first successful key lookup problem

01-first-successful-key-lookup/README.md

@@ -0,0 +1,14 @@
+# First Successful Key Lookup
+
+You are given a function `get(ctx, address, key)`, which retrieves a value for a given key from a remote address. However, you don't know which address has the value, and network calls can fail.
+
+Your task is to implement the Get function that:
+* Calls get for each address in parallel.
+* Returns the first successful response.
+* If all requests fail, returns an error.
+
+## Tags
+`Concurrency`
+
+## Source
+- [Mock-собеседование по Go от Team Lead из Яндекса](https://www.youtube.com/watch?v=x689QxR3AIc) by **it-interview**

01-first-successful-key-lookup/solution/solution.go

@@ -0,0 +1,52 @@
+package solution
+
+import (
+	"context"
+	"errors"
+	"time"
+)
+
+var ErrNotFound = errors.New("key not found")
+
+func get(ctx context.Context, address string, key string) (string, error) {
+	return "", nil
+}
+
+func Get(ctx context.Context, addresses []string, key string) (string, error) {
+	// Creating context with timeout, so we can cancel it when receive first response (or timeout)
+	ctx, cancel := context.WithTimeout(ctx, 10*time.Second)
+	defer cancel()
+
+	// Channels MUST be buffered, in other case there is a goroutine leakage
+	resCh, errCh := make(chan string, len(addresses)), make(chan error, len(addresses))
+
+	for _, address := range addresses {
+		go func() {
+			if val, err := get(ctx, address, key); err != nil {
+				errCh <- err
+			} else {
+				// There is a potential goroutine leak, if channel was unbuffered.
+				// If the result is not first, we WILL NOT read this channel
+				// and this goroutine will stuck forever
+				resCh <- val
+			}
+		}()
+	}
+
+	var errCount int
+	for {
+		select {
+		case err := <-errCh:
+			// If error count is equal to addresses count
+			// it means that no goroutine left and we can return an error
+			errCount++
+			if errCount == len(addresses) {
+				return "", err
+			}
+		case val := <-resCh:
+			return val, nil
+		case <-ctx.Done():
+			return "", context.Canceled
+		}
+	}
+}

01-first-successful-key-lookup/task.go

@@ -0,0 +1,19 @@
+package main
+
+import (
+	"context"
+	"errors"
+)
+
+var ErrNotFound = errors.New("key not found")
+
+func get(ctx context.Context, address string, key string) (string, error) {
+	// Already implemented
+	return "", nil
+}
+
+// Call `get` function for each received address in parallel
+// Return first response or an error if all requests fail
+func Get(ctx context.Context, adresses []string, key string) (string, error) {
+	return "", nil
+}

README.md

@@ -7,6 +7,11 @@ for technical interviews. These problems focus on real-world scenarios, includin
 
 Here is a list of the problems available in the repository. Problems are organized by their respective tags:
 
+### Concurrency
+
+* [First Successful Key Lookup](01-first-successful-key-lookup/) 
+
+
 ## Contributing
 
 We welcome contributions from anyone looking to add more problems, improve solutions, or provide better documentation. Please refer to