diff --git a/lib/CMTD.pm b/lib/CMTD.pm
index ba372a1..50c089a 100644
--- a/lib/CMTD.pm
+++ b/lib/CMTD.pm
@@ -24,6 +24,14 @@ sub startup {
     return $dbh;
   });
 
+  # initial
+  $self->app->maintenance;
+  # periodic
+  my $loop = Mojo::IOLoop->singleton;
+  $loop->recurring(60 => sub {
+    $self->app->maintenance;
+  });
+
   my $r = $self->routes;
 
   $r->get('/')
diff --git a/lib/CMTD/Helpers.pm b/lib/CMTD/Helpers.pm
index 3040d4e..67192c6 100644
--- a/lib/CMTD/Helpers.pm
+++ b/lib/CMTD/Helpers.pm
@@ -5,6 +5,7 @@ use warnings;
 use utf8;
 
 use Mojo::Base 'Mojolicious::Plugin';
+use Mojo::Util qw(b64_encode);
 
 sub register {
   my ($self, $app) = @_;
@@ -31,6 +32,32 @@ sub register {
 
     return {site => $site, pid => $md5, path => $path};
   });
+
+  $app->helper(maintenance => sub {
+    my ($c) = @_;
+    eval {
+      my $table = $c->app->config->{captcha}->{table};
+      my $tries = $c->app->config->{captcha}->{tries};
+      my $limit = $c->app->config->{captcha}->{limit};
+
+      $c->app->db->delete($table, {tries => {'>=' => $tries}});
+      my $count = $c->app->db->select($table, 'count(id)')->list;
+      if ($count < $limit) {
+        $c->app->db->begin;
+        foreach my $n ($count + 1 .. $limit) {
+          $c->app->log->info("generating captcha's: $n of $limit");
+          my ($code, $data) = $c->app->captcha->create;
+          next unless $code and $data;
+          my $b64 = 'data:image/jpeg;base64,' . b64_encode($data, '');
+          $c->app->db->insert($table, { code => $code, data => $b64 });
+        }
+        $c->app->db->commit;
+      } 1;
+    } or do {
+      chomp $@;
+      $c->app->log->error($@);
+    };
+  });
 }
 
 1;