Add metrics API scaler docs

content/blog/keda-2.0-beta.md.draft

@@ -25,6 +25,7 @@ Here are some highlights:
     - Provide more information when quering KEDA resources with `kubectl`
 - **Extensibility**
     - Introduction of External Push scaler ([docs](https://keda.sh/docs/2.0/scalers/external-push/))
+    - Introduction of Metric API scaler ([docs](https://keda.sh/docs/2.0/scalers/metrics-api/))
     - Provide KEDA client-go library
 
 For a full list of changes, we highly recommend going through [our changelog](https://github.com/kedacore/keda/blob/v2/CHANGELOG.md#v200)! With our stable release, we'll provide a full overview of what's released in a new blog post.

content/docs/2.0/scalers/metrics-api.md

@@ -0,0 +1,166 @@
++++
+title = "Metrics API"
+layout = "scaler"
+availability = "v2.0+"
+maintainer = "Community"
+description = "Scale applications based on a metric from an API"
+go_file = "metrics_api_scaler"
++++
+
+### Trigger Specification
+
+This specification describes the `metrics-api` trigger that scales based on a metric value from an API.
+
+The metrics API requires following configuration values:
+- `url` string representing the base URL of the API (i.e `http://app:1317/api/v1/`).
+- `metricName` the metric name to be used to retrieve value. Scaler will look for metric value under
+    `url/metric/{metricName}` endpoint. For more information check next section.
+- `targetValue` the target value is the target value to scale on. When the metric provided by the 
+    API is equal or higher to this value, KEDA will start scaling out.
+
+An example of triggers configuration using metric API scaler:
+
+```yaml
+triggers:
+- type: metric-api
+  metadata:
+    targetValue: "1"
+    url: "http://api:3232/api/v1"
+    metricName: "workersNumber"
+```
+
+**Note**:
+This scaler scales deployment to 0 if and only if the value of the metric is lower or equal to zero.
+
+### Trigger prerequisites
+
+To use an external API it has to comply with:
+
+```yaml
+openapi: 3.0.1
+info:
+  title: KEDA - External Metric Source Petstore
+  description: >-
+    This is the specification to comply with for external KEDA metric sources.
+  termsOfService: 'http://swagger.io/terms/'
+  contact:
+    email: apiteam@swagger.io
+  license:
+    name: Apache 2.0
+    url: 'http://www.apache.org/licenses/LICENSE-2.0.html'
+  version: 1.0.0
+externalDocs:
+  description: Find out more about Swagger
+  url: 'http://swagger.io'
+servers:
+  - url: 'https://samples.keda.sh/'
+tags:
+  - name: Health
+    description: Operations about runtime health
+  - name: Metrics
+    description: Operations about providing metrics to KEDA
+paths:
+  /api/v1/metric/{metric}:
+    get:
+      summary: Get Metric
+      description: Get value for a given metric
+      operationId: GetMetric
+      parameters: 
+       - name: metric
+         in: path
+         required: true
+         schema:
+            type: string
+         description: Name of the metric for which information is requested
+      responses:
+        '200':
+          description: Metric inforation was provided
+          content:
+            application/json:
+              schema:
+                $ref: '#/components/schemas/ReportedMetric'
+        '404':
+          description: Metric is not supported
+        '401':
+          $ref: '#/components/responses/UnauthorizedError'
+        '500':
+          description: Unable to determine metric information
+      tags:
+        - Metrics
+      security:
+        - basic_auth: []
+        - api_key: []
+  /api/v1/metrics:
+    get:
+      summary: Get All Metrics
+      description: Provides a list of all supported metrics
+      operationId: GetMetrics
+      responses:
+        '200':
+          description: Metric inforation was provided
+          content:
+            application/json:
+              schema:
+                $ref: '#/components/schemas/MetricInfo'
+        '401':
+          $ref: '#/components/responses/UnauthorizedError'
+        '500':
+          description: Unable to determine metric information
+      tags:
+        - Metrics
+      security:
+        - basic_auth: []
+        - api_key: []
+  /api/v1/health:
+    get:
+      summary: Get Health
+      description: Provides information about the health of the runtime
+      operationId: GetHealth
+      responses:
+        '200':
+          description: External metric source is healthy
+        '401':
+          $ref: '#/components/responses/UnauthorizedError'
+        '500':
+          description: External metric source is not healthy
+      tags:
+        - Health
+      security:
+        - basic_auth: []
+        - api_key: []
+components:
+  schemas:
+    MetricInfo:
+      type: array
+      items:
+        type: object
+        properties:
+          name:
+            type: string
+    ReportedMetric:
+      type: object
+      required:
+        - name
+        - value
+      properties:
+        name:
+          type: string
+        value:
+          type: number
+  responses:
+    UnauthorizedError:
+      description: Authentication information is missing or invalid
+  securitySchemes:
+    basic_auth:
+      type: http
+      scheme: basic
+    api_key:
+      type: apiKey
+      name: X-API-Key
+      in: header
+security:
+  - basic_auth: []
+  - api_key: []
+```
+
+The `/health` endpoint is used once when creating new `metricsAPIScaler`.