diff --git a/manifests/app.pp b/manifests/app.pp
index f40ab10..05b0f85 100644
--- a/manifests/app.pp
+++ b/manifests/app.pp
@@ -7,6 +7,15 @@ class mediawiki::app ($revision = 'origin/REL1_28'){
     source   => 'https://gerrit.wikimedia.org/r/mediawiki/core.git',
     revision => $revision,
   }
+  exec { 'fetch_external_libraries' :
+    # https://www.mediawiki.org/wiki/Download_from_Git#Fetch_external_libraries
+    # note that the files this creates are tracked in .gitignore
+    command     => '/usr/bin/composer update --no-dev',
+    cwd         => '/srv/mediawiki/w',
+    refreshonly => true,
+    require     => Class['mediawiki::php'],
+    subscribe   => Vcsrepo['/srv/mediawiki/w'],
+  }
 }
 
 # vim:sw=2:ts=2:expandtab:textwidth=79
diff --git a/manifests/php.pp b/manifests/php.pp
index 8bbc6d8..4d790d4 100644
--- a/manifests/php.pp
+++ b/manifests/php.pp
@@ -2,6 +2,7 @@
 #
 class mediawiki::php {
   package { [
+      'composer', # used by fetch_external_libraries in mediawiki::app
       'php',
       'php-apcu',
       'php-cli',