Exponential back-off for process restarting

bin/concurrently.ts

@@ -165,9 +165,9 @@ const args = yargs(argsBeforeSep)
             type: 'number',
         },
         'restart-after': {
-            describe: 'Delay time to respawn the process, in milliseconds.',
+            describe: 'Delay before restarting the process, in milliseconds, or "exponential".',
             default: defaults.restartDelay,
-            type: 'number',
+            type: 'string',
         },
 
         // Input
@@ -223,7 +223,8 @@ concurrently(
         prefix: args.prefix,
         prefixColors: args.prefixColors.split(','),
         prefixLength: args.prefixLength,
-        restartDelay: args.restartAfter,
+        restartDelay:
+            args.restartAfter === 'exponential' ? 'exponential' : Number(args.restartAfter),
         restartTries: args.restartTries,
         successCondition: args.success,
         timestampFormat: args.timestampFormat,

src/flow-control/restart-process.spec.ts

@@ -1,5 +1,5 @@
 import { createMockInstance } from 'jest-create-mock-instance';
-import { TestScheduler } from 'rxjs/testing';
+import { VirtualTimeScheduler } from 'rxjs';
 
 import { createFakeCloseEvent, FakeCommand } from '../fixtures/fake-command';
 import { Logger } from '../logger';
@@ -8,12 +8,14 @@ import { RestartProcess } from './restart-process';
 let commands: FakeCommand[];
 let controller: RestartProcess;
 let logger: Logger;
-let scheduler: TestScheduler;
+let scheduler: VirtualTimeScheduler;
 beforeEach(() => {
     commands = [new FakeCommand(), new FakeCommand()];
-
     logger = createMockInstance(Logger);
-    scheduler = new TestScheduler(() => true);
+
+    // Don't use TestScheduler as it's hardcoded to a max number of "frames" (time),
+    // which don't work for some tests in this suite
+    scheduler = new VirtualTimeScheduler();
     controller = new RestartProcess({
         logger,
         scheduler,
@@ -34,14 +36,31 @@ it('does not restart processes that complete with success', () => {
     expect(commands[1].start).toHaveBeenCalledTimes(0);
 });
 
-it('restarts processes that fail after delay has passed', () => {
+it('restarts processes that fail immediately, if no delay was passed', () => {
+    controller = new RestartProcess({ logger, scheduler, tries: 1 });
+    controller.handle(commands);
+
+    commands[0].close.next(createFakeCloseEvent({ exitCode: 1 }));
+    scheduler.flush();
+
+    expect(scheduler.now()).toBe(0);
+    expect(logger.logCommandEvent).toHaveBeenCalledTimes(1);
+    expect(logger.logCommandEvent).toHaveBeenCalledWith(
+        `${commands[0].command} restarted`,
+        commands[0],
+    );
+    expect(commands[0].start).toHaveBeenCalledTimes(1);
+});
+
+it('restarts processes that fail after delay ms has passed', () => {
     controller.handle(commands);
 
     commands[0].close.next(createFakeCloseEvent({ exitCode: 1 }));
     commands[1].close.next(createFakeCloseEvent({ exitCode: 0 }));
 
     scheduler.flush();
 
+    expect(scheduler.now()).toBe(100);
     expect(logger.logCommandEvent).toHaveBeenCalledTimes(1);
     expect(logger.logCommandEvent).toHaveBeenCalledWith(
         `${commands[0].command} restarted`,
@@ -51,6 +70,23 @@ it('restarts processes that fail after delay has passed', () => {
     expect(commands[1].start).not.toHaveBeenCalled();
 });
 
+it('restarts processes that fail with an exponential back-off', () => {
+    const tries = 4;
+    controller = new RestartProcess({ logger, scheduler, tries, delay: 'exponential' });
+    controller.handle(commands);
+
+    let time = 0;
+    for (let i = 0; i < tries; i++) {
+        commands[0].close.next(createFakeCloseEvent({ exitCode: 1 }));
+        scheduler.flush();
+
+        time += Math.pow(2, i) * 1000;
+        expect(scheduler.now()).toBe(time);
+        expect(logger.logCommandEvent).toHaveBeenCalledTimes(i + 1);
+        expect(commands[0].start).toHaveBeenCalledTimes(i + 1);
+    }
+});
+
 it('restarts processes up to tries', () => {
     controller.handle(commands);
 

src/flow-control/restart-process.ts

@@ -1,18 +1,20 @@
 import * as Rx from 'rxjs';
-import { defaultIfEmpty, delay, filter, map, skip, take, takeWhile } from 'rxjs/operators';
+import { defaultIfEmpty, delayWhen, filter, map, skip, take, takeWhile } from 'rxjs/operators';
 
 import { Command } from '../command';
 import * as defaults from '../defaults';
 import { Logger } from '../logger';
 import { FlowController } from './flow-controller';
 
+export type RestartDelay = number | 'exponential';
+
 /**
  * Restarts commands that fail up to a defined number of times.
  */
 export class RestartProcess implements FlowController {
     private readonly logger: Logger;
     private readonly scheduler?: Rx.SchedulerLike;
-    readonly delay: number;
+    private readonly delay: RestartDelay;
     readonly tries: number;
 
     constructor({
@@ -21,13 +23,13 @@ export class RestartProcess implements FlowController {
         logger,
         scheduler,
     }: {
-        delay?: number;
+        delay?: RestartDelay;
         tries?: number;
         logger: Logger;
         scheduler?: Rx.SchedulerLike;
     }) {
         this.logger = logger;
-        this.delay = delay != null ? +delay : defaults.restartDelay;
+        this.delay = delay ?? 0;
         this.tries = tries != null ? +tries : defaults.restartTries;
         this.tries = this.tries < 0 ? Infinity : this.tries;
         this.scheduler = scheduler;
@@ -38,6 +40,12 @@ export class RestartProcess implements FlowController {
             return { commands };
         }
 
+        const delayOperator = delayWhen((_, index) => {
+            const { delay } = this;
+            const value = delay === 'exponential' ? Math.pow(2, index) * 1000 : delay;
+            return Rx.timer(value, this.scheduler);
+        });
+
         commands
             .map((command) =>
                 command.close.pipe(
@@ -50,7 +58,7 @@ export class RestartProcess implements FlowController {
                     // Delay the emission (so that the restarts happen on time),
                     // explicitly telling the subscriber that a restart is needed
                     failure.pipe(
-                        delay(this.delay, this.scheduler),
+                        delayOperator,
                         map(() => true),
                     ),
                     // Skip the first N emissions (as these would be duplicates of the above),

src/index.ts

@@ -15,7 +15,7 @@ import { LogError } from './flow-control/log-error';
 import { LogExit } from './flow-control/log-exit';
 import { LogOutput } from './flow-control/log-output';
 import { LogTimings } from './flow-control/log-timings';
-import { RestartProcess } from './flow-control/restart-process';
+import { RestartDelay, RestartProcess } from './flow-control/restart-process';
 import { Logger } from './logger';
 
 export type ConcurrentlyOptions = BaseConcurrentlyOptions & {
@@ -59,7 +59,7 @@ export type ConcurrentlyOptions = BaseConcurrentlyOptions & {
      *
      * @see RestartProcess
      */
-    restartDelay?: number;
+    restartDelay?: RestartDelay;
 
     /**
      * How many times commands should be restarted when they exit with a failure.