feat: Add hook-data concept for hooks.

Signed-off-by: Ryan Lamb <[email protected]>

specification/sections/04-hooks.md

@@ -31,7 +31,9 @@ Hooks can be configured to run globally (impacting all flag evaluations), per cl
 
 ### Definitions
 
-**Hook**: Application author/integrator-supplied logic that is called by the OpenFeature framework at a specific stage. **Stage**: An explicit portion of the flag evaluation lifecycle. e.g. `before` being "before the [resolution](../glossary.md#resolving-flag-values) is run. **Invocation**: A single call to evaluate a flag. `client.getBooleanValue(..)` is an invocation. **API**: The global API singleton.
+**Hook**: Application author/integrator-supplied logic that is called by the OpenFeature framework at a specific stage.
+
+**Stage**: An explicit portion of the flag evaluation lifecycle. e.g. `before` being "before the [resolution](../glossary.md#resolving-flag-values) is run. **Invocation**: A single call to evaluate a flag. `client.getBooleanValue(..)` is an invocation. **API**: The global API singleton.
 
 ### 4.1. Hook context
 
@@ -81,75 +83,154 @@ see: [dynamic-context paradigm](../glossary.md#dynamic-context-paradigm)
 
 > Condition: The provider `metadata` field in the `hook context` **MUST** be immutable.
 
-### 4.3. Hook creation and parameters
+### 4.3. Hook data
+
+Hook data exists to allow hook stages to share data for a specific evaluation. For instance a span
+for OpenTelemetry could be created in a `before` stage and closed in an `after` stage.
+
+```Java
+  public Optional<EvaluationContext> before(HookContext context, HookHints hints, HookData data) {
+    SpanBuilder builder = tracer.spanBuilder('sample')
+    .setParent(Context.current().with(Span.current()));
+    Span span = builder.startSpan()
+    data.set("span", span);
+  }
+
+  public void after(HookContext context, FlagEvaluationDetails details, HookHints hints, HookData data) {
+    // Only accessible by this hook for this specific evaluation.
+    Object value = data.get("span");
+    if (value instanceof Span) {
+      Span span = (Span) value;
+      span.end();
+    }
+  }
+```
 
 #### Requirement 4.3.1
 
+> `hook data` **MUST** be a structure supports definition of arbitrary properties, with keys of type `string`, and values of type `boolean | string | number | datetime | structure`.
+
+### 4.4. Hook creation and parameters
+
+#### Requirement 4.4.1
+
 > Hooks **MUST** specify at least one stage.
 
-#### Condition 4.3.2
+#### Requirement 4.4.2
+
+> `Hook data` **MUST** must be created before the first `stage` invoked in a hook for a specific evaluation and propagated between each `stage` of the hook.
+
+Example showing data between `before` and `after` stage for two different hooks.
+```mermaid
+sequenceDiagram
+actor Application
+participant Client
+participant HookA
+participant HookB
+
+Application->>Client: getBooleanValue('my-bool', myContext, false)
+activate Client
+
+Client-->>Client: create hook data for HookA
+
+Client->>HookA: before(hookContext, data: {})
+activate HookA
+
+HookA-->>HookA: data.set('key',' data for A')
+
+HookA-->>Client: (return)
+deactivate HookA
+
+Client-->>Client: create hook data for HookB
+
+Client->>HookB: before(hookContext, hints, data: {})
+activate HookB
+
+HookB-->>HookB: data.set('key', 'data for B')
+deactivate HookB
+
+Client-->>Client: Flag evaluation
+
+Client->>HookB: after(hookContext, detail, hints, data: {key: 'data for B'})
+activate HookB
+
+HookB-->>Client: (return)
+deactivate HookB
+
+Client->>HookA: after(hookContext, data: {'key': 'data for A'})
+activate HookA
+
+HookA-->>Client: (return)
+deactivate HookA
+
+Client-->>Application: true
+deactivate Client
+
+```
+
+#### Condition 4.4.2
 
 > The implementation uses the dynamic-context paradigm.
 
 see: [dynamic-context paradigm](../glossary.md#dynamic-context-paradigm)
 
-##### Conditional Requirement 4.3.2.1
+##### Conditional Requirement 4.4.2.1
 
-> The `before` stage **MUST** run before flag resolution occurs. It accepts a `hook context` (required) and `hook hints` (optional) as parameters and returns either an `evaluation context` or nothing.
+> The `before` stage **MUST** run before flag resolution occurs. It accepts a `hook context` (required), `hook hints` (optional), and `hook data` (required) as parameters and returns either an `evaluation context` or nothing.
 
 ```java
-EvaluationContext | void before(HookContext hookContext, HookHints hints);
+EvaluationContext | void before(HookContext hookContext, HookHints hints, HookData data);
 ```
 
-#### Condition 4.3.3
+#### Condition 4.4.3
 
 [![experimental](https://img.shields.io/static/v1?label=Status&message=experimental&color=orange)](https://github.com/open-feature/spec/tree/main/specification#experimental)
 
 > The implementation uses the static-context paradigm.
 
 see: [static-context paradigm](../glossary.md#static-context-paradigm)
 
-##### Conditional Requirement 4.3.3.1
+##### Conditional Requirement 4.4.3.1
 
-> The `before` stage **MUST** run before flag resolution occurs. It accepts a `hook context` (required) and `hook hints` (optional) as parameters. It has no return value.
+> The `before` stage **MUST** run before flag resolution occurs. It accepts a `hook context` (required), `hook hints` (optional), and `hook data` (required) as parameters. It has no return value.
 
 ```java
-void before(HookContext hookContext, HookHints hints);
+void before(HookContext hookContext, HookHints hints, HookData data);
 ```
 
-#### Requirement 4.3.4
+#### Requirement 4.4.4
 
 > Any `evaluation context` returned from a `before` hook **MUST** be passed to subsequent `before` hooks (via `HookContext`).
 
-#### Requirement 4.3.5
+#### Requirement 4.4.5
 
 > When `before` hooks have finished executing, any resulting `evaluation context` **MUST** be merged with the existing `evaluation context`.
 
 Evaluation context merge order is defined in [Context levels and merging](./03-evaluation-context.md#32-context-levels-and-merging).
 
-#### Requirement 4.3.6
+#### Requirement 4.4.6
 
-> The `after` stage **MUST** run after flag resolution occurs. It accepts a `hook context` (required), `flag evaluation details` (required) and `hook hints` (optional). It has no return value.
+> The `after` stage **MUST** run after flag resolution occurs. It accepts a `hook context` (required), `flag evaluation details` (required), `hook hints` (optional), and `hook data` (required). It has no return value.
 
-#### Requirement 4.3.7
+#### Requirement 4.4.7
 
-> The `error` hook **MUST** run when errors are encountered in the `before` stage, the `after` stage or during flag resolution. It accepts `hook context` (required), `exception` representing what went wrong (required), and `hook hints` (optional). It has no return value.
+> The `error` hook **MUST** run when errors are encountered in the `before` stage, the `after` stage or during flag resolution. It accepts `hook context` (required), `exception` representing what went wrong (required), `hook hints` (optional), and `hook data`. It has no return value.
 
-#### Requirement 4.3.8
+#### Requirement 4.4.8
 
-> The `finally` hook **MUST** run after the `before`, `after`, and `error` stages. It accepts a `hook context` (required) and `hook hints` (optional). There is no return value.
+> The `finally` hook **MUST** run after the `before`, `after`, and `error` stages. It accepts a `hook context` (required), `hook hints` (optional), and `hook data`. There is no return value.
 
-#### Condition 4.3.9
+#### Condition 4.4.9
 
 > `finally` is a reserved word in the language.
 
-##### Conditional Requirement 4.3.9.1
+##### Conditional Requirement 4.4.9.1
 
 > Instead of `finally`, `finallyAfter` **SHOULD** be used.
 
-### 4.4. Hook registration & ordering
+### 4.5. Hook registration & ordering
 
-#### Requirement 4.4.1
+#### Requirement 4.5.1
 
 > The API, Client, Provider, and invocation **MUST** have a method for registering hooks.
 
@@ -166,7 +247,7 @@ client.addHooks(new Hook2());
 client.getValue('my-flag', 'defaultValue', new Hook3());
 ```
 
-#### Requirement 4.4.2
+#### Requirement 4.5.2
 
 > Hooks **MUST** be evaluated in the following order:
 >
@@ -175,27 +256,27 @@ client.getValue('my-flag', 'defaultValue', new Hook3());
 > - error (if applicable): Provider, Invocation, Client, API
 > - finally: Provider, Invocation, Client, API
 
-#### Requirement 4.4.3
+#### Requirement 4.5.3
 
 > If a `finally` hook abnormally terminates, evaluation **MUST** proceed, including the execution of any remaining `finally` hooks.
 
 In languages with try/catch semantics, this means that exceptions thrown in `finally` hooks should be caught, and not propagated up the call stack.
 
-#### Requirement 4.4.4
+#### Requirement 4.5.4
 
 > If an `error` hook abnormally terminates, evaluation **MUST** proceed, including the execution of any remaining `error` hooks.
 
 In languages with try/catch semantics, this means that exceptions thrown in `error` hooks should be caught, and not propagated up the call stack.
 
-#### Requirement 4.4.5
+#### Requirement 4.5.5
 
 > If an error occurs in the `before` or `after` hooks, the `error` hooks **MUST** be invoked.
 
-#### Requirement 4.4.6
+#### Requirement 4.5.6
 
 > If an error occurs during the evaluation of `before` or `after` hooks, any remaining hooks in the `before` or `after` stages **MUST NOT** be invoked.
 
-#### Requirement 4.4.7
+#### Requirement 4.5.7
 
 > If an error occurs in the `before` hooks, the default value **MUST** be returned.
 
@@ -214,14 +295,14 @@ val = client.get_boolean_value('my-key', False, evaluation_options={
 
 see: [Flag evaluation options](./01-flag-evaluation.md#evaluation-options)
 
-#### Requirement 4.5.1
+#### Requirement 4.6.1
 
 > `Flag evaluation options` **MAY** contain `hook hints`, a map of data to be provided to hook invocations.
 
-#### Requirement 4.5.2
+#### Requirement 4.6.2
 
 > `hook hints` **MUST** be passed to each hook.
 
-#### Requirement 4.5.3
+#### Requirement 4.6.3
 
 > The hook **MUST NOT** alter the `hook hints` structure.