Java: flows

java/flows.md

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+---
+synopsis: >
+  Learn about the flow feature in CAP Java.
+status: released
+---
+
+# Flow: Modeling State Transitions <Beta /> <Since version="4.0.0" of="com.sap.cds:cds-feature-flow" />
+<style scoped>
+  h1:before {
+    content: "Java"; display: block; font-size: 60%; margin: 0 0 .2em;
+  }
+</style>
+
+{{ $frontmatter.synopsis }}
+
+Use the flow feature to define flows in your CDS model. The feature ensures that transitions between states are explicitly modeled, validated, and performed in a controlled manner.
+
+## Enabling Flows { #enabling-flows }
+
+To use the flow feature, add a dependency to [cds-feature-flows](TODO) in the `srv/pom.xml` file of your service:
+
+```xml
+<dependency>
+  <groupId>com.sap.cds</groupId>
+  <artifactId>cds-feature-flows</artifactId>
+  <scope>runtime</scope>
+</dependency>
+```
+
+## Modeling Flows { #modeling-flows }
+
+Flows are defined in the CDS model using annotations. Flows consist out of one status element and one or more _flow_ actions. Annotate the status element with `@flow.status`. Define all transitions as bound actions (TODO link /cap/docs/cds/cdl#bound-actions), list the valid entry states using `@flow.from` and the target state using `@flow.to`. 
+
+::: tip Best Practice
+We recommend to always use `@flow.status` in combination with `@readonly`.
+This gives full control over all transitions to the flow feature.
+:::
+
+### Details
+Let's dive into the details of the annotations:
+
+`@flow.status`
+- This annotation is mandatory. 
+- The annotated element must be either an enum or an association to an enum.
+- Only one status element per entity is supported.
+- Draft-enabled entities are supported.
+- `null` is **not** a valid state. Model your empty state explicitly. 
+
+`@flow.from`
+- This annotation is optional. Leave it out and all states are valid entry states.
+- This annotation's value type is an array. Each array element's type must match the status element.
+
+`@flow.to`
+- This annotation is optional. Leave it out and the target state is the entry state.
+- This annotation's value type must match the status element.
+
+Please note, while both `@flow.from` and `@flow.to` are optional, one of them is mandatory to mark the action as a flow action.
+
+## Using and Extending Flows
+When using the flow feature, default handlers (todo link /docs/java/cqn-services/application-services#implement-event-handler) are provided for your flow actions.
+
+The default `Before` handler will validate that your entity instance is in a valid entry state. A validation error will return a 409 Http Status Code.
+
+The default `On` handler will complete the action for void return types.
+
+The default `After` handler will process the transition and update the status to the target state.
+
+### Custom Event Handler { #custom-event-handler}
+You can extend the default flow handlers with an event handler to implement custom event logic that should run on a transition. 
+
+TODO: node example: less than 100, more than 100
+
+TODO: does readonly mean using the persistence service in custom handlers?
+
+
+## Sample { #sample }
+
+Let's take a look at the [CAP SFLIGHT App](https://github.com/SAP-samples/cap-sflight) sample and how we can update it using the flow feature.
+
+Let's add `cds-feature-flow` to the dependencies.
+
+Annotate the travel status element with the `@flow.status` annotation. Following is an extract of the relevant parts of the entity model:
+
+```cds
+entity Travel : managed {
+  key TravelUUID : UUID;
+  TravelID       : Integer default 0 @readonly;
+  @flow.status
+  TravelStatus   : Association to TravelStatus default 'O' @readonly;
+};
+
+entity TravelStatus : CodeList {
+  key code : TravelStatusCode;
+}
+
+type TravelStatusCode : String(1) enum {
+  Open     = 'O';
+  Accepted = 'A';
+  Canceled = 'X';
+};
+```
+
+Now, let's have a look at the added flow annotations in the service model:
+
+```cds
+service TravelService @(path:'/processor') {
+
+  entity Travel as projection on my.Travel actions {
+    @(flow: { 
+        from:[ Open ],                     
+        to: Canceled 
+    })
+    action rejectTravel();
+    @(flow: { 
+        from:[ Open ],                     
+        to: Accepted 
+    })
+    action acceptTravel();
+  };
+
+}
+```
+
+Finally, in the CAP SFLIGHT App, no action handler is needed. All of the logic is taken care of by the flow feature. The flow feature will validate that entry state is `Open`. The flow feature will transit the status to `Accepted` and `Canceled` respectively. 
+If you followed along, with the CAP SFLIGHT App, simply remove the [AcceptRejectHandler](https://github.com/SAP-samples/cap-sflight/blob/main/srv/src/main/java/com/sap/cap/sflight/processor/AcceptRejectHandler.java).
+
+TODO: validate that draft will work the same as in contracts/tests. It looks different here.
+
+
+
+