feat: context propagation for nestjs (#736)

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## This PR

Implements context propagation in the Nest.js SDK.
This relates to the discussions from
#733 (comment).

Open questions are:
- How to scope the `contextFactory`? It seems that we should be able to
define a common and client specific context factories.
- Should the `AsyncLocalStorage` be used in the `FeatureFlagDecorators`
too as it is in this draft?
- Should we merge the injected `OpenFeatureClient` and the
`OpenFeatureContextService` so that you do not have to inject both into
the consuming services as we do in the `TestService`?

@luizgribeiro this is the draft for what we discussed about, what do you
think?

---------

Signed-off-by: Lukas Reining <[email protected]>
Signed-off-by: Luiz Guilherme Ribeiro <[email protected]>
Co-authored-by: Michael Beemer <[email protected]>
Co-authored-by: Luiz Guilherme Ribeiro <[email protected]>

Author: 3 people

package-lock.json

@@ -17,8 +17,8 @@
     "test": "jest --verbose",
     "lint": "eslint ./",
     "clean": "shx rm -rf ./dist",
-    "build:esm": "esbuild src/index.ts --bundle --external:@nestjs/common --external:@nestjs/common --external:@openfeature/server-sdk --sourcemap --target=es2016 --platform=node --format=esm --outfile=./dist/esm/index.js --analyze",
-    "build:cjs": "esbuild src/index.ts --bundle --external:@nestjs/common --external:@nestjs/common --external:@openfeature/server-sdk --sourcemap --target=es2016 --platform=node --format=cjs --outfile=./dist/cjs/index.js --analyze",
+    "build:esm": "esbuild src/index.ts --bundle --external:@nestjs/* --external:@openfeature/server-sdk --sourcemap --target=es2016 --platform=node --format=esm --outfile=./dist/esm/index.js --analyze",
+    "build:cjs": "esbuild src/index.ts --bundle --external:@nestjs/* --external:@openfeature/server-sdk --sourcemap --target=es2016 --platform=node --format=cjs --outfile=./dist/cjs/index.js --analyze",
     "build:rollup-types": "rollup -c ../../rollup.config.mjs",
     "build": "npm run clean && npm run build:esm && npm run build:cjs && npm run build:rollup-types",
     "postbuild": "shx cp ./../../package.esm.json ./dist/esm/package.json",
@@ -56,9 +56,7 @@
     "@nestjs/testing": "^10.2.10",
     "@openfeature/core": "*",
     "@openfeature/server-sdk": "*",
-    "@types/express": "^4.17.21",
     "@types/supertest": "^2.0.16",
-    "express": "^4.18.2",
     "supertest": "^6.3.3"
   }
 }

packages/nest/package.json

@@ -0,0 +1,35 @@
+import { EvaluationContext } from '@openfeature/core';
+import { ExecutionContext, Inject } from '@nestjs/common';
+
+/**
+ * A factory function for creating an OpenFeature {@link EvaluationContext} from Nest {@link ExecutionContext}.
+ * This can be used e.g. to get header info from an HTTP request or information from a gRPC call.
+ *
+ * Example getting an HTTP header value:
+ * ```typescript
+ * async function(context: ExecutionContext) {
+ *   const request = await context.switchToHttp().getRequest();
+ *
+ *   const userId = request.header('x-user-id');
+ *
+ *   if (userId) {
+ *     return {
+ *       targetingKey: userId,
+ *     };
+ *   }
+ *
+ *   return undefined;
+ * }
+ * ```
+ * @param {ExecutionContext} request The {@link ExecutionContext} to get the information from.
+ * @returns {(Promise<EvaluationContext | undefined> | EvaluationContext | undefined)} The {@link EvaluationContext} new.
+ */
+export type ContextFactory = (
+  request: ExecutionContext,
+) => Promise<EvaluationContext | undefined> | EvaluationContext | undefined;
+
+/**
+ * InjectionToken for a {@link ContextFactory}.
+ * @see {@link Inject}
+ */
+export const ContextFactoryToken = Symbol('CONTEXT_FACTORY');

packages/nest/src/context-factory.ts

@@ -0,0 +1,23 @@
+import { CallHandler, ExecutionContext, Inject, Injectable, NestInterceptor } from '@nestjs/common';
+import { ContextFactory, ContextFactoryToken } from './context-factory';
+import { Observable } from 'rxjs';
+import { OpenFeature } from '@openfeature/server-sdk';
+
+@Injectable()
+export class EvaluationContextInterceptor implements NestInterceptor {
+  constructor(@Inject(ContextFactoryToken) private contextFactory?: ContextFactory) {}
+
+  async intercept(executionContext: ExecutionContext, next: CallHandler) {
+    const context = await this.contextFactory?.(executionContext);
+
+    return new Observable((subscriber) => {
+      OpenFeature.setTransactionContext(context ?? {}, async () => {
+        next.handle().subscribe({
+          next: (res) => subscriber.next(res),
+          error: (err) => subscriber.error(err),
+          complete: () => subscriber.complete(),
+        });
+      });
+    });
+  }
+}

packages/nest/src/evaluation-context-interceptor.ts

@@ -0,0 +1,14 @@
+import { TransactionContextPropagator } from '@openfeature/server-sdk';
+import { AsyncLocalStorage } from 'async_hooks';
+import { EvaluationContext } from '@openfeature/server-sdk';
+
+export class AsyncLocalStorageTransactionContext implements TransactionContextPropagator {
+  private asyncLocalStorage = new AsyncLocalStorage<EvaluationContext>();
+
+  getTransactionContext(): EvaluationContext {
+    return this.asyncLocalStorage.getStore() ?? {};
+  }
+  setTransactionContext(context: EvaluationContext, callback: () => void): void {
+    this.asyncLocalStorage.run(context, callback);
+  }
+}