Discount service

Discount stores one coupon per product name and serves it over gRPC. It has no REST controllers. Its only client in the codebase is the Basket service.

Source	src/Services/Discount
Store	PostgreSQL, Coupon table, accessed with Npgsql and Dapper
Protocol	gRPC over HTTP/2 cleartext, Kestrel endpoint http://0.0.0.0:8080
Contract	src/Services/Discount/Discount.Application/Protos/discount.proto
Local port	8002 → container 8080

Layers

Project	What is in it
src/Services/Discount/Discount.API	DiscountService : DiscountProtoServiceBase, which maps each RPC to a mediator request; Program.cs runs the DB migration at startup
src/Services/Discount/Discount.Application	The .proto (compiled with GrpcServices="Server"), commands, a query, handlers, DiscountMapper
src/Services/Discount/Discount.Core	Coupon entity, IDiscountRepository
src/Services/Discount/Discount.Infrastructure	DiscountRepository (Dapper SQL), DbExtension.MigrateDatabase

gRPC contract

service DiscountProtoService {
  rpc GetDiscount(GetDiscountRequest) returns (CouponModel);
  rpc CreateDiscount(CreateDiscountRequest) returns (CouponModel);
  rpc UpdateDiscount(UpdateDiscountRequest) returns (CouponModel);
  rpc DeleteDiscount(DeleteDiscountRequest) returns (DeleteDiscountResponse);
}

message CouponModel {
  int32 id = 1;
  string productName = 2;
  string description = 3;
  int32 amount = 4;
}

RPC	Mediator request	Repository SQL
GetDiscount	GetDiscountQuery	SELECT * FROM Coupon WHERE ProductName = @ProductName
CreateDiscount	CreateDiscountCommand	INSERT INTO Coupon (ProductName, Description, Amount) ...
UpdateDiscount	UpdateDiscountCommand	UPDATE Coupon SET ... WHERE Id = @Id
DeleteDiscount	DeleteDiscountCommand	DELETE FROM Coupon WHERE ProductName = @ProductName

All queries are parameterized through Dapper (src/Services/Discount/Discount.Infrastructure/Repositories/DiscountRepository.cs). Each call opens a new NpgsqlConnection, and Npgsql connection pooling makes that cheap.

Amount is an int32, so discounts are whole currency units. Basket subtracts it from the unit price.

Data model

erDiagram
  COUPON {
    serial Id PK
    varchar ProductName "VARCHAR(500) NOT NULL"
    text Description
    int Amount
  }

Startup migration

app.MigrateDatabase<Program>() (src/Services/Discount/Discount.Infrastructure/Extensions/DbExtension.cs) runs before the host starts:

1. Connects to the postgres maintenance database and creates the target database if pg_database does not list it.
2. In one transaction, takes a PostgreSQL advisory lock (pg_advisory_xact_lock) so replicas starting together cannot race, then runs CREATE TABLE IF NOT EXISTS Coupon(...) and a unique index on ProductName.
3. Seeds two coupons, ASUS ZenBook 13 OLED Ultrabook (500) and ASUS ROG Zephyrus G14 Gaming Laptop (700), only when the table is empty, with INSERT ... ON CONFLICT DO NOTHING. Seed coupons an admin deleted do not come back.
4. Retries transient connection failures with exponential back-off and fails fast (the process exits) if the database is still unreachable, instead of starting a broken service.

Fixed in #562

Startup used to run DROP TABLE IF EXISTS Coupon and reseed, so coupons created through CreateDiscount disappeared on every restart. tests/integration/Discount.IntegrationTests now proves coupons survive a second startup and that four concurrent startups seed exactly once.

Implementation notes

• Missing coupons are not errors. GetDiscountQueryHandler throws RpcException(StatusCode.NotFound) when the repository returns null. The repository never does: it returns a placeholder Coupon { ProductName = "No Discount", Amount = 0 }. The effective behaviour is “no coupon means a zero discount”, which suits Basket, but the NotFound branch is dead code.
• Mapping. DiscountMapper is a Mapperly source-generated mapper registered as a singleton. Create ignores Id, because the database generates it.
• Handlers return protobuf types. Handlers return CouponModel (a generated gRPC type), so Discount.Application depends on the transport contract.
• Nothing reaches it over HTTP. The Ocelot route files still contain /Discount REST routes, and deploy/istio/virtualservices.yaml has a discount VirtualService matching /api/v1/Discount. Discount has no REST controllers, so these routes cannot be served. Coupon administration would need a REST facade, gRPC-JSON transcoding, or a gRPC-capable gateway route.
• No tracing of Npgsql. Only ASP.NET Core instrumentation is registered, so incoming gRPC calls are traced, but database calls are not.