distributed systems - Event Carried State Transfer

Event Carried State Transfer

Event Carried State Transfer is a design pattern in which a service publishes events containing enough information about a change so that other services that receive the event can update their own state, without needing to request additional data.

In distributed systems, services often need to share data. One common way to do this is to make a remote call to the service that owns the data. This, however, can lead to tight coupling between services and can impact performance, as the services must wait for the remote call to complete.

The Event Carried State Transfer pattern offers an alternative approach. Instead of making a call to fetch data when it’s needed, services listen for events that indicate the data has changed. These events contain all the relevant data related to the change. When a service receives an event, it uses the data in the event to update its own local copy of the data.

This approach has several benefits:

1. Decoupling: Services do not need to make synchronous calls to each other, which reduces coupling and increases the autonomy of each service.
2. Performance: Because services are not making remote calls to fetch data, the system can often perform better and be more responsive.
3. Resilience: If a service goes down, it won’t affect other services as they have their own local copy of the data they need.

However, there are also some considerations and potential drawbacks:

1. Data Duplication: This pattern involves replicating data across services, which can lead to increased storage requirements and the potential for inconsistency if not managed carefully.
2. Event Content: Deciding what data to include in events can be challenging. Include too little, and services may still need to make calls to fetch data. Include too much, and you could be sharing data unnecessarily, potentially violating principles of data privacy and security.
3. Data Synchronization: Managing updates to ensure all services have a consistent view of the data can be complex, particularly in systems with high levels of concurrency.

In summary, Event Carried State Transfer can be a powerful pattern for managing data sharing in distributed, event-driven systems. As with all patterns, it’s important to consider the specific needs and constraints of your system to determine if it’s the right approach.