How to sync a MySQL table between two remote databases.

Definitely tricker than you might think.

Seems like it should be trivial using SELECT ... INTO OUTFILE and LOAD DATA INFILE ... to make the transfer via dumping the table into a temporary file. However, SELECT ... INTO OUTFILE creates a file on the remote server rather than locally. This prevents the use of LOAD DATA INFILE for the second step as the file being loaded has to be local or on the destination server.

Following the guidance in the docs, you can create local dump of a table by using the --execute option to output the results of a SELECT ... statement into a local file.

mysql -D database_name -e "SELECT ... " > /path/to/file.txt

This works but has two downsides:

  1. First, running a shell command forces you to step outside the MySQL adapter of your progamming language which means it is a new place where the database credentials need to be passed. Shelling out commands always feels like you’ve failed.
  2. Further, as far as I can tell, you can’t control the field separator or line endings using this technique (in the same way as you can with SELECT ... INTO OUTFILE ...) and so the file includes an unwanted line with the field names and tab-separates the fields.

It’s worth noting the mysqldump isn’t much help here, as the --tab option that allows CSV output to be generated only works with a local database connection.

Now that we’ve got our data locally, we load it into the remote database using LOAD DATA INFILE and make use of the LOCAL keyword which lets us use a local data file:

mysql -h x.x.x.x -u user -D database_name --password=... -e \
"LOAD DATA LOCAL INFILE '/path/to/file.txt' \
REPLACE INTO TABLE table_name \
IGNORE 1 LINES"

Of course, you may want to truncate the table first if you want a clean sync. As this operations locks the destination table, it often makes sense to load the data into a temporary copy of the table, and then perform a RENAME TABLE operation to swap in the new table.

Here’s a quick and dirty PHP implementation:

<?php
$tableName = 'some_table';
$sql =
   "SELECT * 
    FROM $tableName";
$pathToCsv = '/tmp/some-file.csv';
$command = sprintf("mysql -h %s -u %s  --password=%s -D %s -e '%s' > %s",
    '10.0.0.2', 
    'db-user', 
    'db-password', 
    'database_name', 
    $sql, 
    $pathToCsv);
exec($command);

$sql =
   "LOAD DATA LOCAL INFILE '$pathToCsv'
    REPLACE INTO TABLE `$tableName`
    CHARACTER SET 'utf8'
    IGNORE 1 LINES";
$db->execute($sql); // Using your favourite database adapter
----

Tagged with: mysql
Filed in: tips
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