feat: implement dynamic behavior for halfOpenAfter

Closes #68

src/CircuitBreakerPolicy.test.ts

@@ -7,6 +7,7 @@ import { abortedSignal } from './common/abort';
 import { BrokenCircuitError, TaskCancelledError } from './errors/Errors';
 import { IsolatedCircuitError } from './errors/IsolatedCircuitError';
 import { circuitBreaker, handleAll, handleType } from './Policy';
+import { IterableBackoff } from './backoff/IterableBackoff';
 
 class MyException extends Error {}
 
@@ -79,6 +80,40 @@ describe('CircuitBreakerPolicy', () => {
     expect(onReset).calledOnce;
   });
 
+  it('uses the given backof factory to decide whether to enter the half open state', async () => {
+    p = circuitBreaker(handleType(MyException), {
+      halfOpenAfter: new IterableBackoff([1000, 2000]),
+      breaker: new ConsecutiveBreaker(2),
+    });
+    p.onReset(onReset);
+    p.onHalfOpen(onHalfOpen);
+
+    await openBreaker();
+
+    clock.tick(1000);
+
+    const failedAttempt = p.execute(stub().throws(new MyException()));
+    expect(p.state).to.equal(CircuitState.HalfOpen);
+    expect(onHalfOpen).calledOnce;
+    await expect(failedAttempt).to.be.rejectedWith(MyException);
+    expect(p.state).to.equal(CircuitState.Open);
+
+    clock.tick(1000);
+
+    await expect(p.execute(stub().throws(new MyException()))).to.be.rejectedWith(
+      BrokenCircuitError,
+    );
+
+    clock.tick(1000);
+
+    const result = p.execute(stub().resolves(42));
+    expect(p.state).to.equal(CircuitState.HalfOpen);
+    expect(onHalfOpen).calledTwice;
+    expect(await result).to.equal(42);
+    expect(p.state).to.equal(CircuitState.Closed);
+    expect(onReset).calledOnce;
+  });
+
   it('dedupes half-open tests', async () => {
     await openBreaker();
     clock.tick(1000);

src/CircuitBreakerPolicy.ts

@@ -1,3 +1,4 @@
+import { ConstantBackoff, IBackoff, IBackoffFactory } from './backoff/Backoff';
 import { IBreaker } from './breaker/Breaker';
 import { neverAbortedSignal } from './common/abort';
 import { EventEmitter } from './common/Event';
@@ -30,9 +31,31 @@ export enum CircuitState {
   Isolated,
 }
 
+/**
+ * Context passed into halfOpenAfter backoff delegate.
+ */
+export interface IHalfOpenAfterBackoffContext extends IDefaultPolicyContext {
+  /**
+   * The consecutive number of times the circuit has entered the
+   * {@link CircuitState.Open} state.
+   */
+  attempt: number;
+  /**
+   * The result of the last method call that caused the curcuit to enter the
+   * {@link CircuitState.Open}. Either a thrown error, or a value that we
+   * determined should open the circuit.
+   */
+  result: FailureReason<unknown>;
+}
+
 export interface ICircuitBreakerOptions {
   breaker: IBreaker;
-  halfOpenAfter: number;
+  /**
+   * When to (potentially) enter the {@link CircuitState.HalfOpen} state from
+   * the {@link CircuitState.Open} state. Either a duration in milliseconds or a
+   * backoff factory.
+   */
+  halfOpenAfter: number | IBackoffFactory<IHalfOpenAfterBackoffContext>;
 }
 
 type InnerState =
@@ -48,8 +71,11 @@ export class CircuitBreakerPolicy implements IPolicy {
   private readonly resetEmitter = new EventEmitter<void>();
   private readonly halfOpenEmitter = new EventEmitter<void>();
   private readonly stateChangeEmitter = new EventEmitter<CircuitState>();
+  private readonly halfOpenAfterBackoffFactory: IBackoffFactory<IHalfOpenAfterBackoffContext>;
   private innerLastFailure?: FailureReason<unknown>;
   private innerState: InnerState = { value: CircuitState.Closed };
+  private openEnteredCount = 0;
+  private halfOpenAfterBackof: IBackoff<IHalfOpenAfterBackoffContext> | undefined;
 
   /**
    * Event emitted when the circuit breaker opens.
@@ -99,7 +125,12 @@ export class CircuitBreakerPolicy implements IPolicy {
   constructor(
     private readonly options: ICircuitBreakerOptions,
     private readonly executor: ExecuteWrapper,
-  ) {}
+  ) {
+    this.halfOpenAfterBackoffFactory =
+      typeof options.halfOpenAfter === 'number'
+        ? new ConstantBackoff(options.halfOpenAfter)
+        : options.halfOpenAfter;
+  }
 
   /**
    * Manually holds open the circuit breaker.
@@ -152,7 +183,7 @@ export class CircuitBreakerPolicy implements IPolicy {
         } else {
           this.innerLastFailure = result;
           if (this.options.breaker.failure(state.value)) {
-            this.open(result);
+            this.open(result, signal);
           }
         }
 
@@ -167,7 +198,7 @@ export class CircuitBreakerPolicy implements IPolicy {
         return this.execute(fn);
 
       case CircuitState.Open:
-        if (Date.now() - state.openedAt < this.options.halfOpenAfter) {
+        if (Date.now() - state.openedAt < (this.halfOpenAfterBackof?.duration ?? 0)) {
           throw new BrokenCircuitError();
         }
         const test = this.halfOpen(fn, signal);
@@ -197,7 +228,7 @@ export class CircuitBreakerPolicy implements IPolicy {
       } else {
         this.innerLastFailure = result;
         this.options.breaker.failure(CircuitState.HalfOpen);
-        this.open(result);
+        this.open(result, signal);
       }
 
       return returnOrThrow(result);
@@ -209,21 +240,26 @@ export class CircuitBreakerPolicy implements IPolicy {
     }
   }
 
-  private open(reason: FailureReason<unknown>) {
+  private open(reason: FailureReason<unknown>, signal: AbortSignal) {
     if (this.state === CircuitState.Isolated || this.state === CircuitState.Open) {
       return;
     }
 
     this.innerState = { value: CircuitState.Open, openedAt: Date.now() };
     this.breakEmitter.emit(reason);
     this.stateChangeEmitter.emit(CircuitState.Open);
+    const context = { attempt: ++this.openEnteredCount, result: reason, signal };
+    this.halfOpenAfterBackof =
+      this.halfOpenAfterBackof?.next(context) ?? this.halfOpenAfterBackoffFactory.next(context);
   }
 
   private close() {
     if (this.state === CircuitState.HalfOpen) {
       this.innerState = { value: CircuitState.Closed };
       this.resetEmitter.emit();
       this.stateChangeEmitter.emit(CircuitState.Closed);
+      this.halfOpenAfterBackof = undefined;
+      this.openEnteredCount = 0;
     }
   }
 }

src/Policy.ts

@@ -1,7 +1,7 @@
 import { ConstantBackoff, IBackoffFactory } from './backoff/Backoff';
 import { IBreaker } from './breaker/Breaker';
 import { BulkheadPolicy } from './BulkheadPolicy';
-import { CircuitBreakerPolicy } from './CircuitBreakerPolicy';
+import { CircuitBreakerPolicy, IHalfOpenAfterBackoffContext } from './CircuitBreakerPolicy';
 import { Event } from './common/Event';
 import { ExecuteWrapper } from './common/Executor';
 import { FallbackPolicy } from './FallbackPolicy';
@@ -450,7 +450,13 @@ export function retry(
  * @param breaker The circuit breaker to use. This package exports
  * ConsecutiveBreaker and SamplingBreakers for you to use.
  */
-export function circuitBreaker(policy: Policy, opts: { halfOpenAfter: number; breaker: IBreaker }) {
+export function circuitBreaker(
+  policy: Policy,
+  opts: {
+    halfOpenAfter: number | IBackoffFactory<IHalfOpenAfterBackoffContext>;
+    breaker: IBreaker;
+  },
+) {
   return new CircuitBreakerPolicy(
     opts,
     new ExecuteWrapper(policy.options.errorFilter, policy.options.resultFilter),