Setting up PXC reference architecture with HAProxy

This tutorial is a step-by-step guide to set up Percona XtraDB Cluster, in a virtualized test sandbox. This example uses Amazon EC2 micro instances, but the content here is applicable for any kind of virtualization technology (for example VirtualBox). You will need 4 virtual machines. 3 for Percona XtraDB Cluster and 1 for the client, which will have HAProxy. In this how-to CentOS 6 is used as the operating system, the instructions are similar for any Linux distribution.

The client node will have HAProxy installed and it will redirect requests to Percona XtraDB Cluster nodes. This approach works well in real-world scenarios too. Running HAProxy on the application servers instead of having them as dedicated entities gives you benefits like no need for an extra network roundtrip, because loadbalancer and scalability of Percona XtraDB Cluster‘s load balancing layer scales simply with application servers.

We’ll use Percona and EPEL repositories for software installation.

After configuring the repositories you’ll be able to install software that will be used. First, install Percona XtraDB Cluster on the database nodes.

# yum -y install Percona-XtraDB-Cluster-server Percona-XtraDB-Cluster-client percona-xtrabackup

Install HAProxy and sysbench on the client node.

# yum -y install haproxy sysbench

After installing everything, we’ll configure Percona XtraDB Cluster first. On the first node, my.cnf should look something like this on a relatively weak machine.





You can start your first node now. Make sure that you only start second and third nodes when the first node is up and running (it will serve as a donor for SST).

This configuration is for the first node. For the second and third node, you need to change wsrep_cluster_address (alternatively, you can use wsrep_urls in [mysqld_safe] section), which should point to a node in the cluster which is already up, so it will join the cluster. The server_id and wsrep_node_name variables have to be different on each host, for wsrep_node_name, you can use the output of hostname command.

Based on that, for the second node, the differences in the configuration should be the following.

wsrep_cluster_address=gcomm:// # replace this with the IP of your first node

For the third node, the differences look like this.

wsrep_cluster_address=gcomm:// # replace this with the IP of your first node

For SST we use xtrabackup. This means at startup time, the new node will connect to an existing node in the cluster and it takes a backup of that node with xtrabackup and copies it to the new node with netcat. After a successful SST, you should see this in the error log.

120619 13:20:17 [Note] WSREP: State transfer required:
      Group state: 77c9da88-b965-11e1-0800-ea53b7b12451:97
      Local state: 00000000-0000-0000-0000-000000000000:-1
120619 13:20:17 [Note] WSREP: New cluster view: global state: 77c9da88-b965-11e1-0800-ea53b7b12451:97, view# 18: Primary, number of nodes: 3, my index: 0, protocol version 2
120619 13:20:17 [Warning] WSREP: Gap in state sequence. Need state transfer.
120619 13:20:19 [Note] WSREP: Running: 'wsrep_sst_xtrabackup 'joiner' '' '' '/var/lib/mysql/' '/etc/my.cnf' '20758' 2>sst.err'
120619 13:20:19 [Note] WSREP: Prepared |SST| request: xtrabackup|
120619 13:20:19 [Note] WSREP: wsrep_notify_cmd is not defined, skipping notification.
120619 13:20:19 [Note] WSREP: Assign initial position for certification: 97, protocol version: 2
120619 13:20:19 [Warning] WSREP: Failed to prepare for incremental state transfer: Local state UUID (00000000-0000-0000-0000-000000000000) does not match group state UUID (77c9da88-b965-11e1-0800-ea53b7b12451): 1 (Operation not permitted)
       at galera/src/replicator_str.cpp:prepare_for_IST():439. IST will be unavailable.
120619 13:20:19 [Note] WSREP: Node 0 (ip-10-244-33-92) requested state transfer from '*any*'. Selected 1 (ip-10-112-39-98)(SYNCED) as donor.
120619 13:20:19 [Note] WSREP: Shifting PRIMARY -> JOINER (TO: 102)
120619 13:20:19 [Note] WSREP: Requesting state transfer: success, donor: 1
120619 13:20:59 [Note] WSREP: 1 (ip-10-112-39-98): State transfer to 0 (ip-10-244-33-92) complete.
120619 13:20:59 [Note] WSREP: Member 1 (ip-10-112-39-98) synced with group.
120619 13:21:17 [Note] WSREP: |SST| complete, seqno: 105
120619 13:21:17 [Note] Plugin 'FEDERATED' is disabled.
120619 13:21:17 InnoDB: The InnoDB memory heap is disabled
120619 13:21:17 InnoDB: Mutexes and rw_locks use GCC atomic builtins
120619 13:21:17 InnoDB: Compressed tables use zlib 1.2.3
120619 13:21:17 InnoDB: Using Linux native AIO
120619 13:21:17 InnoDB: Initializing buffer pool, size = 400.0M
120619 13:21:17 InnoDB: Completed initialization of buffer pool
120619 13:21:18 InnoDB: highest supported file format is Barracuda.
120619 13:21:18  InnoDB: Waiting for the background threads to start
120619 13:21:19 Percona XtraDB ( 1.1.8-rel25.3 started; log sequence number 246661644
120619 13:21:19 [Note] Recovering after a crash using mysql-bin
120619 13:21:19 [Note] Starting crash recovery...
120619 13:21:19 [Note] Crash recovery finished.
120619 13:21:19 [Note] Server hostname (bind-address): '(null)'; port: 3306
120619 13:21:19 [Note]   - '(null)' resolves to '';
120619 13:21:19 [Note]   - '(null)' resolves to '::';
120619 13:21:19 [Note] Server socket created on IP: ''.
120619 13:21:19 [Note] Event Scheduler: Loaded 0 events
120619 13:21:19 [Note] WSREP: Signalling provider to continue.
120619 13:21:19 [Note] WSREP: Received |SST|: 77c9da88-b965-11e1-0800-ea53b7b12451:105
120619 13:21:19 [Note] WSREP: |SST| received: 77c9da88-b965-11e1-0800-ea53b7b12451:105
120619 13:21:19 [Note] WSREP: 0 (ip-10-244-33-92): State transfer from 1 (ip-10-112-39-98) complete.
120619 13:21:19 [Note] WSREP: Shifting JOINER -> JOINED (TO: 105)
120619 13:21:19 [Note] /usr/sbin/mysqld: ready for connections.
Version: '5.5.24-log'  socket: '/var/lib/mysql/mysql.sock'  port: 3306  Percona XtraDB Cluster (GPL), wsrep_23.6.r340
120619 13:21:19 [Note] WSREP: Member 0 (ip-10-244-33-92) synced with group.
120619 13:21:19 [Note] WSREP: Shifting JOINED -> SYNCED (TO: 105)
120619 13:21:20 [Note] WSREP: Synchronized with group, ready for connections

For debugging information about the SST, you can check the sst.err file and the error log too.

After the SST’s is done, you should check if you have a 3 node cluster.

mysql> show global status like 'wsrep_cluster_size';
| Variable_name      | Value |
| wsrep_cluster_size | 3     |
1 row in set (0.00 sec)

When all nodes are started, you can set up HAProxy on the client. The point of this is that the application will be able to connect to localhost as MySQL server, so although we are using Percona XtraDB Cluster, the application will see this as a single MySQL server running on localhost.

In order to achieve this, you’ll need to configure HAProxy on the client node. There are 2 possible configurations here. First is configuring round robin, which means you will connect and write to all cluster nodes. This can be done, but because of optimistic locking at commit time, rollbacks can happen if you have conflicting writes. In the second configuration, you will configure HAProxy in a way that it writes only to one node, so the application doesn’t have to be prepared about unexpected rollbacks. The first configuration is a good choice in most cases, not handling rollbacks is not healthy in a well behaving application anyway.

HAProxy can be configured in the /etc/haproxy/haproxy.cfg and it should look like this.

log local0
log local1 notice
maxconn 4096
chroot /usr/share/haproxy
user haproxy
group haproxy

log global
mode http
option tcplog
option dontlognull
retries 3
option redispatch
maxconn 2000
contimeout 5000
clitimeout 50000
srvtimeout 50000

frontend pxc-front
bind *:3307
mode tcp
default_backend pxc-back

frontend stats-front
bind *:80
mode http
default_backend stats-back

frontend pxc-onenode-front
bind *:3306
mode tcp
default_backend pxc-onenode-back

backend pxc-back
mode tcp
balance leastconn
option httpchk
server c1 check port 9200 inter 12000 rise 3 fall 3
server c2 check port 9200 inter 12000 rise 3 fall 3
server c3 check port 9200 inter 12000 rise 3 fall 3

backend stats-back
mode http
balance roundrobin
stats uri /haproxy/stats
stats auth pxcstats:secret

backend pxc-onenode-back
mode tcp
balance leastconn
option httpchk
server c1 check port 9200 inter 12000 rise 3 fall 3
server c2 check port 9200 inter 12000 rise 3 fall 3 backup
server c3 check port 9200 inter 12000 rise 3 fall 3 backup

In this configuration, three frontend-backend pairs are defined. The stats pair is for HAProxy statistics page, and the others are for Percona XtraDB Cluster. MySQL will be listening on ports 3306 and 3307. If you connect to port 3306, you’ll connect to pxc-onenode, and you’ll be only using one node at a time (to avoid rollbacks because of optimistic locking). If that node goes off-line, you’ll start using an other one. However if you connect to port 3307, you’ll be using all three nodes for reads and writes too. In this case the leastconn load balancing method is used instead of round robin, which means you always connect to the backend with the least connections established. The statistics page is accessible on the client node with a browser pointed to /haproxy/stats, the stats auth parameter in the configuration has the credentials for that in plain text. You can also use this for monitoring purposes (the CSV version is good for trending and alerting).

Here MySQL is checked via HTTP checks. MySQL won’t serve these requests. As part of Percona XtraDB Cluster packages, we distribute the clustercheck utility which has to be set up. After that, HAProxy will be able to use check MySQL via HTTP. The clustercheck script is a simple shell script, which accepts HTTP requests, and checks MySQL on incoming request. If the Percona XtraDB Cluster node is ok, it will emit a response with HTTP code 200 OK, otherwise, it emits 503. The script examines wsrep_local_state variable.

To set it up, create the clustercheck user.

mysql> grant process on *.* to 'clustercheckuser'@'localhost' identified by 'clustercheckpassword!';
Query OK, 0 rows affected (0.00 sec)

mysql> flush privileges;
Query OK, 0 rows affected (0.00 sec)

If you want to use a different username or password, you have to modify them in the script too. Let’s test.

# clustercheck
HTTP/1.1 200 OK

Content-Type: Content-Type: text/plain

Node is running.

You can use xinetd to daemonize the script. If xinetd is not installed yet, you can install it with yum.

# yum -y install xinetd

The service itself should be configured in /etc/xinetd.d/mysqlchk.

# default: on
# description: mysqlchk
service mysqlchk
# this is a config for xinetd, place it in /etc/xinetd.d/
  disable = no
  flags = REUSE
  socket_type = stream
  port = 9200
  wait = no
  user = nobody
  server = /usr/bin/clustercheck
  log_on_failure += USERID
  only_from =
  # recommended to put the IPs that need
  # to connect exclusively (security purposes)
  per_source = UNLIMITED

Also, you should add the new service to /etc/services.

mysqlchk 9200/tcp # mysqlchk

Clustercheck will now listen on port 9200 after xinetd restart, and HAProxy is ready to check MySQL via HTTP.

# service xinetd restart

If you did everything right so far, the statistics page of HAProxy should look like this.


Testing the cluster with sysbench

You can test the cluster using the sysbench (this example uses one from the EPEL repository). First, you need to create a database and a user for it.

mysql> create database sbtest;
Query OK, 1 row affected (0.01 sec)

mysql> grant all on sbtest.* to 'sbtest'@'%' identified by 'sbpass';
Query OK, 0 rows affected (0.00 sec)

mysql> flush privileges;
Query OK, 0 rows affected (0.00 sec)

Populate the table with data for the benchmark.

# sysbench --test=oltp --db-driver=mysql --mysql-engine-trx=yes --mysql-table-engine=innodb --mysql-host= --mysql-port=3307 --mysql-user=sbtest --mysql-password=sbpass --oltp-table-size=10000 prepare

You can now run the benchmark against the 3307 port.

# sysbench --test=oltp --db-driver=mysql --mysql-engine-trx=yes --mysql-table-engine=innodb --mysql-host= --mysql-port=3307 --mysql-user=sbtest --mysql-password=sbpass --oltp-table-size=10000 --num-threads=8 run

This is the status of pxc-back backend while the sysbench above is running. If you look at Cur column under Session, you can see, that c1 has 2 threads connected, c2 and c3 has 3.

If you run the same benchmark, but against the 3306 backend, HAProxy stats will show us that the all the threads are going to hit the c1 server.

# sysbench --test=oltp --db-driver=mysql --mysql-engine-trx=yes --mysql-table-engine=innodb --mysql-host= --mysql-port=3306 --mysql-user=sbtest --mysql-password=sbpass --oltp-table-size=10000 --num-threads=8 run

This is the status of pxc-onenode-back while sysbench above is running. Here only c1 has 8 connected threads, c2 and c3 are acting as backup nodes.

If you are using HAProxy for MySQL you can break the privilege system’s host part, because MySQL will think that the connections are always coming from the load balancer. You can work this around using T-Proxy patches and some iptables magic for the backwards connections. However in the setup described in this how-to this is not an issue, since each application server has it’s own HAProxy instance, each application server connects to, so MySQL will see that connections are coming from the application servers. Just like in the normal case.

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