What is this Orchestration :
Orchestration is the traditional way of handling interactions between different services in Service-Oriented Architecture (SOA). With orchestration process, there is typically one controller that acts as the “orchestrator” of the overall service interactions. This typically follows a request/response type pattern.
For example, if three services needed to be called in a particular order, the orchestrator makes a call to each one, waiting for a response before calling the next.
As you can identify the differences of two flow as seen in the picture.
PROS:
Provides a good way for controlling the flow of the application when there is synchronous processing. For example, if one Service needs to complete successfully before another Service is invoked.
CONS:
- If there is a central shared instance of the orchestrator for all requests, then the orchestrator is a single point of failure. If it goes down, all processing stops.
- Leverages synchronous processing that blocks requests.
Reactive architecture pattern is an event-driven architecture pattern applied to microservices. Instead of having a central orchestrator that controls the logic of what steps happen when, that logic is built into each service ahead of time.
Services use an event stream for asynchronous communication of events. Multiple services can consume the same events, do some processing, and then produce their own events back into the event stream, all at the same time. The event stream does not have any logic and is intended to be a dumb pipe.
The asynchronous nature of a reactive architecture removes the blocking, or waiting, that happens with orchestration (request/response) type processing. Services can produce events and keep processing.
Pros:
- Enables faster end-to-end processing as services can be executed in parallel/asynchronously.
- Easier to add/update services as they can be plugged in/out of the event stream easily.
- Aligns well with an agile delivery model as teams can focus on particular services instead of the entire application.
- Control is distributed, so there is no longer a single orchestrator serving as a central point of failure.
- Several patterns can be used with a reactive architecture to provide additional benefits. For example, Event Sourcing is when the Event Stream stores all of the events and enables event replay. This way, if a service went down while events were still being produced, when it came back online it could replay those events to catch back up.
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