What is the difference between Gateway and Service Activator? - spring-integration

Question

What is the difference between Gateway and Service Activator as Message Endpoints (in terms of Enterprise Integration Patterns)?

Answer 1

http://eaipatterns.com/
Typically, a service activator is used to invoke a local service, in such a manner that the service doesn't know it's being invoked from a messaging system.
A gateway s typically an entry or exit point for the messaging system.

Answer 2

The service activator calls a method on an object where the application developer provides the implementation. Spring Integration takes care of calling the method with messages from the input channel and shunting the results off to some output channel. The application-provided code can do some arbitrary work.
For the gateway the application developer provides only an interface, its implementation is provided by Spring.
An appendix to the Spring Integration documentation includes a Cafe example where Barista is a service called through a service activator, and Cafe is a gateway.
The application's main method looks up a Cafe object from the Spring application context and calls placeOrder, on it, passing an Order in as an argument:
public static void main(String[] args) {
AbstractApplicationContext context = null;
if (args.length > 0) {
context = new FileSystemXmlApplicationContext(args);
}
else {
context = new ClassPathXmlApplicationContext(
"cafeDemo.xml", CafeDemo.class);
}
Cafe cafe = (Cafe) context.getBean("cafe");
for (int i = 1; i <= 100; i++) {
Order order = new Order(i);
order.addItem(DrinkType.LATTE, 2, false);
order.addItem(DrinkType.MOCHA, 3, true);
cafe.placeOrder(order);
}
}
The Cafe is an interface that the application does not provide an implementation for. Spring generates an implementation that sends the Orders passed into it down the input channel called "orders".
Further down the pipeline, there are two service-activators that have a reference to the Barista. The Barista is a POJO that has code for creating a Drink like this:
public Drink prepareHotDrink(OrderItem orderItem) {
try {
Thread.sleep(this.hotDrinkDelay);
System.out.println(Thread.currentThread().getName()
+ " prepared hot drink #" + hotDrinkCounter.incrementAndGet()
+ " for order #" + orderItem.getOrder().getNumber()
+ ": " + orderItem);
return new Drink(orderItem.getOrder().getNumber(),
orderItem.getDrinkType(),
orderItem.isIced(), orderItem.getShots());
}
catch (InterruptedException e) {
Thread.currentThread().interrupt();
return null;
}
}
The Barista receives drink orders from the service-activator's input channel and has a method called on it that returns a Drink, which gets sent down the service-activator's output channel, "preparedDrinks".

Answer 3

For me the gateway is used for making an abstraction and provide a normalised API for one or more back-end services.
E.g You have 5 providers which are using different ways to interface with you (SOAP, REST, XML/http, whatever), but your client want only one way to get the data (let says json/REST).
The gateway will convert the json request form your client and convert them to the right backend with its own protocol, after it will convert the backend response to json to give the response to your client.
The service activator acts more as a trigger on an incoming message. Let say your activator poll a database for incoming message and then when the condition meet the "activation" it calls the underlying service.
Info for gateway here.
Info for Activator here.