diff --git a/contracts/Marketplace.sol b/contracts/Marketplace.sol
index b531784..3787d51 100644
--- a/contracts/Marketplace.sol
+++ b/contracts/Marketplace.sol
@@ -11,11 +11,13 @@ import "./libs/DAL.sol";
 contract Marketplace is Collateral, Proofs {
   using DAL for DAL.Database;
   using EnumerableSet for EnumerableSet.Bytes32Set;
+  using DAL for EnumerableSet.Bytes32Set;
 
   uint256 public immutable collateral;
   MarketplaceFunds private funds;
   mapping(DAL.RequestId => RequestContext) private requestContexts;
   DAL.Database private db;
+  uint16 private constant MAX_SLOTS = 256;
 
   constructor(
     IERC20 _token,
@@ -45,6 +47,8 @@ contract Marketplace is Collateral, Proofs {
     public
     marketplaceInvariant
   {
+    require(request.ask.slots <= MAX_SLOTS, "Max slots exceeded");
+    require(request.ask.maxSlotLoss <= MAX_SLOTS, "Max slot loss exceeded");
     require(request.client == msg.sender, "Invalid client address");
 
     DAL.RequestId requestId = _toRequestId(request);
@@ -152,7 +156,14 @@ contract Marketplace is Collateral, Proofs {
 
       DAL.Client storage client = db.select(request.client);
       db.remove(client.requests, requestId);
-      db.remove(host.requests, requestId);
+
+      // Remove all request's slots from hosts.slots. This is an expensive
+      // operation as it iterates all slots and removes them one-by-one.
+      // An alternative is to use EnumerableExtensions.ClearableBytes32Set
+      // for host.slots instead, however this would not free any storage
+      // but instead use a new EnumerableSet.Bytes32Set each time it is cleared.
+      db.clearSlots(host, requestId, MAX_SLOTS);
+
       emit RequestFailed(requestId);
 
       // TODO: burn all remaining slot collateral (note: slot collateral not
diff --git a/contracts/libs/DAL.sol b/contracts/libs/DAL.sol
index b5ab72a..8cc34a4 100644
--- a/contracts/libs/DAL.sol
+++ b/contracts/libs/DAL.sol
@@ -3,7 +3,6 @@
 pragma solidity ^0.8.8;
 
 import "@openzeppelin/contracts/utils/structs/EnumerableSet.sol";
-import "../Marketplace.sol";
 import "./Utils.sol";
 
 library DAL {
@@ -18,18 +17,18 @@ library DAL {
   struct Client {
     ClientId id; // PK
 
-    EnumerableSet.Bytes32Set requests;
+    EnumerableSet.Bytes32Set requests; // FKs
   }
 
   struct Host {
     HostId id; // PK
 
-    EnumerableSet.Bytes32Set slots;
-    EnumerableSet.Bytes32Set requests;
+    EnumerableSet.Bytes32Set slots; // FKs
+    EnumerableSet.Bytes32Set requests; // FKs
   }
 
   struct Request {
-    RequestId id;
+    RequestId id; // PK
     ClientId client;
     Ask ask;
     Content content;
@@ -40,10 +39,10 @@ library DAL {
   }
 
   struct Slot {
-    SlotId id;
-    HostId host;
+    SlotId id; // PK
+    HostId host; // FK
     bool hostPaid;
-    RequestId requestId;
+    RequestId requestId; // FK
   }
 
   struct Ask {
@@ -246,6 +245,8 @@ library DAL {
     Client storage client = db.clients[request.client];
     bytes32 bRequestId = RequestId.unwrap(request.id);
     require(!client.requests.contains(bRequestId), "active request refs");
+    // TODO: iterate all request's hosts and check that host.requests doesn't
+    // have requestId
 
     delete db.requests[request.id];
   }
@@ -279,6 +280,8 @@ library DAL {
     internal
   {
     require(exists(db, requestId), "request does not exist");
+    // TODO: check that host.slots doesn't have a slot belonging to the request
+    // being removed
 
     requests.remove(RequestId.unwrap(requestId));
   }
@@ -293,6 +296,26 @@ library DAL {
     slots.remove(SlotId.unwrap(slotId));
   }
 
+  // WARNING: this may cause a transaction to run out of gas!
+  function clearSlots(Database storage db,
+                      Host storage host,
+                      RequestId requestId,
+                      uint256 maxIterations)
+    internal
+  {
+    require(maxIterations > 0, "maxIterations must be > 0");
+    require(host.slots.length() <= maxIterations, "iterations out of bounds");
+
+    for (uint256 i = 0; i < host.slots.length(); i++) {
+      bytes32 slotId = host.slots.at(i);
+      Slot storage slot = select(db, SlotId.wrap(slotId));
+      if (equals(slot.requestId, requestId)) {
+        host.slots.remove(slotId);
+      }
+    }
+    remove(db, host.requests, requestId);
+  }
+
 
 
   /// *** CALCULATED PROPERTIES *** ///