geek-cookbook/docs/docker-swarm/design.md

---
title: Design a secure, scalable Docker Swarm
description: Presenting a Docker Swarm design to create your own container-hosting platform, which is highly-available, scalable, portable, secure and automated! 💪
---

# Highly Available Docker Swarm Design

In the design described below, our "private cloud" platform is:

* **Highly-available** (_can tolerate the failure of a single component_)
* **Scalable** (_can add resource or capacity as required_)
* **Portable** (_run it on your garage server today, run it in AWS tomorrow_)
* **Secure** (_access protected with [LetsEncrypt certificates](/docker-swarm/traefik/) and optional [OIDC with 2FA](/docker-swarm/traefik-forward-auth/)_)
* **Automated** (_requires minimal care and feeding_)

## Design Decisions

### Where possible, services will be highly available.**

This means that:

* At least 3 docker swarm manager nodes are required, to provide fault-tolerance of a single failure.
* [Ceph](/docker-swarm/shared-storage-ceph/) is employed for share storage, because it too can be made tolerant of a single failure.

!!! note
    An exception to the 3-nodes decision is running a single-node configuration. If you only **have** one node, then obviously your swarm is only as resilient as that node. It's still a perfectly valid swarm configuration, ideal for starting your self-hosting journey. In fact, under the single-node configuration, you don't need ceph either, and you can simply use the local volume on your host for storage. You'll be able to migrate to ceph/more nodes if/when you expand.

**Where multiple solutions to a requirement exist, preference will be given to the most portable solution.**

This means that:

* Services are defined using docker-compose v3 YAML syntax
* Services are portable, meaning a particular stack could be shut down and moved to a new provider with minimal effort.

## Security

Under this design, the only inbound connections we're permitting to our docker swarm in a **minimal** configuration (*you may add custom services later, like UniFi Controller*) are:

### Network Flows

* **HTTP (TCP 80)** : Redirects to https
* **HTTPS (TCP 443)** : Serves individual docker containers via SSL-encrypted reverse proxy

### Authentication

* Where the hosted application provides a trusted level of authentication (*i.e., [NextCloud](/recipes/nextcloud/)*), or where the application requires public exposure (*i.e. [Privatebin](/recipes/privatebin/)*), no additional layer of authentication will be required.
* Where the hosted application provides inadequate (*i.e. [NZBGet](/recipes/autopirate/nzbget/)*) or no authentication (*i.e. [Gollum](/recipes/gollum/)*), a further authentication against an OAuth provider will be required.

## High availability

### Normal function

Assuming a 3-node configuration, under normal circumstances the following is illustrated:

* All 3 nodes provide shared storage via Ceph, which is provided by a docker container on each node.
* All 3 nodes participate in the Docker Swarm as managers.
* The various containers belonging to the application "stacks" deployed within Docker Swarm are automatically distributed amongst the swarm nodes.
* Persistent storage for the containers is provide via cephfs mount.
* The **traefik** service (*in swarm mode*) receives incoming requests (*on HTTP and HTTPS*), and forwards them to individual containers. Traefik knows the containers names because it's able to read the docker socket.
* All 3 nodes run keepalived, at varying priorities. Since traefik is running as a swarm service and listening on TCP 80/443, requests made to the keepalived VIP and arriving at **any** of the swarm nodes will be forwarded to the traefik container (*no matter which node it's on*), and then onto the target backend.

![HA function](../images/docker-swarm-ha-function.png){ loading=lazy }

### Node failure

In the case of a failure (or scheduled maintenance) of one of the nodes, the following is illustrated:

* The failed node no longer participates in Ceph, but the remaining nodes provide enough fault-tolerance for the cluster to operate.
* The remaining two nodes in Docker Swarm achieve a quorum and agree that the failed node is to be removed.
* The (*possibly new*) leader manager node reschedules the containers known to be running on the failed node, onto other nodes.
* The **traefik** service is either restarted or unaffected, and as the backend containers stop/start and change IP, traefik is aware and updates accordingly.
* The keepalived VIP continues to function on the remaining nodes, and docker swarm continues to forward any traffic received on TCP 80/443 to the appropriate node.

![HA function](../images/docker-swarm-node-failure.png){ loading=lazy }

### Node restore

When the failed (*or upgraded*) host is restored to service, the following is illustrated:

* Ceph regains full redundancy
* Docker Swarm managers become aware of the recovered node, and will use it for scheduling **new** containers
* Existing containers which were migrated off the node are not migrated backend
* Keepalived VIP regains full redundancy

![HA function](../images/docker-swarm-node-restore.png){ loading=lazy }

### Total cluster failure

A day after writing this, my environment suffered a fault whereby all 3 VMs were unexpectedly and simultaneously powered off.

Upon restore, docker failed to start on one of the VMs due to local disk space issue[^1]. However, the other two VMs started, established the swarm, mounted their shared storage, and started up all the containers (services) which were managed by the swarm.

In summary, although I suffered an **unplanned power outage to all of my infrastructure**, followed by a **failure of a third of my hosts**... ==all my platforms are 100% available[^1] with **absolutely no manual intervention**==.

[^1]: Since there's no impact to availability, I can fix (or just reinstall) the failed node whenever convenient.

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