Merge pull request #773 from smallrye/document-connector-less

Explain how applications without connectors work

documentation/src/main/doc/modules/ROOT/pages/concepts.adoc

@@ -1,15 +1,15 @@
 = Concepts
 
-When dealing with event-driven or data streaming application, there are a few concepts and vocabulary to introduce.
+When dealing with event-driven or data streaming applications, there are a few concepts and vocabulary to introduce.
 
 [#messages]
 == Messages, Payload, Metadata
 
 A `Message` is an _envelope_ around a `payload`.
-Your application is going to receive, process and send `Messages`.
+Your application is going to receive, process, and send `Messages`.
 
-These `Messages` can be generated by your application, or are retrieved from a message broker.
-They can also be consumed by your application, or sent to a message broker.
+Your application's logic can generate these `Messages` or receive them from a message broker.
+They can also be consumed by your application or sent to a message broker.
 
 image::messages.png[]
 
@@ -22,12 +22,12 @@ include::example$messages/MessageExamples.java[tag=message]
 ----
 
 As you can see in the previous snippet, `Messages` can also have _metadata_.
-_Metadata_ are a way to extend messages with additional data.
+_Metadata_ is a way to extend messages with additional data.
 It can be metadata related to the message broker ({javadoc-base}/apidocs/io/smallrye/reactive/messaging/kafka/KafkaMessageMetadata.html[`KafkaMessageMetadata`] for instance), or contain operational data (such as tracing metadata), or business-related data.
 
 NOTE: When retrieving metadata, you get an `Optional` as it may not be present.
 
-TIP: Metadata are also used to influence the outbound dispatching (how the message is going to be sent to the broker).
+TIP: Metadata is also used to influence the outbound dispatching (how the message will be sent to the broker).
 
 [#channels]
 == Channels and Streams
@@ -38,7 +38,7 @@ A _channel_ is a virtual destination identified by a name.
 
 image::channels.png[]
 
-SmallRye Reactive Messaging connects the component to channel they read and to channel they populate.
+SmallRye Reactive Messaging connects the component to the channel they read and to the channel they populate.
 The resulting structure is a stream: Messages flow between components through channels.
 
 [NOTE]
@@ -58,16 +58,21 @@ It also means that <<Connectors>> are intended to use non-blocking IO to interac
 Your application is interacting with messaging brokers or event backbone using _connectors_.
 A _connector_ is a piece of code that connects to a broker and:
 
-1. subscribe / poll / receive messages from the broker and propagate them to the application
-2. send / write / dispatch messages provided by the application to the broker
+1. subscribe/poll/receive messages from the broker and propagate them to the application
+2. send/write/dispatch messages provided by the application to the broker
 
-To achieve this, connectors are configured to map incoming messages to a specific _channel_ (consumed by the application) and to collect outgoing messages sent to a specific channel by the application.
+Connectors are configured to map incoming messages to a specific _channel_ (consumed by the application) and collect outgoing messages sent to a specific channel.
 These collected messages are sent to the external broker.
 
 image::connectors.png[]
 
-Each connector is dedicated to a technology.
-For example a Kafka Connector only deal with Kafka.
+Each connector is dedicated to a specific technology.
+For example, a Kafka Connector only deals with Kafka.
 
+You don't necessarily need a connector.
+When your application does not use connectors, everything happens _in-memory_, and the streams are created by chaining methods altogether.
+Each chain is still a reactive stream and enforces the back-pressure protocol.
+When you don't use connectors, you need to make sure the chain is complete, meaning it starts with a message source, and it ends with a sink.
+In other words, you need to generate messages from within the application (using a method with only `@Outgoing`, or an `Emitter`) and consume the messages from within the application (using a method with only `@Incoming` or using an unmanaged stream).