diff --git a/src/interfaces/IStablecoinDEX.sol b/src/interfaces/IStablecoinDEX.sol
index 44ee229..fa0d06b 100644
--- a/src/interfaces/IStablecoinDEX.sol
+++ b/src/interfaces/IStablecoinDEX.sol
@@ -58,6 +58,7 @@ interface IStablecoinDEX {
     error BelowMinimumOrderSize(uint128 amount);
     error InvalidBaseToken();
     error OrderNotStale();
+    error IndexAlreadySet();
 
     event OrderCancelled(uint128 indexed orderId);
     event OrderFilled(
@@ -110,6 +111,10 @@ interface IStablecoinDEX {
         view
         returns (address base, address quote, int16 bestBidTick, int16 bestAskTick);
 
+    function bookIndexForKey(bytes32 key) external view returns (bool exists, uint32 index);
+
+    function bookKeyForIndex(uint32 index) external view returns (bytes32 key);
+
     function cancel(uint128 orderId) external;
 
     function cancelStaleOrder(uint128 orderId) external;
@@ -143,6 +148,8 @@ interface IStablecoinDEX {
         view
         returns (uint128 amountIn);
 
+    function setBookIndex(uint32 index) external;
+
     function swapExactAmountIn(address tokenIn, address tokenOut, uint128 amountIn, uint128 minAmountOut)
         external
         returns (uint128 amountOut);