--- instance A: three links
 id |  code   | len 
----+---------+-----
  1 | 6MelkkB |   7
  2 | 7qqChen |   7
  3 | J4Wn4LP |   7
(3 rows)

--- instance A stopped and restarted (ids 4..100 of its first block are never used)
 id  |  code   | len 
-----+---------+-----
   1 | 6MelkkB |   7
   2 | 7qqChen |   7
   3 | J4Wn4LP |   7
 101 | IZKi2wO |   7
(4 rows)

--- two instances running at once never overlap: B starts while A is alive
 id  |  code   
-----+---------
 102 | DDCfjej
 201 | Globq13
(2 rows)