# Select Default Case

The `default` case in a `select` statement executes *immediately* if no other channel has a value ready. A `default` case stops the `select` statement from blocking.

```go
select {
  case v := <-ch:
    // use v
  default:
    // receiving from ch would block
    // so do something else
}
```

## Tickers

* [time.Tick()](https://golang.org/pkg/time/#Tick) is a standard library function that returns a channel that sends a value on a given interval.
* [time.After()](https://golang.org/pkg/time/#After) sends a value once after the duration has passed.
* [time.Sleep()](https://golang.org/pkg/time/#Sleep) blocks the current goroutine for the specified amount of time.

## Read-only Channels

A channel can be marked as read-only by casting it from a `chan` to a `<-chan` type. For example:

```go
func main(){
  ch := make(chan int)
  readCh(ch)
}

func readCh(ch <-chan int) {
  // ch can only be read from
  // in this function
}
```

## Write-only Channels

The same goes for write-only channels, but the arrow's position moves.

```go
func writeCh(ch chan<- int) {
  // ch can only be written to
  // in this function
}
```

## Assignment

Like all good back-end engineers, we frequently save backup snapshots of the Mailio database.

Complete the `saveBackups` function.

It should read values from the `snapshotTicker` and `saveAfter` channels simultaneously.

* If a value is received from `snapshotTicker`, call `takeSnapshot()`
* If a value is received from `saveAfter`, call `saveSnapshot()` and `return` from the function: you're done.
* If neither channel has a value ready, call `waitForData()` and then [time.Sleep()](https://pkg.go.dev/time#example-Sleeps) for 500 milliseconds. After all, we want to show in the logs that the snapshot service is running.