Add toString method to Sampler API (#79)

* Add toString method to Sampler API

* Add comment

* Fix review comment

* fix review comment

* add cap on _probability value

* Normalize _probability in constructor

* Non-number types check, add more tests

* Make default _probability to 0

packages/opentelemetry-core/src/trace/sampler/ProbabilitySampler.ts

@@ -18,15 +18,26 @@ import { Sampler, SpanContext } from '@opentelemetry/types';
 
 /** Sampler that samples a given fraction of traces. */
 export class ProbabilitySampler implements Sampler {
-  readonly description = 'ProbabilitySampler';
-
-  constructor(private readonly _probability: number = 1) {}
+  constructor(private readonly _probability: number = 0) {
+    this._probability = this._normalize(_probability);
+  }
 
   shouldSample(parentContext?: SpanContext) {
     if (this._probability >= 1.0) return true;
     else if (this._probability <= 0) return false;
     return Math.random() < this._probability;
   }
+
+  toString(): string {
+    // TODO: Consider to use `AlwaysSampleSampler` and `NeverSampleSampler`
+    // based on the specs.
+    return `ProbabilitySampler{${this._probability}}`;
+  }
+
+  private _normalize(probability: number): number {
+    if (typeof probability !== 'number' || isNaN(probability)) return 0;
+    return probability >= 1 ? 1 : probability <= 0 ? 0 : probability;
+  }
 }
 
 export const ALWAYS_SAMPLER = new ProbabilitySampler(1);

packages/opentelemetry-core/test/trace/ProbabilitySampler.test.ts

@@ -23,7 +23,7 @@ import {
 
 describe('ProbabilitySampler', () => {
   it('should return a always sampler for 1', () => {
-    const sampler = new ProbabilitySampler();
+    const sampler = new ProbabilitySampler(1);
     assert.strictEqual(
       sampler.shouldSample({
         traceId: 'd4cda95b652f4a1592b449d5929fda1b',
@@ -36,6 +36,7 @@ describe('ProbabilitySampler', () => {
   it('should return a always sampler for >1', () => {
     const sampler = new ProbabilitySampler(100);
     assert.strictEqual(sampler.shouldSample(), true);
+    assert.strictEqual(sampler.toString(), 'ProbabilitySampler{1}');
   });
 
   it('should return a never sampler for 0', () => {
@@ -52,16 +53,37 @@ describe('ProbabilitySampler', () => {
     Math.random = () => 1 / 10;
     const sampler = new ProbabilitySampler(0.2);
     assert.strictEqual(sampler.shouldSample(), true);
+    assert.strictEqual(sampler.toString(), 'ProbabilitySampler{0.2}');
 
     Math.random = () => 5 / 10;
     assert.strictEqual(sampler.shouldSample(), false);
   });
 
   it('should return true for ALWAYS_SAMPLER', () => {
     assert.strictEqual(ALWAYS_SAMPLER.shouldSample(), true);
+    assert.strictEqual(ALWAYS_SAMPLER.toString(), 'ProbabilitySampler{1}');
   });
 
   it('should return false for NEVER_SAMPLER', () => {
     assert.strictEqual(NEVER_SAMPLER.shouldSample(), false);
+    assert.strictEqual(NEVER_SAMPLER.toString(), 'ProbabilitySampler{0}');
+  });
+
+  it('should handle NaN', () => {
+    const sampler = new ProbabilitySampler(NaN);
+    assert.strictEqual(sampler.shouldSample(), false);
+    assert.strictEqual(sampler.toString(), 'ProbabilitySampler{0}');
+  });
+
+  it('should handle -NaN', () => {
+    const sampler = new ProbabilitySampler(-NaN);
+    assert.strictEqual(sampler.shouldSample(), false);
+    assert.strictEqual(sampler.toString(), 'ProbabilitySampler{0}');
+  });
+
+  it('should handle undefined', () => {
+    const sampler = new ProbabilitySampler(undefined);
+    assert.strictEqual(sampler.shouldSample(), false);
+    assert.strictEqual(sampler.toString(), 'ProbabilitySampler{0}');
   });
 });

packages/opentelemetry-types/src/trace/Sampler.ts

@@ -22,11 +22,6 @@ import { SpanContext } from './span_context';
  * samples of traces collected and sent to the backend.
  */
 export interface Sampler {
-  /**
-   * A string that uniquely describes the sampling behavior of this instance.
-   */
-  readonly description: string;
-
   /**
    * Checks whether span needs to be created and tracked.
    *
@@ -36,4 +31,7 @@ export interface Sampler {
    * @returns whether span should be sampled or not.
    */
   shouldSample(parentContext?: SpanContext): boolean;
+
+  /** Returns the sampler name or short description with the configuration. */
+  toString(): string;
 }