[Openstack] Instance IDs and Multiple Zones

Ed Leafe ed at leafe.com
Tue Mar 22 16:40:21 UTC 2011

	I want to get some input from all of you on what you think is the best way to approach this problem: the RS API requires that every instance have a unique ID, and we are currently creating these IDs by use of an auto-increment field in the instances table. The introduction of zones complicates this, as each zone has its own database.

	The two obvious solutions are a) a single, shared database and b) using a UUID instead of an integer for the ID. Both of these approaches have been discussed and rejected, so let's not bring them back up now.

	Given integer IDs and separate databases, the only obvious choice is partitioning the numeric space so that each zone starts its auto-incrementing at a different point, with enough room between starting ranges to ensure that they would never overlap. This would require some assumptions be made about the maximum number of instances that would ever be created in a single zone in order to determine how much numeric space that zone would need. I'm looking to get some feedback on what would seem to be reasonable guesses to these partition sizes.

	The other concern is more aesthetic than technical: we can make the numeric spaces big enough to avoid overlap, but then we'll have very large ID values; e.g., 10 or more digits for an instance. Computers won't care, but people might, so I thought I'd at least bring up this potential objection.

-- Ed Leafe

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