diff --git a/src/Catalog/Catalog.Api/Registrations.cs b/src/Catalog/Catalog.Api/Registrations.cs
index a98e7cf..4a7f940 100644
--- a/src/Catalog/Catalog.Api/Registrations.cs
+++ b/src/Catalog/Catalog.Api/Registrations.cs
@@ -89,6 +89,8 @@ public static void AddEventuous(this IServiceCollection services, IConfiguration
                 .AddEventHandler<OfferStateProjection>()
                 .WithPartitioningByStream(2));
 
+        // TODO: add additional mongo, postgresql, and other custom projections
+
         // services.AddSubscription<AllStreamSubscription, AllStreamSubscriptionOptions>(
         //     "ProductDraftsProjections",
         //     builder => builder
diff --git a/src/Retail/ShoppingCart.Api/HttpApi/Carts/CartCommandApi.cs b/src/Retail/ShoppingCart.Api/HttpApi/Carts/CartCommandApi.cs
index 0ab2aa1..1343c74 100644
--- a/src/Retail/ShoppingCart.Api/HttpApi/Carts/CartCommandApi.cs
+++ b/src/Retail/ShoppingCart.Api/HttpApi/Carts/CartCommandApi.cs
@@ -39,14 +39,6 @@ public async Task<ActionResult<Result>> OpenCart([FromBody] CartCommands.V1.Remo
         return Ok(result);
     }
 
-    [HttpPost]
-    [Route("prepare-checkout")]
-    public async Task<ActionResult<Result>> PrepareForCheckout([FromBody] CartCommands.V1.ConfirmCartForCheckout cmd, CancellationToken ct)
-    {
-        var result = await _service.Handle(cmd, ct);
-        return Ok(result);
-    }
-
     [HttpPost]
     [Route("confirm")]
     public async Task<ActionResult<Result>> OpenCart([FromBody] CartCommands.V1.ConfirmCart cmd, CancellationToken ct)
diff --git a/src/Retail/ShoppingCart.Api/Infrastructure/Mongo.cs b/src/Retail/ShoppingCart.Api/Infrastructure/Mongo.cs
new file mode 100644
index 0000000..d57d722
--- /dev/null
+++ b/src/Retail/ShoppingCart.Api/Infrastructure/Mongo.cs
@@ -0,0 +1,35 @@
+using MongoDb.Bson.NodaTime;
+using MongoDB.Driver;
+using MongoDB.Driver.Core.Extensions.DiagnosticSources;
+
+namespace ShoppingCart.Api.Infrastructure;
+
+public static class Mongo
+{
+    public static IMongoDatabase ConfigureMongo(IConfiguration configuration)
+    {
+        NodaTimeSerializers.Register();
+        var config = configuration.GetSection("Mongo").Get<MongoSettings>();
+
+        var settings = MongoClientSettings.FromConnectionString(config!.ConnectionString);
+
+        if (config.User != null && config.Password != null) {
+            settings.Credential = new MongoCredential(
+                null,
+                new MongoInternalIdentity("admin", config.User),
+                new PasswordEvidence(config.Password)
+            );
+        }
+
+        settings.ClusterConfigurator = cb => cb.Subscribe(new DiagnosticsActivityEventSubscriber());
+        return new MongoClient(settings).GetDatabase(config.Database);
+    }
+
+    public record MongoSettings
+    {
+        public string ConnectionString { get; init; } = null!;
+        public string Database { get; init; } = null!;
+        public string? User { get; init; }
+        public string? Password { get; init; }
+    }
+}
diff --git a/src/Retail/ShoppingCart.Api/Queries/Carts/UserCartDocument.cs b/src/Retail/ShoppingCart.Api/Queries/Carts/UserCartDocument.cs
new file mode 100644
index 0000000..62b90ad
--- /dev/null
+++ b/src/Retail/ShoppingCart.Api/Queries/Carts/UserCartDocument.cs
@@ -0,0 +1,13 @@
+using Eventuous.Projections.MongoDB.Tools;
+
+namespace ShoppingCart.Api.Queries.Carts;
+
+public record UserCartDocument : ProjectedDocument
+{
+    public UserCartDocument(string Id) : base(Id)
+    {
+    }
+
+    public string CustomerId { get; set; } = null!;
+    public string Status { get; set; } = null!;
+}
diff --git a/src/Retail/ShoppingCart.Api/Queries/Carts/UserCartProjection.cs b/src/Retail/ShoppingCart.Api/Queries/Carts/UserCartProjection.cs
new file mode 100644
index 0000000..73a7f8b
--- /dev/null
+++ b/src/Retail/ShoppingCart.Api/Queries/Carts/UserCartProjection.cs
@@ -0,0 +1,38 @@
+using Eventuous.Projections.MongoDB;
+using Eventuous.Subscriptions.Context;
+using MongoDB.Driver;
+using ShoppingCart.Carts;
+
+namespace ShoppingCart.Api.Queries.Carts;
+
+[Obsolete("Obsolete per Eventuous; use new API instead (TODO)")]
+public class UserCartProjection : MongoProjection<UserCartDocument>
+{
+    public UserCartProjection(IMongoDatabase database) : base(database)
+    {
+        On<CartEvents.V1.CartOpened>(stream => stream.GetId(), Handle);
+
+        On<CartEvents.V1.CartConfirmed>(builder => builder
+            .UpdateOne
+            .DefaultId()
+            .Update((evt, update) =>
+                update.Set(x => x.Status, nameof(CartStatus.Confirmed))));
+
+        On<CartEvents.V1.CartCancelled>(builder => builder
+            .UpdateOne
+            .DefaultId()
+            .Update((evt, update) =>
+                update.Set(x => x.Status, nameof(CartStatus.Cancelled))));
+    }
+
+    private static UpdateDefinition<UserCartDocument> Handle(
+        IMessageConsumeContext<CartEvents.V1.CartOpened> ctx,
+        UpdateDefinitionBuilder<UserCartDocument> update)
+    {
+        var evt = ctx.Message;
+
+        return update.SetOnInsert(x => x.Id, ctx.Stream.GetId())
+            .Set(x => x.CustomerId, evt.CustomerId)
+            .Set(x => x.Status, nameof(CartStatus.Opened));
+    }
+}
diff --git a/src/Retail/ShoppingCart.Api/Registrations.cs b/src/Retail/ShoppingCart.Api/Registrations.cs
index e270624..aa47721 100644
--- a/src/Retail/ShoppingCart.Api/Registrations.cs
+++ b/src/Retail/ShoppingCart.Api/Registrations.cs
@@ -3,13 +3,19 @@
 using Eventuous;
 using Eventuous.Diagnostics.OpenTelemetry;
 using Eventuous.EventStore;
+using Eventuous.EventStore.Subscriptions;
+using Eventuous.Projections.MongoDB;
+using Eventuous.Subscriptions.Registrations;
 using Microsoft.Extensions.Diagnostics.HealthChecks;
 using OpenTelemetry.Metrics;
 using OpenTelemetry.Resources;
 using OpenTelemetry.Trace;
+using ShoppingCart.Api.Infrastructure;
+using ShoppingCart.Api.Queries.Carts;
 using ShoppingCart.Carts;
 using ShoppingCart.Inventories;
 using ShoppingCart.Prices;
+using ShoppingCart.Products;
 
 #pragma warning disable CS0618 // Type or member is obsolete
 
@@ -18,7 +24,6 @@ namespace ShoppingCart.Api;
 public static class Registrations
 {
     private const string OTelServiceName = "shoppingcart";
-    private const string PostgresSchemaName = "shoppingcart";
 
     public static void AddEventuous(this IServiceCollection services, IConfiguration configuration)
     {
@@ -37,8 +42,23 @@ public static void AddEventuous(this IServiceCollection services, IConfiguration
 
         // other internal and core services
         services.AddSingleton<ICombIdGenerator, CombIdGenerator>();
-        services.AddSingleton<ICheckInventoryService, CheckInventoryService>();
-        services.AddSingleton<IQuotePriceService, QuotePriceService>();
+        services.AddSingleton<IProductValidator, ProductValidator>();
+        services.AddSingleton<IInventoryChecker, InventoryChecker>();
+        services.AddSingleton<IPriceQuoter, PriceQuoter>();
+
+        // subscriptions: checkpoint stores
+        services.AddSingleton(Mongo.ConfigureMongo(configuration));
+        services.AddCheckpointStore<MongoCheckpointStore>();
+
+        // subscriptions: projections
+        services.AddSubscription<AllStreamSubscription, AllStreamSubscriptionOptions>(
+            "UserCartProjections",
+            builder => builder
+                .UseCheckpointStore<MongoCheckpointStore>()
+                .AddEventHandler<UserCartProjection>()
+                .WithPartitioningByStream(2));
+
+        // TODO: add additional mongo, postgresql, and other custom projections
 
         // health checks for subscription service
         services
diff --git a/src/Retail/ShoppingCart.Api/ShoppingCart.Api.csproj b/src/Retail/ShoppingCart.Api/ShoppingCart.Api.csproj
index edea4fd..869ade7 100644
--- a/src/Retail/ShoppingCart.Api/ShoppingCart.Api.csproj
+++ b/src/Retail/ShoppingCart.Api/ShoppingCart.Api.csproj
@@ -9,9 +9,12 @@
         <PackageReference Include="Eventuous.Application" Version="$(EventuousVersion)" />
         <PackageReference Include="Eventuous.EventStore" Version="$(EventuousVersion)" />
         <PackageReference Include="Eventuous.Extensions.DependencyInjection" Version="$(EventuousVersion)" />
+        <PackageReference Include="Eventuous.Projections.MongoDB" Version="$(EventuousVersion)" />
         <PackageReference Include="Eventuous.AspNetCore.Web" Version="$(EventuousVersion)" />
         <PackageReference Include="Eventuous.Spyglass" Version="$(EventuousVersion)" />
         <PackageReference Include="Microsoft.AspNetCore.Mvc.NewtonsoftJson" Version="8.0.6" />
+        <PackageReference Include="MongoDb.Bson.NodaTime" Version="3.0.0" />
+        <PackageReference Include="MongoDB.Driver.Core.Extensions.OpenTelemetry" Version="1.0.0" />
         <PackageReference Include="NodaTime.Serialization.SystemTextJson" Version="1.2.0" />
         <PackageReference Include="OpenTelemetry.Exporter.Prometheus.AspNetCore" Version="1.4.0-rc.4" />
         <PackageReference Include="OpenTelemetry.Exporter.Zipkin" Version="1.9.0" />
diff --git a/src/Retail/ShoppingCart.Api/appsettings.Development.json b/src/Retail/ShoppingCart.Api/appsettings.Development.json
index e3c3837..8707552 100644
--- a/src/Retail/ShoppingCart.Api/appsettings.Development.json
+++ b/src/Retail/ShoppingCart.Api/appsettings.Development.json
@@ -11,5 +11,16 @@
     },
     "EventStore": {
         "ConnectionString": "esdb://localhost:2113?tls=false"
+    },
+    "Mongo": {
+        "ConnectionString": "mongodb://localhost:27017",
+        "User": "mongoadmin",
+        "Password": "secret",
+        "Database": "ShoppingCart"
+    },
+    "Postgres": {
+        "ConnectionString": "Server=127.0.0.1;Port=5432;Database=postgres;User Id=postgres;Password=Password123!;Include Error Detail=true;",
+        "Schema": "shopping_cart",
+        "InitializeDatabase": true
     }
 }
diff --git a/src/Retail/ShoppingCart/Carts/CartCommands.cs b/src/Retail/ShoppingCart/Carts/CartCommands.cs
index e512997..9d039d9 100644
--- a/src/Retail/ShoppingCart/Carts/CartCommands.cs
+++ b/src/Retail/ShoppingCart/Carts/CartCommands.cs
@@ -20,11 +20,11 @@ public record RemoveProductFromCart(
             int Quantity
         );
 
-        public record ConfirmCartForCheckout(
+        public record ConfirmCart(
             string CartId
         );
 
-        public record ConfirmCart(
+        public record CancelCart(
             string CartId
         );
     }
diff --git a/src/Retail/ShoppingCart/Carts/CartEvents.cs b/src/Retail/ShoppingCart/Carts/CartEvents.cs
index cf7a885..5a50c53 100644
--- a/src/Retail/ShoppingCart/Carts/CartEvents.cs
+++ b/src/Retail/ShoppingCart/Carts/CartEvents.cs
@@ -31,6 +31,11 @@ public record CartConfirmed(
             string CartId
         );
 
+        [EventType("V1.CartCancelled")]
+        public record CartCancelled(
+            string CartId
+        );
+
         [EventType("V1.EmptyCartDetected")]
         public record EmptyCartDetected(
             string CartId
diff --git a/src/Retail/ShoppingCart/Carts/CartFuncService.cs b/src/Retail/ShoppingCart/Carts/CartFuncService.cs
index e96cbdc..401573c 100644
--- a/src/Retail/ShoppingCart/Carts/CartFuncService.cs
+++ b/src/Retail/ShoppingCart/Carts/CartFuncService.cs
@@ -24,8 +24,11 @@ public CartFuncService(
         OnExisting<Commands.RemoveProductFromCart>(cmd
             => GetStream(cmd.CartId), RemoveProductFromCart);
 
-        OnExisting<Commands.ConfirmCartForCheckout>(cmd
-            => GetStream(cmd.CartId), ConfirmCartForCheckout);
+        OnExisting<Commands.ConfirmCart>(cmd
+            => GetStream(cmd.CartId), ConfirmCart);
+
+        OnExisting<Commands.CancelCart>(cmd
+            => GetStream(cmd.CartId), CancelCart);
 
         static StreamName GetStream(string id) => new($"Cart-{id}");
 
@@ -66,13 +69,21 @@ static IEnumerable<object> RemoveProductFromCart(
                 yield return new Events.EmptyCartDetected(cmd.CartId);
         }
 
-        static IEnumerable<object> ConfirmCartForCheckout(
+        static IEnumerable<object> ConfirmCart(
             CartState state,
             object[] originalEvents,
-            Commands.ConfirmCartForCheckout cmd)
+            Commands.ConfirmCart cmd)
         {
             if (state.CanProceedToCheckout())
                 yield return new Events.CartConfirmed(cmd.CartId);
         }
+
+        static IEnumerable<object> CancelCart(
+            CartState state,
+            object[] originalEvents,
+            Commands.CancelCart cmd)
+        {
+            yield return new Events.CartCancelled(cmd.CartId);
+        }
     }
 }
diff --git a/src/Retail/ShoppingCart/Carts/CartState.cs b/src/Retail/ShoppingCart/Carts/CartState.cs
index 4498209..af4339c 100644
--- a/src/Retail/ShoppingCart/Carts/CartState.cs
+++ b/src/Retail/ShoppingCart/Carts/CartState.cs
@@ -1,5 +1,6 @@
 using Ecommerce.Eventuous.Exceptions;
 using Eventuous;
+using ShoppingCart.Products;
 using static ShoppingCart.Carts.CartEvents.V1;
 
 namespace ShoppingCart.Carts;
@@ -19,6 +20,7 @@ public CartState()
         On<ProductAddedToCart>(Handle);
         On<ProductRemovedFromCart>(Handle);
         On<CartConfirmed>(Handle);
+        On<CartCancelled>(Handle);
     }
 
     private static CartState Handle(CartState state, CartOpened @event) => state with
@@ -37,19 +39,27 @@ private static CartState Handle(CartState state, CartOpened @event) => state wit
 
     private static CartState Handle(CartState state, ProductRemovedFromCart @event) => state.Status switch
     {
-        CartStatus.Confirmed => throw InvalidStateChangeException.For<CartState, ProductAddedToCart>(state.Id, CartStatus.Confirmed),
+        CartStatus.Confirmed => throw InvalidStateChangeException.For<CartState, ProductRemovedFromCart>(state.Id, CartStatus.Confirmed),
         _ => state with { ProductItems = state.ProductItems.Remove(ProductItem.From(@event.ProductId, @event.Quantity)) }
     };
 
     private static CartState Handle(CartState state, CartConfirmed @event) => state.Status switch
     {
-        CartStatus.Confirmed => throw InvalidStateChangeException.For<CartState, ProductAddedToCart>(state.Id, CartStatus.Confirmed),
+        CartStatus.Confirmed => throw InvalidStateChangeException.For<CartState, CartConfirmed>(state.Id, CartStatus.Confirmed),
         _ => state with { Status = CartStatus.Confirmed }
     };
 
+    private static CartState Handle(CartState state, CartCancelled @event) => state.Status switch
+    {
+        CartStatus.Confirmed => throw InvalidStateChangeException.For<CartState, CartCancelled>(state.Id, CartStatus.Confirmed),
+        CartStatus.Cancelled => throw InvalidStateChangeException.For<CartState, CartCancelled>(state.Id, CartStatus.Cancelled),
+        _ => state with { Status = CartStatus.Cancelled }
+    };
+
     public bool CanProceedToCheckout() => Status switch
     {
-        CartStatus.Unset or CartStatus.Opened => false,
+        CartStatus.Unset => false,
+        CartStatus.Confirmed => false,
         _ => HasProductItems
     };
 }
diff --git a/src/Retail/ShoppingCart/Carts/CartStatus.cs b/src/Retail/ShoppingCart/Carts/CartStatus.cs
index 5e7bbad..db8226a 100644
--- a/src/Retail/ShoppingCart/Carts/CartStatus.cs
+++ b/src/Retail/ShoppingCart/Carts/CartStatus.cs
@@ -4,5 +4,6 @@ public enum CartStatus
 {
     Unset = 0,
     Opened = 1,
-    Confirmed = 2
+    Confirmed = 2,
+    Cancelled = 4
 }
diff --git a/src/Retail/ShoppingCart/Inventories/ICheckInventoryService.cs b/src/Retail/ShoppingCart/Inventories/IInventoryChecker.cs
similarity index 62%
rename from src/Retail/ShoppingCart/Inventories/ICheckInventoryService.cs
rename to src/Retail/ShoppingCart/Inventories/IInventoryChecker.cs
index 611bcf2..8ba8f61 100644
--- a/src/Retail/ShoppingCart/Inventories/ICheckInventoryService.cs
+++ b/src/Retail/ShoppingCart/Inventories/IInventoryChecker.cs
@@ -1,8 +1,8 @@
-using ShoppingCart.Carts;
+using ShoppingCart.Products;
 
 namespace ShoppingCart.Inventories;
 
-public interface ICheckInventoryService
+public interface IInventoryChecker
 {
     IReadOnlyList<ProductItem> Check(params ProductId[] productIds);
 }
diff --git a/src/Retail/ShoppingCart/Inventories/CheckInventoryService.cs b/src/Retail/ShoppingCart/Inventories/InventoryChecker.cs
similarity index 83%
rename from src/Retail/ShoppingCart/Inventories/CheckInventoryService.cs
rename to src/Retail/ShoppingCart/Inventories/InventoryChecker.cs
index 1fb3cc9..2f42f61 100644
--- a/src/Retail/ShoppingCart/Inventories/CheckInventoryService.cs
+++ b/src/Retail/ShoppingCart/Inventories/InventoryChecker.cs
@@ -1,8 +1,8 @@
-using ShoppingCart.Carts;
+using ShoppingCart.Products;
 
 namespace ShoppingCart.Inventories;
 
-public class CheckInventoryService : ICheckInventoryService
+public class InventoryChecker : IInventoryChecker
 {
     public IReadOnlyList<ProductItem> Check(params ProductId[] productIds)
     {
diff --git a/src/Retail/ShoppingCart/Prices/IQuotePriceService.cs b/src/Retail/ShoppingCart/Prices/IPriceQuoter.cs
similarity index 65%
rename from src/Retail/ShoppingCart/Prices/IQuotePriceService.cs
rename to src/Retail/ShoppingCart/Prices/IPriceQuoter.cs
index a7e0498..d872448 100644
--- a/src/Retail/ShoppingCart/Prices/IQuotePriceService.cs
+++ b/src/Retail/ShoppingCart/Prices/IPriceQuoter.cs
@@ -1,8 +1,8 @@
-using ShoppingCart.Carts;
+using ShoppingCart.Products;
 
 namespace ShoppingCart.Prices;
 
-public interface IQuotePriceService
+public interface IPriceQuoter
 {
     IReadOnlyList<PricedProductItem> Quote(params ProductItem[] productItems);
 }
diff --git a/src/Retail/ShoppingCart/Prices/QuotePriceService.cs b/src/Retail/ShoppingCart/Prices/PriceQuoter.cs
similarity index 86%
rename from src/Retail/ShoppingCart/Prices/QuotePriceService.cs
rename to src/Retail/ShoppingCart/Prices/PriceQuoter.cs
index 515fba9..7927fd8 100644
--- a/src/Retail/ShoppingCart/Prices/QuotePriceService.cs
+++ b/src/Retail/ShoppingCart/Prices/PriceQuoter.cs
@@ -1,8 +1,8 @@
-using ShoppingCart.Carts;
+using ShoppingCart.Products;
 
 namespace ShoppingCart.Prices;
 
-public class QuotePriceService : IQuotePriceService
+public class PriceQuoter : IPriceQuoter
 {
     public IReadOnlyList<PricedProductItem> Quote(params ProductItem[] productItems)
     {
diff --git a/src/Retail/ShoppingCart/Products/IProductValidator.cs b/src/Retail/ShoppingCart/Products/IProductValidator.cs
new file mode 100644
index 0000000..5d577a0
--- /dev/null
+++ b/src/Retail/ShoppingCart/Products/IProductValidator.cs
@@ -0,0 +1,6 @@
+namespace ShoppingCart.Products;
+
+public interface IProductValidator
+{
+    IReadOnlyList<(ProductId productId, bool isValid)> Quote(params ProductId[] productIds);
+}
diff --git a/src/Retail/ShoppingCart/Carts/PricedProductItem.cs b/src/Retail/ShoppingCart/Products/PricedProductItem.cs
similarity index 98%
rename from src/Retail/ShoppingCart/Carts/PricedProductItem.cs
rename to src/Retail/ShoppingCart/Products/PricedProductItem.cs
index 6a6669b..2c85c48 100644
--- a/src/Retail/ShoppingCart/Carts/PricedProductItem.cs
+++ b/src/Retail/ShoppingCart/Products/PricedProductItem.cs
@@ -1,4 +1,4 @@
-namespace ShoppingCart.Carts;
+namespace ShoppingCart.Products;
 
 public class PricedProductItem
 {
diff --git a/src/Retail/ShoppingCart/Carts/ProductId.cs b/src/Retail/ShoppingCart/Products/ProductId.cs
similarity index 67%
rename from src/Retail/ShoppingCart/Carts/ProductId.cs
rename to src/Retail/ShoppingCart/Products/ProductId.cs
index 78af650..0041ebe 100644
--- a/src/Retail/ShoppingCart/Carts/ProductId.cs
+++ b/src/Retail/ShoppingCart/Products/ProductId.cs
@@ -1,5 +1,5 @@
 using Eventuous;
 
-namespace ShoppingCart.Carts;
+namespace ShoppingCart.Products;
 
 public record ProductId(string Value) : Id(Value);
diff --git a/src/Retail/ShoppingCart/Carts/ProductItem.cs b/src/Retail/ShoppingCart/Products/ProductItem.cs
similarity index 98%
rename from src/Retail/ShoppingCart/Carts/ProductItem.cs
rename to src/Retail/ShoppingCart/Products/ProductItem.cs
index 83bbfa5..e5e4298 100644
--- a/src/Retail/ShoppingCart/Carts/ProductItem.cs
+++ b/src/Retail/ShoppingCart/Products/ProductItem.cs
@@ -1,4 +1,4 @@
-namespace ShoppingCart.Carts;
+namespace ShoppingCart.Products;
 
 public record ProductItem
 {
diff --git a/src/Retail/ShoppingCart/Products/ProductValidator.cs b/src/Retail/ShoppingCart/Products/ProductValidator.cs
new file mode 100644
index 0000000..4bc0e70
--- /dev/null
+++ b/src/Retail/ShoppingCart/Products/ProductValidator.cs
@@ -0,0 +1,16 @@
+namespace ShoppingCart.Products;
+
+public class ProductValidator : IProductValidator
+{
+    public IReadOnlyList<(ProductId productId, bool isValid)> Quote(params ProductId[] productIds)
+    {
+        if (productIds.Length == 0)
+            throw new ArgumentOutOfRangeException(nameof(productIds.Length));
+
+        // TODO: access cache or make a gRPC call
+
+        return productIds
+            .Select(pi => new ValueTuple<ProductId, bool>(pi, true))
+            .ToList();
+    }
+}