mash-playbook/docs/running-multiple-instances.md

Running multiple instances of the same service on the same host

The way this playbook is structured, each Ansible role can only be invoked once and made to install one instance of the service it's responsible for.

If you need multiple instances (of whichever service), you'll need some workarounds as described below.

The example below focuses on hosting multiple Redis instances, but you can apply it to hosting multiple instances or whole stacks of any kind.

Let's say you're managing a host called mash.example.com which installs both PeerTube and NetBox. Both of these services require a Redis instance. If you simply add redis_enabled: true to your mash.example.com host's vars.yml file, you'd get a Redis instance (mash-redis), but it's just one instance. As described in our Redis documentation, this is a security problem and potentially fragile as both services may try to read/write the same data and get in conflict with one another.

We propose that you don't add redis_enabled: true to your main mash.example.com file, but do the following:

Re-do your inventory to add supplementary hosts

Create multiple hosts in your inventory (inventory/hosts) which target the same server, like this:

[mash_servers]
[mash_servers:children]
mash_example_com

[mash_example_com]
mash.example.com-netbox-deps ansible_host=1.2.3.4
mash.example.com-peertube-deps ansible_host=1.2.3.4
mash.example.com ansible_host=1.2.3.4

This creates a new group (called mash_example_com) which groups all 3 hosts:

• (new) mash.example.com-netbox-deps - a new host, for your NetBox dependencies
• (new) mash.example.com-peertube-deps - a new host, for your PeerTube dependencies
• (old) mash.example.com - your regular inventory host

When running Ansible commands later on, you can use the -l flag to limit which host to run them against. Here are a few examples:

• just install-all - runs the installation process on all hosts (3 hosts in this case)
• just install-all -l mash_example_com - runs the installation process on all hosts in the mash_example_com group (same 3 hosts as just install-all in this case)
• just install-all -l mash.example.com-netbox-deps - runs the installation process on the mash.example.com-netbox-deps host

Adjust the configuration of the supplementary hosts to use a new "namespace"

Multiple hosts targetting the same server as described above still causes conflicts, because services will use the same paths (e.g. /mash/redis) and service/container names (mash-redis) everywhere.

To avoid conflicts, adjust the vars.yml file for the new hosts (mash.example.com-netbox-deps and mash.example.com-peertube-deps) and set non-default and unique values in the mash_playbook_service_identifier_prefix and mash_playbook_service_base_directory_name_prefix variables. Examples below:

inventory/host_vars/mash.example.com-netbox-deps/vars.yml:

---

########################################################################
#                                                                      #
# Playbook                                                             #
#                                                                      #
########################################################################

# Put a strong secret below, generated with `pwgen -s 64 1` or in another way
# Various other secrets will be derived from this secret automatically.
mash_playbook_generic_secret_key: ''

# Override service names and directory path prefixes
mash_playbook_service_identifier_prefix: 'mash-netbox-'
mash_playbook_service_base_directory_name_prefix: 'netbox-'

########################################################################
#                                                                      #
# /Playbook                                                            #
#                                                                      #
########################################################################


########################################################################
#                                                                      #
# redis                                                                #
#                                                                      #
########################################################################

redis_enabled: true

########################################################################
#                                                                      #
# /redis                                                               #
#                                                                      #
########################################################################

inventory/host_vars/mash.example.com-peertube-deps/vars.yml:

---

########################################################################
#                                                                      #
# Playbook                                                             #
#                                                                      #
########################################################################

# Put a strong secret below, generated with `pwgen -s 64 1` or in another way
# Various other secrets will be derived from this secret automatically.
mash_playbook_generic_secret_key: ''

# Override service names and directory path prefixes
mash_playbook_service_identifier_prefix: 'mash-peertube-'
mash_playbook_service_base_directory_name_prefix: 'peertube-'

########################################################################
#                                                                      #
# /Playbook                                                            #
#                                                                      #
########################################################################


########################################################################
#                                                                      #
# redis                                                                #
#                                                                      #
########################################################################

redis_enabled: true

########################################################################
#                                                                      #
# /redis                                                               #
#                                                                      #
########################################################################

The above configuration will create 2 Redis instances:

• mash-netbox-redis with its base data path in /mash/netbox-redis
• mash-peertube-redis with its base data path in /mash/peertube-redis

These instances reuse the mash user and group and the /mash data path, but are not in conflict with each other.

Adjust the configuration of the base host

Now that we've created separate Redis instances for both PeerTube and NetBox, we need to put them to use by editing the vars.yml file of the main host (the one that installs PeerTbue and NetBox) to wire them to their Redis instances.

You'll need configuration (inventory/host_vars/mash.example.com/vars.yml) like this:

########################################################################
#                                                                      #
# netbox                                                               #
#                                                                      #
########################################################################

netbox_enabled: true

# Other NetBox configuration here

# Point NetBox to its dedicated Redis instance
netbox_environment_variable_redis_host: mash-netbox-redis
netbox_environment_variable_redis_cache_host: mash-netbox-redis

# Make sure the NetBox service (mash-netbox.service) starts after its dedicated Redis service (mash-netbox-redis.service)
netbox_systemd_required_services_list_custom:
  - mash-netbox-redis.service

# Make sure the NetBox container is connected to the container network of its dedicated Redis service (mash-netbox-redis)
netbox_container_additional_networks_custom:
  - mash-netbox-redis

########################################################################
#                                                                      #
# /netbox                                                              #
#                                                                      #
########################################################################



########################################################################
#                                                                      #
# peertube                                                             #
#                                                                      #
########################################################################

# Other PeerTube configuration here

# Point PeerTube to its dedicated Redis instance
peertube_config_redis_hostname: mash-peertube-redis

# Make sure the PeerTube service (mash-peertube.service) starts after its dedicated Redis service (mash-peertube-redis.service)
peertube_systemd_required_services_list_custom:
  - "mash-peertube-redis.service"

# Make sure the PeerTube container is connected to the container network of its dedicated Redis service (mash-peertube-redis)
peertube_container_additional_networks_custom:
  - "mash-peertube-redis"

########################################################################
#                                                                      #
# /peertube                                                            #
#                                                                      #
########################################################################

Questions & Answers

Can't I just use the same Redis instance for multiple services?

You may or you may not. See the Redis documentation for why you shouldn't do this.

Can't I just create one host and a separate stack for each service (e.g. Nextcloud + all dependencies on one inventory host; PeerTube + all dependencies on another inventory host; with both inventory hosts targetting the same server)?

That's a possibility which is somewhat clean. The downside is that each "full stack" comes with its own Postgres database which needs to be maintained and upgraded separately.