11th November 2010 under PHP

Generating Even and Odd PostgreSQL Databases for Replication

I was recently faced with a problem of replicating two PostgreSQL databases with bucardo. The replication was to be in a master-master fashion so that the databases could be updated on both ends. Although this task can easily be handled by bucardo it requires that the two different systems generate completely different sets of primary keys. According to the Bucardo website (http://bucardo.org/wiki/Bucardo/Sequences) there are three ways of achieving this difference. Their approaches were:

If you are using a swap sync, the best practice is to not replicate sequences, but to make sure that they are different on both sides, such that an insert on database A will never conflict with an insert on database B. There are three general ways to do this:

  1. Use interleaving sequences. On database A, define the sequence as START WITH 1 INCREMENT BY 2. On database B, define the sequence as START WITH 2 INCREMENT BY 2. Thus, the two sequences will never have the same value.
  2. Use different ranges. For example, database A would use a sequence of START WITH 1, while database B uses START WITH 100000000. This is not foolproof, as A can eventually catch up with B, although you can define A as MAXVALUE 99999999.
  3. Use a common sequence. This relies on one or both of the databases using a function that makes a call to an external sequence

After reading this I decided to go by the first approach so I wrote a script to alter the sequences.

// Add the sequences you dont want affected here. The schema doesn't
// really matter.
$exclude = array(

for ($i = 1; $i &lt; $argc; $i++)
    if(substr($argv[$i], 0, 2) == "--")
        $parameter = substr($argv[$i], 2, strlen($argv[$i]) - 2);
        $$parameter = $argv[$i];
        $tables[] = $argv[$i];

$conn = pg_connect("host=$host port=5432 dbname=$db user=$user

$sequences_result = pg_query($conn, "SELECT * FROM
$sequences = array();
while($sequence = pg_fetch_assoc($sequences_result))
    if(array_search($sequence['sequence_name'], $exclude) !== false)
        echo "Skipping " . $sequence['sequence_schema'] . '.' .$sequence['sequence_name'] . "\n";
    $sequence = $sequence['sequence_schema'] . '.' . $sequence['sequence_name'];
    $value = pg_query($conn, "SELECT last_value from $sequence");
    $value = pg_fetch_assoc($value);

    pg_query($conn, "ALTER SEQUENCE $sequence INCREMENT BY 2");

    if($value["last_value"] % 2 == 0 && $mode == 'odd')
        pg_query($conn, "ALTER SEQUENCE $sequence RESTART WITH " .($value['last_value'] + 1));
    elseif(($value["last_value"] % 2 == 1 || $value["last_value"] == 1) && $mode == 'even')
        pg_query($conn, "ALTER SEQUENCE $sequence RESTART WITH " . ($value['last_value'] + 1));

This script takes five required parameters. The parameters are not validated so if it is wrongly specified the consequences are unknown. PLEASE BE CAREFUL! The parameters are:

  1. The hostname
  2. The username
  3. The password
  4. The database name
  5. The mode of the change (even or odd)

An example usage could be

php update_sequences.php --host db1.accra.srv --user postgres --password somepass --db appcore --mode even

Hope that helps. Happy Programming!