Running the Projection


This example requires a Cassandra database to run. If you do not have a Cassandra database then you can run one locally as a Docker container. To run a Cassandra database locally you can use docker-compose to run the docker-compose.yaml found in the Projections project root. The docker-compose.yml file references the latest Cassandra Docker Image.

Change directory to the directory of the docker-compose.yml file and manage a Cassandra container with the following commands.

Action Docker Command
Run docker-compose --project-name getting-started up -d cassandra
Stop docker-compose --project-name getting-started stop
Delete container state docker-compose --project-name getting-started rm -f
CQL shell (when running) docker run -it --network getting-started_default --rm cassandra cqlsh cassandra

To use a different Cassandra database update the Cassandra driver’s contact-points configuration found in ./examples/src/resources/guide-shopping-cart-app.conf.

To run the Projection we must setup our Cassandra database to support the Cassandra Projection offset store as well as the new table we are projecting into with the ItemPopularityProjectionHandler.

Create a Cassandra keyspace.

CREATE KEYSPACE IF NOT EXISTS akka_projection WITH REPLICATION = { 'class' : 'SimpleStrategy','replication_factor':1 };

Create the Cassandra Projection offset store table. The DDL can be found in the Cassandra Projection, Schema section.

Create the ItemPopularityProjectionHandler projection table with the DDL below

CREATE TABLE IF NOT EXISTS akka_projection.item_popularity (
item_id text,
count counter,
PRIMARY KEY (item_id));

Source events are generated with the EventGeneratorApp. This app is configured to use Akka Persistence Cassandra and Akka Cluster Sharding to persist random ShoppingCartApp.Events to a journal. It will checkout a shopping cart with random items and quantities every 1 second. The app will automatically create all the Akka Persistence infrastructure tables in the akka keyspace. We won’t go into any further detail about how this app functions because it falls outside the scope of Akka Projections. To learn more about the writing events with Akka Persistence see the Akka documentation.

Add the Akka Cluster Sharding library to your project:

libraryDependencies += "com.typesafe.akka" %% "akka-cluster-sharding-typed" % "2.6.18"
def versions = [
  ScalaBinary: "2.13"
dependencies {
  implementation "com.typesafe.akka:akka-cluster-sharding-typed_${versions.ScalaBinary}:2.6.18"

Add the EventGeneratorApp to your project.

Run EventGeneratorApp:

sbt "runMain"
mvn compile exec:java -Dexec.mainClass=""

If you don’t see any connection exceptions then you should eventually see log lines produced indicating that events are written to the journal.


[2020-08-13 15:20:05,583] [INFO] [$] [] [] - id [cb52b] tag [shopping-cart] event: ItemQuantityAdjusted(cb52b,skis,1,1) MDC: {persistencePhase=persist-evt, akkaAddress=akka://[email protected]:25520, akkaSource=akka://EventGenerator/system/sharding/shopping-cart-event/678/cb52b, sourceActorSystem=EventGenerator, persistenceId=cb52b}

Finally, we can run ShoppingCartApp in a new terminal:

sbt "runMain"
mvn compile exec:java -Dexec.mainClass=""

After a few seconds you should see the ItemPopularityProjectionHandler logging that displays the current checkouts for the day:

[2020-08-12 12:16:34,216] [INFO] [] [] [] - ItemPopularityProjectionHandler(shopping-cart) item popularity for 'bowling shoes': [58]

Use the CQL shell to observe the full information in the item_popularity table.

cqlsh> SELECT item_id, count FROM akka_projection.item_popularity;

 item_id       | count
  akka t-shirt |    37
   cat t-shirt |    34
          skis |    33
 bowling shoes |    65

(4 rows)
Found an error in this documentation? The source code for this page can be found here. Please feel free to edit and contribute a pull request.