first working version

defaults/main.yml

-mongodb: {}
+mongodb:
+  version: "4.2"

handlers/main.yml

@@ -2,3 +2,6 @@
   systemd:
     state: restarted
     name: mongod.service
+
+- name: start mongodb replication
+  shell: sleep 3; mongo --eval "printjson(rs.initiate())"

tasks/main.yml

@@ -3,9 +3,18 @@
     pkg:
       - mongodb-org
 
+- name: enable replication sets
+  notify:
+  - restart mongodb
+  - start mongodb replication
+  lineinfile:
+    path: /etc/mongod.conf
+    regexp: "^#replication:"
+    line: "replication:\n  replSetName: rs01"
+
 - name: start and enable mongodb
   systemd:
     state: started
-    name: mongodb
+    name: mongod
     daemon_reload: yes
     enabled: yes

vars/main.yml

 packages:
   repos:
     mongodb:
-      url: "deb http://repo.mongodb.org/apt/debian {{ ansible_distribution|lower }}/mongodb-org/4.0 main"
-      keyid: "9DA31620334BD75D9DCB49F368818C72E52529D4"
-      keyserver: keyserver.ubuntu.com
+      url: "deb http://repo.mongodb.org/apt/debian {{ ansible_distribution_release|lower }}/mongodb-org/{{ mongodb.version }} main"
+      keyurl: "https://www.mongodb.org/static/pgp/server-{{ mongodb.version }}.asc"