diff --git a/raft/diff_test.go b/raft/diff_test.go
new file mode 100644
index 000000000..d8173cd0a
--- /dev/null
+++ b/raft/diff_test.go
@@ -0,0 +1,51 @@
+package raft
+
+import (
+	"fmt"
+	"io"
+	"io/ioutil"
+	"os"
+	"os/exec"
+	"strings"
+)
+
+func diffu(a, b string) string {
+	if a == b {
+		return ""
+	}
+	aname, bname := mustTemp("base", a), mustTemp("other", b)
+	defer os.Remove(aname)
+	defer os.Remove(bname)
+	cmd := exec.Command("diff", "-u", aname, bname)
+	buf, err := cmd.CombinedOutput()
+	if err != nil {
+		if _, ok := err.(*exec.ExitError); ok {
+			// do nothing
+			return string(buf)
+		}
+		panic(err)
+	}
+	return string(buf)
+}
+
+func mustTemp(pre, body string) string {
+	f, err := ioutil.TempFile("", pre)
+	if err != nil {
+		panic(err)
+	}
+	_, err = io.Copy(f, strings.NewReader(body))
+	if err != nil {
+		panic(err)
+	}
+	f.Close()
+	return f.Name()
+}
+
+func ltoa(l *log) string {
+	s := fmt.Sprintf("committed: %d\n", l.committed)
+	s += fmt.Sprintf("applied:  %d\n", l.applied)
+	for i, e := range l.ents {
+		s += fmt.Sprintf("#%d: %+v\n", i, e)
+	}
+	return s
+}