first

course/13-channels/exercises/8-select_default/code.go

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+package main
+
+import (
+	"fmt"
+	"time"
+)
+
+func saveBackups(snapshotTicker, saveAfter <-chan time.Time) {
+	// ?
+}
+
+// TEST SUITE - Don't touch below this line
+
+func takeSnapshot() {
+	fmt.Println("Taking a backup snapshot...")
+}
+
+func saveSnapshot() {
+	fmt.Println("All backups saved!")
+}
+
+func waitForData() {
+	fmt.Println("Nothing to do, waiting...")
+}
+
+func test() {
+	snapshotTicker := time.Tick(800 * time.Millisecond)
+	saveAfter := time.After(2800 * time.Millisecond)
+	saveBackups(snapshotTicker, saveAfter)
+	fmt.Println("===========================")
+}
+
+func main() {
+	test()
+}

course/13-channels/exercises/8-select_default/complete.go

@@ -0,0 +1,46 @@
+package main
+
+import (
+	"fmt"
+	"time"
+)
+
+func saveBackups(snapshotTicker, saveAfter <-chan time.Time) {
+	for {
+		select {
+		case <-snapshotTicker:
+			takeSnapshot()
+		case <-saveAfter:
+			saveSnapshot()
+			return
+		default:
+			waitForData()
+			time.Sleep(500 * time.Millisecond)
+		}
+	}
+}
+
+// TEST SUITE - Don't touch below this line
+
+func takeSnapshot() {
+	fmt.Println("Taking a backup snapshot...")
+}
+
+func saveSnapshot() {
+	fmt.Println("All backups saved!")
+}
+
+func waitForData() {
+	fmt.Println("Nothing to do, waiting...")
+}
+
+func test() {
+	snapshotTicker := time.Tick(800 * time.Millisecond)
+	saveAfter := time.After(2800 * time.Millisecond)
+	saveBackups(snapshotTicker, saveAfter)
+	fmt.Println("===========================")
+}
+
+func main() {
+	test()
+}

course/13-channels/exercises/8-select_default/expected.txt

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+Nothing to do, waiting...
+Nothing to do, waiting...
+Taking a backup snapshot...
+Nothing to do, waiting...
+Nothing to do, waiting...
+Taking a backup snapshot...
+Nothing to do, waiting...
+Taking a backup snapshot...
+Nothing to do, waiting...
+All backups saved!
+===========================

course/13-channels/exercises/8-select_default/readme.md

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+# 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.