diff --git a/apps/app-runner/log.go b/apps/app-runner/log.go
new file mode 100644
index 0000000..2e5f524
--- /dev/null
+++ b/apps/app-runner/log.go
@@ -0,0 +1,24 @@
+package main
+
+import (
+	"strings"
+	"sync"
+)
+
+type Log struct {
+	l sync.Mutex
+	d strings.Builder
+}
+
+func (l *Log) Write(p []byte) (n int, err error) {
+	l.l.Lock()
+	defer l.l.Unlock()
+	// TODO(gio): Reset s.logs periodically
+	return l.d.Write(p)
+}
+
+func (l *Log) Contents() string {
+	l.l.Lock()
+	defer l.l.Unlock()
+	return l.d.String()
+}