Oleg Zhurakousky
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### Introduction
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Spring Cloud Function allows you to invoke function via [RSocket](https://rsocket.io/). While you can read more about RSocket and it’s java
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implementation [here](https://github.com/rsocket/rsocket-java), this section will describe the parts relevant to Spring Cloud Function integration.
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Spring Cloud Function allows you to invoke function via [gRPC](https://grpc.io/).
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TBD
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### Programming model
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From the user perspective bringing RSocket does not change the implementation of functions or any of its features, such as type conversion,
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composition, POJO functions etc.
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And while RSocket allows first class reactive interaction over the network supporting important reactive features such as back pressure,
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users of Spring Cloud Function still have freedom to implement their business logic using reactive or imperative functions delegating any
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adjustment needed to apply proper invocation model to the framework.
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To use RSocket integration all you need is to add `spring-cloud-function-rsocket` dependency to your classpath
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```
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<dependency>
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<groupId>org.springframework.cloud</groupId>
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<artifactId>spring-cloud-function-rsocket</artifactId>
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</dependency>
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```
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To interact with functions via RSocket we rely on Spring Boot support for RSocket and `RSocketRequester.Builder` API.
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The code below shows the key parts and you can get more details on various interaction models
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from [this test case](https://github.com/spring-cloud/spring-cloud-function/blob/master/spring-cloud-function-rsocket/src/test/java/org/springframework/cloud/function/rsocket/RSocketAutoConfigurationTests.java).
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```
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@Bean
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public Function<String, String> uppercase() {
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return v -> v.toUpperCase();
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}
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. . .
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RSocketRequester.Builder rsocketRequesterBuilder =
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applicationContext.getBean(RSocketRequester.Builder.class);
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rsocketRequesterBuilder.tcp("localhost", port)
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.route(“uppercase")
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.data("\"hello\"")
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.retrieveMono(String.class)
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.subscribe(System.out::println);
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```
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Once connected to RSocket we use `route` operation to specify which function we want to invoke providing the actual
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payload via `data` operation. Then we use one of the `retrieve` operations that best suits our desired interaction
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(RSocket supports multiple interaction models such as fire-and-forget, request-reply etc.)
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#### Order of priority for routing instructions
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As you can see from the preceding examples, we provide function definition as a value to `route(..)` operator of `RSocketRequester.Builder`.
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However that is not the only way. You can also use standard `spring.cloud.function.definition` property as well as `spring.cloud.function.routing-expression` or property or `MessageRoutingCallback` on the server side of the RSocket interaction (see "Function Routing and Filtering" section of reference manual).
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This raises a question of _order_ and _priorities_ when it comes to reconsiling a conflict in the event several ways of providing definition are used. So it is a mater of clearly stating the rule whcih is:
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***1 - MessageRoutingCallback***
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The `MessageRoutingCallback` takes precedence over all other ways of providing function definition resolution.
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***2 - spring.cloud.function.routing-expression***
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The `spring.cloud.function.routing-expression` property takes next precedence. So, in the event you may have also use `route(..)` operator or `spring.cloud.function.definition` property, they will be ignored if `spring.cloud.function.routing-expression` property is provided.
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***3 - route(..)***
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The next in line is `route(..)` operator. So in the event there are no `spring.cloud.function.routing-expression` property but you defined `spring.cloud.function.definition` property, it will be ignored in favor of definition provided by the `route(..)` operator.
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***4 - spring.cloud.function.definition***
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The `spring.cloud.function.definition` property is the last in the list allowing you to simply `route("")` to empty string.
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### Messaging
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If you want to provide and/or receive additional information that you would normally communicate via Message headers you can send and receive Spring `Message`.
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For example, the following tests case demonstrates how you can accomplish that.
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```
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Person p = new Person();
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p.setName("Ricky");
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Message<Person> message = MessageBuilder.withPayload(p).setHeader("someHeader", "foo").build();
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Message<Employee> result = rsocketRequesterBuilder.tcp("localhost", port)
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.route("pojoMessageToPojo")
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.data(message)
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.retrieveMono(new ParameterizedTypeReference<Message<Employee>>() {})
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.block();
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```
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Aside from sending `Message`, note the usage of `ParameterizedTypeReference` to specify that we want not only `Message` in return but also `Message` with specific payload type.
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### Function Composition over RSocket (Distributed Function Composition)
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By now you shoudl be familiar with the standard function composition feature (e.g., `functionA|functionB|functionC`). This feature allows you to compose several co-located functions into one. But what if these functions are not co-located and instead separated by the network?
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With RSocket and our _distributed function composition_ feature you can still do it. So let's look at the example.
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Let's say we have `uppercase` function available to you locally and `reverse` function exposed via separate RSocket and you wan to compose `uppercase` and `reverse` into a single function. Had they been both available locally it would have been as simple as `uppercase|reverse`, but given that `reverse` function is not locally available we need a way to specify that in our composition instruction. For that we're using _redirect_ operator. So it woudl look like this `uppercase>localhost:2222`, where `localhost:2222` is the host/port combination where `reverse` function is hosted.
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What's interesting is that remote function can in itself be a result of function composition (local or remote), so effectively you are composing `uppercase` with whatever function definition (which could be composition) that is running on `localhost:2222`.
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The complete example is available in [this test case](https://github.com/spring-cloud/spring-cloud-function/blob/0e3a27a392f5c69727d909db26c2ba6aa0344cfd/spring-cloud-function-rsocket/src/test/java/org/springframework/cloud/function/rsocket/RSocketAutoConfigurationTests.java#L371).
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And as you can see it is a bit more complex to showcase this feature. In this test we are composing `reverse` function with `uppercase|concat` function running remotely and then with `wrap` function running locally as if `reverse|uppercase|concat|wrap`.
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So you can see `--spring.cloud.function.definition=reverse>localhost:" + portA + "|wrap"` where `localhost:" + portA` points to another application context instance with `--spring.cloud.function.definition=uppercase|concat`. The result of the `reverse` function are sent to `uppercase|concat` function via RSocket and the result of that are fed into `wrap` function.
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### Samples
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You can also look at one of the [RSocket samples](https://github.com/spring-cloud/spring-cloud-function/tree/master/spring-cloud-function-samples/function-sample-cloudevent-rsocket) that is also introduces you to Cloud Events
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TBD
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