feat(tracer): adopted Utility class & updated unit tests (#549)

* feat: adopted Utility class & updated unit tests

* docs: added notice in docs

* build: update to [email protected]

Author: dreamorosi

Diff for: docs/core/tracer.md

@@ -48,6 +48,8 @@ For a **complete list** of supported environment variables, refer to [this secti
 
 #### Example using AWS Serverless Application Model (SAM)
 
+The `Tracer` utility is instantiated outside of the Lambda handler. In doing this, the same instance can be used across multiple invocations inside the same execution environment. This allows `Metrics` to be aware of things like whether or not a given invocation had a cold start or not.
+
 === "handler.ts"
 
     ```typescript hl_lines="1 4"

Diff for: packages/tracing/package-lock.json

@@ -64,7 +64,7 @@
     "url": "https://github.com/awslabs/aws-lambda-powertools-typescript/issues"
   },
   "dependencies": {
-    "@aws-lambda-powertools/commons": "^0.2.0",
+    "@aws-lambda-powertools/commons": "^0.6.0",
     "aws-xray-sdk-core": "^3.3.3"
   }
 }

Diff for: packages/tracing/package.json

@@ -1,4 +1,5 @@
 import { Handler } from 'aws-lambda';
+import { Utility } from '@aws-lambda-powertools/commons';
 import { TracerInterface } from '.';
 import { ConfigServiceInterface, EnvironmentVariablesService } from './config';
 import { HandlerMethodDecorator, TracerOptions, MethodDecorator } from './types';
@@ -109,9 +110,7 @@ import { Segment, Subsegment } from 'aws-xray-sdk-core';
  * }
  * ```
  */
-class Tracer implements TracerInterface {
-  
-  public static coldStart: boolean = true;
+class Tracer extends Utility implements TracerInterface {
 
   public provider: ProviderServiceInterface;
 
@@ -128,6 +127,8 @@ class Tracer implements TracerInterface {
   private tracingEnabled: boolean = true;
 
   public constructor(options: TracerOptions = {}) {
+    super();
+
     this.setOptions(options);
     this.provider = new ProviderService();
     if (!this.isTracingEnabled()) {
@@ -196,10 +197,7 @@ class Tracer implements TracerInterface {
    */
   public annotateColdStart(): void {
     if (this.isTracingEnabled()) {
-      this.putAnnotation('ColdStart', Tracer.coldStart);
-    }
-    if (Tracer.coldStart) {
-      Tracer.coldStart = false;
+      this.putAnnotation('ColdStart', this.getColdStart());
     }
   }
 
@@ -430,27 +428,6 @@ class Tracer implements TracerInterface {
       return descriptor;
     };
   }
-
-  /**
-   * Retrieve the current value of `ColdStart`.
-   * 
-   * If Tracer has been initialized outside the Lambda handler then the same instance
-   * of Tracer will be reused throughout the lifecycle of that same Lambda execution environment
-   * and this method will return `false` after the first invocation.
-   * 
-   * @see https://docs.aws.amazon.com/lambda/latest/dg/runtimes-context.html
-   * 
-   * @returns boolean - `true` if is cold start, otherwise `false`
-   */
-  public static getColdStart(): boolean {
-    if (Tracer.coldStart) {
-      Tracer.coldStart = false;
-
-      return true;
-    }
-
-    return false;
-  }
 
   /**
    * Get the active segment or subsegment in the current scope.

Diff for: packages/tracing/src/Tracer.ts

@@ -40,7 +40,6 @@ describe('Class: Tracer', () => {
   };
 
   beforeEach(() => {
-    Tracer.coldStart = true;
     jest.clearAllMocks();
     jest.resetModules();
     process.env = { ...ENVIRONMENT_VARIABLES };
@@ -259,20 +258,6 @@ describe('Class: Tracer', () => {
 
   });
 
-  describe('Method: getColdStart', () => {
-
-    test('when called, it returns false the first time and always true after that', () => {
-    
-      // Assess
-      expect(Tracer.getColdStart()).toBe(true);
-      expect(Tracer.getColdStart()).toBe(false);
-      expect(Tracer.getColdStart()).toBe(false);
-      expect(Tracer.getColdStart()).toBe(false);
-    
-    });
-    
-  });
-
   describe('Method: getSegment', () => {
 
     test('when called outside of a namespace or without parent segment, and tracing is enabled, it throws an error', () => {

Diff for: packages/tracing/tests/unit/Tracer.test.ts

@@ -32,7 +32,6 @@ describe('Middy middleware', () => {
   };
 
   beforeEach(() => {
-    Tracer.coldStart = true;
     jest.clearAllMocks();
     jest.resetModules();
     process.env = { ...ENVIRONMENT_VARIABLES };