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Updated details on VM builds

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David Young
2017-07-16 22:59:04 +12:00
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docs/ha-docker-swarm/design.md
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The "private cloud" platform is:
# Introduction
* **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)
* **Automated** (requires minimal care and feeding)
In the design described below, the "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_)
* **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.
* GlusterFS is employed for share filesystem, because it too can be made tolerant of a single failure.
**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.
## High availability
### Normal function
Assuming 3 nodes, under normal circumstances the following is illustrated:
* All 3 nodes provide shared storage via GlusterFS, which is provided by a docker container on each node. (i.e., not running in swarm mode)
* 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 GlusterFS 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 access the docker socket.
* All 3 nodes run keepalived, at different 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)
### 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 GlusterFS, 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)
### Node restore
When the failed (or upgraded) host is restored to service, the following is illustrated:
* GlusterFS 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)