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Percona XtraDB Cluster reference architecture with HaProxy

 | June 20, 2012 |  Posted In: MySQL, Percona Software, XtraDB Cluster

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This post is a step-by-step guide to set up Percona XtraDB Cluster (PXC) in a virtualized test sandbox. I used Amazon EC2 micro instances, but the content here is applicable for any kind of virtualization technology (for example VirtualBox). The goal is to give step by step instructions, so the setup process is understandable and it could serve as a reference architecture.

You will need 4 virtual machines. 3 for PXC and 1 for the client, which will have HaProxy. I used CentOS 6 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 PXC 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 you don’t need an extra network roundtrip because of a loadbalancer and scalability of PXC’s load balancing layer scales simply with application servers.

We will use Percona and EPEL repositories for software installation.
To add these follow the instructions for
Percona and EPEL to enable those.

After configuring the repositories you will be able to install software that we will use. First, install PXC on the database nodes.

Install haproxy and sysbench on the client node.

After installing everything, let’s configure PXC 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.

For the third node, the differences look like this.

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.

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

After the SSTs are done, you should check if you have a 3 node cluster.

When all nodes are started, we will 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 PXC, the application will see this as a single MySQL server running on localhost.

In order to achieve this, we will configure HaProxy on the client node. There are 2 possible configurations here.
First is configuring round robin, which means we will connect and write to all cluster nodes. This can be done, but because of optimistic locking at commit time, rollbacks can happen if we have conflicting writes. In the second configuration, we 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.

In this configuration, we defined 3 frontend-backend pairs. The stats pair is for haproxy’s statistics page, and the others are for PXC. We will have MySQL listening on 3306 and 3307. If we connect to port 3306, we will connect to pxc-onenode, so we will only use one node at a time (to avoid rollbacks bcause of optimistic locking). If that node goes offline, we will start using an other one.
However if we connect to port 3307, we will use all the 3 nodes for reads and writes too. We are using the leastconn load balancing method instead of round robin, which means we 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 PXC 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 PXC node is ok, it will emit a reposnse with HTTP code 200 OK, otherwise, it emits 503. The script examines wsrep_local_state variable.

To set it up, create the clustercheck user.

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

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

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

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

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

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

Next we will try to use the cluster with sysbench. We will use sysbench from the EPEL repo. First, let’s create a database and a userfor it.

Populate the table with data for the benchmark.

Let’s run the bechmark against the 3307 port.

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 we 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.

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 post this is not an issue, since each application server has it’s own HaProxy instance, each application server connects to 127.0.0.1, so MySQL will see that connections are coming from the application servers. Just like in the normal case.

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Peter Boros

Peter joined the European consulting team in May 2012. Before joining Percona, among many other things, he worked at Sun Microsystems, specialized there in performance tuning and was a DBA at Hungary's largest social networking site. He also taught many Oracle University MySQL courses. He has been using and working with open source software from early 2000s. Peter's first and foremost professional interest is performance tuning. He currently lives in Budapest, Hungary with his wife and son.

71 Comments

  • Running ha-proxy on the app server is a good one as you’re still able to use the security features for connections. And I assume it scales quite well. It also removes a possible spof.

    Why do you use ‘FLUSH PRIVILEGES’ after a GRANT?

    Reply

  • Running in Ubuntu Server 12.04 i get the message:

    [ALERT] 173/171337 (13252) : Starting frontend pxc-onenode-front: cannot bind socket

    for

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

    Reply

  • Hi,

    Thanks for this useful document.

    Could you please detailed the choice/reason of having a 3306 port to have only one node being used for R/W queries and above all the mention : (to avoid rollbacks bcause of optimistic locking) ?
    In a production environment with a reasonable traffic (500 req/sec on each node), would you recommend to use 3307 port to have all requests R/W balanced through all nodes, would it be a problem concerning conflicts (higher risk of queries that can be conflictual from node to node and so more queries ‘rejected’) please ?

    Thanks for your answers.

    Regards,

    Laurent

    Reply

  • Laurent: this is a very good question, would actually worth an other post.
    If you lock records in InnoDB with PXC, records are only locked on the local node. Let’s say you open up 2 mysql sessions in HaProxy, and you get to machines 2 and 3, and let’s suppose you have a table called t, with a column called id which is the primary key. You start a new transaction on both sessions and do update t set name=’foo’ where id=1, this won’t block on the second session, because you are on a different database instance. However, at commit time cluster communication happens. One of these transaction has to roll back, because the same record was changed on both nodes. This can be handled on the application level by retrying transaction. This is called optimistic locking, because here we are being optimistic. This means we assume that for most of the cases, we won’t write conflicting data on 2 nodes. A rollback is a much more expensive operation than a commit, so ideally, we want these to be rare. On the other side, pessimistic locking across nodes would involve much more network roundtrips, which would limit performance.

    I think this kind of answers your second question. It is not the request/sec which matters here, but the possbility of writing the same data on multiple nodes. If you are suffering from this, the configuration for port 3307 can be a solution for you, in which case you will use PXC only as a HA solution.

    Panagiotis: You are right. In fact, you don’t even need clustercheck at all. You just need something which checks you PXC nodes, and emits HTTP codes for HaProxy based on status. It can be written in any language, we just wanted to provide something which does this as part of the software. If you take a look at the script, it only examines wsrep status variables.

    Reply

  • Hi Peter,

    Thanks for your answer, this helps to clarify the situation for me, so choice of solution A (R/W to a single node) or solution B (R/W to all nodes) would mainly depend of what can be done or not at the application level if I understand well ? If I consider pros and cons of these two solutions :

    With Solution A, I can still have the advantage of a High Availability solution without having to deal with rollbacked transaction that needs to be retried at application level because as I can R/W to/from a single node, behaviour will be the same as if I were having only one single standard MySQL server in front ? Cons of this solution is maybe that traffic cannot be load balanced through all nodes so maybe a lower throughput than solution B ? Is it really the case or even measurable ?

    With Solution B, I do not have to worry about load balancing queries across the nodes so consequently maybe a higher number of queries and throughput that can be managed ? But for the cons, I almost “have to” modify my applications in front of the cluster so they can manage the case where transaction will be rejected by the cluster because of conflictual queries (probability is higher in this solution I think?)

    So maybe solution A would be the best one because it provides less problems at query level, benefits of cluster for High Availability and maybe a good throughput if I’m able to combined it with an API that is able to manage load balancing Reads to 3 nodes so reads ? like this one : http://dev.mysql.com/doc/refman/5.5/en/apis-php-mysqlnd-ms.setup.html#apis-php-mysqlnd-ms.installation

    Thks for sharing your experience, skills and opinion on the subject ! 😉

    Regards,

    Laurent

    Reply

  • Hi Laurent,

    With solution B, according to our benchmarks you will have a higher throughput. If an application doesn’t handle rollbacks well, it can bite you anyway, even in a single server case.

    You can create an other haproxy frontend-backend pair just for reads with solution A, or use any other load balancing solution too. In this case you have to do R/W splitting somehow, either with mysqlnd, or with your custom solution from your application.

    Reply

  • Hey

    This might sound like a stupid question from a guy trying to read up on ha-proxy, but I can’t see what makes pxc-onenode-back and pxc-back different in how they would handle a rollback depending on if the connection goes to port 3306/3307.

    Anyone care to explain it to me? 🙂

    Reply

  • Hi Tom,

    Onenode is the keyword for doing writes only on one node. When you issue an update command in innodb, record is locked, so concurrent updates won’t happen until you commit. If an other transaction tries to update that very same record, it will be blocked until the former transaction ends. This is not necessarily the case with pxc-back. The behaviour described previously is pessimistic locking, which means that if we “work” on a records, we lock it, because there will be some other transaction which wants to mess with it anyway. InnoDB uses that within the instance. However between the cluster nodes, PXC uses optimistic locking, which means we don’t lock the record, because most likely transactions won’t be conflicting (this would require inter-node communication inside a transaction, not just commit time). So, 2 conflicting update transactions updating the same record issued on pxc-back won’t necessarily block each other (because they are issued on different nodes in reality), so one of them will be rolled back at commit time.

    I hope this helps, I will try to write a separate blogpost about this with examples this week.

    Reply

  • Hi Peter,

    we have implemented your reference architecture on some virtual machines and were wondering if you really need the fourth machine. With HaProxy installed on the three PXC-nodes and UCARP to provide one IP address for HaProxy everything works fine.

    Are there any reasons against this kind of setup? Of course, one node would experience some additional load (and it would be always the same node until it fails).

    Regards
    Jens

    Reply

  • Hi Jens,

    The fourth node in the example is not a dedicated load balancing node, but rather one application server. You will have many of this in your environment, serving your wordpress/drupal/whatever application. With this setup, on each application server the application can connect to localhost as mysql server.

    Technically, nothing is wrong with your setup, although it has a few caveats.
    – Using a floating IP address is not always possible (thinking about cloud).

    – You will break mysql’s privilege system.
    This is because mysql will see that everybody is coming from localhost (from the haproxy to be more precise). If you install haproxy on the application servers, and connect to localhost, mysql will identify that those connections are from the application server and privilege system will work as expected, because nothing is changed here from mysql’s perspective.

    – It is more likely that you may hit haproxy performance bottlenecks, like one haproxy process can use only 1 cpu core, spreading your database load balancing layer between your application servers (typically you have many of those anyway), it’s less likely you will run into this problem. Not to mention that people like to use cpus in the database server to run database workload.

    Reply

  • What would you say about running a Percona MySQL server on each application server instead of haproxy (if localhost security credentials mentioned above is acceptable and application is chrooted)?

    Reply

  • Hi Tom,

    If you mean running a PXC node on each application server, that will probably bite you sooner or later, because in that case you are replicating synchronously to a relatively large number of nodes.

    If you mean a standalone Percona server, than I think I don’t get what you mean.

    Reply

  • How do I separate read vs write traffic for different algorithms (i.e.RR(read) or Master/failover(write)) in case of typical jdbc web application ?

    Reply

  • Thanks for your answer! I did’nt know about the 1-core-bottleneck.

    Just played a bit with a four-node-cluster. If one node is shut down and starts again it gets synced automatically. But if the first node fails (the one with wsrep_cluster_address=gcomm://) it obviously can’t connect to the cluster after a restart.

    Starting the cluster once, changing the cluster_address and restarting the first node solved the problem but does not seem right. Is there another way to ensure that every node can reconnect to the cluster?

    Reply

  • Hi Jens,

    See the updated docs on https://www.percona.com/doc/percona-xtradb-cluster/howtos/singlebox.html for a better way to set cluster addresses.

    Reply

  • Hi Vanne,

    thank you for the link!
    Since I really use four machines I tried to adopt the configuration mentioned there and came up with this:
    http://paste.jdsv.de/view/raw/66778546
    This is for the first node with the IP addresse 10.2.10.128

    My error-log shows the following:
    http://paste.jdsv.de/view/raw/65552680

    Any clues?

    Reply

  • Jens,

    Can you try adding back the :4567 to all IPs in the wsrep_urls?
    It’s the default, i know, but if i remember correctly i made the same “mistake”.

    cheers

    Reply

  • Hi Peter, Thanks for writing such a detailed and articulate document. Very much helpful. i have a few question though on the configurations
    on node 1 you had
    wsrep_cluster_address=gcomm://
    and node2 and node3 you have:
    wsrep_cluster_address=gcomm://

    Question 1: Dont we need to add the ip addresses of nodes 2 and nodes 3 in the config, if so how? and i

    Question 2: If you are doing a in-place upgrade on a server that was working as a master, do we need to ommit some configurations or add anything extra?

    Question 3: Here was my test scenario, tell me if i was missing something,
    I used the same configurations as mentioned by you
    i created a schema on node1 (sbtest) it propagated fine to node2 and node3
    then created table on node1, inserted some rows, saw those records on node2 and node3
    then i shutdown node1,
    created a table on node2, i see the table on node3 too
    inserted some records and they are on both the node2 and node3
    then i started node1, it started fine, but as the document mentions, that paramert(wsrep_sst_method=xtrabackup) should pickup the changes and just apply them to this node….”that did not happen” and
    now node1 is out of sync with node 2 and node3
    so no changes are getting propagated from node1 to node2 and 3 and vice-versa

    any clues? is it the gcomm://config that is incorrect?

    Reply

  • If you use Xtrabackup as SST, the current clustercheck script will see the donor as down (503). If you have 2 nodes only (which is of course not recommended at all) you will have then your database completely unavailable the time of the SST.

    You can then modify clustercheck to allow connections to the donor anyway (only if you use a non blocking SST method, xtrabackup):

    -if [ “$WSSREP_STATUS” == “4” ]
    +if [ “$WSSREP_STATUS” == “4” ] || [ “$WSSREP_STATUS” == “2” ]

    Reply

  • Hi mandm,

    For node2 and node3 I had wsrep_cluster_address configured to node1’s IP. Basically, this option tells the node where can it find an existing cluster member (this is when it has an IP address), or if it should form a new cluster (so, this is the first node in the cluster, the configuration in this case is wsrep_cluster_address=gcomm://). Since you restarted node1 with gcomm://, it forms a new cluster, so, it won’t pick up the changes from the other nodes. If you want node1 to join the cluster, you should specify wsrep_cluster_address=gcomm://(node2 or node3 ip here), or you can use wsrep_urls in [mysqld_safe] section of the configuration file if you don’t want to change configuration.

    Reply

  • @Frederic, i have 3 cluster node..

    @ Peter thanks for your response, yes so after researching a bit, i have my changed my config on all the 3 nodes as

    # This was formally known as [safe_mysqld]. Both versions are currently parsed.
    [mysqld_safe]
    socket = /var/run/mysqld/mysqld.sock
    nice = 0
    wsrep_urls = gcomm://10.1.1.1:4567,gcomm://10.2.2.2:4567,gcomm://10.3.3.3:4567,gcomm://

    [mysqld]
    #
    # * Basic Settings
    #
    server_id=1
    binlog_format=ROW
    wsrep_provider=/usr/lib64/libgalera_smm.so /
    wsrep_slave_threads=2
    wsrep_cluster_name=dev_cluster
    wsrep_sst_method=rsync
    wsrep_node_name=node1 (this variable changes as per different nodes)
    innodb_locks_unsafe_for_binlog=1
    innodb_autoinc_lock_mode=2
    log_slave_updates
    wsrep_replicate_myisam=1

    now by adding the value in mysqld_safe section the only concern i have on ubuntu 10.04 LTS is that when i reboot the machine and let the rsync process complete….the synced node does not let me make connections…so i have to stop the db as “service mysql stop” and then restart as /etc/init.d/mysql start….and then after resynching and letting the rsync process finish the node start correctly…have you see this behaviour?

    and is their a way to modify the on startup of VM to start using /etc/init.d/mysql start..?

    Thanks again…i am just taking small steps and trying to understand this cluster setup and your suggestions are valuable..

    Reply

  • Hi

    I’ve tried this configuration on a 2 node (third is garbd).
    I’ve notice that when a 2 node join the cluster with the xtrabackup sst method enable the haproxy stops responding to sql query because the 1 node is busy to be a donor.

    Is it a normal behaviour ? this means that this kind of configuration requires at least 3 operational nodes in order to be reliable.

    Thx

    Reply

  • Hi Davide,

    The supplied monitoring script for HaProxy takes the node out if it’s a donor for xtrabackup SST. In some cases this might make sense, since the IO pressure is increased on this node. However, if you want to allow queries on a node, which is a donor for xtrabackup SST, you can just follow Frederic’s advice above and modify clustercheck in the following way.

    -if [ “$WSSREP_STATUS” == “4” ]
    +if [ “$WSSREP_STATUS” == “4” ] || [ “$WSSREP_STATUS” == “2” ]

    Reply

  • Hi mandm,

    No, I haven’t seen anything like this. Can you describe it in a little more detail? What version is this? Did you install from packages? What does the error log say and what exactly does does not let me make connections mean? Also, this discussion might be better moved to percona-discussion@googlegroups.com mailing list.

    Reply

  • Hi Peter

    Thx for help .

    I’ve tried to apply the same concept of using the script clustercheck to check the mysql health with keepalived and vip.
    Basically by assuming 1 node is master and the 2 is slave each keepalived instance on both nodes checks the health of node 1.

    When health is not ok the vip is switched on node 2.
    Node 3 is always garbd.

    Is it a reliable config this kind of vip management ?

    Thx.

    Reply

  • Hi Peter,
    When i use the sys bench tests…

    sysbench –test=oltp –db-driver=mysql –mysql-engine-trx=yes –mysql-table-engine=innodb –mysql-host=127.0.0.1 –mysql-port=3306 –mysql-user=sbtest –mysql-password=sbpass –oltp-table-size=10000 –num-threads=8 run

    and if i run some other application at the same time accessing a different schema other that subtest…i see that the other application has lot of sessions disconnected…with the error connecting to mysql..

    have you seen anything like this?…i am trying hard to debug this issue but no luck so far
    does the cluster have a limit to any commits? ..or does it wait on any sessions to complete..?

    Reply

  • Hi mandm,

    I can confirm what you’re saying because I’m also seeing the same behaviour only by running sysbench on an empty cluster which have no traffic at all ( 3 cluster nodes each with 32GB RAM, Intel(R) Xeon(R) CPU E31270 @ 3.40GHz and 146GB 10k rpm HDD ).
    I think this could be a problem for us in our environment because this cannot be explained at all, no errors on node’s log and only these errors displayed by sysbench when doing the bench :

    FATAL: error 2003: Can’t connect to MySQL server on ‘172.16.29.238’ (99)
    FATAL: failed to connect to database server!
    FATAL: unable to connect to MySQL server, aborting…
    FATAL: error 2003: Can’t connect to MySQL server on ‘172.16.29.238’ (99)
    FATAL: failed to connect to database server!
    FATAL: unable to connect to MySQL server, aborting…
    FATAL: error 2003: Can’t connect to MySQL server on ‘172.16.29.238’ (99)
    FATAL: failed to connect to database server!

    Any informations or advises about this behaviour would be very appreciated.

    Regards,

    Laurent

    Reply

  • @Laurent, @mandm,

    Can you provide more details on this? I tested the same setup described in blog post but with 3 cluster nodes on 3 KVM guests (VDE + dnsmasq) and then running haproxy from the host (with necessary privs).

    I tested with two different sysbench on two different tables to test this and I didn’t see any errors or disconnects.

    Reply

  • Hi Raghavendra,

    Sure, thanks for your interest in this problem, you will find below my informations :

    HAProxy configuration :
    http://pastebin.com/Gc1hcxBc

    Percona XtraDB my.cnf for 1 node, others are almost the same except for ip addresses :
    http://pastebin.com/d7U1Je0d

    And finally my sysbench command line and outputs :
    http://pastebin.com/VS3H8kHy

    Don’t hesitate if you need others informations.
    Thanks, regards

    Laurent

    Reply

  • ok so after debugging for a few days i think the issue was with the machine settings..
    in the /var/log/messages i saw these messages “nf_conntrack: table full, dropping packet”

    and i updated my server config to

    $ cat /proc/sys/net/ipv4/netfilter/ip_conntrack_generic_timeout
    $ echo 120 > /proc/sys/net/ipv4/netfilter/ip_conntrack_generic_timeout

    $ cat /proc/sys/net/ipv4/netfilter/ip_conntrack_tcp_timeout_established
    $ echo 54000 > /proc/sys/net/ipv4/netfilter/ip_conntrack_tcp_timeout_established

    $ cat /sys/module/nf_conntrack/parameters/hashsize
    $ echo 32768 > /sys/module/nf_conntrack/parameters/hashsize

    $ echo 131072 > /proc/sys/net/ipv4/netfilter/ip_conntrack_max
    $ cat /proc/sys/net/ipv4/netfilter/ip_conntrack_max
    131072

    and that seems to have resolved the issue…

    so to help you debug…please check the values during the load test…do this from both the machines (db and load balancer )

    sudo sysctl net.netfilter.nf_conntrack_count net.netfilter.nf_conntrack_max

    once you see the number reach a certain limit , you will see the error for connection timeout..

    Let me know if that fixes it

    Reply

  • Hi!
    I’ve got a question about mysql + haproxy. Here, the mysql user is always granted access since it was created using something like this: ‘sbtest’@’%’, am I right?
    But how can I grant access to, for example, ‘sbtest’@’10.5.4.20’? In this case, won’t Mysql get the haproxy’s IP instead of the client’s IP (10.5.4.20)?

    Thanks!

    Reply

  • Hi Matias,

    You are right. If you run haproxy on each application server and your application connects to localhost everything remains the same from the privilege system’s perspective, otherwise, you will see connections from haproxy’s IP address. This also means that you lose client level access control granularity in MySQL, but in my opinion, this is not that big of a limitation. You can still implement access control for example on the TCP level on haproxy. In most cases haproxy will run on each application or mysql is on a trusted network, where the haproxy instance responsible for database connection is most likely on the same network as the application servers with the same host part and MySQL has that network in the host part of user account.

    Reply

  • Hello, Piter!
    Laurens told that there is solution A (using 3306 port) and B (using 3307 port) with their pluses and minuses.
    But what if to combine both of this methods, when all write operations goes to 3306 port (to one specific PXC node) and all read operations goes to 3307 port (to all PXC nodes). As I read, Percona Cluster will not give much performance in write operations, but read operations will gain much spead. That scheme will slightly reduces write speed (because it will also execute read operations while reading from 3307 port), but no rollback transactions will be done at all, because write executes only on one node.
    In such scheme HAProxy probably should be placed to separate node so the 3306 port points to the same PXE node for all application servers (HA for that node should be also implemented in case of one HAProxy node crash). But as HAProxy a very light, no powerfull servers needed for it.
    Does such scheme make sense these scheme?

    P.S. Sorry for my english, I’m not a native speaker.

    Reply

  • Just completed the setup, and restored a database. Everything looked OK, until I use pt-log-player, and discovered that after 30 seconds, HAProxy drops the session. I have never used HAProxy before so am a little in the dark. I looked at mandm’s suggestion, but that does not help me.
    Basically, if I log into the cluster directly from a remote machine wit mysql then the session stays alive, however, if I log in to the 127.0.0.1 host with port 3306 or 3307, the session stays alive for 30 seconds and then haproxy debug says :

    00000001:pxc-front.accept(0004)=0008 from [127.0.0.1:59633]
    00000001:pxc-back.srvcls[0008:0009]
    00000001:pxc-back.clicls[0008:0009]
    00000001:pxc-back.closed[0008:0009]

    This causes errors with pt-log-player, as sometimes the queries on a perticular thread are more than 30 seconds apart, nd then all other queries on that thread fail.
    pt-log-player says :
    in proc 5 session 233461088 caused an error: MySQL server has gone away
    just after the session has closed. Not all pt-log-player-threads close, but it does get annoying with the error messages clogging up the log files

    Can anyone help me with a keepalive solution?

    Reply

  • I have the same problem with HAProxy.
    A large bulk of queries (reads and writes) causes ‘MySQL server has gone away’, but really mysqld process is alive.
    Error log does not contain anything about process crash.
    If connection via local socket used everything is OK, so problem with HAProxy.

    Debug in the same as Mike has:
    00000000:pxc-referencenode-front.accept(0006)=0009 from [10.0.0.70:34658]
    00000000:pxc-onenode-back.srvcls[0009:000a]
    00000000:pxc-onenode-back.clicls[0009:000a]
    00000000:pxc-onenode-back.closed[0009:000a]

    My suggestion was to increase memory limit for haproxy process, I have increased it to 1GB (bulk size = 700M) but still have the error. My /etc/default/haproxy (I use Ubuntu):

    ENABLED=1
    EXTRAOPTS=”-d -V -de -m 1024″

    percona-xtradb-cluster version is the latest – 5.5-28

    Is here anyone who has already experienced in this and have a solution?
    Thanks in advance.

    Reply

  • A little late, but nonetheless: If you are running into the ‘MySQL server has gone away’ problem, you should change the “timeout server” and “timeout client” values in the haproxy.cfg file according to your “wait_timeout” value of MySQL (28800 by default).

    In my case, I have set the MySQL “wait_timeout” to 3600. So in HAProxy haproxy.cfg, I would add 1 second extra to the “timeout client” and “timeout server” values:

    —
    frontend pxc-front
    timeout client 3601s
    … etc

    frontend pxc-onenode-front
    timeout cilent 3601s
    … etc

    backend pxc-back
    timeout server 3601s
    … etc

    backend pxc-onenode-back
    timeout server 3601s
    … etc
    —

    Again, just make sure it corresponds to your MySQL timeout value.

    Also, if you’re running into problems with exhausted sockets, for one set “net.ipv4.tcp_tw_reuse” to 1 in your /etc/sysctl.conf file (don’t forget to apply with “sysctl -p” 😉 ). You can also add to the “pxc-back” and “pxc-onenode-back” a “source” parameter that includes a very wide port range that overrides the automatic kernel selection of a free port (which often is set to about 28K sockets per source), ie:

    server c1 10.116.39.76:3306 check port 9200 inter 12000 rise 3 fall 3 source 10.116.39.76:10240-65000

    Reply

  • Hello Peter,

    Can you please provide an example how would I need to set up the haproxy for separating the traffic via mysql read for roundrobin and the mysql write failover configuration.

    It would help me alot.
    Thanks

    Reply

  • Bulk answer for the last few comments, sorry for being late on that one.
    For haproxy and mysql server has gone away: this is usually timeout/maxconn mismatch on either haproxy or mysql side. If you are not using persistent connections, it’s relatively easy to use haproxy for queueing with the appropriate maxconn/queue values so your database won’t meet a workload that’s over the desired degree of parallelism.

    Reply

  • Zoltan,

    Haproxy won’t split read and write traffic for you. You need to do that from the application. Haproxy is only a tcp level load balancer here. Just it can make sense to write only to 1 node if you have conflicting writes. With PXC, conflicting writes are causing rollbacks, not locks waits like in a single node case. Rollbacks are io intensive, so they are much more expensive. If you don’t have conflicting write transactions (parallel threads updating the same record for example), you can use just the 3307 port backend in the case above.

    Reply

  • Hi,

    I have implemented the HA proxy and with the XtraDB cluster. I was using your B configuration everything seamed to work just fine but after two days I have find out that certain mysql updates and inserts are not sent to the database by the application via HAproxy. When I have disabled the HAproxy and I have set the application to work directly with one of the Xtradb nodes everything works properly. Would you have information about this how can I fix this behaviour?

    Thanks for your help in advance.

    Reply

  • Hi Zoltan,

    With B configuration, only 1 node is written at a time. If you lost inserts from the application, there can be many reasons for that, but none of this is haproxy. Haproxy in this setup doesn’t know what mysql is, it only acts as a tcp level load balancer. So, one tcp session comes in, it goes to one node, if an other TCP session comes in, it goes to an other node, and that’s it. What I think could happen is that your application tried to use a connection, which was already established (which can be fairly common if you are using persistent connections), when haproxy decided to use an other node. From the other node, the application got a TCP RST, and reconnected, but it did not retry the failed insert. I can also imagine that for example 2 nodes were down from that cluster and the third one decided to shut down galera, in order to prevent split brain (because it was in minority). In this case, writes to this node will produce an error, but the application has to handle that.

    There can be other reasons, only from this, I don’t know why certain inserts and updates are failing, maybe haproxy maxconn, maxqueue and timeout options are too strict, and these sessions fail on the TCP level, maybe haproxy is using one core completely, and it is just not able to cope with your load with a single thread, and you have to use nbproc to have multiple.

    Only from this information, it’s very hard to tell what is happening exactly, but I hope I could give you a few pointers.

    Reply

  • Hello,

    Thanks for your quick reply. As I read your comment I can tell that all the nodes were up but the reason I can see would be because there is a certain time between the mysql connection is made and the time when the insert or update is done since this is a voip application and these are for the cdrs therefore the application waits until the conversation is done and only then it inserts the proper values into the database. I could see that the inserts/updates are not happening if the application waits for at least 60 seconds. Is there a way I can increase the wait time? There might be cases where it has to be up to an hour or so.

    Thanks,

    Reply

  • Hi Zoltan,

    That might cause an issue if there is a failover during that time, and the application doesn’t handle that well. A good practice here would be not having the connection made, until it’s necessary. In this haproxy configuration, the timeout on the client and on the server side is 50000 seconds, it very unlikely that you hit that. You can check wait timeout at the mysql level, or timeout values in the TCP stack for the database, haproxy and the application also.

    Reply

  • I forgot to mention that this connection reset has to come from the Haproxy since we don’t experience it when we connect directly to the xtradb cluster without the haproxy.

    Reply

  • Hi,

    I followed your suggestion and I found out that the problem wasn’t at all the cluster but more like the haproxy configuration. By default it has the maxconn set for 2000 and the haproxy stopped sending the queries after it has reached that 2000 connections By disabling that setting the system started to work as expected without dropping anymore inserts.

    Thanks for your help and suggestions.

    Reply

  • Thanks for this really nice tutorial but when running sysbench like you do I get

    $ sysbench –test=oltp –db-driver=mysql –mysql-engine-trx=yes –mysql-table-engine=innodb –mysql-host=haproxy –mysql-port=3307 –mysql-user=sbtest –mysql-password=sbpass –oltp-table-size=10000 –num-threads=8 run
    sysbench 0.4.12: multi-threaded system evaluation benchmark

    Running the test with following options:
    Number of threads: 8

    Doing OLTP test.
    Running mixed OLTP test
    Using Special distribution (12 iterations, 1 pct of values are returned in 75 pct cases)
    Using “BEGIN” for starting transactions
    Using auto_inc on the id column
    Maximum number of requests for OLTP test is limited to 10000
    Threads started!
    ALERT: failed to execute mysql_stmt_execute(): Err1317 Query execution was interrupted
    FATAL: database error, exiting…
    Done.

    Another test with mysqlslap :

    $ mysqlslap -vv -u root -h haproxy -P 3307 –number-of-queries=150 –iterations=50 –concurrency=5 –engine=innodb –auto-generate-sqlBuilding Create Statements for Auto
    Building Query Statements for Auto
    Parsing engines to use.
    Starting Concurrency Test
    Loading Pre-data
    Generating primary key list
    mysqlslap: Cannot run query INSERT INTO t1 VALUES (100669,’qnMdipW5KkXdTjGCh2PNzLoeR0527frpQDQ8uw67Ydk1K06uuNHtkxYBxT5w8plb2BbpzhwYBgPNYX9RmICWGkZD6fAESvhMzH3yqzMtXoH4BQNylbK1CmEIPGYlC6′) ERROR : Deadlock found when trying to get lock; try restarting transaction

    So I am having this obscure deadlock problem… Why have your tests been successful ?

    kind regards

    Reply

  • Hi jzee,

    That is not obscure, it’s coming from conflicting record updates because of optimistic locking. If you write on multiple nodes, if 2 nodes are trying to write the same record, one of them has to roll back, and you will get the error message you are getting.

    Wether you hit this or not is dependent on the speed of your infrastructure and the size of your table in this benchmark. In order to avoid that, make the table size bigger. In production, make sure you retry failed transactions, if you are writing on all nodes, and make sure you don’t have too much conflicting writes.

    Reply

  • hi all, so lately i have noticed some errors similar to this “blocked because of many connection errors; unblock with ‘mysqladmin flush-hosts’ on a lot of my clusters

    what could be a reason if any? i am guess its something to do with the mysql haproxy connections
    and i am using the haproxy configuration exactly mentioned in this post, will it have to do with any haproxy connection timeouts or any other server parameter..

    from the forum i understand that i can increase the max_connection_error timeout to be huge ? but i think thats just neglecting the issue

    do you guys have any comments?

    Reply

  • Hello Peter,

    I have set up a n xtradb cluster and after about two months I have observed that one of the nodes are down. When I tried to start it up the server does not starts and I can see in the xtradb.err the following error:
    WSREP_SST: [ERROR] Error while getting st data from donor node: 1, 2 (20130318 11:02:37.926)
    130318 11:02:37 [ERROR] WSREP: Process completed with error: wsrep_sst_xtrabackup –role ‘joiner’ –address ‘10.0.0.11’ –auth ” –datadir ‘/var/lib/mysql/’ –defaults-file ‘/etc/my.cnf’ –parent ‘26835’: 32 (Broken pipe)
    130318 11:02:37 [ERROR] WSREP: Failed to read uuid:seqno from joiner script.
    130318 11:02:37 [ERROR] WSREP: SST failed: 32 (Broken pipe)
    130318 11:02:37 [ERROR] Aborting

    Could you please give me some hints how to fix this?

    regards,
    Peter

    Reply

  • Hi Peter,
    Great directions. However, I’ve run into a bit of an issue…

    I have a 3 node cluster with a dedicated server running haproxy. All are VMs.

    Clustercheck says each node is synched when running utility locally on each. However, on the /haproxy/stats it says all nodes are down. Any suggestions what might cause this?

    All nodes are listening on port 9200 for mysqlchk. Please let me know. Thanks!

    Moody

    Reply

  • Moody,

    I had this just before, and fixed it by editing /etc/services and restarting xinetd. Maybe that helps.

    Reply

  • Peter,

    First, Thank you for all the great research and work.

    Some minor problem when we running the xinetd cluster checker, it seems that different application server that has haproxy installed will fail when we try to use curl to check servers in different availability zones.

    I don’t know if this is necessary, but in case some one needs it, we, TOTVS Inc., like to contribute this for people to use:

    1. create init script that will run this cluster check:

    $ more /etc/init.d/mysqlchk

    #!/bin/bash -e
    ### BEGIN INIT INFO
    # Provides: mysqlchk
    # Required-Start:
    # Required-Stop:
    # Default-Start: 2 3 4 5
    # Default-Stop: 0 1 6
    # Short-Description:
    # Description: script to start/stop mysql cluster status check server
    ### END INIT INFO

    case “$1” in
    start)
    nohup python /etc/mysql/clustercheck.py > /dev/null 2>&1 &
    ;;
    stop)
    ps ax | grep -i “clustercheck.py” 2>/dev/null | grep -v grep | awk ‘{print $1}’ | xargs kill -15 2>/dev/null
    sleep 5
    ;;
    *)
    echo “Usage: $0 {start|stop}” >&2
    exit 1
    ;;
    esac

    2, create a small python program that serves the request:

    $ more /etc/mysql/clustercheck.py

    import sys
    import time
    import BaseHTTPServer
    from SimpleHTTPServer import SimpleHTTPRequestHandler
    from subprocess import Popen, PIPE

    HOST_NAME = ‘0.0.0.0’
    PORT_NUMBER = 9200

    class MySQLClusterCheckHandler(BaseHTTPServer.BaseHTTPRequestHandler):
    def do_HEAD(s):
    s.send_response(200)
    s.send_header(“Content-type”, “text/plain”)
    s.end_headers()
    def do_OPTIONS(self):
    self.send_response(200, “ok”)
    self.send_header(‘Access-Control-Allow-Origin’, “*”)
    self.send_header(‘Access-Control-Allow-Methods’, ‘HEAD, GET, POST, OPTIONS’)
    def do_GET(s):
    “””Respond to a GET request.”””
    “””if you want to use path parameter like …/user/password, then s.path will be /user/password”””
    MYSQL_USERNAME=”clustercheckuser”
    MYSQL_PASSWORD=”X52m83S792J72Q4″
    AVAILABLE_WHEN_DONOR=0
    ERR_FILE=”/dev/null”
    “””Timeout exists for instances where mysqld may be hung”””
    TIMEOUT=10

    proc=Popen([ \
    “/usr/bin/mysql”, \
    “-nNE”, \
    “–connect-timeout=” + str(TIMEOUT), \
    “–user=” + MYSQL_USERNAME, \
    “–password=” + MYSQL_PASSWORD, \
    “-e”, \
    “SHOW STATUS LIKE \’wsrep_local_state\’;” \
    ], stdout=PIPE)

    try:
    WSREP_STATUS=proc.stdout.read().split(“\n”)[-2]
    except IndexError:
    WSREP_STATUS=””

    if (WSREP_STATUS == “4”) or (WSREP_STATUS == “2” and AVAILABLE_WHEN_DONOR == 1):
    s.send_response(200)
    s.send_header(“Content-type”, “text/plain”)
    s.end_headers()
    s.wfile.write(“Percona XtraDB Cluster Node is synced.\r\n”)
    else:
    s.send_response(503)
    s.send_header(“Content-type”, “text/plain”)
    s.end_headers()
    s.wfile.write(“Percona XtraDB Cluster Node is not synced.\r\n”)
    s.connection.shutdown(1)

    if __name__ == ‘__main__’:
    HandlerClass = MySQLClusterCheckHandler
    ServerClass = BaseHTTPServer.HTTPServer
    Protocol = “HTTP/1.0”

    if sys.argv[1:]:
    port = int(sys.argv[1])
    else:
    port = PORT_NUMBER

    server_address = (HOST_NAME, port)

    HandlerClass.protocol_version = Protocol
    httpd = ServerClass(server_address, HandlerClass)

    sa = httpd.socket.getsockname()
    print time.asctime(), “Server Starts – %s:%s” % (HOST_NAME, port)
    try:
    httpd.serve_forever()
    except KeyboardInterrupt:
    pass
    httpd.server_close()
    print time.asctime(), “Server Stops – %s:%s” % (HOST_NAME, port)

    3. uninstall xinetd if you don’t need it, then run service mysqlchk start

    Reply

  • 131013 01:24:33 mysqld_safe Starting mysqld daemon with databases from /data/perconacluster
    131013 01:24:33 mysqld_safe Skipping wsrep-recover for empty datadir: /data/perconacluster
    131013 01:24:33 mysqld_safe Assigning 00000000-0000-0000-0000-000000000000:-1 to wsrep_start_position
    131013 1:24:33 [Note] WSREP: wsrep_start_position var submitted: ‘00000000-0000-0000-0000-000000000000:-1’
    131013 1:24:33 [Note] WSREP: Read nil XID from storage engines, skipping position init
    131013 1:24:33 [Note] WSREP: wsrep_load(): loading provider library ‘/usr/lib64/libgalera_smm.so’
    131013 1:24:33 [Note] WSREP: wsrep_load(): Galera 2.7(r157) by Codership Oy loaded succesfully.
    131013 1:24:33 [Warning] WSREP: Could not open saved state file for reading: /data/perconacluster//grastate.dat
    131013 1:24:33 [Note] WSREP: Found saved state: 00000000-0000-0000-0000-000000000000:-1
    131013 1:24:33 [Note] WSREP: Preallocating 134219048/134219048 bytes in ‘/data/perconacluster//galera.cache’…
    131013 1:24:33 [Note] WSREP: Passing config to GCS: base_host = 192.168.47.136; base_port = 4567; cert.log_conflicts = no; gcache.dir = /data/perconacluster/; gcache.keep_pages_size = 0; gcache.mem_size = 0; gcache.name = /data/perconacluster//galera.cache; gcache.page_size = 128M; gcache.size = 128M; gcs.fc_debug = 0; gcs.fc_factor = 1; gcs.fc_limit = 16; gcs.fc_master_slave = NO; gcs.max_packet_size = 64500; gcs.max_throttle = 0.25; gcs.recv_q_hard_limit = 9223372036854775807; gcs.recv_q_soft_limit = 0.25; gcs.sync_donor = NO; replicator.causal_read_timeout = PT30S; replicator.commit_order = 3
    131013 1:24:34 [Note] WSREP: Assign initial position for certification: -1, protocol version: -1
    131013 1:24:34 [Note] WSREP: wsrep_sst_grab()
    131013 1:24:34 [Note] WSREP: Start replication
    131013 1:24:34 [Note] WSREP: Setting initial position to 00000000-0000-0000-0000-000000000000:-1
    131013 1:24:34 [Note] WSREP: protonet asio version 0
    131013 1:24:34 [Note] WSREP: backend: asio
    131013 1:24:34 [Note] WSREP: GMCast version 0
    131013 1:24:34 [Note] WSREP: (29630c5c-3363-11e3-af54-4383bb91a7fc, ‘tcp://0.0.0.0:4567’) listening at tcp://0.0.0.0:4567
    131013 1:24:34 [Note] WSREP: (29630c5c-3363-11e3-af54-4383bb91a7fc, ‘tcp://0.0.0.0:4567’) multicast: , ttl: 1
    131013 1:24:34 [Note] WSREP: EVS version 0
    131013 1:24:34 [Note] WSREP: PC version 0
    131013 1:24:34 [Note] WSREP: gcomm: connecting to group ‘my_centos_cluster’, peer ‘192.168.47.135:,192.168.47.136:,192.168.47.137:’
    131013 1:24:34 [Warning] WSREP: (29630c5c-3363-11e3-af54-4383bb91a7fc, ‘tcp://0.0.0.0:4567’) address ‘tcp://192.168.47.136:4567’ points to own listening address, blacklisting
    131013 1:24:37 [Warning] WSREP: no nodes coming from prim view, prim not possible
    131013 1:24:37 [Note] WSREP: view(view_id(NON_PRIM,29630c5c-3363-11e3-af54-4383bb91a7fc,1) memb {
    29630c5c-3363-11e3-af54-4383bb91a7fc,
    } joined {
    } left {
    } partitioned {
    })
    131013 1:24:37 [Warning] WSREP: last inactive check more than PT1.5S ago (PT3.50887S), skipping check
    131013 1:25:07 [Note] WSREP: view((empty))
    131013 1:25:07 [ERROR] WSREP: failed to open gcomm backend connection: 110: failed to reach primary view: 110 (Connection timed out)
    at gcomm/src/pc.cpp:connect():139
    131013 1:25:07 [ERROR] WSREP: gcs/src/gcs_core.c:gcs_core_open():195: Failed to open backend connection: -110 (Connection timed out)
    131013 1:25:07 [ERROR] WSREP: gcs/src/gcs.c:gcs_open():1291: Failed to open channel ‘my_centos_cluster’ at ‘gcomm://192.168.47.135,192.168.47.136,192.168.47.137’: -110 (Connection timed out)
    131013 1:25:07 [ERROR] WSREP: gcs connect failed: Connection timed out
    131013 1:25:07 [ERROR] WSREP: wsrep::connect() failed: 6
    131013 1:25:07 [ERROR] Aborting

    131013 1:25:07 [Note] WSREP: Service disconnected.
    131013 1:25:08 [Note] WSREP: Some threads may fail to exit.
    131013 1:25:08 [Note] /usr/sbin/mysqld: Shutdown complete

    131013 01:25:08 mysqld_safe mysqld from pid file /data/perconacluster/percona-cluster-node02.pid ended

    what the fuck

    Reply

  • While running HAProxy, my WebApp does not look to the second or third node when the first node fails.

    I have it set to:

    backend pxc-onenode-back
    mode tcp
    balance leastconn
    option httpchk
    server server1 192.168.35.11:3306 check port 9260 inter 12000 rise 3 fall 3
    server server2 192.168.35.12:3306 check port 9260 inter 12000 rise 3 fall 3 backup
    server server3 192.168.35.13:3306 check port 9260 inter 12000 rise 3 fall 3 backup

    Cluster is 100% happy, but when one node goes down, the App does not choose the second node, it stays on the first node and just waits.

    Any ideas?

    Reply

  • Hi Jusin,

    I have 2 ideas for you:
    – The app is using persistent connections. Haproxy does the load balancing at the TCP level, and once a connection is in established state, it has nothing to do with haproxy any more. The cure for this can be haproxy 1.5, which can kill all connections when a backend goes down.
    – You are not connecting to haproxy, but the cluster node directly.

    Let us know how it goes.

    Reply

  • Peter,

    I am struggling with the decision on where to place HAProxy. I like the idea of placing HAProxy on the application server, but I worry that it would quickly become unmanageable as we scale our app servers, and all the checks might become overwhelming. The other option I’ve considered is having 2 HAProxy servers that all the app servers connect to. I like this approach better, since it seems more manageable, but how much impact will the 2xRTT have? Would doing the HAProxy on the application server be worth the extra configuration and management, or does the RTT have minimal impact?

    Any information would be helpful. Thanks!

    Dan

    Reply

  • Hi Daniel,

    You are right about the overwhelming checks with a large number of application servers, perhaps the topic desires it’s own blog post. The trick what you can do here is making the checks smart, meaning that haproxy only expcets a 200 or 300 response code from a backend to be considered up, or a 400 or 500 to be considered down. You can write a small daemon which runs on the database servers, and checks the local databases every 300 – 500 ms. After that, it can serve the previous check result until the next check to hundreds of application servers via http. The cost of the check itself can be greatly reduced this way, while the functionality doesn’t change much.

    Reply

  • Hi Peter
    Thanks for your post. I have the following issue when setting up the client (CentOS 7.1) and starting the haproxy
    haproxy-systemd-wrapper[4301]: [ALERT] 273/111148 (4302) : Starting frontend pxc-front: cannot bind socket [0.0.0.0:3306]

    I can connect from client to the cluster nodes as clusteruser with no problem.

    Is it something else I should do?

    Thanks,

    George

    Reply

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