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Batch Swaps

Batch Swap Overview

Balancer V2 allows powerful multi-hop trades, or "batch swaps", which pull the best prices from all the pools registered with the Vault.
The Vault exposes the batchSwap function to allow multi-hop trades with the the interface below.
batchSwap(SwapKind kind,
BatchSwapStep[] swaps,
IAsset[] assets,
FundManagement funds,
int256[] limits,
uint256 deadline) returns (int256[] assetDeltas)
To simplify the inputs to this function, we have grouped related fields into a number of structs which are explained below.

BatchSwapStep struct

struct BatchSwapStep {
bytes32 poolId;
uint256 assetInIndex;
uint256 assetOutIndex;
uint256 amount;
bytes userData;
}
  • poolId: The id of the pool to trade with.
  • assetInIndex: The index of the token within assets which to use as an input of this step.
  • assetOutIndex: The index of the token within assets which is the output of this step.
  • amount: The meaning of amount depends on the value of kind which passed to the batchSwap function.
    • GIVEN_IN: The amount of tokens we are sending to the pool in this step.
    • GIVEN_OUT: The amount of tokens we want to receive from the pool in this step.
  • userData: Any additional data which the pool requires to perform the swap. This allows pools to have more flexible swapping logic in future - for all current Balancer pools this can be left empty.
When performing multi-hop trades, it's not always possible to know the value of amountexactly. For example, consider a case where we want to trade USDC for ETH but the trade is being routed through a DAI-USDC pool and then a ETH-DAI pool. It's not possible to know exactly how much DAI we'll receive from this first step so we can't set amount to that value at the time we send the transaction.
For this reason setting amount to 0 will be interpreted to use the full output of the previous trade. We can then trade USDC for DAI and then all of the DAI we receive will be traded for ETH.

FundManagement struct

The FundManagement struct defines where the input tokens for the first swap are coming from and where any tokens received from swaps should be sent. The FundManagement struct is defined as below.
struct FundManagement {
address sender;
bool fromInternalBalance;
address payable recipient;
bool toInternalBalance;
}
  • sender: The address from which tokens will be taken to perform the trade
  • fromInternalBalance: Whether the trade should use tokens owned by the sender which are already stored in the Vault.
  • recipient: The address to which tokens will be sent to after the trade.
  • toInternalBalance: Whether the tokens should be sent to the recipient or stored within their internal balance within the Vault.
For more information on internal balances see Core Concepts.

BatchSwap function

batchSwap(SwapKind kind,
BatchSwapStep[] swaps,
IAsset[] assets,
FundManagement funds,
int256[] limits,
uint256 deadline) returns (int256[] assetDeltas)
  • kind: The type of batch swap we want to perform - either "Out Given In" or "In Given Out." We either know the amount of tokens we're sending to the pool and want to know how many we'll receive, or vice versa.
  • assets: An array of tokens which are used in the batch swap. This is referenced from within swaps
  • limits: An array of maximum amounts of each asset to be transferred. For tokens going in to the Vault, the limit shall be a positive number. For tokens going out of the Vault, the limit shall be a negative number. If the amount to be transferred for a given asset is greater than its limit, the trade will fail with error BAL#507: SWAP_LIMIT.
    • How do you determine what your limits should be? If you want to compute limits, it is recommended to use queryBatchSwap and then add a slippage tolerance.
  • deadline: The UNIX timestamp at which our trade must be completed by - if the transaction is confirmed after this time, the transaction will fail.

queryBatchSwap

queryBatchSwap is an extremely useful function in the Vault contract. With queryBatchSwap, you can get the exact amounts for a given swap with the on-chain state. You can use these amounts to calculate input/output limits based on a slippage tolerance.
You should NOT use queryBatchSwap to calculate limits from a smart contract that is executing a swap. This will leave you vulnerable to sandwich attacks.
You should only use queryBatchSwap before sending a batchSwap transaction, when calculating your batchSwap arguments off-chain.
Calling queryBatchSwap is very similar to calling batchSwap itself, just without the limit and deadline arguments.
queryBatchSwap(SwapKind kind,
BatchSwapStep[] swaps,
IAsset[] assets,
FundManagement funds)
returns (int256[] assetDeltas)
To use queryBatchSwap, you must use eth_call.
You may notice that queryBatchSwap shows up on Etherscan as a write function, but this is simply due to the fact that the function fully executes a batchSwap before reverting.

Adding a Slippage Tolerance

Once you have received your assetDeltas from calling queryBatchSwap, you can calculate limits for a batchSwap by applying your slippage tolerance.

GIVEN_IN

If we are performing a GIVEN_IN batchSwap and wanted to apply a 1% slippage tolerance, we would multiple our negative assetDeltas by 0.99. We do not need to modify our positive amounts because we know the exact amount we are putting in.

GIVEN_OUT

If we are performing a GIVEN_OUT batchSwap and wanted to apply a 1% slippage tolerance, we would multiple our positive assetDeltas by 1.01. We do not need to modify our negative amounts because we know the exact amount we are getting out.

Multi-hop Examples

In these examples, we’re trading token A for token C, through the intermediate token B (we could illustrate this as A->B->C). Tokens A, B, and C could be in different pools, or in the same pool.
"Given In" means the caller knows the exact amount of the incoming token, and is asking the pool to calculate the tokenOut amount. The opposite is true of "Given Out."

Example 1 ("Given In")

The first case is a "Given In" Batch Swap: say we have 10 A and want to know how much C we can get for it. We can accomplish this with a two-step multi-hop swap: A for B, then B for C.
Since we know we have 10 A to start, the swap kind is GIVEN_IN, and the amount for the first swap is 10. The first swap will produce some output amount of B, but we don’t know in advance how much.
Since we don’t know the amount of B when constructing the multi-hop, we initialize the amount in the second swap to 0, which instructs the multi-hop logic to use the calculated output amount from the first swap as input to the second.
Parameter
Swap 1
Swap 2
Amount
10
0
Token In
A
B
Token Out
B
C
Say we get 5 B from the first swap. The amount of the second swap is then set to 5 in the Vault logic, and the second swap produces some output amount of C. (The caller would then validate the overall swap by comparing this value to the minimum amountOut of C.)

Example 2 ("Given Out")

The second case is a “Given Out” Batch Swap: say we want 20 C, and want to know how much A that will cost. Here we need to do the swaps “backwards,” first trading C for B, then B for A.
Since we know we want to get 20 C out, the swap kind is GIVEN_OUT, and the amount for the first swap is 20. The first swap will produce some require input amount of token B, but as before, we don’t know how much in advance. So again we set the amount of the second swap to zero.
After the first swap, the amount of B will be known, and the zero amount in the second swap instructs the multi-hop logic to substitute the calculated amount from the first swap. Since this is a “Given In” Batch Swap, the result will be the required input amount of token A. (The caller would then validate the overall swap by comparing this value to the maximum amountIn of A.)
Parameter
Swap 1
Swap 2
Amount
20
0
Token Out
C
B
Token In
B
A
So in both cases, setting the amount of a swap within a batch to zero causes the multi-hop logic to substitute the calculated amount from the previous swap. If the batch swap kind is “Given In,” the calculated amount will be the “output” of the previous step. If the batch swap kind is “Given Out,” the calculated amount will be the “input” from the previous step.
Of course, the amount of the first swap in a batch cannot be zero. The batch swap must begin with a known piece of data: for a "Given In," the input amount; or for a "Given Out," the output amount.

Parallel Examples

As described in the examples above, batch swaps are most commonly used for multi-hop trades. Although much less common, it is also possible to use batch swaps for a set of unrelated swaps to be performed in parallel.

Example 3 (Parallel Single Swaps - "Given In")

In this case, the input amount of each swap must be provided explicitly. In this GIVEN_IN batch swap, the user will supply 99 A, 42 C, and 5 E, and will be returned computed amounts of B, D, and F.
Parameter
Swap 1
Swap 2
Swap 3
Amount
99
42
5
Token In
A
C
E
Token Out
B
D
F

Example 4 (Combined Swaps - "Given Out")

And of course, it is also possible to combine multi-hop trades and single-hop swaps in parallel. This example performs two trades in GIVEN_OUT fashion: A->B->C->D and E->F. The final outputs will be 100 D and 50 F, if the user can supply the computed amounts of A and E.
Parameter
Swap 1
Swap 2
Swap 3
Swap 4
Amount
100
0
0
50
Token Out
D
C
B
F
Token In
C
B
A
E