Implementing Views in Java or Scala

You can access a single Entity with its Entity key. You might want to retrieve multiple Entities, or retrieve them using an attribute other than the key. Kalix Views allow you achieve this. By creating multiple Views, you can optimize for query performance against each one.

Views can be defined from any of the following:

The remainder of this page describes:

Be aware that Views are not updated immediately when Entity state changes. Kalix does update Views as quickly as possible. It is not instant but eventually all changes will become visible in the query results. View updates might also take more time during failure scenarios than during normal operation.

Creating a View from a Value Entity

Consider an example of a Customer Registry service with a customer Value Entity. When customer state changes, the entire state is emitted as a value change. Value changes update any associated Views. To create a View that lists customers by their name:

This example assumes the following customer state is defined in a customer_domain.proto file:

Java
src/main/proto/customer/domain/customer_domain.proto
syntax = "proto3";
package customer.domain;
option java_outer_classname = "CustomerDomain";
message CustomerState {
  string customer_id = 1;
  string email = 2;
  string name = 3;
  Address address = 4;
}

message Address {
  string street = 1;
  string city = 2;
}
Scala
src/main/proto/customer/domain/customer_domain.proto
syntax = "proto3";
package customer.domain;
message CustomerState {
  string customer_id = 1;
  string email = 2;
  string name = 3;
  Address address = 4;
}

message Address {
  string street = 1;
  string city = 2;
}

As well as a Value Entity service that will produce the state changes consumed by the View:

Java
src/main/proto/customer/api/customer_api.proto
service CustomerService {
  option (kalix.codegen) = {
    value_entity: {
      name: "customer.domain.CustomerValueEntity"
      entity_type: "customers"
      state: "customer.domain.CustomerState"
    }
  };

  rpc Create(Customer) returns (google.protobuf.Empty) {}
  rpc ChangeName(ChangeNameRequest) returns (google.protobuf.Empty) {}
  rpc ChangeAddress(ChangeAddressRequest) returns (google.protobuf.Empty) {}
  rpc GetCustomer(GetCustomerRequest) returns (Customer) {}
  rpc Delete(DeleteCustomerRequest) returns (google.protobuf.Empty) {}
}
Scala
src/main/proto/customer/api/customer_api.proto
service CustomerService {
  option (kalix.codegen) = {
    value_entity: {
      name: "customer.domain.CustomerValueEntity"
      entity_type: "customers"
      state: "customer.domain.CustomerState"
    }
  };

  rpc Create(Customer) returns (google.protobuf.Empty) {}
  rpc ChangeName(ChangeNameRequest) returns (google.protobuf.Empty) {}
  rpc ChangeAddress(ChangeAddressRequest) returns (google.protobuf.Empty) {}
  rpc GetCustomer(GetCustomerRequest) returns (Customer) {}
  rpc Delete(DeleteCustomerRequest) returns (google.protobuf.Empty) {}
}

Define the View service descriptor

To get a View of multiple customers by their name, define the View as a service in Protobuf:

Java
src/main/proto/customer/view/customer_view.proto
syntax = "proto3";

package customer.view;

option java_outer_classname = "CustomerViewModel";

import "customer/domain/customer_domain.proto";
import "kalix/annotations.proto";
import "google/protobuf/any.proto";
import "google/protobuf/empty.proto";

service CustomerByName {
  option (kalix.codegen) = {
    view: {} (1)
  };

  rpc UpdateCustomer(domain.CustomerState) returns (domain.CustomerState) {(2)
    option (kalix.method).eventing.in = {(3)
      value_entity: "customers"
    };
    option (kalix.method).view.update = {(4)
      table: "customers"
    };
  }

  rpc DeleteCustomer(google.protobuf.Empty) returns (domain.CustomerState) {
    option (kalix.method).eventing.in = {
      value_entity: "customers"
      handle_deletes: true (5)
    };
    option (kalix.method).view.update = {
      table: "customers"
    };
  }

  rpc GetCustomers(ByNameRequest) returns (stream domain.CustomerState) {(6)
    option (kalix.method).view.query = {(7)
      query: "SELECT * FROM customers WHERE name = :customer_name"
    };
  }
}

message ByNameRequest {
  string customer_name = 1;
}
Scala
src/main/proto/customer/view/customer_view.proto
syntax = "proto3";

package customer.view;

import "customer/domain/customer_domain.proto";
import "kalix/annotations.proto";
import "google/protobuf/empty.proto";

service CustomerByName {
  option (kalix.codegen) = {
    view: {} (1)
  };

  rpc UpdateCustomer(domain.CustomerState) returns (domain.CustomerState) { (2)
    option (kalix.method).eventing.in = { (3)
      value_entity: "customers"
    };
    option (kalix.method).view.update = { (4)
      table: "customers"
    };
  }

  rpc DeleteCustomer(google.protobuf.Empty) returns (domain.CustomerState) {
    option (kalix.method).eventing.in = {
      value_entity: "customers"
      handle_deletes: true (5)
    };
    option (kalix.method).view.update = {
      table: "customers"
    };
  }

  rpc GetCustomers(ByNameRequest) returns (stream domain.CustomerState) { (6)
    option (kalix.method).view.query = { (7)
      query: "SELECT * FROM customers WHERE name = :customer_name"
    };
  }
}

message ByNameRequest {
  string customer_name = 1;
}
1 The protobuf option (kalix.codegen) is specific to code-generation as provided by the Kalix Maven plugin. This annotation indicates to the code-generation that a View must be generated.
2 The UpdateCustomer method defines how Kalix will update the view.
3 The source of the View is the "customers" Value Entity. This identifier is defined in the entity_type: "customers" property of the (kalix.codegen).value_entity option in the customer_api.proto file.
4 The (kalix.method).view.update annotation defines that this method is used for updating the View. You must define the table attribute for the table to be used in the query. Pick any name and use it in the query SELECT statement.
5 The second (kalix.method).eventing.in annotation with handle_deletes: true and (required) google.protobuf.Empty as a parameter defines that this method is used for handling Value Entity deletes. Kalix will automatically delete the View state, but you can also write your own delete handler when transform_updates flag is true. If you don’t provide such method then, by default, your View state will not be deleted.
6 The GetCustomers method defines the query to retrieve a stream of customers.
7 The (kalix.method).view.query annotation defines that this method is used as a query of the View.
In this sample we use the internal domain.CustomerState as the state of the view. This is convenient since it allows automatic updates of the view without any logic but has the draw back that it implicitly makes the domain.CustomerState type a part of the public service API. Transforming the state to another type than the incoming update to avoid this can be seen in Creating a View from an Event Sourced Entity.

If the query should only return one result, remove the stream from the return type:

Java
rpc GetCustomer(ByEmailRequest) returns (domain.CustomerState) { (1)
  option (kalix.method).view.query = {
    query: "SELECT * FROM customers WHERE email = :email"
  };
}
Scala
rpc GetCustomer(ByEmailRequest) returns (domain.CustomerState) { (1)
  option (kalix.method).view.query = {
    query: "SELECT * FROM customers WHERE email = :email"
  };
}
1 Without stream when expecting single result.

When no result is found, the request fails with gRPC status code NOT_FOUND. A streamed call completes with an empty stream when no result is found.

Registering a View

Once you’ve defined a View, register it with Kalix by invoking the KalixFactory.withComponents method in the Main class.

Java
src/main/java/customer/Main.java
public static Kalix createKalix() {
  return KalixFactory.withComponents(
    CustomerValueEntity::new,
    CustomerActionImpl::new,
    CustomerByEmailView::new,
    CustomerByNameView::new,
    CustomerSummaryByNameView::new,
    CustomersResponseByNameView::new);
}
Scala
src/main/scala/customer/Main.scala
def createKalix(): Kalix = {
  KalixFactory.withComponents(
    new CustomerValueEntity(_),
    new CustomerActionImpl(_),
    new CustomerByEmailView(_),
    new CustomerByNameView(_),
    new CustomerSummaryByNameView(_),
    new CustomersResponseByNameView(_))
}

Creating a View from an Event Sourced Entity

Create a View from an Event Sourced Entity by using events that the Entity emits to build a state representation. Using a Customer Registry service example, to create a View for querying customers by name:

  1. Define a View descriptor to consume events

  2. Create a transformation class

  3. Register the View

The example assumes a customer_domain.proto file that defines the events that will update the View on name changes:

Java
src/main/proto/customer/domain/customer_domain.proto
syntax = "proto3";
package customer.domain;
option java_outer_classname = "CustomerDomain";
message CustomerState {
  string customer_id = 1;
  string email = 2;
  string name = 3;
  Address address = 4;
}

message Address {
  string street = 1;
  string city = 2;
}
message CustomerCreated {
  CustomerState customer = 1;
}

message CustomerNameChanged {
  string new_name = 1;
}

message CustomerAddressChanged {
  Address new_address = 1;
}
Scala
src/main/proto/customer/domain/customer_domain.proto
syntax = "proto3";
package customer.domain;
message CustomerState {
  string customer_id = 1;
  string email = 2;
  string name = 3;
  Address address = 4;
}

message Address {
  string street = 1;
  string city = 2;
}
message CustomerCreated {
  CustomerState customer = 1;
}

message CustomerNameChanged {
  string new_name = 1;
}

message CustomerAddressChanged {
  Address new_address = 1;
}

As well as an Event Sourced Entity service that will produce the events consumed by the View:

Java
src/main/proto/customer/api/customer_api.proto
service CustomerService {
  option (kalix.codegen) = {
    event_sourced_entity: {
      name: "customer.domain.CustomerEntity"
      entity_type: "customers"
      state: "customer.domain.CustomerState"
      events: [
        "customer.domain.CustomerCreated",
        "customer.domain.CustomerNameChanged",
        "customer.domain.CustomerAddressChanged"]
    }
  };
  rpc Create(Customer) returns (google.protobuf.Empty) {}
  rpc ChangeName(ChangeNameRequest) returns (google.protobuf.Empty) {}
  rpc ChangeAddress(ChangeAddressRequest) returns (google.protobuf.Empty) {}
  rpc GetCustomer(GetCustomerRequest) returns (Customer) {}
}
Scala
src/main/proto/customer/api/customer_api.proto
service CustomerService {
  option (kalix.codegen) = {
    event_sourced_entity: {
      name: "customer.domain.CustomerEntity"
      entity_type: "customers"
      state: "customer.domain.CustomerState"
      events: [
        "customer.domain.CustomerCreated",
        "customer.domain.CustomerNameChanged",
        "customer.domain.CustomerAddressChanged"]
    }
  };

  rpc Create(Customer) returns (google.protobuf.Empty) {}
  rpc ChangeName(ChangeNameRequest) returns (google.protobuf.Empty) {}
  rpc ChangeAddress(ChangeAddressRequest) returns (google.protobuf.Empty) {}
  rpc GetCustomer(GetCustomerRequest) returns (Customer) {}
}

Define a View descriptor to consume events

The following lines in the .proto file define a View to consume the CustomerCreated and CustomerNameChanged events:

Java
src/main/proto/customer/customer_view.proto
syntax = "proto3";

package customer.view;

option java_outer_classname = "CustomerViewModel";

import "customer/domain/customer_domain.proto";
import "customer/api/customer_api.proto";
import "kalix/annotations.proto";
import "google/protobuf/any.proto";

message ByNameRequest {
  string customer_name = 1;
}

service CustomerByName {
  option (kalix.codegen) = { (1)
    view: {}
  };

  rpc ProcessCustomerCreated(domain.CustomerCreated) returns (api.Customer) { (2)
    option (kalix.method).eventing.in = {
      event_sourced_entity: "customers" (3)
    };
    option (kalix.method).view.update = {
      table: "customers"
      transform_updates: true (4)
    };
  }

  rpc ProcessCustomerNameChanged(domain.CustomerNameChanged) returns (api.Customer) { (2)
    option (kalix.method).eventing.in = {
      event_sourced_entity: "customers" (5)
    };
    option (kalix.method).view.update = {
      table: "customers"
      transform_updates: true (6)
    };
  }

  rpc ProcessCustomerAddressChanged(domain.CustomerAddressChanged) returns (api.Customer) {
    option (kalix.method).eventing.in = {
      event_sourced_entity: "customers"
    };
    option (kalix.method).view.update = {
      table: "customers"
      transform_updates: true
    };
  }

  rpc IgnoreOtherEvents(google.protobuf.Any) returns (api.Customer) {
    option (kalix.method).eventing.in = {
      event_sourced_entity: "customers"
      ignore: true
     };
  };

  rpc GetCustomers(ByNameRequest) returns (stream api.Customer) {
    option (kalix.method).view.query = {
      query: "SELECT * FROM customers WHERE name = :customer_name"
    };
  }
}
1 The kalix.codegen option configures code generation to provide base classes and an initial implementation for the class transforming events to updates of the state.
2 Define an update method for each event.
3 The source of the View is from the journal of the "customers" Event Sourced Entity. This identifier is defined in the entity_type: "customers"` property of the (kalix.codegen).event_sourced_entity option in the customer_api.proto file.
4 Enable transform_updates to build the View state from the events.
5 The same event_sourced_entity for all update methods. Note the required table attribute. Use any name, which you will reference in the query SELECT statement.
6 Enable transform_updates for all update methods.
Scala
src/main/proto/customer/customer_view.proto
syntax = "proto3";

package customer.view;

import "customer/domain/customer_domain.proto";
import "customer/api/customer_api.proto";
import "kalix/annotations.proto";
import "google/protobuf/any.proto";

message ByNameRequest {
  string customer_name = 1;
}

service CustomerByName {
  option (kalix.codegen) = { (1)
    view: {}
  };

  rpc ProcessCustomerCreated(domain.CustomerCreated) returns (api.Customer) { (2)
    option (kalix.method).eventing.in = {
      event_sourced_entity: "customers" (3)
    };
    option (kalix.method).view.update = {
      table: "customers"
      transform_updates: true (4)
    };
  }

  rpc ProcessCustomerNameChanged(domain.CustomerNameChanged) returns (api.Customer) { (2)
    option (kalix.method).eventing.in = {
      event_sourced_entity: "customers" (5)
    };
    option (kalix.method).view.update = {
      table: "customers"
      transform_updates: true (6)
    };
  }

  rpc ProcessCustomerAddressChanged(domain.CustomerAddressChanged) returns (api.Customer) {
    option (kalix.method).eventing.in = {
      event_sourced_entity: "customers"
    };
    option (kalix.method).view.update = {
      table: "customers"
      transform_updates: true
    };
  }

  rpc IgnoreOtherEvents(google.protobuf.Any) returns (api.Customer) {
    option (kalix.method).eventing.in = {
      event_sourced_entity: "customers" (5)
      ignore: true
     };
  };

  rpc GetCustomers(ByNameRequest) returns (stream api.Customer) {
    option (kalix.method).view.query = {
      query: "SELECT * FROM customers WHERE name = :customer_name"
    };
  }
}
1 The kalix.codegen option configures code generation to provide base classes and an initial implementation for the class transforming events to updates of the state.
2 Define an update method for each event.
3 The source of the View is from the journal of the "customers" Event Sourced Entity. This identifier is defined in the entity_type: "customers"` property of the (kalix.codegen).event_sourced_entity option in the customer_api.proto file.
4 Enable transform_updates to build the View state from the events.
5 The same event_sourced_entity for all update methods. Note the required table attribute. Use any name, which you will reference in the query SELECT statement.
6 Enable transform_updates for all update methods.

See Query syntax reference for more examples of valid query syntax.

Create a transformation class

Next, you need to define how to transforms events to state that can be used in the View. An Event Sourced entity can emit many types of events. You need to define a method for each event type. If a View does not use all events, you need to ignore unneeded events as shown in the IgnoreOtherEvents update method

The code-generation will generate an implementation class with an initial empty implementation which we’ll discuss below.

View update handlers are implemented in the CustomerByNameView class as methods that override abstract methods from AbstractCustomerByNameView. The methods take the current view state as the first parameter and the event as the second parameter. They return an UpdateEffect, which describes next processing actions, such as updating the view state.

When adding or changing the rpc definitions, including name, parameter and return messages, in the .proto files the corresponding methods are regenerated in the abstract class (AbstractCustomerByNameView). This means that the compiler will assist you with such changes. The IDE can typically fill in missing method signatures and such.

Java
src/main/java/customer/view/CustomerByNameView.java
import kalix.javasdk.view.ViewContext;
import com.google.protobuf.Any;
import customer.domain.CustomerDomain;
import customer.api.CustomerApi;

public class CustomerByNameView extends AbstractCustomerByNameView { (1)

  public CustomerByNameView(ViewContext context) {}

  @Override
  public CustomerApi.Customer emptyState() { (2)
    return null;
  }

  @Override (3)
  public UpdateEffect<CustomerApi.Customer> processCustomerCreated(
    CustomerApi.Customer state,
    CustomerDomain.CustomerCreated customerCreated) {
    if (state != null) {
      return effects().ignore(); // already created
    } else {
      return effects().updateState(convertToApi(customerCreated.getCustomer()));
    }
  }

  @Override (3)
  public UpdateEffect<CustomerApi.Customer> processCustomerNameChanged(
    CustomerApi.Customer state,
    CustomerDomain.CustomerNameChanged customerNameChanged) {
    return effects().updateState(
        state.toBuilder().setName(customerNameChanged.getNewName()).build());
  }

  @Override (3)
  public UpdateEffect<CustomerApi.Customer> processCustomerAddressChanged(
    CustomerApi.Customer state,
    CustomerDomain.CustomerAddressChanged customerAddressChanged) {
    return effects().updateState(
        state.toBuilder().setAddress(convertToApi(customerAddressChanged.getNewAddress())).build());
  }

  private CustomerApi.Customer convertToApi(CustomerDomain.CustomerState s) {
    CustomerApi.Address address = CustomerApi.Address.getDefaultInstance();
    if (s.hasAddress()) {
      address = convertToApi(s.getAddress());
    }
    return CustomerApi.Customer.newBuilder()
        .setCustomerId(s.getCustomerId())
        .setEmail(s.getEmail())
        .setName(s.getName())
        .setAddress(address)
        .build();
  }

  private CustomerApi.Address convertToApi(CustomerDomain.Address a) {
    return CustomerApi.Address.newBuilder()
        .setStreet(a.getStreet())
        .setCity(a.getCity())
        .build();
  }
}
1 Extends the generated AbstractCustomerByNameView, which extends View new tab.
2 Defines the initial, empty, state that is used before any updates.
3 One method for each event.
Scala
src/main/scala/customer/view/CustomerByNameView.scala
import kalix.scalasdk.view.View.UpdateEffect
import kalix.scalasdk.view.ViewContext
import com.google.protobuf.any.{Any => ScalaPbAny}
import customer.api
import customer.api.Customer
import customer.domain
import customer.domain.CustomerAddressChanged
import customer.domain.CustomerCreated
import customer.domain.CustomerNameChanged
import customer.domain.CustomerState

class CustomerByNameView(context: ViewContext) extends AbstractCustomerByNameView { (1)

  override def emptyState: Customer = Customer.defaultInstance (2)

  override def processCustomerCreated(
      state: Customer,
      customerCreated: CustomerCreated): UpdateEffect[Customer] = (3)
    if (state != emptyState) effects.ignore() // already created
    else effects.updateState(convertToApi(customerCreated.customer.get))

  override def processCustomerNameChanged(
      state: Customer,
      customerNameChanged: CustomerNameChanged): UpdateEffect[Customer] = (3)
    effects.updateState(state.copy(name = customerNameChanged.newName))

  override def processCustomerAddressChanged(
      state: Customer,
      customerAddressChanged: CustomerAddressChanged): UpdateEffect[Customer] = (3)
    effects.updateState(state.copy(address = customerAddressChanged.newAddress.map(convertToApi)))

  private def convertToApi(customer: CustomerState): Customer =
    Customer(
      customerId = customer.customerId,
      name = customer.name,
      email = customer.email,
      address = customer.address.map(convertToApi))

  private def convertToApi(address: domain.Address): api.Address =
    api.Address(street = address.street, city = address.city)
}
1 Extends the generated AbstractCustomerByNameView, which extends View new tab.
2 Defines the initial, empty, state that is used before any updates.
3 One method for each event.
This type of update transformation is a natural fit for Events emitted by an Event Sourced Entity, but it can also be used for Value Entities. For example, if the View representation is different from the Entity state you might want to transform it before presenting the View to the client.

Ignoring events

When consuming events, each event must be matched by a Protobuf service method. In case your View is only interested in certain events, you may declare a method to receive all events that are not received by the other methods. If an event type is not handled, the View will fail. Views are designed to restart, but since the handler is missing, it will fail again. Therefore, it’s important to define methods for all events or define a catch-all method in case you want to discard some events.

Java
src/main/proto/customer/customer_view.proto
rpc IgnoreOtherEvents(google.protobuf.Any) returns (api.Customer) {
  option (kalix.method).eventing.in = {
    event_sourced_entity: "customers"
    ignore: true
   };
};
Scala
src/main/proto/customer/customer_view.proto
rpc IgnoreOtherEvents(google.protobuf.Any) returns (api.Customer) {
  option (kalix.method).eventing.in = {
    event_sourced_entity: "customers"
    ignore: true
  };
};

The IgnoreOtherEvents method here is defined as a catch-all because it has input type Any. Instead of using a catch-all it can be better to define concrete methods for all known event types that should be ignored because then there is no risk of accidentally ignoring events that are added in later evolution of the service.

When adding the ignore: true annotation the corresponding implementation is not needed in the component. It is more efficient to use ignore: true than implementing the method with an immediate reply.

Register the View

Register the View class with Kalix:

Java
src/main/java/customer/Main.java
public static Kalix createKalix() {
  return KalixFactory.withComponents(
    CustomerEntity::new,
    CustomerByNameView::new
  );
}
Scala
src/main/scala/customer/Main.scala
def createKalix(): Kalix = {
  KalixFactory.withComponents(
    new CustomerEntity(_),
    new CustomerByNameView(_)
  )
}

Creating a View from a topic

The source of a View can be an eventing topic. You define it in the same way as shown in Creating a View from an Event Sourced Entity or Creating a View from a Value Entity, but leave out the eventing.in annotation in the Protobuf file.

Java
src/main/proto/customer/view/customer_view.proto
syntax = "proto3";

package customer.view;

option java_outer_classname = "CustomerViewModel";

import "customer/domain/customer_domain.proto";
import "customer/api/customer_api.proto";
import "kalix/annotations.proto";
import "google/protobuf/any.proto";

service CustomerByNameFromTopic {
  rpc ProcessCustomerCreated(domain.CustomerCreated) returns (api.Customer) {
    option (kalix.method).eventing.in = {
      topic: "customers" (1)
    };
    option (kalix.method).view.update = {
      table: "customers"
      transform_updates: true
    };
  }

  rpc ProcessCustomerNameChanged(domain.CustomerNameChanged) returns (api.Customer) {
    option (kalix.method).eventing.in = {
      topic: "customers"
    };
    option (kalix.method).view.update = {
      table: "customers"
      transform_updates: true
    };
  }

  rpc ProcessCustomerAddressChanged(domain.CustomerAddressChanged) returns (api.Customer) {
    option (kalix.method).eventing.in = {
      topic: "customers"
    };
    option (kalix.method).view.update = {
      table: "customers"
      transform_updates: true
    };
  }

  rpc IgnoreOtherEvents(google.protobuf.Any) returns (api.Customer) {
    option (kalix.method).eventing.in = {
      topic: "customers"
      ignore: true
     };
  };

  rpc GetCustomers(ByNameRequest) returns (stream api.Customer) {
    option (kalix.method).view.query = {
      query: "SELECT * FROM customers WHERE name = :customer_name"
    };
  }
}
1 This is the only difference from Creating a View from an Event Sourced Entity.
Scala
src/main/proto/customer/view/customer_view.proto
syntax = "proto3";

package customer.view;

import "customer/domain/customer_domain.proto";
import "customer/api/customer_api.proto";
import "kalix/annotations.proto";
import "google/protobuf/any.proto";

service CustomerByNameFromTopic {
  rpc ProcessCustomerCreated(domain.CustomerCreated) returns (api.Customer) {
    option (kalix.method).eventing.in = {
      topic: "customers" (1)
    };
    option (kalix.method).view.update = {
      table: "customers"
      transform_updates: true
    };
  }

  rpc ProcessCustomerNameChanged(domain.CustomerNameChanged) returns (api.Customer) {
    option (kalix.method).eventing.in = {
      topic: "customers"
    };
    option (kalix.method).view.update = {
      table: "customers"
      transform_updates: true
    };
  }

  rpc ProcessCustomerAddressChanged(domain.CustomerAddressChanged) returns (api.Customer) {
    option (kalix.method).eventing.in = {
      topic: "customers"
    };
    option (kalix.method).view.update = {
      table: "customers"
      transform_updates: true
    };
  }

  rpc IgnoreOtherEvents(google.protobuf.Any) returns (api.Customer) {
    option (kalix.method).eventing.in = {
      topic: "customers"
      ignore: true
     };
  };

  rpc GetCustomers(ByNameRequest) returns (stream api.Customer) {
    option (kalix.method).view.query = {
      query: "SELECT * FROM customers WHERE name = :customer_name"
    };
  }
}
1 This is the only difference from Creating a View from an Event Sourced Entity.

How to transform results

When creating a View, you can transform the results as a relational projection instead of using a SELECT * statement.

Relational projection

Instead of using SELECT * you can define what columns that will be used in the response message:

Java
message CustomerSummary {
  string id = 1;
  string name = 2;
}

service CustomerSummaryByName {
  option (kalix.codegen) = {
    view: {}
  };

  rpc GetCustomers(ByNameRequest) returns (stream CustomerSummary) {
    option (kalix.method).view.query = {
      query: "SELECT customer_id AS id, name FROM customers WHERE name = :customer_name"
    };
  }

  rpc UpdateCustomer(domain.CustomerState) returns (domain.CustomerState) {
    option (kalix.method).eventing.in = {
      value_entity: "customers"
    };
    option (kalix.method).view.update = {
      table: "customers"
    };
  }
}
Scala
message CustomerSummary {
  string id = 1;
  string name = 2;
}

service CustomerSummaryByName {
  option (kalix.codegen) = {
    view: {}
  };

  rpc GetCustomers(ByNameRequest) returns (stream CustomerSummary) {
    option (kalix.method).view.query = {
      query: "SELECT customer_id AS id, name FROM customers WHERE name = :customer_name"
    };
  }

  rpc UpdateCustomer(domain.CustomerState) returns (domain.CustomerState) {
    option (kalix.method).eventing.in = {
      value_entity: "customers"
    };
    option (kalix.method).view.update = {
      table: "customers"
    };
  }
}

In a similar way, you can include values from the request message in the response, for example :request_id:

SELECT :request_id, customer_id as id, name FROM customers WHERE name = :customer_name

Response message including the result

Instead of streamed results you can include the results in a repeated field in the response message:

Java
message CustomersResponse {
  repeated domain.CustomerState results = 1; (1)
}

service CustomersResponseByName {
  option (kalix.codegen) = {
    view: {}
  };

  rpc GetCustomers(ByNameRequest) returns (CustomersResponse) { (2)
    option (kalix.method).view.query = {
      query: "SELECT * AS results FROM customers WHERE name = :customer_name" (3)
    };
  }

  rpc UpdateCustomer(domain.CustomerState) returns (domain.CustomerState) {
    option (kalix.method).eventing.in = {
      value_entity: "customers"
    };
    option (kalix.method).view.update = {
      table: "customers"
    };
  }
}
Scala
message CustomersResponse {
  repeated domain.CustomerState results = 1; (1)
}

service CustomersResponseByName {
  option (kalix.codegen) = {
    view: {}
  };

  rpc GetCustomers(ByNameRequest) returns (CustomersResponse) { (2)
    option (kalix.method).view.query = {
      query: "SELECT * AS results FROM customers WHERE name = :customer_name" (3)
    };
  }

  rpc UpdateCustomer(domain.CustomerState) returns (domain.CustomerState) {
    option (kalix.method).eventing.in = {
      value_entity: "customers"
    };
    option (kalix.method).view.update = {
      table: "customers"
    };
  }
}
1 The response message contains a repeated field.
2 The return type is not streamed.
3 The repeated field is referenced in the query with * AS results.

How to modify a View

Kalix creates indexes for the View based on the query. For example, the following query will result in a View with an index on the name column:

SELECT * FROM customers WHERE name = :customer_name

If the query is changed, Kalix might need to add other indexes. For example, changing the above query to filter on the city would mean that Kalix needs to build a View with the index on the city column.

SELECT * FROM customers WHERE address.city = :city

Such changes require you to define a new View. Kalix will then rebuild it from the source event log or value changes.

Views from topics cannot be rebuilt from the source messages, because it’s not possible to consume all events from the topic again. The new View will be built from new messages published to the topic.

Rebuilding a new View may take some time if there are many events that have to be processed. The recommended way when changing a View is multi-step, with two deployments:

  1. Define the new View, and keep the old View intact. A new View is defined by a new service in Protobuf. The viewId is the same as the service name, i.e. it will be a different viewId than the old View. Keep the old register of the old service in Main.

  2. Deploy the new View, and let it rebuild. Verify that the new query works as expected. The old View can still be used.

  3. Remove the old View definition and rename the new service to the old name if the public API is compatible, but keep the new viewId by defining it as shown below.

  4. Deploy the second change.

This is how to define a custom viewId:

Java
src/main/java/customer/Main.java
public static Kalix createKalix() {
  Kalix kalix = new Kalix();
  kalix.register(CustomerByNameViewProvider.of(CustomerByNameView::new)
      .withViewId("CustomerByNameV2"));
  kalix.register(CustomerEntityProvider.of(CustomerEntity::new));
  return kalix;
}
Scala
src/main/scala/customer/Main.scala
def createKalix(): Kalix =
  Kalix()
    .register(
      CustomerByNameViewProvider(new CustomerByNameView(_))
        .withViewId("CustomerByNameV2"))
    .register(CustomerEntityProvider(new CustomerEntity(_)))

The View definitions are stored and validated when a new version is deployed. There will be an error message if the changes are not compatible.

Streaming view updates

A query can provide a near real time stream of results for the query, emitting new entries matching the query as they are added or updated in the view.

This is done by adding the option stream_updates to a query method with a stream reply:

Java
message ByCityRequest {
  string city = 1;
}
service CustomerByCityStreaming {
  option (kalix.codegen) = {
    view: {}
  };

  // update methods omitted
  rpc GetCustomers(ByCityRequest) returns (stream api.Customer) { (1)
    option (kalix.method).view.query = {
      query: "SELECT * FROM customers WHERE address.city = :city", (2)
      stream_updates: true (3)
    };
  }
}
1 Return type must be a stream
2 A regular query, in this case showing all customers from a specific city
3 The stream_updates option set to true
Scala
message ByCityRequest {
  string city = 1;
}
service CustomerByCityStreaming {
  option (kalix.codegen) = {
    view: {}
  };

  // update methods omitted
  rpc GetCustomers(ByCityRequest) returns (stream api.Customer) { (1)
    option (kalix.method).view.query = {
      query: "SELECT * FROM customers WHERE address.city = :city", (2)
      stream_updates: true (3)
    };
  }
}
1 Return type must be a stream
2 A regular query, in this case showing all customers from a specific city
3 The stream_updates option set to true

This will first list the complete result for the query and then keep the response stream open, emitting new or updated entries matching the query as they are added to the view. The stream does not complete until the client closes it.

Note: This is not intended as transport for service to service propagation of updates and does not guarantee delivery, for such use cases you should instead publish events to a topic, see Publishing and Subscribing with Actions

Query syntax reference

Define View queries in a language that is similar to SQL. The following examples illustrate the syntax. To retrieve:

  • All customers without any filtering conditions (no WHERE clause):

    SELECT * FROM customers

  • Customers with a name matching the customer_name property of the request message:

    SELECT * FROM customers WHERE name = :customer_name

  • Customers matching the customer_name AND city properties of the request message:

    SELECT * FROM customers WHERE name = :customer_name AND address.city = :city

  • Customers in a city matching a literal value:

    SELECT * FROM customers WHERE address.city = 'New York'

Filter predicates

Use filter predicates in WHERE conditions to further refine results.

Comparison operators

The following comparison operators are supported:

  • = equals

  • != not equals

  • > greater than

  • >= greater than or equals

  • < less than

  • <= less than or equals

Logical operators

Combine filter conditions with the AND operator, and negate using the NOT operator. Group conditions using parentheses.

OR support is currently disabled, until it can be more efficiently indexed. 
SELECT * FROM customers WHERE
  name = :customer_name AND NOT (address.city = 'New York' AND age > 65)

Array operators

Use IN or = ANY to check whether a value is contained in a group of values or in an array column or parameter (a repeated field in the Protobuf message).

Use IN with a list of values or parameters:

SELECT * FROM customers WHERE email IN ('bob@example.com', :some_email)

Use = ANY to check against an array column (a repeated field in the Protobuf message):

SELECT * FROM customers WHERE :some_email = ANY(emails)

Or use = ANY with a repeated field in the request parameters:

SELECT * FROM customers WHERE email = ANY(:some_emails)

Pattern matching

Use LIKE to pattern match on strings. The standard SQL LIKE patterns are supported, with _ (underscore) matching a single character, and % (percent sign) matching any sequence of zero or more characters.

SELECT * FROM customers WHERE name LIKE 'Bob%'
For index efficiency, the pattern must have a non-wildcard prefix or suffix. A pattern like '%foo%' is not supported. Given this limitation, only constant patterns with literal strings are supported; patterns in request parameters are not allowed.

Text search

Use the text_search function to search text values for words, with automatic tokenization and normalization based on language-specific configuration. The text_search function takes the text column to search, the query (as a parameter or literal string), and an optional language configuration.

text_search(<column>, <query parameter or string>, [<configuration>])

If the query contains multiple words, the text search will find values that contain all of these words (logically combined with AND), with tokenization and normalization automatically applied.

The following text search language configurations are supported: 'danish', 'dutch', 'english', 'finnish', 'french', 'german', 'hungarian', 'italian', 'norwegian', 'portuguese', 'romanian', 'russian', 'simple', 'spanish', 'swedish', 'turkish'. By default, a 'simple' configuration will be used, without language-specific features.

SELECT * FROM customers WHERE text_search(profile, :search_words, 'english')
Text search is currently only available for deployed services, and can’t be used in local testing.

Data types

The following data types are supported, for their corresponding Protobuf types. Arrays are created for a repeated field in a Protobuf message. Timestamps can be stored and compared using the google.protobuf.Timestamp message type.

Data type Protobuf type

Text

string

Integer

int32

Long (Big Integer)

int64

Float (Real)

float

Double

double

Boolean

bool

Byte String

bytes

Array

repeated fields

Timestamp

google.protobuf.Timestamp

Optional fields

Fields in a Protobuf message that were not given a value are handled as [the default value](https://developers.google.com/protocol-buffers/docs/proto3#default) of the field data type.

In some use cases it is important to explicitly express that a value is missing, doing that in a view column can be done in three ways:

  • mark the message field as optional

  • use one of the Protobuf "wrapper" types for the field (messages in the package google.protobuf ending with Value)

  • make the field a part of a nested message and omit that whole nested message, for example address.street where the lack of an address message implies there is no street field.

Optional fields with values present can be queried just like regular view fields:

SELECT * FROM customers WHERE phone_number = :number

Finding results with missing values can be done using IS NULL:

SELECT * FROM customers WHERE phone_number IS NULL

Finding entries with any value present can be queried using IS NOT NULL:

SELECT * FROM customers WHERE phone_number IS NOT NULL

Optional fields in query requests messages are handled like normal fields if they have a value, however missing optional request parameters are seen as an invalid request and lead to a bad request response.

Sorting

Results for a view query can be sorted. Use ORDER BY with view columns to sort results in ascending (ASC, by default) or descending (DESC) order.

If no explicit ordering is specified in a view query, results will be returned in the natural index order, which is based on the filter predicates in the query.

SELECT * FROM customers WHERE name = :name AND age > :min_age ORDER BY age DESC
Some orderings may be rejected, if the view index cannot be efficiently ordered. Generally, to order by a column it should also appear in the WHERE conditions.

Paging

Splitting a query result into one "page" at a time rather than returning the entire result at once is possible in two ways:

  • with a count based offset or

  • a token based offset.

In both cases OFFSET and LIMIT are used.

OFFSET specifies at which offset in the result to start

LIMIT specifies a maximum number of results to return

Count based offset

The values can either be static, defined up front in the query:

SELECT * FROM customers LIMIT 10

Or come from fields in the request message:

SELECT * FROM customers OFFSET :start_from LIMIT :max_customers

Note: Using numeric offsets can lead to missing or duplicated entries in the result if entries are added to or removed from the view between requests for the pages.

Token based offset

The count based offset requires that you keep track of how far you got by adding the page size to the offset for each query.

An alternative to this is to use a string token emitted by Kalix identifying how far into the result set the paging has reached using the functions next_page_token() and page_token_offset().

When reading the first page, an empty token is provided to page_token_offset. For each returned result page a new token that can be used to read the next page is returned by next_page_token(), once the last page has been read, an empty token is returned (see also has_more for determining if the last page was reached).

The size of each page can optionally be specified using LIMIT, if it is not present a default page size of 100 is used.

With a request and response message for the view like this:

message Request {
    string page_token = 1;
}

message Response {
    repeated Customer customers = 1;
    string next_page_token = 2;
}

A query such as the one below will allow for reading through the view in pages, each containing 10 customers:

SELECT * AS customers, next_page_token() AS next_page_token
FROM customers
OFFSET page_token_offset(:page_token)
LIMIT 10

The token value is not meant to be parseable into any meaningful information other than being a token for reading the next page.

Total count of results

To get the total number of results that will be returned over all pages, use COUNT(*) in a query that projects its results into a field. The total count will be returned in the aliased field (using AS) or otherwise into a field named count.

SELECT * AS customers, COUNT(*) AS total, has_more() AS more FROM customers LIMIT 10

Check if there are more pages

To check if there are more pages left, you can use the function has_more() providing a boolean value for the result. This works both for the count and token based offset paging, and also when only using LIMIT without any OFFSET:

SELECT * AS customers, has_more() AS more_customers FROM customers LIMIT 10

This query will return more_customers = true when the view contains more than 10 customers.