Containers and orchestration frameworks like Rancher will soon allow every organization to have access to efficient cluster management. This brave new world frees operations from managing application configuration and allows development to focus on writing code; containers abstract complex dependency requirements, which enables ops to deploy immutable containerized applications and allows devs a consistent runtime for their code. If the benefits are so clear, then why do companies with existing infrastructure practices not switch? One of the key issues is risk. The risk of new unknowns brought by an untested technology, the risk of inexperience operating a new stack, and the risk of downtime impacting the brand. Planning for risks and demonstrating that the ops team can maintain a resilient workload whilst moving into a containerized world is the key social aspect of a container migration project. Especially since, when done correctly, Docker and Rancher provide a solid framework for quickly iterating on infrastructure improvements, such as [Rancher
quickly spinning up popular distributed applications like ElasticSearch. In regard to risk management, we will look into identifying the five keys to running a resilient workload on Rancher and Docker. The topics that will be covered are as follows:
- Running Rancher in HA Mode (covered in this post)
- Using Service Load Balancers in Rancher
- Setting up Rancher service health checks and monitoring
- Providing developers with their own Rancher setup
- Discussing Convoy for data resiliency
I had originally hoped to perform experiments on a Rancher cluster built on a laptop using Docker Machine with a Rancher Server and various Rancher Agents on Raspberry Pis. Setup instructions here. The problem is that most Docker images are made for Intel based CPUs, so nothing works properly on Pi’s ARM processors. Instead I will directly use AWS for our experiments with resilient Rancher clusters. With our initial setup, we have 1 Rancher Server and 1 Agent. Let’s deploy a simple multiple container application. The above diagram illustrates the setup I am going to use to experiment with Rancher. I chose AWS because I am familiar with the service, but you can choose any other provider for setting up Rancher according to the Quick Start Guide. Let’s test our stack with the WordPress compose described in the Rancher Quick Start instructions. So now our application is up and running, the one scenario is what happens if the Rancher Server malfunctions? Or a network issue occurs? What happens to our application? Will it still continue serving requests? For this experiment, I will perform the following and document the results.
- Cutting the Internet from Rancher Agent to Rancher Server
- Stopping the Rancher Server Container
- Peeking under the hood of the Rancher Server Container
Afterwards we will address each of these issues, and then look at Rancher HA as a means of addressing these risks.
Cutting the Internet from Rancher Agent to Rancher Server
So let’s go onto AWS and block all access to the Rancher Server from my Rancher Agents.
- Block access from Rancher Server to Rancher Agent
- Note down what happens
- Kill a few WordPress containers
- Re-instantiate the connection
Firstly, after a few seconds our Rancher hosts end up in a reconnecting state. Browsing to my WordPress URL I can still access all my sites properly. There is no service outage as the containers are still running on the remote hosts. The IPSec tunnel between my two agents is still established, thus allowing my lone WordPress container to still connect to the DB. Now let’s kill a WordPress container and see what happens. Since I can’t access my Rancher Agents from the UI, I will be SSHing into the agent hosts to run Docker commands. (Instructions for SSHing into Rancher-created hosts can be found here) The WordPress container does not get restarted. This is troublesome, we will need our Rancher Server back online. Let’s re-establish the network connection and see if the Rancher Server notices that one of our WordPress services is down. After a few moments, our Rancher Server re-establishes connection with the agents and restarts the WordPress container. Excellent. So the takeaway here is that Rancher Server can handle intermittent connection issues and reconnect to the agents and continue on as usual. Although, for reliable uptime of our containers we would need multiple instances of Rancher Server on different hosts for resiliency against networking issues in the data center. Now, what would happen if the Rancher Server dies? Would we lose all of our ability to manage our hosts after it comes back? Let’s find out!
Killing the Rancher Server
In this second experiment I will go into the Rancher Server host and manually terminate the process. Generally a failure will result in the Docker process restarting due to --restart=always being set. Though let’s assume that either your host ran out of disk space or otherwise borked itself.
Let’s simulate catastrophic failure, and nuke our Rancher container.
sudo docker stop rancher-server As with the network experiment our
WordPress applications still run on the agents and serve traffic
normally. The Rancher UI and any semblance of control is now gone. We
don’t like this world, so we will start the rancher-server back up.
sudo docker start rancher-server After starting up again, the Rancher
server picks up where it left off. Wow, that is cool, how does this
Peeking under the hood of the Rancher Server Container
So how does the Rancher Server operate? Let’s take a brief tour into the inner working of the Rancher server container to get a sense of what makes it tick. Taking a look at the Rancher Server Docker build file found here.
# Dockerfile contents FROM ... ... ... CMD ["/usr/bin/s6-svscan", "/service"]
What is s6-svscan? It is a supervisor process that keeps a process running based on commands found in files in a folder; these key files are named as Run, Down, and Finish. If we look inside the service directory we can see that the container will install dependencies and use s6-svscan to start up 2 services. The Cattle service, which is the core Rancher scheduler, and a MySQL instance. Inside our container the following services are being run.
PID TTY STAT TIME COMMAND 1 ? Ss 0:00 /usr/bin/s6-svscan /service 7 ? S 0:00 s6-supervise cattle 8 ? S 0:00 s6-supervise mysql 9 ? Ssl 0:57 java -Xms128m -Xmx1g -XX:+HeapDumpOnOutOfMemoryError -XX:HeapDumpPath=/var/lib/cattle/logs -Dlogback.bootstrap.level=WARN -cp /usr/share/cattle/1792f92ccdd6495127a28e16a685da7 135 ? Sl 0:01 websocket-proxy 141 ? Sl 0:00 rancher-catalog-service -catalogUrl library=https://github.com/rancher/rancher-catalog.git,community=https://github.com/rancher/community-catalog.git -refreshInterval 300 142 ? Sl 0:00 rancher-compose-executor 143 ? Sl 0:00 go-machine-service 1517 ? Ss 0:00 bash 1537 ? R+ 0:00 ps x
We see that our Rancher brain is a Java application named Cattle, which uses a MySQL database embedded within its container to store state. This is quite convenient, but it would seem that we found the single point of failure on our quick-start setup. All the state for our cluster lives in one MySQL instance which no one knows existed. What happens if I nuke some data files?
Corrupting the MySQL Store
Inside my Rancher server container I executed MySQL commands. There is a
certain rush of adrenaline as you execute commands you know will break
docker exec -it rancher-server bash $ > mysql mysql> use cattle; mysql> SET FOREIGN_KEY_CHECKS = 0; mysql> truncate service; mysql> truncate network;
Lo and behold, my Rancher service tracking is broken, even when I kill
my WordPress containers they do not come back up, because Rancher no
longer remembers them.
Since I also truncated the network setup tables, my WordPress
application no longer knows how to route to its DB.
Clearly, to have confidence in running Rancher in production, we need a
way to protect our Rancher Server’s data integrity. This is where
Rancher HA comes in.
Rancher HA Setup Process
The first order of business is we need to secure the cluster data. I chose AWS RDS for this because it is what I am familiar with -- you can manage your own MySQL or choose another managed provider. We will proceed assuming we have a trusted MySQL management system with backups and monitoring. Following the HA setup steps documented in Rancher: As per the setup guide, we create an AWS RDS instance to be our data store. Once we have our database’s public endpoint, the next step is to dump your current Rancher installation’s data, and export it to the new database. For this I created an RDS instance with a public IP address. For your first Rancher HA setup I recommend just making the database public, then secure it later with VPC rules. Since Rancher provides an easy way to dump the state, you can move it around to a secured database at a later time. Next we will set up our Rancher Server to use the new database. After Rancher detects that it is using an external database, it will open up 2 more options as part of setting up HA mode. (At this point, we have already solved our point of failure, but for larger scale deployments, we need to go bigger to lower risk of failure.) Oh no, decision! -- but no worries, let’s go through each of these options and their implications. Cluster size, notice how everything is odd? Behind the scenes, Rancher HA sets up a ZooKeeper Quorum to keep locks in sync (More on this in the appendix). ZooKeeper recommends odd numbers because an even number of servers does not provide additional fault tolerance. Let’s pick 3 hosts to test out the feature, as it is a middle ground between usefulness and ease of setup. Host registration URL, well this section is asking us to provide the Fully Qualified Domain Name (FQDN) of our Rancher HA cluster. The instructions recommend an external loadbalancer or a DNS record that round robins between the 3 hosts. The examples would be to use a SRV Record on your DNS provider to balance between the 3 hosts; or an ELB on AWS with the 3 Rancher EC2 instances attached; or just a plain old DNS record pointing to 3 hosts. I choose the DNS record for my HA setup as it is the simplest to setup and debug. Now anytime I hit https://rancher.example.com my DNS hosting provider will round robin requests between the 3 Rancher hosts that I defined above. SSL Certificate is the last item on the list. If you have your own SSL certificate on your domain then you can use it here. Otherwise Rancher will provide a self-signed certificate instead. Once all options are filled, Rancher will update fields in its database to prepare for HA setup. You will then be prompted to download a rancher-ha.sh script.
WARNING Be sure to kill the Rancher container you used to generate the rancher-ha.sh script. It will be using ports that are needed by the Rancher-HA container that will be spun up by the script.
Next up, copy the rancher-ha.sh script onto each of the participating instances in the cluster and then execute them on the nodes to setup HA.
Caveat! Docker v1.10.3 is required at the time of writing. Newer version of Docker is currently unsupported for the rancher-ha.sh script.
You can provision the correct Docker version on your hosts with the following commands:
#!/bin/bash apt-get install -y -q apt-transport-https ca-certificates apt-key adv --keyserver hkp://p80.pool.sks-keyservers.net:80 --recv-keys 58118E89F3A912897C070ADBF76221572C52609D echo "deb https://apt.dockerproject.org/repo ubuntu-trusty main" > /etc/apt/sources.list.d/docker.list apt-get update apt-get install -y -q docker-engine=1.10.3-0~trusty # run the command below to show all available versions # apt-cache showpkg docker-engine
After Docker, we need to make sure that our instances can talk to each other so make sure the ports listed on the Rancher multi-node requirements page are open.
Advice! For your first test setup, I recommend opening all ports to avoid networking-related blockers.
Once you have the correct prerequisites, you can run the rancher-ha.sh script on each participating host. You will see the following output.
... ed5d8e75b7be: Pull complete ed5d8e75b7be: Pull complete 7ebc9fcbf163: Pull complete 7ebc9fcbf163: Pull complete ffe47ea37862: Pull complete ffe47ea37862: Pull complete b320962f9dbe: Pull complete b320962f9dbe: Pull complete Digest: sha256:aff7c52e52a80188729c860736332ef8c00d028a88ee0eac24c85015cb0e26a7 Status: Downloaded newer image for rancher/server:latest Started container rancher-ha c41f0fb7c356a242c7fbdd61d196095c358e7ca84b19a66ea33416ef77d98511 Run the below to see the logs docker logs -f rancher-ha
This is where the rancher-ha.sh script creates additional images that support the HA feature. Due to the addition of components to the Rancher Server, it is recommended to run a host with at least 4 GB of memory. A docker ps of what is running after running the rancher-ha.sh script is shown here.
Common Problems and Solutions
You may see some connection errors, so try to run the script on all 3 hosts first. You should see logs showing members being added to the Rancher HA Cluster.
time="2016-07-22T04:13:22Z" level=info msg="Cluster changed, index=0, members=[172.30.0.209, 172.30.0.111, ]" component=service ... time="2016-07-22T04:13:34Z" level=info msg="Cluster changed, index=3, members=[172.30.0.209, 172.30.0.111, 172.30.0.69]" component=service
Sometimes you will see a stream of the following error lines.
time="2016-07-23T14:37:02Z" level=info msg="Waiting for server to be available" component=cert time="2016-07-23T14:37:02Z" level=info msg="Can not launch agent right now: Server not available at http://172.17.0.1:18080/ping:" component=service
This is the top level symptom of many issues. Here are some other issues I have identified by going through the GitHub issues list and various forum posts: Security Group Network issues Sometimes your nodes are binding on the wrong IP so you would want to coerce Rancher to broadcast the correct IP. ZooKeeper not being up It is possible that the ZooKeeper Docker container is not able to communicate with the other nodes, so you would want to verify ZooKeeper and you should expect to see this sample output. Leftover files in the /var/lib/rancher/state directory from previous HA attempt If you ran the rancher-ha.sh multiple times then you may need to clean up old state files. Broken Rancher HA setup state from multiple reattempts Drop Database and try again. There is a previous issue with detailed steps to try to surface the issue. Insufficient Resources on the machine Since Rancher HA runs multiple Java processes on the machine, you will want to have at least 4 GB of memory. While testing with a t2.micro instance with 1 GB the instance became inaccessible due to being memory constrained. Another issue is that your database host needs to support 50 connections per HA node. You will see these messages when you attempt to spin up additional nodes.
time="2016-07-25T11:01:02Z" level=fatal msg="Failed to create manager" err="Error 1040: Too many connections"
Mismatched rancher/server:version By default the rancher-ha.sh
script pulls in rancher/server:latest, but this kicked me in the back
because during my setup, Rancher pushed out rancher/server:1.1.2 so I
had two hosts running rancher/server:1.1.1, and my third host was
rancher/server:1.1.2. This caused quite a headache, but a good takeaway
is to always specify the version of rancher/server when running the
rancher-ha.sh script on subsequent hosts.
./rancher-ha.sh rancher/server: Docker virtual network bridge was
returning wrong IP This was the issue I ran into – my HA setup was
trying to check agent health on the wrong Docker interface.
curl localhost:18080/ping > pong curl http://172.17.0.1:18080/ping > curl: (7) Failed to connect to 172.17.0.1 port 18080: Connection refused
The error line is found on
And the offending error call is found here in
What the code is doing is to locate the Docker Bridge and attempt to
ping the :18080 port on the exposed Docker port. Since my Docker bridge
is actually set up on 172.17.42.1 this will always fail. To resolve it I
re-instantiated the host because the multiple Docker installation seemed
to have caused the wrong bridge IP to be fetched. After restarting the
instance and setting the correct Docker bridge, I now see the expected
log lines for HA.
After Setting Up HA
time="2016-07-24T19:51:53Z" level=info msg="Waiting for 3 host(s) to be active" component=cert
Excellent. With one node up and ready, repeat the procedure for the rest of the hosts. After 3 hosts are up, you should be able to access the Rancher UI on the URL you specified for step 3 of the setup.
time="2016-07-24T20:00:11Z" level=info msg="[0/10] [zookeeper]: Starting " time="2016-07-24T20:00:12Z" level=info msg="[1/10] [zookeeper]: Started " time="2016-07-24T20:00:12Z" level=info msg="[1/10] [tunnel]: Starting " time="2016-07-24T20:00:13Z" level=info msg="[2/10] [tunnel]: Started " time="2016-07-24T20:00:13Z" level=info msg="[2/10] [redis]: Starting " time="2016-07-24T20:00:14Z" level=info msg="[3/10] [redis]: Started " time="2016-07-24T20:00:14Z" level=info msg="[3/10] [cattle]: Starting " time="2016-07-24T20:00:15Z" level=info msg="[4/10] [cattle]: Started " time="2016-07-24T20:00:15Z" level=info msg="[4/10] [go-machine-service]: Starting " time="2016-07-24T20:00:15Z" level=info msg="[4/10] [websocket-proxy]: Starting " time="2016-07-24T20:00:15Z" level=info msg="[4/10] [rancher-compose-executor]: Starting " time="2016-07-24T20:00:15Z" level=info msg="[4/10] [websocket-proxy-ssl]: Starting " time="2016-07-24T20:00:16Z" level=info msg="[5/10] [websocket-proxy]: Started " time="2016-07-24T20:00:16Z" level=info msg="[5/10] [load-balancer]: Starting " time="2016-07-24T20:00:16Z" level=info msg="[6/10] [rancher-compose-executor]: Started " time="2016-07-24T20:00:16Z" level=info msg="[7/10] [go-machine-service]: Started " time="2016-07-24T20:00:16Z" level=info msg="[8/10] [websocket-proxy-ssl]: Started " time="2016-07-24T20:00:16Z" level=info msg="[8/10] [load-balancer-swarm]: Starting " time="2016-07-24T20:00:17Z" level=info msg="[9/10] [load-balancer-swarm]: Started " time="2016-07-24T20:00:18Z" level=info msg="[10/10] [load-balancer]: Started " time="2016-07-24T20:00:18Z" level=info msg="Done launching management stack" component=service time="2016-07-24T20:00:18Z" level=info msg="You can access the site at https://" component=service
To get around issues regarding the self-signed HTTPS certificate, you will need to add it to your trusted certificates. After waiting and fixing up resource constraints on the DB, I then see all 3 hosts up and running.
Wow, that was a lot more involved than originally thought. This is why scalable distributed systems is a realm of PhD study. After resolving all the failure points, I think setting up and getting to know Rancher HA is a great starting point to touching state-of-the-art distributed systems. I will eventually script this out into Ansible provisioning to make provisioning Rancher HA a trivial task. Stay tuned!
For any distributed system, there is an explicit way to manage state and changes. Multiple servers need a process to coordinate between updates. Rancher’s management process works by keeping state and desired state in the database; then emitting events to be handled by processing entities to realize the desired state. When an event is being processed, there is a lock on it, and it is up to the processing entity to update the state in the database. In the single server setup, all of the coordination happens in memory on the host. Once you go to a multi server setup, the additional components like ZooKeeper and Redis are needed. Nick Ma is an Infrastructure Engineer who blogs about Rancher and Open Source. You can visit Nick’s blog, CodeSheppard.com, to catch up on practical guides for keeping your services sane and reliable with open-source solutions.