The SQL Relay drop-in replacement library for MySQL clients allows you to run an application that was written using the native MySQL client API against SQL Relay without rewriting the application.
The drop-in replacement library is a shared object library that implements MySQL client library functions as calls to similar SQL Relay API functions which then map the results back into native MySQL client data structures.
There are a variety of reasons that you might want to use the SQL Relay drop-in replacement library for MySQL clients. An application written for MySQL could, for instance, be made to run queries against an Oracle or Microsoft SQL Server database without modification. Or you could simply put SQL Relay between an application and the MySQL database that it normally runs against to take advantage of SQL Relay's persistence, load balancing or throttling facilities.
To use the SQL Relay drop-in replacement library for MySQL clients, you first need to determine what version of MySQL your existing application was compiled against. There are 4 different drop-in replacement libraries. One each for MySQL versions 3.xx, 4.0, 4.1 and 5.0. MySQL's native data structures changed a bit between the different versions. If you use the wrong library your program will most likely crash.
You can find out which library your program uses by running the following command. In this example, the command checks the "mysql" program; the command line client that comes with the MySQL distribution.
ldd /usr/bin/mysql
It should return something like the following:
libreadline.so.4 => /usr/lib/libreadline.so.4 (0x00625000) libncurses.so.5 => /usr/lib/libncurses.so.5 (0x00976000) libmysqlclient.so.10 => /usr/lib/mysql/libmysqlclient.so.10 (0x009b8000) libz.so.1 => /usr/lib/libz.so.1 (0x00963000) libcrypt.so.1 => /lib/libcrypt.so.1 (0x003dc000) libnsl.so.1 => /lib/libnsl.so.1 (0x003c5000) libstdc++.so.5 => /usr/lib/libstdc++.so.5 (0x002ff000) libm.so.6 => /lib/tls/libm.so.6 (0x008ba000) libgcc_s.so.1 => /lib/libgcc_s.so.1 (0x003bb000) libc.so.6 => /lib/tls/libc.so.6 (0x0077f000) libgpm.so.1 => /usr/lib/libgpm.so.1 (0x006d1000) /lib/ld-linux.so.2 => /lib/ld-linux.so.2 (0x00767000)
The line in bold above is the important line.
The 10 at the end of libmysqlclient.so.10 means MySQL protocol version 10. If your application links against version 10 or lower then you can use the MySQL 3.xx version of the drop-in library. If it links against version 11 or 12 then you need to use the MySQL 4.0 version of the drop-in library. If it links against version 13 then you need to use the MySQL 4.1 version of the drop-in library. If it links against version 14 then you may need to use the MySQL 4.1 or 5.0 version of the drop-in library. MySQL 4.1 and 5.0 both use the same protocol version (so far) but there are differences in the data structures, so you'll have to figure out which version it uses some other way.
Once you've figured out which version of MySQL your program was compiled against, you can load the appropriate drop in library and run your program.
The parameters that would ordinarily indicate which host, port, socket, username and password to use to connect to MySQL will be used as parameters to connect to SQL Relay. The parameter that would ordinarily indicate which database to connect to will be ignored. Instances of SQL Relay are configured to connect to a single database, and that database will be used by the client program.
In the following example, we're running the "mysql" program against an instance of SQL Relay running on the localhost, port 8009 against an Oracle database. This instance of SQL Relay is configured with a username/password of oracle8user/oracle8pass.
For sh-based shells:
LD_PRELOAD=/usr/local/firstworks/lib/libmysql3sqlrelay.so export LD_PRELOAD mysql -h localhost -P 8009 --user=oracle8user --password=oracle8pass
For csh-based shells:
setenv LD_PRELOAD /usr/local/firstworks/lib/libmysql3sqlrelay.so mysql -h localhost -P 8009 --user=oracle8user --password=oracle8pass
The LD_PRELOAD environment variable instructs the dynamic loader to load libmysql3sqlrelay.so before loading any other libraries for any programs. The mysql client program will still load the native MySQL client library, but since it loaded the SQL Relay drop-in replacement library first, function calls that would normally be fulfilled by the native MySQL client library are fulfilled by the SQL Relay drop-in replacement library instead.
If your application was compiled against a different version of MySQL, you will need to replace libmysql3sqlrelay.so with a different library. The following table indicates which library is required for each version of MySQL.
Version of MySQL | Library |
---|---|
3.x | libmysql3sqlrelay.so |
4.0.x | libmysql40sqlrelay.so |
4.1.x | libmysql41sqlrelay.so |
5.0.x | libmysql50sqlrelay.so |
Below is a sample session using the mysql command line client against an Oracle database through SQL Relay.
Welcome to the MySQL monitor. Commands end with ; or \g. Your MySQL connection id is 0 to server version: 3.23.58 Type 'help;' or '\h' for help. Type '\c' to clear the buffer. mysql> create table testtable (col1 varchar2(60), col2 number(5,2)); Empty set (0.21 sec) mysql> insert into testtable values ('hello',123.45); Empty set (0.00 sec) mysql> select * from testtable; +-------+--------+ | COL1 | COL2 | +-------+--------+ | hello | 123.45 | +-------+--------+ 1 row in set (0.09 sec) mysql> drop table testtable; Empty set (0.21 sec) mysql> quit;
Using the SQL Relay drop-in replacement library for MySQL with daemons is simlar to using it on the command line. You just need to add the LD_PRELOAD command to the startup script for the daemon before the command that starts the daemon itself.
Inetd and xinetd are daemons that listen on ports and run helper programs to service requests on those ports. The helper programs die off after the request is serviced.
The easist way to get an inetd helper program to use the SQL Relay drop-in replacement library for MySQL is to add the LD_PRELOAD command to the startup script for inetd/xinetd. Any command that inetd/xinetd runs will also preload the library.
However, if some of the helper programs need to actually run against MySQL and not against SQL Relay, then you will have to do something different. The easiest thing to do is create a script for each helper program that needs to run against SQL Relay that runs the LD_PRELOAD command and then runs the actual helper program, passing it all the necessary command line arguments.
For example, lets say you have a pop3 server called pop3d that uses MySQL for user identification and you wanted to use SQL Relay instead of MySQL. The inetd.conf entry might look like this:
pop3 stream tcp nowait root /usr/local/bin/pop3d
An /etc/xinetd.d entry might look like this:
service pop3 { socket_type = stream wait = no user = root server = /usr/local/bin/pop3d }
You could write the a script called /usr/local/bin/pop3d-sqlrelay as follows:
#!/bin/sh LD_PRELOAD=/usr/local/firstworks/lib/libmysql3sqlrelay.so export LD_PRELOAD /usr/local/bin/pop3d $@
And modify the entries to call the script instead of pop3d as follows:
pop3 stream tcp nowait root /usr/local/bin/pop3d-sqlrelay
Or for xinetd:
service pop3 { socket_type = stream wait = no user = root server = /usr/local/bin/pop3d-sqlrelay }
You may want to use the SQL Relay drop-in replacement library for MySQL clients with a program that isn't compiled against the native MySQL client library but rather loads it as a module such as a program that uses ODBC or Perl DBI, or an Apache/PHP application.
Using the SQL Relay drop-in replacement library with programs that load the native MySQL client library as a module is simlar to using it on the command line. You just need to make sure that the LD_PRELOAD command is run before the program starts.
If the program is a command line program, then run the LD_PRELOAD command before running your program. Even though the program ultimately loads the native MySQL client library, all of it's functions will be overriden by the SQL Relay drop-in replacement library.
If the program is a daemon then add the LD_PRELOAD command to the startup script for the daemon.
If the program runs in the address space of a daemon, such as a PHP application running under Apache's mod_php, then add the LD_PRELOAD command to the startup script for the daemon. The caveat here is that all applications running in the address space of the daemon will use the drop-in replacement library instead of the native MySQL library. It is not possible, for example for a web server to run one PHP application directly against MySQL and another PHP application against SQL Relay using the drop-in replacement library; if the drop-in replacement library is loaded, both applications will end up using it.
If the program is spawned by a daemon, such as a cgi spawned by a web-server or an inetd/xinetd helper program, then you can either add the LD_PRELOAD command to the daemon's startup script or write a script to run the LD_PRELOAD command and pass along the command line arguments (see the section Using the drop-in replacement library with inetd/xinetd helper programs above)).
The SQL Relay drop-in replacement library for MySQL implements most of the native MySQL client library's functions, but there are a few functions that aren't implemented because SQL Relay doesn't have a good way to support them. These functions return safe values or a failure condition.
Here is a list of functions that are implemented and functions that are not. If your application uses one of the functions that is not implemented, you may or may not be able to use it with the SQL Relay drop-in replacement library for MySQL.
Function | Implemented? |
---|---|
mysql_thread_safe | always returns 1 |
mysql_init | yes |
mysql_set_server_option | does nothing but always returns success |
mysql_options | does nothing but always returns success |
mysql_ssl_set | does nothing but always returns success |
mysql_connect | yes |
mysql_real_connect | yes |
mysql_close | yes |
mysql_ping | yes |
mysql_stat | returns a string similar to the real mysql_stat but with empty values |
mysql_shutdown | always returns failure |
mysql_reload | yes |
mysql_refresh | only implemented for refresh_options==REFRESH_GRANT |
mysql_thread_id | always returns 0 |
mysql_list_processes | always returns NULL |
mysql_kill | always returns failure |
mysql_get_client_info | returns the version string of the client that the library emulates |
mysql_get_client_version | returns the version number of the client that the library emulates |
mysql_get_host_info | returns an empty string |
mysql_get_proto_info | returns the protocol version number of the client that the libary emulates |
mysql_get_server_info | returns the version string of the client that the library emulates |
mysql_get_server_version | returns the version number of the client that the libary emulates |
mysql_change_user | yes |
mysql_character_set_name | always returns latin1 |
mysql_debug | does nothing |
mysql_dump_debug_info | always returns failure |
mysql_create_db | always returns failure |
mysql_select_db | always returns failure |
mysql_drop_db | always returns failure |
mysql_list_dbs | always returns NULL |
mysql_list_tables | always returns NULL |
mysql_escape_string | yes |
mysql_odbc_escape_string | always returns NULL |
myodbc_remove_escape | does nothing |
mysql_query | yes |
mysql_send_query | yes |
mysql_read_query_result | yes |
mysql_real_escape_string | implemented but only for utf-8 character set |
mysql_real_query | yes |
mysql_info | always returns an empty string |
mysql_insert_id | always returns 0 |
mysql_store_result | yes |
mysql_use_result | yes |
mysql_free_result | yes |
mysql_more_results | always returns false |
mysql_next_result | always returns -1 |
mysql_list_fields | always returns NULL |
mysql_num_fields | yes |
mysql_fetch_field | yes |
mysql_fetch_fields | yes |
mysql_fetch_field_direct | yes |
mysql_fetch_lengths | yes |
mysql_field_count | yes |
mysql_field_seek | yes |
mysql_field_tell | yes |
mysql_num_rows | yes |
mysql_affected_rows | yes |
mysql_row_seek | yes |
mysql_row_tell | yes |
mysql_data_seek | yes |
mysql_fetch_row | yes |
mysql_eof | yes |
mysql_warning_count | always returns 0 |
mysql_errno | yes |
mysql_error | returns the SQL Relay error string |
mysql_sqlstate | always returns an empty string |
mysql_commit | yes |
mysql_rollback | yes |
mysql_autocommit | yes |
mysql_prepare | yes |
mysql_bind_param | yes, for all types except INT24 and GEOMETRY |
mysql_bind_result | yes |
mysql_execute | yes |
mysql_param_count | yes |
mysql_param_result | no, the MySQL docs don't even explain what this should do |
mysql_fetch | yes |
mysql_fetch_column | no, the MySQL docs don't even explain what this should do |
mysql_get_metadata | yes |
mysql_send_long_data | always returns false |
mysql_stmt_num_rows | yes |
mysql_stmt_affected_rows | yes |
mysql_stmt_row_seek | yes |
mysql_stmt_row_tell | yes |
mysql_stmt_data_seek | yes |
mysql_stmt_close | yes |
mysql_stmt_errno | yes |
mysql_stmt_error | returns the SQL Relay error string |
mysql_stmt_sqlstate | always returns an empty string |
mysql_stmt_store_result | yes |
mysql_stmt_free_result | yes |
mysql_stmt_reset | yes |
The native MySQL client API also defines several data structures. Applications are supposed to access fields of the MYSQL_FIELD and MYSQL_BIND structures directly. Applications also access MYSQL_ROW's directly, but MYSQL_ROW is just a typedef for (char **). The SQL Relay drop-in replacement library for MySQL supports MYSQL_ROW and the following fields of MYSQL_FIELD and MYSQL_BIND. Note that the MYSQL_BIND structure is only available when emulating MySQL 4.1 or 5.0 and that the fields of MYSQL_FIELD that are available depend on whether the drop-in library is emulating MySQL 3, 4.0, 4.1 or 5.0
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