191 lines
6.9 KiB
Markdown
191 lines
6.9 KiB
Markdown
# Failover with Keepalived
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## Introduction
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[Keepalived](http://www.keepalived.org/index.html) is a routing software for
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load balancing and high-availability. It has several applications, but for this
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tutorial the goal is to set up a simple IP failover between two servers running
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MaxScale. If the main server fails the backup machine takes over, receiving any
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new connections. The Keepalived settings used in this tutorial follow the
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example given in [Simple keepalived failover setup on Ubuntu 14.04](
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https://raymii.org/s/tutorials/Keepalived-Simple-IP-failover-on-Ubuntu.html).
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Two hosts and one client machine are used, all in the same LAN. Hosts run
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MaxScale and Keepalived. The backend servers may be running on one of the hosts,
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e.g. in docker containers, or on separate machines for a more realistic setup.
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Clients connect to the virtual IP (VIP), which is claimed by the current master
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host.
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Once configured and running, the different Keepalived nodes continuously
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broadcast their status to the network and listen for each other. If a node does
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not receive a status message from another node with a higher priority than
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itself, it will claim the VIP, effectively becoming the master. Thus, a node can
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be put online or removed by starting and stopping the Keepalived service.
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If the current master node is removed (e.g. by stopping the service or pulling
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the network cable) the remaining nodes will quickly elect a new master and
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future traffic to the VIP will be directed to that node. Any connections to the
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old master node will naturally break. If the old master comes back online, it
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will again claim the VIP, breaking any connections to the backup machine.
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MaxScale has no knowledge of this even happening. Both MaxScales are running
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normally, monitoring the backend servers and listening for client connections.
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Since clients are connecting through the VIP, only the machine claiming the VIP
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will receive incoming connections. The connections between MaxScale and the
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backends are using real IPs and are unaffected by the VIP.
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## Configuration
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MaxScale does not require any specific configuration to work with Keepalived in
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this simple setup, it just needs to be running on both hosts. The MaxScale
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configurations should be similar to the extent that both look identical to
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connecting clients. In practice the listening ports and related services should
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be the same. Setting the service-level setting “version_string” to different
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values on the MaxScale nodes is recommended, as it will be printed to any
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connecting clients indicating which node was connected to.
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Keepalived requires specific setups on both machines. On the **primary host**,
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the */etc/keepalived/keepalived.conf*-file should be as follows.
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```
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vrrp_instance VI_1 {
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state MASTER
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interface eth0
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virtual_router_id 51
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priority 150
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advert_int 1
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authentication {
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auth_type PASS
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auth_pass mypass
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}
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virtual_ipaddress {
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192.168.1.123
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}
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}
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```
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The *state* must be MASTER on both hosts. *virtual_router_id* and *auth_pass*
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must be identical on all hosts. The *interface* defines the network interface
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used. This depends on the system, but often the correct value is *eth0*,
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*enp0s12f3* or similar. *priority* defines the voting strength between different
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Keepalived instances when negotiating on which should be the master. The
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instances should have different values of priority. In this example, the backup
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host(s) could have priority 149, 148 and so on. *advert_int* is the interval
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between a host “advertising” its existence to other Keepalived host. One second
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is a reasonable value.
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*virtual_ipaddress* (VIP) is the IP the different Keepalived hosts try to claim
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and must be identical between the hosts. For IP negotiation to work, the VIP
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must be in the local network address space and unclaimed by any other machine
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in the LAN. An example *keepalived.conf*-file for a **backup host** is listed
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below.
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```
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vrrp_instance VI_1 {
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state MASTER
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interface eth0
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virtual_router_id 51
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priority 100
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advert_int 1
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authentication {
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auth_type PASS
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auth_pass mypass
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}
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virtual_ipaddress {
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192.168.1.123
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}
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}
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```
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Once the Keepalived service is running, recent log entries can be printed with
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the command `service keepalived status`.
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```
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Aug 11 10:27:59 maxscale2 Keepalived_vrrp[27369]: VRRP_Instance(VI_1) Received higher prio advert
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Aug 11 10:27:59 maxscale2 Keepalived_vrrp[27369]: VRRP_Instance(VI_1) Entering BACKUP STATE
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Aug 11 10:27:59 maxscale2 Keepalived_vrrp[27369]: VRRP_Instance(VI_1) removing protocol VIPs.
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```
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## MaxScale health check
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So far, none of this tutorial has been MaxScale-specific and the health of the
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MaxScale process has been ignored. To ensure that MaxScale is running on the
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current master host, a *check script* should be set. Keepalived runs the script
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regularly and if the script returns an error value, the Keepalived node will
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assume that it has failed, stops broadcasting its state and relinquishes the
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VIP. This allows another node to take the master status and claim the VIP. To
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define a check script, modify the configuration as follows. The example is for
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the primary node. See [Keepalived Check and Notify Scripts](
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https://tobrunet.ch/2013/07/keepalived-check-and-notify-scripts/) for more
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information.
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```
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vrrp_script chk_myscript {
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script "/home/scripts/is_maxscale_running.sh"
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interval 2 # check every 2 seconds
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fall 2 # require 2 failures for KO
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rise 2 # require 2 successes for OK
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}
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vrrp_instance VI_1 {
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state MASTER
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interface wlp2s0
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virtual_router_id 51
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priority 150
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advert_int 1
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authentication {
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auth_type PASS
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auth_pass mypass
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}
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virtual_ipaddress {
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192.168.1.13
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}
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track_script {
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chk_myscript
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}
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}
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```
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An example script, *is_maxscale_running.sh*, is listed below. The script uses
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MaxAdmin to try to contact the locally running MaxScale and request a server
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list, then check that the list has at least some expected elements. The timeout
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command ensures the MaxAdmin call exits in reasonable time. The script detects
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if MaxScale has crashed, is stuck or is totally overburdened and no longer
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responds to connections.
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```
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#!/bin/bash
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fileName="maxadmin_output.txt"
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rm $fileName
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timeout 2s maxadmin list servers > $fileName
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to_result=$?
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if [ $to_result -ge 1 ]
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then
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echo Timed out or error, timeout returned $to_result
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exit 3
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else
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echo MaxAdmin success, rval is $to_result
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echo Checking maxadmin output sanity
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grep1=$(grep server1 $fileName)
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grep2=$(grep server2 $fileName)
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if [ "$grep1" ] && [ "$grep2" ]
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then
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echo All is fine
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exit 0
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else
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echo Something is wrong
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exit 3
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fi
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fi
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```
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```
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Aug 11 10:51:56 maxscale2 Keepalived_vrrp[20257]: VRRP_Script(chk_myscript) failed
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Aug 11 10:51:57 maxscale2 Keepalived_vrrp[20257]: VRRP_Instance(VI_1) Entering FAULT STATE
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Aug 11 10:51:57 maxscale2 Keepalived_vrrp[20257]: VRRP_Instance(VI_1) removing protocol VIPs.
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Aug 11 10:51:57 maxscale2 Keepalived_vrrp[20257]: VRRP_Instance(VI_1) Now in FAULT state
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```
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