I read swarm routing mesh
I create a simple service which uses tomcat server and listens at 8080.
docker swarm init I created a node manager at node1.
docker swarm join /tokens I used the token provided by the manager at node 2 and node 3 to create workers.
docker node ls shows 5 instances of my service, 3 running at node 1, 1 running at node 2, another one is at node 3.
docker service create image I created the service.
docker service scale imageid=5 scaled it.
My application uses atomic number which is maintained at JVM level.
If I hit http://node1:8080/service 25 times, all requests goes to node1. How dose it balance node?
If I hit http://node2:8080/service, it goes to node 2.
Why is it not using round-robin?
Doubts:
Is anything wrong in the above steps?
Did I miss something?
I feel I am missing something. Like common service name http://domain:8080/service, then swarm will work in round robin fashion.
I would like to understand only swarm mode. I am not interested external load balancer as of now.
How do I see swarm load balance in action?
Docker does round robin load balancing per connection to the port. As long as a connection is up, it will continue to go to the same instance.
Http allows a connection to be kept alive and reused. Browsers take advantage of this behavior to speed up later requests by leaving connections open. To test the round robin load balancing, you'd need to either disable that keep alive setting or switch to a command line tool like curl or wget.
Related
As we know that in docker swarm we create multiple worker and one manager.
The conatiner is running in mutliple worker. So we can access that in the browser by putting that worker node ip and then port like (ip:80). We can access another worker node by putting their ip and port. But What if I want that I run put one commone IP and run the container. So if anyh one of the nodes goes down then It my site does not goes down. it use another runnig worker.
worker1: 192.168.99.100:80
wokere2: 192.168.99.100:80
worker3: 192.168.99.100:80
I want one common IP so that if any one goes down the it should not goes down.
You basically have primarily two ways of doing this:
You can put an HTTP proxy in front of the docker swarm, and then this proxy has health check routes on the node if any goes down the proxy removes the IP of the downed node from rotation until it comes back up (traefik, Nginx, caddy, ...).
You can use keepalived, and with this approach, you point the domain to your virtual VRRP IP which then is "floating" around.
I know a very good ops person and they use keepalived in their company without any issues and complications. In our company, we decided to go with the proxy because we also route other "traffic" over them to different systems (legacy, ...) and we have VMWare's top licence with veeam and we can solve real-time duplications (in case of VM going down and such) with that.
So both methods are proven and tested :)
This question illustrates the theoretical differences between docker run and docker service.
What I don't understand is when would one need to use the exact same container replicated multiple times (as per the Docker documentation example)?
There, they run the same web app replicated 5 times.
Is deployment on Kubernetes (for example) a potential use case, where the developer does not want to centralize the app on one host, in order to make it more resilient, hence why 5 replicas are created?
To understand, can someone please please with an example use case, where the docker service is useful?
swarm is an orchestrator just like kubernetes. docker service deploys services to swarm just as you deploy your services to kubernetes using kubectl.
swarm is essentially built-in primitive orchestrator. One possible case for replicas is running a proxy that directs requests to proper containers. You could expose multiple machines and have one take place of another in case another fails. Or any other high availability case you could think of.
Your question could be rephrased as "What's the difference between running a single container and running containers in a cluster?", which would be another question altogether, but that rephrasing might help illustrate what docker service does.
If you want to scale your application, you can run multiple instances of it (horizontal scaling) or you beef up the machine(s) that it runs on (vertical scaling). For the first, you would have to put a load balancer in front of your application so that the traffic is evenly distributed between the different instances. The idea is that those instances run on different hosts, so if one goes down, your application is still up. Some controlling instance (a Kubernetes service, for example) will notice that one of your instances has gone south and won't direct any more traffic to it. Nowadays, with all the cloud stuff going on, this is typically the way to go.
You don't need Kubernetes for such a setup, but you're right, this would be a typical use case for it. At least if you run your application in a Docker container.
Once use case is running on Docker swarm which consists of n number of nodes in your swarm cluster. You can run replicas of your application on the swarm cluster with a load balancer/reverse proxy to load balance your setup. If any one of the nodes goes down the application can still run.
But the exact use case for running multiple instances is scalabilty. Suppose you know that one instance of your app can serve 10000 users (Assume Bank authentication) at a time.
If you want your application to serve 50K users just run 5 replicas(using docker service create) .
I'm hoping that there are some docker swarm experts out there who have configured a load balancer to front a docker swarm multi-node setup. In such a simplified architecture, if the load balancer needs to detect if a manager node is down and stop routing traffic to it, what is the "best practice" for that? Does Docker swarm provide a health endpoint (api) that can be tested for each manager node? I'm new to some of this and there doesn't seem to be a lot out there that describes what I'm looking for. Thanks in advance
There is the metrics endpoint of the engine, and then the engine api, but I don't think that's what you want by an application load balancer.
What I see most people do is put a load balancer in front of the Swarm nodes they want to handle incoming traffic for specific apps running in services, and since that LB needs to know if the containers are responding (not just the node's engine health) they should hit the apps health endpoint, and take nodes in and out of that apps LB based on the app response.
This is how AWS ELB's work out of the box, for example.
If you had a published service on port 80 in the Swarm, you would setup your ELB to point to the nodes you want to handle incoming traffic, and have them expect a healthy 200/300 return on those nodes. It'll remove nodes from the pool if they return something else or don't respond.
Then you could use a full monitoring solution that checks node health and optionally respond to issues like replacing nodes.
I've working with Docker containers. What Ive done is lunching 5 containers running the same application, I use HAProxy to redirect requests to them, I added a volume to preserve data and set restart policy as Always.
It works. (So far this is my load balancing aproach)But sometimes I need another container to join the pool as there might be more requests, or maybe at first I don't need 5 containers.
This is provided by the Swarm Mode addition in Docker 1.12. It includes orchestration that lets you not only scale your service up or down, but recover from an outage by automatically rescheduling the jobs to run on other nodes.
If you don't want to use Docker 1.12 (yet!), you can also use a Service Discovery like Consul, register your containers inside and use a tool like Consul Template to regenerate your load balancer configuration accordingly.
I made a talk 6 months ago about it. You can find the code and the configuration I used during my demo here: https://github.com/bargenson/dockerdemo
I'm building an Apache mesos cluster with 3 masters and 3 slaves. I installed docker on the slave nodes and it's able to create instances which are vissible in Marathon. Now i tried to install the HAproxy server on top of it but that didn't worked out that well so I deleted it.
The problem is, since then i'm only able to scale my application to a maximum of 3 instances, the exact number of nodes When I want to scale to 5, there are 2 instances that are stuck at the 'deploying' stage.
Does anyone know how to fix this issue so i'm back able to create more instances?
Thank you
To perform that, you trully need to setup Marathon ServiceDiscovery with HAProxy as unknown ports on the same slave machine will be binded to your containers.
First, install HAProxy on every slave. If you need SSL, you will need to make build HAProxy to support SSL.
Then, when HAProxy service is running, you need to follow this very well explain tutorial to enable Marathon service discovery on every Slave
HAProxy marathon Service discovery
Pay well attention to the tutorial, it is very well explained and straightforward.