Introduction
AWS Step Functions and Amazon EventBridge are both used to connect the parts of an event-driven system, but they perform different roles. Step Functions provides orchestration, which is the coordination of ordered steps by a central workflow. EventBridge provides event routing, which is the delivery of events to interested targets. This guide compares the two services and explains when each should be used. The broader idea of connecting components through events is covered in the guide on event-driven architecture.
What is being compared?
Step Functions runs a workflow whose sequence is defined in advance as a state machine. It controls the order of the steps, makes decisions, handles errors, and carries data from one step to the next. EventBridge is an event bus, which is a channel that receives events and matches them to rules. A rule describes which events should be delivered and to which targets. EventBridge does not control any overall sequence; each target reacts on its own.
Why the choice matters
The two services correspond to two ways of connecting components. In orchestration, a central controller directs the steps. In choreography, each component reacts to events independently and no central controller exists. Choosing the wrong approach can make a process either harder to track or unnecessarily rigid, so the distinction is important.
How each service works
In Step Functions, an execution begins with an input and moves through the states of the workflow until it ends. The progress of the execution is tracked, retries and errors are handled, and data is passed between states. The order and the logic are fixed by the definition.
In EventBridge, a producer sends an event to an event bus. The bus evaluates its rules and delivers a copy of the event to each matching target, such as a Lambda function, a queue, or a Step Functions workflow. The producer does not know which targets will receive the event, and the targets do not depend on one another.
Comparison diagram
Where Step Functions is stronger
- Ordered steps. A defined sequence with decisions and branches is enforced.
- State and progress. Data is carried between steps and the progress of each execution is tracked.
- Error handling. Retries and catches are built in.
- Auditability. Standard workflows record the full history of an execution.
Where EventBridge is stronger
- Broadcasting. A single event can be delivered to many independent consumers.
- Loose coupling. Producers and consumers do not depend on one another.
- Scheduling. Targets can be triggered on a fixed schedule.
- Extensibility. New consumers can be added without existing components being changed.
When to choose each
- Step Functions suits a multi-step process with a defined order, decisions, retries, and a need to track progress.
- EventBridge suits the distribution of an event to independent consumers, and the scheduling of tasks.
- The two are often combined, with EventBridge receiving an event and starting a Step Functions workflow.
Best practices
- Orchestration should be used when the order of steps and the tracking of progress matter, and routing when components should react independently.
- EventBridge should be used as the entry point that starts a workflow in response to an event.
- A schedule in EventBridge should be used to start recurring workflows rather than custom timing code.
Common mistakes
- EventBridge is used to build a multi-step process by chaining rules, which becomes difficult to track and debug.
- Step Functions is used to broadcast a single event to many independent consumers, which is unnecessarily rigid.
- The two services are treated as alternatives rather than as complementary tools.
Related AWS services
- AWS Lambda is a common target of EventBridge and a common task within Step Functions.
- Amazon SNS and Amazon SQS also distribute messages in event-driven systems.
- Amazon CloudWatch records the metrics of both services.
Frequently Asked Questions
- What is the core difference between Step Functions and EventBridge?
- Step Functions is an orchestrator that runs an ordered workflow whose sequence is defined in advance. EventBridge is an event router that delivers events to targets according to rules, without controlling any overall sequence.
- Is one a replacement for the other?
- No. They solve different problems and are frequently used together. EventBridge often receives an event and starts a Step Functions workflow, which then coordinates the ordered steps.
- Which one maintains state?
- Step Functions maintains the state of an execution and carries data between steps. EventBridge does not maintain workflow state, because it only routes individual events.
- Which should be chosen for a multi-step process?
- A process with a defined order, decisions, retries, and a need to track progress is best handled by Step Functions. EventBridge is better suited to broadcasting an event to independent consumers.
- Can EventBridge schedule a workflow?
- Yes. EventBridge can trigger a target on a schedule, which allows a Step Functions workflow to be started at fixed times without additional code.
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