了解docker swarm的高可用性

Question

I am currently trying to understand what would be necessary to create a docker swarm to make some service highly available. I read through a lot of the docker swarm documentation, but if my understanding is correct, docker swarm will just execute a service on any host. What would happen if a host fails? Would the swarm manager restart the service(s) running on that host/node on another one? Is there any better explanation of this than in the original documentation found here ?

Answer 1

Nothing more complex than that really. Like it says, Swarm (and kubernetes, and most other tooling in this space) is declarative , which means that you tell it the state that you want (ie 'I want 4 instances of redis') and Swarm will converge the system to that state. If you have 3 nodes, then it will schedule 1 redis on Node 1, 1 on Node 2, and 2 on Node 3. If Node 2 dies, then the system is now not 'compliant' with your declared state, and Swarm will schedule another redis on Node 1 or 3 (depending on strategy, etc...).

Now this dynamism of container / task / instance scheduling brings another problem, discovery. Swarm deals with this by maintaining an internal DNS registry and by creating VIP (virtual IPs) for each service. Instead of having to address / keep track of each redis instance, I can instead point to a service alias and Swarm will automatically route traffic to where it needs to go.

Of course there are also other considerations:

• Can your service support multiple backend instances? Is it stateless? Sessions? Cache? Etc...
• What is 'HA'? Multi-node? Multi-AZ? Multi-region? Etc...