design_proposal: Component and Trait Dependency with Parameter Passing

design/20200305-spec-v1alpha2-dependency-parampassing.md

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+# Component and Trait Dependency with Parameter Passing
+
+* Author: @wonderflow @hongchaodeng
+
+## Background
+
+In https://github.com/oam-dev/spec/issues/171, we have discussed our dependency stories and started to explore solutions.
+Now that we have implemented and evolved the design to solve real world problems, we are proposing the design to OAM spec.
+
+## Goals
+
+In this design, we want to help applications reduce the burden of handling fault tolerance and dependency health.
+By putting this work in the OAM platform layer, OAM users can define explicit dependency and parameter passing in a standard way
+without carign about the implementation details.
+
+## Use Cases
+
+### Case 1: MySQL and Web
+
+Two components, MySQL and Web exists in the same AppConfig and Web depends on MySQL that:
+
+1. MySQL needs to start first;
+2. Web depends on connection credentials from MySQL Component. Once MySQL has started, the credentials will be injected into Web component's environment variables via service binding.
+
+### Case 2: NAS FileSystem and MountTarget
+
+Two components, NAS_FileSystem and NAS_MountTarget exists in the same _ApplicationConfiguration_ and NAS_MountTarget depends on NAS_FileSystem that:
+
+1. NAS_FileSystem needs to be created first;
+2. NAS_MountTarget depends on filesystemID from NAS_FileSystem to fill in NAS_MountTarget's spec.
+
+## Proposal
+
+### Component Dependency
+
+For use case 1 and 2, we are proposing to add `dependencies` in _ApplicationConfiguration_ which a component can describe its dependent components.
+
+```yaml
+apiVersion: core.oam.dev/v1alpha2
+kind: ApplicationConfiguration
+spec:
+- components:
+  - componentName: mysql
+  - componentName: web
+    dependencies:
+      - mysql
+---
+kind: ApplicationConfiguration
+spec:
+- components:
+  - componentName: nas-file-system
+  - componentName: nas-mount-target
+    dependencies:
+    - nas-file-system
+```
+
+### Trait Ordering
+
+For use case 1, we are using a trait called [ServiceBinding](https://github.com/oam-dev/trait-injector) to auto-bind credentials. ServiceBinding trait should be applied before component starts. We propose to add `stage` in TraitDefinition to express ordering relationship with the workload:
+
+```yaml
+apiVersion: core.oam.dev/v1alpha2
+kind: TraitDefinition
+spec:
+  appliesToWorkloads:
+  - core.oam.dev/v1alpha2.ContainerizedWorkload
+  stage: preStart # other options: postStart | preStop | postStop
+```
+
+### Parameter Passing
+
+For use case 2, the filesystemID from NasFileSystem's status needs to be passed to NasMountTarget's spec. To solve this problem, we propose to extend _ParameterValue_ to accept value from another component's field:
+
+```yaml
+kind: ApplicationConfiguration
+spec:
+  components:
+  - componentName: nas-file-system # The Workload of nas-file-system output a filesystemID at .status.filesystemID
+
+  - componentName: nas-mount-target
+    dependencies:
+    - nas-file-system
+    parameterValues:
+    - name: filesystem-id
+      from:
+        compoent: nas-file-system
+        fieldPath: ".status.filesystemID" # take the field's value from nas-file-system's workload
+---
+apiVersion: core.oam.dev/v1alpha2
+kind: Component
+metadata:
+  name: nas-mount-target
+spec:
+  workload:
+    apiVersion: ros.aliyun.com/v1alpha2
+    kind: NASMountTarget
+    spec:
+      filesystemID: "" # to be filled
+  parameters:
+  - name: filesystem-id
+    fieldPath: ".spec.filesystemID"
+```