Ansible: Start role if service present - service

Question

I want to run a role, but before i want to check if the program is already installed.
I guessed I should use the module "service" ? In Ubuntu the service is pointed with [ ? ] arkeia
I tried:
tasks:
- name: check arkeia
service: name=arkeia state=started
register: result
roles:
- { role: arkeia, when: result==true }
Otherwise if it is not possible to check if a program is already installed, i would check if the program folder is present and next start the role.
EDIT:
Thanks for your fast answer!
I think my flaw is the "when:" command.
But how can I check if the result is false?
I am getting following error:
> PLAY [all] ********************************************************************
GATHERING FACTS ***************************************************************
ok: [managed-centos]
ok: [managed-ubuntu]
ok: [ansible]
TASK: [arkeia | check arkeia] *************************************************
ok: [ansible]
ok: [managed-ubuntu]
ok: [managed-centos]
TASK: [arkeia | create directory] *********************************************
skipping: [managed-centos]
skipping: [managed-ubuntu]
changed: [ansible]
TASK: [arkeia | download sources debian] **************************************
skipping: [managed-centos]
skipping: [managed-ubuntu]
changed: [ansible]
TASK: [arkeia | download sources cent_os] *************************************
skipping: [ansible]
skipping: [managed-ubuntu]
fatal: [managed-centos] => error while evaluating conditional: ansible_os_family == "RedHat" and p.stat.exist == false
TASK: [arkeia | install Arkeia debian] ****************************************
fatal: [ansible] => error while evaluating conditional: ansible_os_family == "Debian" and p.stat.exist == false
fatal: [managed-ubuntu] => error while evaluating conditional: ansible_os_family == "Debian" and p.stat.exist == false
FATAL: all hosts have already failed -- aborting
PLAY RECAP ********************************************************************
to retry, use: --limit #/home/vagrant/arkeia.retry
ansible : ok=4 changed=2 unreachable=1 failed=0
managed-centos : ok=2 changed=0 unreachable=1 failed=0
managed-ubuntu : ok=2 changed=0 unreachable=1 failed=0
This is my Playbook:
- name: check arkeia
stat: path=/etc/init.d/arkeia
register: p
- name: create directory
file: state=directory path={{ install_directory }}
when: p.stat.exists == False
- name: download sources debian
get_url: url={{ arkeia_deb_url }} dest={{ install_directory }}/{{ deb_filename }}
when: ansible_os_family == "Debian" and p.stat.exists == false
- name: download sources cent_os
get_url: url={{ arkeia_centos_url }} dest={{ install_directory }}/{{ centos_filename }}
when: ansible_os_family == "RedHat" and p.stat.exist == false
- name: install Arkeia debian
sudo: yes
apt: deb={{ install_directory }}/{{ deb_filename }}
when: ansible_os_family == "Debian" and p.stat.exist == false
- name: install Arkeia centos
sudo: yes
yum: name={{ install_directory }}/{{ centos_filename }}
when: ansible_os_family == "RedHat" and p.stat.exist == false
Debug result:
TASK: [arkeia | debug msg="{{ p }}"] ******************************************
ok: [ansible] => {
"msg": "{'invocation': {'module_name': u'stat', 'module_args': u'path=/etc/init.d/arkeia'}, 'stat': {'exists': False}, 'changed': False}"
}
ok: [managed-ubuntu] => {
"msg": "{'invocation': {'module_name': u'stat', 'module_args': u'path=/etc/init.d/arkeia'}, 'stat': {'exists': False}, 'changed': False}"
}
ok: [managed-centos] => {
"msg": "{'invocation': {'module_name': u'stat', 'module_args': u'path=/etc/init.d/arkeia'}, 'stat': {'uid': 0, 'exists': True, 'woth': False, 'mtime': 1421377097.0, 'inode': 67318822, 'isgid': False, 'size': 1224, 'wgrp': False, 'isuid': False, 'isreg': True, 'pw_name': 'root', 'gid': 0, 'ischr': False, 'wusr': True, 'xoth': True, 'rusr': True, 'nlink': 1, 'issock': False, 'rgrp': True, 'gr_name': 'root', 'path': '/etc/init.d/arkeia', 'xusr': True, 'atime': 1427813706.6750975, 'md5': '52de32627765e8f6e9ff8c5eee7ba232', 'isdir': False, 'ctime': 1427453340.1586156, 'isblk': False, 'xgrp': True, 'dev': 64769, 'roth': True, 'isfifo': False, 'mode': '0755', 'checksum': 'ddd459110d743402d9e18cfe4b3e032faaf54ba1', 'islnk': False}, 'changed': False}"
}

Answer 1

service module doesn't allow you to get current status. If the service doesn't exist module execution would fail.
You can either run shell: service arkeia status or shell: ps -eaf | grep -w arkeia or check for file's existance with stat
Other thing you are doing wrong is using the registered variable incorrectly. It's value is a dict (or map) with keys like rc, stdout, stdout_lines, stderr, .... So e.g. to use stat module you would do this:
- stat: path=/etc/init.d/arkeia
register: p
- debug: msg="/etc/init.d/arkeia exists and is a file"
when: p.stat.isreg is defined and p.stat.isreg
If you don't know the contents of result then debug and print it so you know what you're accessing:
- name: check arkeia
service: name=arkeia state=started
register: result
- debug: msg="result={{ result }}"

Answer 2

The solution ... so simply :)
- name: create directory
file: state=directory path={{ install_directory }}
when: p.stat.exists == False
- name: download sources debian
get_url: url={{ arkeia_deb_url }} dest={{ install_directory }}/{{ deb_filename }}
when: ansible_os_family == "Debian" and p.stat.**exists** == **f**alse
- name: download sources cent_os
get_url: url={{ arkeia_centos_url }} dest={{ install_directory }}/{{ centos_filename }}
when: ansible_os_family == "RedHat" and p.stat.**exist** == **f**alse
Write exists and false with a upper "F" and it works...