with functions to collect a wager, refund a wager, and pay winnings, in which
mustprefixes indicate either a user-side error (leading to a failed transaction) or a failure of the application's escrow account (requiring the whole application be terminated). - How to interact with the CLI to check account balances to test that wagers are being withheld and paid.