Writing gRPC descriptors
Kalix SDKs support protobuf descriptors in the Proto3 
Protocol Buffers language. You define command messages, data associated with Entities, and events in .proto files. From these definitions, the gRPC compiler creates client and server side code that saves work for you and enables Kalix to serialize message data.
We recommend that you define your service API and events and data associated with components separately. This allows business logic to evolve independently of the public interface. This page walks you through elements in an example shoppingcart_api.proto file and the associated shoppingcart_domain.proto file.
The service proto file
The first line of the example shoppingcart_api.proto file defines the version of Protocol Buffer syntax:
syntax = "proto3";The following imports provide Kalix and gRPC functionality:
import "google/protobuf/empty.proto";
import "kalix/annotations.proto";
import "google/api/annotations.proto";The package specifies a namespace for this proto file and its imports—names must be unique within each namespace:
package com.example.shoppingcart;It is recommended that you align the proto directory structure with the package structure, just like with Java classes, so that files defining services and messages in com.example.shoppingcart can be found in src/main/proto/com/example/shoppingcart.
Messages
You define messages that can be sent to or returned from the service.
Messages for entities
Each input message for an Entity that is input to a RPC command, must be associated with an Entity key, so that Kalix can identify which entity the message is for. In the example, this includes AddLineItem, RemoveLineItem, and GetShoppingCart where the user_id is the (kalix.field).entity_key. Kalix extracts the value of these fields in order to route messages to the right Entity. If more than one field is specified as an Entity key, the fields are concatenated together. Kalix serializes Entity keys to strings.
message CreateCart {
string cart_id = 1 [(kalix.field).entity_key = true];
}
message AddLineItem {
string cart_id = 1 [(kalix.field).entity_key = true];
string product_id = 2;
string name = 3;
int32 quantity = 4;
}
message RemoveLineItem {
string cart_id = 1 [(kalix.field).entity_key = true];
string product_id = 2;
}
message GetShoppingCart {
string cart_id = 1 [(kalix.field).entity_key = true];
}
message RemoveShoppingCart {
string cart_id = 1 [(kalix.field).entity_key = true];
}
message LineItem {
string product_id = 1;
string name = 2;
int32 quantity = 3;
}
message Cart {
repeated LineItem items = 1;
int64 creation_timestamp = 2;
}To use a multi-field key, add the entity_key notation to each field. For example, the following SomeMessage element defines both first_field and second_field as part of the key:
message SomeMessage {
string first_field = 1 [(kalix.field).entity_key = true];
string second_field = 2 [(kalix.field).entity_key = true];
}Generated entity keys
In some cases, you may wish to generate an Entity key, this is typically done when an RPC method creates an entity, and the key is a surrogate key. To indicate to Kalix that an Entity key should be generated, the incoming message must not have any (kalix.field).entity_key annotated field. Instead, the rpc method should be annotated with (kalix.method).entity.key_generator, for example:
rpc CreateCart(CreateCartRequest) returns (CreateCartResponse) {
option (kalix.method).entity.key_generator = VERSION_4_UUID;
};This will generate a version 4 (random) UUID for the Entity. Only version 4 UUIDs are currently supported for generated Entity keys.
It will often be necessary to access the generated entity key from inside the entities code. This can be done using the EntityContext.entityId method.
Service
This section of the .proto file declares the API of the service itself, along with each function or method and their parameters and return types. When a command is received for a given Entity key, Kalix will establish a gRPC streamed call to the service implementation using that Entity’s type’s protocol—if one isn’t already established. Any commands received for the Entity key will be sent through that call.
The AddItem and RemoveItem methods have no return value (the Empty type).
Most importantly, in this file we instruct the Kalix code generation tooling (codegen) which kind of component we want to create. The codegen will generate all stubs for your entity/service and corresponding tests, as well as an abstract class for your implementation to extend.
service ShoppingCartService {
// Describes how this domain relates to a value entity
option (kalix.codegen) = {
value_entity: { (1)
name: "com.example.shoppingcart.domain.ShoppingCart" (2)
entity_type: "shopping-cart" (3)
state: "com.example.shoppingcart.domain.Cart" (4)
}
};
rpc Create (CreateCart) returns (google.protobuf.Empty) {
}
rpc AddItem (AddLineItem) returns (google.protobuf.Empty) {
}
rpc RemoveItem (RemoveLineItem) returns (google.protobuf.Empty) {
}
rpc GetCart (GetShoppingCart) returns (Cart) {
}
rpc RemoveCart (RemoveShoppingCart) returns (google.protobuf.Empty) {
}
}| 1 | Indicates to the codegen that we want to generate a Value Entity. |
| 2 | The package and name of our Value Entity. |
| 3 | Unique identifier of the "state storage" for this entity. The entity name may be changed even after data has been created, the entity_type can’t. |
| 4 | The Entity state using a fully-qualified name. Note, the package and name follow the definition in the domain.proto file (see below). |
The domain proto file
The shoppingcart_domain.proto file specifies the state and messages for a Value Entity.
syntax = "proto3"; (1)
package com.example.shoppingcart.domain; (2)
option java_outer_classname = "ShoppingCartDomain";
message LineItem {
string product_id = 1;
string name = 2;
int32 quantity = 3;
}
// The shopping cart state.
message Cart { (3)
repeated LineItem items = 1;
int64 creation_timestamp = 2;
}| 1 | Defines the gRPC version. |
| 2 | The proto package for the state. |
| 3 | The Entity’s state defined as a proto message. |
Transcoding HTTP
Kalix supports transcoding gRPC to HTTP/JSON, using the Google transcoding annotations described here. You can use transcoding to consume your Entities' gRPC interfaces using HTTP/JSON.
In the example below, the optional transcoding of the service to bind the various endpoints to HTTP is highlighted with annotations.
rpc Create (CreateCart) returns (google.protobuf.Empty) {
option (google.api.http) = { (1)
post: "/cart/{cart_id}/create"
};
}
rpc AddItem (AddLineItem) returns (google.protobuf.Empty) {
option (google.api.http) = { (1)
post: "/cart/{cart_id}/items/add"
body: "*"
};
}
rpc RemoveItem (RemoveLineItem) returns (google.protobuf.Empty) {
option (google.api.http) = { (2)
post: "/cart/{cart_id}/items/{product_id}/remove"
};
}
rpc GetCart (GetShoppingCart) returns (Cart) {
option (google.api.http) = { (3)
get: "/carts/{cart_id}"
additional_bindings: {
get: "/carts/{cart_id}/items"
response_body: "items"
}
};
}
rpc RemoveCart (RemoveShoppingCart) returns (google.protobuf.Empty) {
option (google.api.http).post = "/carts/{cart_id}/remove";
}| 1 | This extra annotation specifies that you can call this endpoint using the POST method with the URI /cart/{user_id}/items/add, where {user_id} is the actual user id we want the cart for. |
| 2 | A URL that accepts a POST method to remove a line item. |
| 3 | A more complex example where the first get URI retrieves the whole cart, and the second retrieves the items in the cart. |