Stager

Stager is a library to help write code where you are in control of start and shutdown of concurrent operations. I.e. you know when goroutines start and stop and in which order.

An example is below:

package main

import (
  "context"
  "log"
  "time"

  "github.com/ash2k/stager"
)

func main() {
  defer log.Print("Exiting main")
  st := stager.New()

  s := st.NextStage()
  s.Go(func(ctx context.Context) error {
    log.Print("Start 1.1")
    defer log.Print("Stop 1.1")
    <-ctx.Done()
    return nil
  })
  s.Go(func(ctx context.Context) error {
    log.Print("Start 1.2")
    defer log.Print("Stop 1.2")
    <-ctx.Done()
    return nil
  })

  s = st.NextStage()
  s.Go(func(ctx context.Context) error {
    log.Print("Start 2")
    defer log.Print("Stop 2")
    <-ctx.Done()
    return nil
  })

  s = st.NextStage()
  s.Go(func(ctx context.Context) error {
    log.Print("Start 3")
    defer log.Print("Stop 3")
    <-ctx.Done()
    return nil
  })

  ctx, cancel := context.WithTimeout(context.Background(), 5*time.Second)
  defer cancel()
  err := st.Run(ctx)
  if err != nil {
    log.Fatal(err)
  }
}

Output:

2020/12/15 15:34:41 Start 3
2020/12/15 15:34:41 Start 1.2
2020/12/15 15:34:41 Start 1.1
2020/12/15 15:34:41 Start 2
2020/12/15 15:34:46 Stop 3
2020/12/15 15:34:46 Stop 2
2020/12/15 15:34:46 Stop 1.2
2020/12/15 15:34:46 Stop 1.1
2020/12/15 15:34:46 Exiting main

Note the following:

Shutdown order is deterministic - 3, 2, and then 1.
Shutdown order within a stage is not deterministic - 1.1 and 1.2 are not ordered.