Scriptable State Snapshot Transfers¶
When a node sends and receives a State Snapshot Transfer, it manage it through processes that run external to the database server. If you need more from these processes than the default behavior provides, Galera Cluster provides an interface for custom shell scripts to manage state snapshot transfers on the node.
Using the Common SST Script
Galera Cluster includes a common script for managing a State Snapshot Transfer, which you can use as a starting point in building your own custom script. The filename is wsrep_sst_common. For Linux users, the package manager typically installs it for you in /usr/bin.
The common SST script provides ready functions for parsing argument lists, logging errors, and so on. There are no constraints on the order or number of parameters it takes. You can add new parameters and ignore any of the existing ones as you prefer.
It assumes that the storage engine initialization on the receiving node takes place only after the state transfer is complete. Meaning that it copies the contents of the source data directory to the destination data directory (with possible variations).
State Transfer Script Parameters
When Galera Cluster starts an external process for state snapshot transfers, it passes a number of parameters to the script, which you can use in configuring your own state transfer script.
General Parameters
These parameters are passed to all state transfer scripts, regardless of method or whether the node is sending or receiving:
--role The script is given a string, either donor or joiner, to indicate whether the node is using it to send or receive a state snapshot transfer.
--address The script is given the IP address of the Joiner Node.
When the script is run by the joiner, the node uses the value of either the wsrep_sst_receive_address parameter or a sensible default formatted as <ip_address>:<port>. When the script is run by the donor, the node uses the value from the state transfer request.
--auth The script is given the node authentication information.
When the script is run by the joiner, the node uses the value given to the wsrep_sst_auth parameter. When the script is run by the donor, it uses the value given by the state transfer request.
--datadir The script is given the path to the data directory. The value is drawn from the mysql_real_data_home parameter.
--defaults-file The script is given the path to the my.cnf configuration file.
The values the node passes to these parameters varies depending on whether the node calls the script to send or receive a state snapshot transfer. For more information, see Calling Conventions below.
Donor-specific Parameters
These parameters are passed only to state transfer scripts initiated by a node serving as the Donor Node, regardless of the method being used:
--gtid The node gives the Global Transaction ID, which it forms from the state UUID and the sequence number, or seqno, of the last committed transaction.
--socket The node gives the local server socket for communications, if required.
--bypass The node specifies whether the script should skip the actual data transfer and only pass the Global Transaction ID to the receiving node. That is, whether the node should initiate an Incremental State Transfer.
Logical State Transfer-specific Parameters
These parameters are passed only to the wsrep_sst_mysqldump state transfer script by both the sending and receiving nodes:
--user The node gives to the script the database user, which the script then uses to connect to both donor and joiner database servers. Meaning, this user must be the same on both servers, as defined by the wsrep_sst_auth parameter.
--password The node gives to the script the password for the database user, as configured by the wsrep_sst_auth paraemter.
--host The node gives to the script the IP address of the joiner node.
--port The node gives to the script the port number to use with the joiner node.
--local-port The node gives to the script the port number to use in sending the state transfer.
Calling Conventions
In writing your own custom script for state snapshot transfers, there are certain conventions that you need to follow in order to accommodate how Galera Cluster calls the script.
Receiver
When the node calls for a state snapshot transfer as a joiner, it begins by passing a number of arguments to the state transfer script, as defined in General Parameters above. For your own script you can choose to use or ignore these arguments as suits your needs.
After the script receives these arguments, prepare the node to accept a state snapshot transfer. For example, in the case of wsrep_sst_rsync, the script starts rsync in server mode.
To signal that the node is ready to receive the state transfer, print the following string to standard output: ready <address>:port\n. Use the IP address and port at which the node is waiting for the state snapshot. For example:
ready 192.168.1.1:4444
The node responds by sending a state transfer request to the donor node. The node forms the request with the address and port number of the joiner node, the values given to wsrep_sst_auth, and the name of your script. The donor receives the request and uses these values as input parameters in running your script on that node to send back the state transfer.
When the joiner node receives the state transfer and finishes applying it, print to standard output the Global Transaction ID of the received state. For example:
e2c9a15e-5485-11e0-0800-6bbb637e7211:8823450456
Then exit the script with a 0 status, to indicate that the state transfer was successful.
Sender
When the node calls for a state snapshot transfer as a donor, it begins by passing a number of arguments to the state transfer script, as defined in General Parameters above. For your own script, you can choose to use or ignore these arguments as suits your needs.
While your script runs, Galera Cluster accepts the following signals. You can trigger them by printing to standard output:
flush tables\n Optional signal that asks the database server to run FLUSH TABLES. When complete, the database server creates a tables_flushed file in the data directory.
continue\n Optional signal that tells the database server that it can continue to commit transactions.
done\n Mandatory signal that tells the database server that the state transfer is complete and successful.
After your script sends the done\n signal, exit with a 0 return code.
In the event of failure, Galera Cluster expects your script to return a code that corresponds to the error it encountered. The donor node returns this code to the joiner through group communication. Given that its data directory now holds an inconsistent state, the joiner node then leaves the cluster and aborts the state transfer.
Note
Without the continue\n signal, your script runs in Total Order Isolation, which guarantees that no further commits occur until the script exits.
Enabling Scriptable SST’s
Whether you use wsrep_sst_common directly or decide to write a script of your own from scratch, the process for enabling it remains the same. The filename must follow the convention of wsrep_sst_<name>, with <name> being the value that you give for the wsrep_sst_method parameter in the configuration file.
For example, if you write a script with the filename wsrep_sst_galera-sst, you would add the following line to your my.cnf:
wsrep_sst_method = galera-sst
When the node starts, it uses your custom script for state snapshot transfers.
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