Tag Archives: Galera

Securing MariaDB Galera Cluster using SSL

In MariaDB Galera cluster, the data (writeset) is replicated across all the nodes over network. This data traffic can be secured by enabling SSL connection between the nodes. In order to achieve this, one needs to have SSL certificate/key pair (Here’s how to generate one). The certificate and key can then be copied to all the nodes. Once done, instruct the nodes to establish SSL connection on startup by simply pointing to the certificate/key files using wsrep_provider_options system variable.


Following is the list of SSL options that Galera supports (details : Galera parameters:

  • socket.ssl : Enable/disable SSL, explicitly
  • socket.ssl_ca : SSL CA file
  • socket.ssl_cert : SSL certificate file
  • socket.ssl_cipher : SSL cipher list
  • socket.ssl_compression : Enable/disable SSL compression
  • socket.ssl_key : SSL key file
  • socket.ssl_password_file : SSL password file, in case the key is encrypted

At the time of writing, there is no way to check if galera connection is encrypted using SQL (issue#165). The only way is to look into the node’s error log for the following :

150516 14:22:03 [Note] WSREP: SSL handshake successful, remote endpoint ssl:// local endpoint ssl:// cipher: AES128-SHA compression: 
150516 14:22:03 [Note] WSREP: (6f49f928, 'ssl://') turning message relay requesting on, nonlive peers: 
150516 14:22:03 [Note] WSREP: declaring 73263ae6 at ssl:// stable

Lastly, it is important to note that SST (snapshot state transfer) traffic is not affected, whatsoever, by the use of galera SSL options.

MariaDB 10.1 : A new version of GRA_X_X.log file

When a MariaDB Galera cluster node fails to apply a binary log (or writeset, as we call it), the node dumps it into a file (GRA_X_X.log) under the data directory for further investigation. This process has been explained fairly well in this Percona blog). Since the dumped log file is header-less, one has to first prepend a binary log header to it in order to open it using mysqlbinlog tool.

Starting MariaDB 10.1.4, the node will automatically prepend the binlog header to the writeset before dumping it into the GRA_ log file. In order to differentiate it from the older log files, the file has been renamed to GRA_X_X_v2.log.

$ ./bin/mysqlbinlog data2/GRA_1_1_v2.log

/*!40019 SET @@session.max_insert_delayed_threads=0*/;
# at 4
#150414 17:50:20 server id 0  end_log_pos 248  Start: binlog v 4, server v 10.1.4-MariaDB-wsrep-debug created 150414 17:50:20 at startup
# at 248
#150414 17:50:20 server id 0  end_log_pos 76   Query   thread_id=4 exec_time=0 error_code=0
use `test`/*!*/;
SET TIMESTAMP=1429048220/*!*/;
SET @@session.pseudo_thread_id=4/*!*/;
SET @@session.foreign_key_checks=1, @@session.sql_auto_is_null=0, @@session.unique_checks=1, @@session.autocommit=1/*!*/;
SET @@session.sql_mode=0/*!*/;
SET @@session.auto_increment_increment=1, @@session.auto_increment_offset=1/*!*/;
/*!\C utf8 *//*!*/;
SET @@session.character_set_client=33,@@session.collation_connection=33,@@session.collation_server=8/*!*/;
SET @@session.lc_time_names=0/*!*/;
SET @@session.collation_database=DEFAULT/*!*/;
drop table t1
# End of log file
ROLLBACK /* added by mysqlbinlog */;

Understanding split brain in a Galera cluster

Split brain is a condition when a cluster gets partitioned and each part is operating independently. This is an unwanted situation that one would always want to avoid. So, how is it handled in a MariaDB Galera cluster? In order to understand this, let’s first start by looking into the logs of a node from a split-brain(ed) cluster.

In the event of a network partition, some nodes of the cluster may no longer be reachable from the other nodes. They try to reconnect to these suspecting nodes and later move them to partitioned list by marking them as inactive when no response is received. A voting for quorum is then taken on each node to see if they belong to the majority partition (Primary Component) using the following formula :


  • : members of the last seen primary component,
  • : members that are known to have left gracefully,
  • : current components members, and
  • : member’s weight

In a Galera cluster, nodes outside the primary component are not allowed to process queries. It is mainly done preserve data consistency.

MariaDB [test]> select 1;
ERROR 1047 (08S01): WSREP has not yet prepared node for application use

Now, as shown in the logs above, when the cluster gets split into two partitions of equal size, (i.e. both the partitions get equal weight, split-brain), the quorum algorithm fails find the the primary component. As a result, the cluster has no primary component and can no longer process any queries. This can be resolved by finding the node with most recent updates and bootstraping the cluster using that node.

Reference: http://galeracluster.com/documentation-webpages/weightedquorum.html

New Information schema tables for Galera membership and status

MariaDB Galera server logs all the cluster related information like node status, cluster status, membership, etc. in the error log. MariaDB 10.1.2 introduces a new INFORMATION SCHEMA plugin WSREP_INFO that enables querying these information via INFORMATION SCHEMA tables. The WSREP_INFO plugin adds two new tables to the Information Schema, WSREP_MEMBERSHIP and WSREP_STATUS. The plugin is not enabled by default, so in order to use it, it needs to be installed first :

MariaDB [test]> INSTALL SONAME 'wsrep_status';
Query OK, 0 rows affected (0.04 sec)

MariaDB [test]> SHOW PLUGINS;
| Name                        | Status   | Type               | Library         | License |
| WSREP_MEMBERSHIP            | ACTIVE   | INFORMATION SCHEMA | wsrep_status.so | GPL     |
| WSREP_STATUS                | ACTIVE   | INFORMATION SCHEMA | wsrep_status.so | GPL     |
52 rows in set (0.01 sec)

*************************** 1. row ***************************
  `INDEX` int(11) NOT NULL DEFAULT '0',
  `UUID` varchar(36) NOT NULL DEFAULT '',
  `NAME` varchar(32) NOT NULL DEFAULT '',
  `ADDRESS` varchar(256) NOT NULL DEFAULT ''
1 row in set (0.00 sec)

*************************** 1. row ***************************
       Table: WSREP_STATUS
  `NODE_STATUS` varchar(16) NOT NULL DEFAULT '',
  `GAP` varchar(10) NOT NULL DEFAULT '',
1 row in set (0.00 sec)

MariaDB [test]> SELECT @@wsrep_provider;
| @@wsrep_provider                 |
| /usr/lib/galera/libgalera_smm.so |
1 row in set (0.00 sec)

Now that WSREP_INFO plugin is installed, lets look into the contents of these tables on a 3-node cluster.

| INDEX | UUID                                 | NAME     | ADDRESS         |
|     0 | 19058073-8940-11e4-8570-16af7bf8fced | my_node1 | |
|     1 | 19f2b0e0-8942-11e4-9cb8-b39e8ee0b5dd | my_node3 | |
|     2 | d85e62db-8941-11e4-b1ef-4bc9980e476d | my_node2 | |
3 rows in set (0.00 sec)

*************************** 1. row ***************************
         NODE_INDEX: 0
        NODE_STATUS: Synced
     CLUSTER_STATUS: Primary
       CLUSTER_SIZE: 3
 CLUSTER_STATE_UUID: 190604d7-8940-11e4-a084-ebee5211c190
                GAP: NO
1 row in set (0.00 sec)

As seen above, WSREP_MEMBERSHIP table shows information about current members in the cluster which includes node’s name and incoming address. WSREP_STATUS table, on the other hand, shows status information about the node and cluster as a whole.

SHOW command can also be used to query these tables. Its quick and reduces the number of columns for WSREP_STATUS to fit to the screen.

| Index | Uuid                                 | Name     | Address         |
|     0 | 19058073-8940-11e4-8570-16af7bf8fced | my_node1 | |
|     1 | 19f2b0e0-8942-11e4-9cb8-b39e8ee0b5dd | my_node3 | |
|     2 | d85e62db-8941-11e4-b1ef-4bc9980e476d | my_node2 | |
3 rows in set (0.00 sec)

| Node_Index | Node_Status | Cluster_Status | Cluster_Size |
|          0 | Synced      | Primary        |            3 |
1 row in set (0.00 sec)

The curious case of lost data

There is no doubt that Galera makes a fine clustering solution. The ease at which one can setup a cluster is incredible. Recently, a query from a user on #mariadb@freenode, however, revealed a caveat which I feel is quite important and worth sharing.

Sep 10 14:16:09 <____> nirbhay: executive summary, my cluster was out of sync, and when i brought it back into sync the DB is now 3 weeks old. it updated the new node with the old node’s contents.
Sep 10 14:16:23 <____> nirbhay: galera + maria 5.5

Let me first try to explain the problem. The user has a MariaDB Galera cluster – up and running – but with stale data (3 weeks old to be precise) and another MariaDB Galera node running outside the cluster storing latest updates. Now, the user takes this node and adds to the cluster, thinking that out-of-sync cluster nodes would receive latest changes from this joining node. Unfortunately, it turned out that opposite happened – and this joiner node instead became stale (losing 3 weeks worth of updates!) when into synced with the cluster.

Actually, this is expected. When a node joins an existing Galera cluster, it receives data from one of the nodes (donor) of the cluster in order to get in sync with the cluster. During this process (aka snapshot state transfer) the node itself loses it own data (if any). This is precisely the reason why the joiner node (with latest updates) lost the recent changes in the above mentioned scenario.

So, how to handle such situations? In cases like this, one should discard the existing stale cluster and bootstrap a new cluster using the node having latest updates.

Snapshot State Transfer (SST) methods in MariaDB Galera Cluster

In a Galera cluster, when a node (joiner) joins the cluster, it receives a copy of entire data from one of the nodes (donor) in the cluster. This process is called Snapshot state transfer (SST). MariaDB Galera Cluster provides various methods for snapshot state transfer which can be configured using wsrep_sst_method option. MariaDB Galera cluster distribution currently includes the following SST methods :

  • rsync
  • It is the default method which uses rsync to transfer data files across the cluster. The donor node itself becomes READ-ONLY during the transfer by executing FLUSH TABLES WITH READ LOCK. It is important to note that even though the data transfer is fast, this method requires no authentication (wsrep_sst_auth).

  • mysqldump
  • This method uses mysqldump tool to get a dump of all the databases on donor node which is then played/executed on the joiner node. This method requires wsrep_sst_auth to be set with credentials to connect to the donor as well as joiner node.

  • xtrabackup/xtrabackup-v2
  • This method uses Percona XtraBackup tool to take a backup (snapshot) of donor’s data directory which is then restored on the joiner node. This method requires wsrep_sst_auth to be set with credentials to connect to the donor node.

Configuring SELinux for Galera cluster

Setting up a MariaDB Galera cluster can easily become tedious if its being setup on a Linux system with SELinux enabled.

140805 16:16:20 [Note] WSREP: gcomm: connecting to group 'my_wsrep_cluster', peer ''
140805 16:16:20 [ERROR] WSREP: Permission denied
140805 16:16:20 [ERROR] WSREP: failed to open gcomm backend connection: 13: error while trying to listen 'tcp://', asio error 'Permission denied': 13 (Permission denied)
         at gcomm/src/asio_tcp.cpp:listen():814
140805 16:16:20 [ERROR] WSREP: gcs/src/gcs_core.c:gcs_core_open():202: Failed to open backend connection: -13 (Permission denied)
140805 16:16:20 [ERROR] WSREP: gcs/src/gcs.c:gcs_open():1291: Failed to open channel 'my_wsrep_cluster' at 'gcomm://': -13 (Permission denied)
140805 16:16:20 [ERROR] WSREP: gcs connect failed: Permission denied
140805 16:16:20 [ERROR] WSREP: wsrep::connect() failed: 7
140805 16:16:20 [ERROR] Aborting

In a test environment one can disable/enable SELinux for mysqld using the following commands (Thanks to Daniel Black for the tip!) :

$ sudo semanage permissive -a mysqld_t
$ sudo semanage permissive -d mysqld_t

Disabling SELinux for mysql works, but its certainly not the best solution. So, I tried to configure SELinux for a 2-node MariaDB Galera cluster on CentOS by using some tools provided to manage SELinux policies. The basic idea is to let the MariaDB Galera nodes run under permissive mode in order to get all possible operations (which SELinux would have otherwise denied) logged into the audit log and then create a policy module using allow2audit tool after carefully analyzing the “denials”. The resulting module can then be installed before enabling (enforcing) SELinux for mysqld again.

Prepare the hosts
* Install MariaDB Galera server packages ($ sudo yum install MariaDB-Galera-server)
* Setup MariaDB configuration options ($sudo vi /etc/my.cnf.d/server.cnf)
* Install SELinux policy management tools ($ sudo yum install policycoreutils-python)
* Firewall settings (see resources below)

Generate the policy module
* Disable SELinux for mysqld on both the hosts. With SELinux in permissive mode, it logs all the denial operations as warnings instead of enforcing them.

$ sudo semanage permissive -a mysqld_t
  • Once mysqld on both the hosts are in permissive mode, our goal is to trigger all sorts of events that can happen on a node in a MariaDB Galera cluster (the more extensive, the better!) like, starting the node as donor/joiner with different snapshot state transfer (SST) methods and incremental state transfer (IST). The idea is to let all possible denials get logged into the audit log, which we later use to generate the policy module.
  • Carefully analyze (sealert Messages) all the “denials” logged in the audit log (/var/log/audit/audit.log). If the denials are expected, create a local policy module using allow2audit.
  $ sudo grep mysql audit.log | audit2allow -M mariadb-galera
  • Install the policy module.
  $ sudo semodule -i mariadb-galera.pp
  • Put mysqld back to enforcing mode.
$ sudo semanage permissive -d mysqld_t


mariadb-galera-server deb dependency graph

The following commands can be used to generate dependency graph for a debian package:

# Get relationship between packages (.dot)
$ debtree --no-conflicts --no-provides --with-suggests --max-depth=1 mariadb-galera-server-5.5 > mariadb-galera-server-5.5.dot
# Generate the image
$ dot -T png -o mariadb-galera-server-5.5.png mariadb-galera-server-5.5.dot

Reference: http://askubuntu.com/a/261808

Galera: runtime adjustment of applier threads

In a MariaDB Galera node, writesets received from other node(s) can be applied parallely through multiple applier threads. The number of slave applier threads is controlled by server’s wsrep_slave_threads system variable. Its a dynamic variable and thus the number of slave applier threads can be adjusted in runtime. The current number of slave applier threads can be checked through “SHOW PROCESSLIST” or wsrep_thread_count status variable (MDEV-6206).

One interesting point to note here is that when the number of @@global.wsrep_slave_threads is increased at runtime, the additional requested applier threads gets spawned immediately. However, when the number is decreased, the effect can not be noticed immediately. What happens internally is that when the number is decreased, the extra applier threads are not killed right away. The process is deferred, and the extra threads exit gracefully only after each apply one last writeset (transaction) after receiving it. So, one will not notice the number of applier threads decreasing on an idle node. The thread count will decrease only after the node starts receiving writesets to apply.

Here are some snippets to help you understand this further :

1. Calculate the change.

  wsrep_slave_count_change += (var->value->val_int() - wsrep_slave_threads);

2a: If its positive, spawn new applier threads.

  if (wsrep_slave_count_change > 0)
    wsrep_slave_count_change = 0;

2b: else mark the thread as “done” after it has applied (commits or rollbacks) the given last writeset.

wsrep_cb_status_t wsrep_commit_cb(void*         const     ctx,
                                  uint32_t      const     flags,
                                  const wsrep_trx_meta_t* meta,
                                  wsrep_bool_t* const     exit,
                                  bool          const     commit)
  if (commit)
    rcode = wsrep_commit(thd, meta->gtid.seqno);
    rcode = wsrep_rollback(thd, meta->gtid.seqno);


  if (wsrep_slave_count_change < 0 && commit && WSREP_CB_SUCCESS == rcode)
    if (wsrep_slave_count_change < 0)
      *exit = true;

And the thread exits :

static void wsrep_replication_process(THD *thd)
  rcode = wsrep->recv(wsrep, (void *)thd);
  DBUG_PRINT("wsrep",("wsrep_repl returned: %d", rcode));

  WSREP_INFO("applier thread exiting (code:%d)", rcode);