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Debian integration of Patroni and vip-manager

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Michael Banck

Patroni is a clustering solution for PostgreSQL that is getting more and more popular in the cloud and Kubernetes sector due to its operator pattern and integration with Etcd or Consul. Some time ago we wrote a blog post about the integration of Patroni into Debian. Recently, the vip-manager project which is closely related to Patroni has been uploaded to Debian by us. We will present vip-manager and how we integrated it into Debian in the following.

To recap, Patroni uses a distributed consensus store (DCS) for leader-election and failover. The current cluster leader periodically updates its leader-key in the DCS. As soon the key cannot be updated by Patroni for whatever reason it becomes stale. A new leader election is then initiated among the remaining cluster nodes.

PostgreSQL Client-Solutions for High-Availability

From the user’s point of view it needs to be ensured that the application is always connected to the leader, as no write transactions are possible on the read-only standbys. Conventional high-availability solutions like Pacemaker utilize virtual IPs (VIPs) that are moved to the primary node in the case of a failover.

For Patroni, such a mechanism did not exist so far. Usually, HAProxy (or a similar solution) is used which does periodic health-checks on each node’s Patroni REST-API and routes the client requests to the current leader.

An alternative is client-based failover (which is available since PostgreSQL 10), where all cluster members are configured in the client connection string. After a connection failure the client tries each remaining cluster member in turn until it reaches a new primary.

vip-manager

A new and comfortable approach to client failover is vip-manager. It is a service written in Go that gets started on all cluster nodes and connects to the DCS. If the local node owns the leader-key, vip-manager starts the configured VIP. In case of a failover, vip-manager removes the VIP on the old leader and the corresponding service on the new leader starts it there. The clients are configured for the VIP and will always connect to the cluster leader.

Debian-Integration of vip-manager

For Debian, the pg_createconfig_patroni program from the Patroni package has been adapted so that it can now create a vip-manager configuration:

pg_createconfig_patroni 11 test --vip=10.0.3.2

Similar to Patroni, we start the service for each instance:

systemctl start vip-manager@11-test

The output of patronictl shows that pg1 is the leader:

# patronictl -c /etc/patroni/11-test.yml status
+---------+--------+------------+--------+---------+----+-----------+
| Cluster | Member |    Host    |  Role  |  State  | TL | Lag in MB |
+---------+--------+------------+--------+---------+----+-----------+
| 11-test |  pg1   | 10.0.3.247 | Leader | running |  1 |           |
| 11-test |  pg2   | 10.0.3.94  |        | running |  1 |         0 |
| 11-test |  pg3   | 10.0.3.214 |        | running |  1 |         0 |
+---------+--------+------------+--------+---------+----+-----------+

In journal of ‘pg1’ it can be seen that the VIP has been configured:

Jan 19 14:53:38 pg1 vip-manager[9314]: 2020/01/19 14:53:38 IP address 10.0.3.2/24 state is false, desired true
Jan 19 14:53:38 pg1 vip-manager[9314]: 2020/01/19 14:53:38 Configuring address 10.0.3.2/24 on eth0
Jan 19 14:53:38 pg1 vip-manager[9314]: 2020/01/19 14:53:38 IP address 10.0.3.2/24 state is true, desired true

If LXC containers are used, one can also see the VIP in the output of lxc-ls -f:

NAME    STATE   AUTOSTART GROUPS IPV4                 IPV6 UNPRIVILEGED
pg1     RUNNING 0         -      10.0.3.2, 10.0.3.247 -    false
pg2     RUNNING 0         -      10.0.3.94            -    false
pg3     RUNNING 0         -      10.0.3.214           -    false

The vip-manager packages are available for Debian testing (bullseye) and unstable, as well as for the upcoming 20.04 LTS Ubuntu release (focal) in the official repositories. For Debian stable (buster), as well as for Ubuntu 19.04 and 19.10, packages are available at `apt.postgresql.org’ maintained by credativ, along with the updated Patroni packages with vip-manager integration.

Switchover Behaviour

In case of a planned switchover, e.g. pg2 becomes the new leader:

# patronictl -c /etc/patroni/11-test.yml switchover --master pg1 --candidate pg2 --force
Current cluster topology
+---------+--------+------------+--------+---------+----+-----------+
| Cluster | Member |    Host    |  Role  |  State  | TL | Lag in MB |
+---------+--------+------------+--------+---------+----+-----------+
| 11-test |  pg1   | 10.0.3.247 | Leader | running |  1 |           |
| 11-test |  pg2   | 10.0.3.94  |        | running |  1 |         0 |
| 11-test |  pg3   | 10.0.3.214 |        | running |  1 |         0 |
+---------+--------+------------+--------+---------+----+-----------+
2020-01-19 15:35:32.52642 Successfully switched over to "pg2"
+---------+--------+------------+--------+---------+----+-----------+
| Cluster | Member |    Host    |  Role  |  State  | TL | Lag in MB |
+---------+--------+------------+--------+---------+----+-----------+
| 11-test |  pg1   | 10.0.3.247 |        | stopped |    |   unknown |
| 11-test |  pg2   | 10.0.3.94  | Leader | running |  1 |           |
| 11-test |  pg3   | 10.0.3.214 |        | running |  1 |         0 |
+---------+--------+------------+--------+---------+----+-----------+

The VIP has now been moved to the new leader:

NAME    STATE   AUTOSTART GROUPS IPV4                 IPV6 UNPRIVILEGED
pg1     RUNNING 0         -      10.0.3.247          -    false
pg2     RUNNING 0         -      10.0.3.2, 10.0.3.94 -    false
pg3     RUNNING 0         -      10.0.3.214          -    false

This can also be seen in the journals, both from the old leader:

Jan 19 15:35:31 pg1 patroni[9222]: 2020-01-19 15:35:31,634 INFO: manual failover: demoting myself
Jan 19 15:35:31 pg1 patroni[9222]: 2020-01-19 15:35:31,854 INFO: Leader key released
Jan 19 15:35:32 pg1 vip-manager[9314]: 2020/01/19 15:35:32 IP address 10.0.3.2/24 state is true, desired false
Jan 19 15:35:32 pg1 vip-manager[9314]: 2020/01/19 15:35:32 Removing address 10.0.3.2/24 on eth0
Jan 19 15:35:32 pg1 vip-manager[9314]: 2020/01/19 15:35:32 IP address 10.0.3.2/24 state is false, desired false

As well as from the new leader pg2:

Jan 19 15:35:31 pg2 patroni[9229]: 2020-01-19 15:35:31,881 INFO: promoted self to leader by acquiring session lock
Jan 19 15:35:31 pg2 vip-manager[9292]: 2020/01/19 15:35:31 IP address 10.0.3.2/24 state is false, desired true
Jan 19 15:35:31 pg2 vip-manager[9292]: 2020/01/19 15:35:31 Configuring address 10.0.3.2/24 on eth0
Jan 19 15:35:31 pg2 vip-manager[9292]: 2020/01/19 15:35:31 IP address 10.0.3.2/24 state is true, desired true
Jan 19 15:35:32 pg2 patroni[9229]: 2020-01-19 15:35:32,923 INFO: Lock owner: pg2; I am pg2

As one can see, the VIP is moved within one second.

Updated Ansible Playbook

Our Ansible-Playbook for the automated setup of a three-node cluster on Debian has also been updated and can now configure a VIP if so desired:

# ansible-playbook -i inventory -e vip=10.0.3.2 patroni.yml

Questions and Help

Do you have any questions or need help? Feel free to write to info@credativ.com.

This blog post originally appeared on the german credativ blog.

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