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[feat] Multinode Guide So Far #331

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docs/abra/swarm.md
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| Feature | Explanation |
| ----------- | ----------- |
| Multi-node | It is possible but it doesn't seem like anyone in our community is really doing this? Please report in to `#coop-cloud-tech-future-brain:autonomic.zone` to discuss your usage if you are using multi-node swarm! We believe the majority of Co-op Cloud installs are single node. There is also a lack of [CSI](https://github.com/olljanat/csi-plugins-for-docker-swarm?tab=readme-ov-file) support for coordinating storage across multiple hosts when using Swarm mode. This means we kind of throw out [a bunch](https://docs.docker.com/engine/swarm/#feature-highlights) of the features of Swarm mode. |
| Multi-node | The vast majority of Co-op Cloud installs are single node. There is a lack of [CSI](https://github.com/olljanat/csi-plugins-for-docker-swarm?tab=readme-ov-file) support for coordinating storage across multiple hosts when using Swarm mode, which means we kind of throw out [a bunch](https://docs.docker.com/engine/swarm/#feature-highlights) of the features of Swarm mode. However, there is growing interest in multinode setups and a push to improve our understanding. [Here's what we know so far, and some hints on getting started.](/operators/multinode) |
## Limitations
docs/operators/handbook.md
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This may be possible to overcome if someone really needs it, we encourage people to investigate. We've found that often there are limitations in the actual software which don't support this anyway and several of the current operators simply use a new domain per app.
## Can I add worker nodes to my docker swarm?
At time of writing (Sep 2026), we officially only support single-node swarms. However, we are working on improving our tools and understanding to support multi-node in the future. If you'd like to get involved, [here is what we know so far](/operators/multinode).
## How do I bootstrap a server for running Co-op Cloud apps?
The requirements are:
docs/operators/multinode.md
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---
title: Multinode Best Practices
---
## Who This How-To Guide is For
Configuring coop cloud on multiple nodes poses some difficult problems, and we have not yet streamlined this process. For the sake of security and sanity, you should have a good understanding of the following before you attempt this:
- VPN configuration, understanding how authentication and encryption are handled.
- Firewalls, and `iptables` in particular.
- Managing and synchronizing files on a network (e.g. sftp, nfs, rsync)
What follows is our best knowledge to date. Tread carefully, for here be dragons.
## Before You Start
**1. *On every node*, make sure docker is installed and running.
```bash
docker run hello-world
```
**2. *On every node*, ensure kernel modules `ip_vs`[^ip_vs] and `br_netfilter`[^br_netfilter] are loaded.
```bash
lsmod | grep -E "^ip_vs |^br_netfilter "
```
??? Why?
- `ip_vs` enables multiple Linux kernels to act as *one* virtual server, coordinating request handling and processing with eachother over IP. [kernelconfig.io](https://www.kernelconfig.io/config_ip_vs)
- `br_netfilter` enables the kernelspace netfilter capability for bridge network interfaces. Without this kernel module, docker needs to route swarm packets via a slow and insecure userland proxy to communicate with the other docker networks. [Serverfault answer](https://serverfault.com/a/964491), [kernelconfig.io](https://www.kernelconfig.io/CONFIG_BRIDGE_NETFILTER?q=br_netfilter&kernelversion=7.2.4&arch=x86)
**3. Choose one node to be the manager node.
Ensure you can connect to this node with `ssh`.
!!! info "If your server isn't reachable"
If your manager node is not your main traefik proxy, `abra` may complain about the server not being reachable from the Internet. You can use the `-D` flag to ignore this warning.
??? warning "Do you need multiple manager nodes?"
It's easiest to start your setup by choosing only one node in your swarm to be the manager (the others will be worker nodes). If you need multiple manager nodes, e.g. to improve uptime, [read this first](https://docs.docker.com/engine/swarm/how-swarm-mode-works/nodes/#manager-nodes).
**4. establish a trusted network connection between each worker node and the manager.
This can be
- A standard "hub-and-spokes" VPN like strongswan, where worker nodes are connected to the manager node,
- Or a mesh VPN like tailscale, where all nodes are connected together.
??? info "Use a route-based VPN"
There are two types of VPN implementations: policy-based and route-based. They differ in that route-based VPNs create a distinct network interface for trusted traffic. We'll be using this interface in the next step.
**5. *On every node*, using `iptables`[^iptables], block ingress on ports `2377/tcp`, `4789/udp`, `7946/udp`, `7946/tcp` except when it comes from a trusted interface. [Docker docs](https://docs.docker.com/engine/swarm/swarm-tutorial/#open-protocols-and-ports-between-the-hosts).
E.g., if trusted packets arrive on `vpn0`:
```bash
iptables -A INPUT -p tcp --dport 2377 -i !vpn0 -j DROP
iptables -A INPUT -p udp --dport 4789 -i !vpn0 -j DROP
iptables -A INPUT -p udp --dport 7946 -i !vpn0 -j DROP
iptables -A INPUT -p tcp --dport 7946 -i !vpn0 -j DROP
```
??? warning "Use iptables, not nftables"
All hosts must use `iptables` as the firewall backend, not `nftables`. Nftables support in Docker is experimental, and docker swarm mode hasn't been migrated to support nftables yet. [docker docs](https://docs.docker.com/engine/network/firewall-nftables).
## Configuring the Swarm and Abra
**1. In `abra`, create a server using the manager node. (Refer to the [New Operators' Tutorial](https://docs.coopcloud.tech/operators/tutorial/) for a refresher on how to do this.)
When calling `docker swarm init`, include `--advertise-addr` and point to the trusted interface of your VPN.
**2. On the *manager node*, call
```bash
docker swarm join-token worker
```
**3. On a *worker node*, call the command provided, e.g.
```bash
docker swarm join --token MYTOKEN M.Y.I.P:2377
```
**4. On the *manager node*, verify the node is in the swarm:
```bash
docker node ls
```
## Volume Duplication
!!! warning "Docker swarm does not synchronize volume data between nodes."
- If services on the same recipe deploy on different nodes, they will not have access to any volumes shared between them.
- If a service dies or is stopped, and docker swarm deploys it to a different node, it will not have access to its existing volume data.
You must choose one of these strategies:
**1. Using placement constraints, restrict apps to always run on specific nodes.
**2. Choose a networked filesystem (e.g. nfs, rclone) and use its *docker volume plugin* to synchronize volume data between worker nodes.
Both strategies are described below.
### How to restrict apps to specific nodes
**1. In your *abra config*, create a new compose file and set appropriate deploy.placement.constraints for each service. For example:
`~/.abra/compose/compose.restrict-nextcloud.yml`
```yml
---
version: "3.8"
services:
web:
deploy:
placement:
constraints:
- node.labels.nextcloud_node == true
app:
deploy:
placement:
constraints:
- node.labels.nextcloud_node == true
cron:
deploy:
placement:
constraints:
- node.labels.nextcloud_node == true
cache:
deploy:
placement:
constraints:
- node.labels.redis_node == true
# nb: redis does not share volumes with other services,
# so it could be deployed on a separate node.
```
**2. On your *worker node*, assign the appropriate labels:
```bash
docker node update --label-add nextcloud_node=true --label-add redis_node=true <worker_hostname>
```
**3. On the *abra server*, Add your compose file to your app's config. E.g.:
```bash
abra app config my.app
```
```yml
...
COMPOSE_FILE="compose.yml"
COMPOSE_FILE="$COMPOSE_FILE:../../compose/compose.restrict-nextcloud.yml"
...
```
**4. Deploy the app.
**5. Confirm all the services were correctly assigned:
```bash
docker node ps <worker_hostname> | grep "Running"
```
??? info "You can make much more complex setups with placement constraints and node configurations"
- [Placement constraints (about)](https://docs.docker.com/engine/swarm/services/#control-service-placement)
- [Placement constraints (compose file reference)](https://docs.docker.com/reference/compose-file/deploy/#placement)
- [Node labels (how to)](https://docs.docker.com/engine/swarm/manage-nodes/#add-or-remove-label-metadata)
### How to Use docker volume plugins
!!! warning "Do not use this strategy to synchronize database volumes."
Instead:
- optionally configure a distributed database across your nodes (unknown what tools are best).
- be careful when using `db` services in your recipes. Ignore them in favor of a dedicated database instance, or restrict the services to a dedicated node.
- in your app configuration, use `DB_HOST` and similar environment variables to point to your network's database host.
!!! warning "Use the S3 protocol wherever a recipe allows it."
S3 is much more efficient at synchronizing large amounts of data. We recommend [garage](https://git.coopcloud.tech/coop-cloud/garage).
On the other hand, avoid using `s3fs` to store docker volumes - this can cause race conditions because S3 does not allow file locking.
**1. Choose a machine to be your dedicated volume data store.
**2. On your *data storage machine*, choose and install a network-accessible file store. Just about anything can be made to work (with tradeoffs): ssh, nfs, NextCloud, WebDav, ProtonDrive, etc.
**3. Ensure your file store can be securely accessed from every worker node.
**4. *On every node in your swarm*, install a dedicated docker volume plugin for your file store, or use a generic middleware like [Rclone](https://rclone.org/docker/).
**5. In your *abra config*, create a custom compose file and configure the plugin on all of your recipe's volumes. E.g.:
`~/.abra/compose/compose.nextcloud-rclone.yml`
```yml
---
version: "3.8"
volumes:
nextcloud:
driver: rclone
driver_opts:
...
nextapps:
driver: rclone
driver_opts:
...
nextdata: # note: you should use S3 for the majority of your data.
driver: rclone
driver_opts:
...
nextconfig:
driver: rclone
driver_opts:
...
# here we skip the redis volume, because its cache data
# doesn't need to be persisted between nodes
```
**6. Include the compose file in your instance's config. E.g.
```bash
abra app config my.app
```
```yml
...
COMPOSE_FILE="compose.yml"
COMPOSE_FILE="$COMPOSE_FILE:../../compose/compose.nextcloud-rclone.yml"
...
```
**7. Deploy the app
**8. Optionally, verify the volumes are no longer on disk:
```bash
docker volume ls
sudo ls /var/lib/docker/volumes/
```
**9. Optionally, destroy any volumes that were previously created:
```bash
docker volume prune -a
```
## Additional Notes
- Manager nodes should be placed on your most reliable machines, since without them workers cannot deploy services. Because of this, you may want to constrain critical apps to run only on managers: `node.role == manager`.
- If an app needs certain resources to be available (e.g. 4 GiB RAM), you can set a resource constraint, which ensures the app can only be scheduled on nodes with that resource available. [See docker docs](https://docs.docker.com/reference/compose-file/deploy/#resources)
- A Docker swarm cluster should have an odd number of manager nodes. 1 manager is sufficient, and 3 managers is the minimum for redundancy. [See docker docs](https://docs.docker.com/engine/swarm/how-swarm-mode-works/nodes/#manager-nodes)
- When using a swarm with worker nodes, some information will be invisible to abra. In particular, `abra app logs` and `abra app ps` will (at time of writing) give incomplete information. Here are some workarounds:
- Instead of `abra app logs example.com app`, call `docker service logs example_com_app` on a manager node.
- In addition to `abra app ps example.com`, try: `docker node ps <node_name> | grep example_com`, `docker service ps example_com_app`, `docker service ls -f name=example_com`, `docker stats`
- Abra can only interact with manager nodes; worker nodes lack permissions to run most of abra's commands. When creating an `abra server`, remember to connect to a manager node.
### Additional Resources
- [nix-config by papiris](https://codeberg.org/papiris/nix-config/src/branch/main/overlays/virtualisation/docker.nix) (multi-node Co-op Cloud, docker within systemd-nspawn)
- [SweHarris blog post](https://www.sweharris.org/post/2017-07-30-docker-placement/) about docker swarm placement
- [OneUpTime article](https://oneuptime.com/blog/post/2026-03-20-portainer-service-placement-constraints/view) by @nawazdhandala about docker swarm placement