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Events and Commands are often used side-by-side in event-driven architectures.

Knowing their differences is important so you can choose the right approach to handling them and choose the right technology stack.

Event vs. Command

An event is a notification that something has already happened. It's a fact, and it’s immutable. e.g. a customer placed an order. Events are not directed at anyone, and we don't care how many (if any) subscribers are listening.

A command is a request to perform a specific action. It’s not just information; it’s an instruction, such as a command to process a payment. Commands are directed at an intended recipient who is responsible for fulfilling the requested action.

We often want to know the result of the action. As such, a command is often followed by an event to report the outcome of the action. e.g. a "process_payment" command is followed by a "payment_succeeded" event upon completion.

Pros & Cons

Events promote loose coupling but make systems less predictable and complex, especially when it comes to handling failures.

Commands simplify your control flow as you know where each command is going, making them easier to debug too. But you increase the coupling between components.

What to use as the bus

Regarding technology choices, EventBridge and SNS are good choices for the event bus, which must broadcast events to multiple subscribers.

SQS is not suitable here because it doesn't do broadcasts.

But SQS is a great fit for the command bus precisely because it’s a targeted delivery where you would have only one subscriber per queue.

Also, messages stay on the queue until they are either expired or processed, which is an important requirement for commands.

Event Sourcing & CQRS

Finally, patterns such as event sourcing & CQRS combine both events and commands to great effect.
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Event Sourcing ensures that every change is captured as an event, as you model your system’s state as a series of events rather than snapshots.
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CQRS separates read and write operations to optimize performance, where the write operations are modelled as commands.
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More on event sourcing and CQRS another time!