Exercise4.md

Exercise 4

First checkout the branch exercise_4_specs with Git: git checkout exercise_4_specs

In this exercise, we are going to handle chat room messages

The MessageEntity is already provided with appropriate annotations.

Please notice the usage of @CompoundPrimaryKey annotation. For type safety reason, Achilles requires a separate class to map a compound primary key from CQL into Java. Inside this class, you can use @PartitionKey and @ClusteringColumn annotations to specify which attribute is a partition key or clustering column.

For a chat application, we want to display the latest messages first so we order the clustering column messageId by reversed order. This is achieved through @ClusteringColumn(value = 1, reversed = true)

Please notice the usage of @TimeUUID annotation on the messageId property. Indeed in Java the type java.util.UUID is used to represents both version 1 UUID (based on MAC address and timestamp) and version 4 UUID for random number. CQL distinguish between both types (uuid and timeuuid). This annotation is a marker to map a version 1 Java UUID into CQL timeuuid type.

For more detail on clustered entities (optional reading) : https://github.com/doanduyhai/Achilles/wiki/Entity-Mapping#clustered-entity

This entity mapping is equivalent to the following CQL script:

CREATE TABLE killrchat.chat_room_messages(
    room_name text,
    message_id timeuuid,
    content text,
    author text,
    system_message boolean,
    PRIMARY KEY(room_name, message_id)
) WITH CLUSTERING ORDER BY (message_id DESC);

To complete the exercise, open the MessageService and implement the remaining methods to make the tests greens. Follow the instructions and read the documentation link ( if necessary, don't spend too much time on it )

Solution

The solution is given in the branch exercise_4_solution.

Do not forget to save your work (if you want to keep track of your solution) in a branch and then check out with Git: git checkout exercise_4_solution