UserNames (Redis Transactional)

Here is a projection that handles UserCreated and UserDeleted events. It solves the same problem as the example we’ve seen in Spring transactional projections. However, this time we use Redis as our data store and Redisson as the access API.

Configuration

The @RedisTransactional annotation provides various configuration options:

Constructing

Since we decided to use a managed projection, we extended the AbstractRedisManagedProjection class. To configure the connection to Redis via Redisson, we injected RedissonClient in the constructor, calling the parent constructor.

@ProjectionMetaData(serial = 1)
@RedisTransactional
public class UserNames extends AbstractRedisManagedProjection {

  public UserNames(RedissonClient redisson) {
    super(redisson);
  }
    ...

FactStreamPosition and Lock-Management are automatically taken care of by the underlying AbstractRedisManagedProjection.

In contrast to non-atomic projections, when applying Facts to the Redis data structure, the instance variable userNames cannot be used as this would violate the transactional semantics. Instead, accessing and updating the state is carried out on a transaction derived data-structure (Map here) inside the handler methods.

Updating the projection

Applying Events

Received events are processed inside the methods annotated with @Handler (the handler methods). To participate in the transaction, these methods have an additional RTransaction parameter which represents the current transaction.

Let’s have a closer look at the handler for the UserCreated event:

@Handler
void apply(UserCreated e,RTransaction tx){
        Map<UUID, String> userNames=tx.getMap(getRedisKey());
        userNames.put(e.getAggregateId(),e.getUserName());
}

In the previous example, the method getRedisKeys() was used to retrieve the Redis key of the projection. Let’s have a closer look at this method in the next section.

Default redisKey

The data structures provided by Redisson all require a unique identifier which is used to store them in Redis. The method getRedisKey() provides an automatically generated name, assembled from the class name of the projection and the serial number configured with the @ProjectionMetaData.

Additionally, an AbstractRedisManagedProjection or a AbstractRedisSubscribedProjection maintain the following keys in Redis:

• getRedisKey() + "_state_tracking" - contains the UUID of the last position of the Fact stream
• getRedisKey() + "_lock" - shared lock that needs to be acquired to update the projection.

Redisson API Datastructures vs. Java Collections

As seen in the above example, some Redisson data structures also implement the appropriate Java Collections interface. For example, you can assign a Redisson RMap also to a standard Java Map:

// 1) use specific Redisson type
RMap<UUID, String> =tx.getMap(getRedisKey());

// 2) use Java Collections type
        Map<UUID, String> =tx.getMap(getRedisKey());

There are good reasons for either variant, 1) and 2):

Full Example

@ProjectionMetaData(serial = 1)
@RedisTransactional
public class UserNames extends AbstractRedisManagedProjection {

  private final Map<UUID, String> userNames;

  public UserNames(RedissonClient redisson) {
    super(redisson);

     userNames = redisson.getMap(getRedisKey());
  }

  public List<String> getUserNames() {
    return new ArrayList<>(userNames.values());
  }

  @Handler
  void apply(UserCreated e, RTransaction tx) {
    tx.getMap(getRedisKey()).put(e.getAggregateId(), e.getUserName());
  }

  @Handler
  void apply(UserDeleted e, RTransaction tx) {
    tx.getMap(getRedisKey()).remove(e.getAggregateId());
  }
}

To study the full example, see

• the UserNames projection using @RedisTransactional and
• example code using this projection