Patroni has dramatically simplified the setup of the PostgreSQL High-Availability cluster. Personally I consider Patroni to be by far the best tool out there to cluster PostgreSQL and to ensure HA in the most reliable and straightforward way possible.
The main questions asked by many people are: How can you make a PostgreSQL cluster transparent? How does the application figure out where the primary database is? The solution to the problem is a tool we implemented some time ago: Let me introduce you to vip-manager (Virtual IP Manager), a simple to use tool to handle service IPs in a PostgreSQL Patroni cluster.
vip-manager: Automatic IP failover
Before we dig a bit deeper into vip-manager, I want to explain the basic architecture of Patroni itself briefly. As you might know, Patroni is a tool developed initially by Zalando, which was a milestone in the development of PostgreSQL High-Availability. Over the years, we (CYBERTEC) have contributed countless patches to Patroni. We hope that we have made a difference in this crucial area relevant to so many people relying on PostgreSQL HA.
Patroni uses a key-value store (usually, etcd, zookeeper or consul) to handle consensus. The idea is to keep a leader key in the key-value store, which knows where the current master is. In other words: the cluster knows who happens to be primary and who happens to be secondary. Why not use this information to move a service IP around inside the cluster and assign it to the active primary? This is precisely what vip-manager does. It runs on every database node and checks if the node it runs on happens to be the “chosen one”. In case vip-manager sees that it runs on the current primary it grabs the IP and creates an IP alias. In case the vip-manager sees that the node it is running on is not the master, it ensures that the IP is removed.
In a cluster consisting of 3 database servers, you will therefore need 4 IPs. 1 IP per server will be static – one IP will be moved around and follow the primary. Of course, the floating IP is what you should use in your applications.
A typical node in a Patroni cluster has to run two services: Patroni and vip-manager. Patroni controls PostgreSQL (stop, start, sync, etc.). vip-manager is a simple Go application doing nothing else but control the floating IP pointing to the master.
Configuring vip-manager is simple. Here is an example:
# config for vip-manager by Cybertec PostgreSQL International GmbH # time (in milliseconds) after which vip-manager wakes up and checks # if it needs to register or release ip addresses. interval: 1000 # the etcd or consul key which vip-manager will regularly poll. key: "/service/pgcluster/leader" # if the value of the above key matches the NodeName (often the hostname of this host), # vip-manager will try to add the virtual ip address to the interface specified in Iface nodename: "pgcluster_member1" ip: 192.168.0.123 # the virtual ip address to manage mask: 24 # netmask for the virtual ip iface: enp0s3 #interface to which the virtual ip will be added # how the virtual ip should be managed. we currently support "ip addr add/remove" # through shell commands or the Hetzner api hosting_type: basic # possible values: basic, or hetzner. endpoint_type: etcd # etcd or consul # a list that contains all endpoints to which etcd could talk. endpoints: - http://127.0.0.1:2379 - http://192.168.0.42:2379 # A single list-item is also fine. # consul will always only use the first entry from this list. # For consul, you'll obviously need to change the port to 8500. Unless you're # using a different one. Maybe you're a rebel and are running consul on port 2379? # Just to confuse people? Why would you do that? Oh, I get it. etcd_user: "patroni" etcd_password: "Julian's secret password" # don't worry about parameter with a prefix that doesn't match the endpoint_type. # You can write anything there, I won't even look at it. consul_token: "Julian's secret token" # how often things should be retried and how long to wait between retries. # (currently only affects arpClient) retry_num: 2 retry_after: 250 #in milliseconds
Basically, the configuration is simple: First of all, vip-manager has to know where etcd is. Then it has to know where it can find the leader key (URL). Finally, we want to know which IP has to be bound to which network interface. The rest is the simple login information for etcd or some retry configuration. The core is really: Where is my etcd and how can I log in? Where in etcd is my leader key and which IP is assigned to which device. That is the entire magic.
vip-manager helps to make a cluster fully transparent. It is available as binary packages for the most common Linux distributions and Windows. We maintain the package and add improvements as needed. One improvement worth pointing out is that vip-manager is also able to work on Hetzner (a leading German hosting company). We are able to talk to the Hetzner API to move IPs around. The reason I am mentioning this is that if you’ve got special requirements vip-manager can be adjusted to your needs with reasonable effort.
I hope you enjoyed this posting about virtual IPs and PostgreSQL clustering. If you want to know more about recent software released, I want to point you to Scalefield, a solution to automate PostgreSQL deployments using Kubernetes.
If you want to read something right now, I want to tell you about one more tool we have recently implemented in Go (golang). pg_timetable is a cutting-edge job scheduler for PostgreSQL, which allows you to run SQL code as well as built-ins (email, etc.) and other executables. Check out our posts about that here.