Redis Transactional

A Redis transactional projection is a transactional projection based on Redisson RTransaction.

Compared to a Spring transactional projection, a Redis transactional projection is more lightweight since

• transactionality is directly provided by RTransaction. There is no need to deal with Spring’s PlatformTransactionManager
• the fact stream position is automatically managed (see example below)

Motivation

You would want to use Redis Transactional for two reasons:

• atomicity of factStreamPosition updates and your projection state updates
• increased fact processing throughput

The performance bit is achieved by skipping unnecessary factStreamPosition updates and (more importantly) by reducing the number of operations on your Redis backend by using bulkSize updates with one redisson transsaction instead of single writes. The bulkSize is configurable per projection via the @RedisTransactional annotation.

Working with a Redis transactional projection you can read your own uncommitted write. For this reason, a Redis transactional projection is best used for projections which need to access the projection’s data during the handling of an event.

If this is not necessary, you could also use a better performing alternative: Redis batch projection

Configuration

In order to make use of redisson RTransaction support, the necessary dependency has to be included in your project:

    <dependency>
        <groupId>org.factcast</groupId>
        <artifactId>factcast-factus-redis</artifactId>
    </dependency>

Structure

A Redis transactional projection can be a managed- or a subscribed projection and is defined as follows:

• it is annotated with @RedisTransactional
• it implements RedisProjection revealing the RedisClient used
• it provides the serial number of the projection via the @ProjectionMetaData annotation
• the handler methods receive an additional RTransaction parameter

Example

@Handler
void apply(SomethingHappened fact, RTransaction tx) {
    myMap = tx.getMap( ... ).put( fact.getKey() , fact.getValue() );
}

a full example can be found here