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On Heroku, as soon as you push new code, the web-serving instances restart... even if the underlying database schema additions/changes (via syncdb or south migrate) haven't yet been applied. In many cases, this might just cause harmless errors undtil the syncdb/migrate is run soon afterward. But I'm concerned that in some cases, new code might half-work making unexpected changes in the pre-migration database. What's the right way to be safe against this risk? One technique might be to add the syncdb/migrate to the Procfile so it's run before web restart. But, in the case of multiple instances, or maybe even a case where the one old-code-instance is left running until the moment the one new-code-instance is known-up, there's still a variant of the issue where code is talking to a DB with a mismatched schema. Is there a 'hold all web instances' feature (or common best practice) for letting the migrate complete without web traffic? Or am I being overly concerned about a risk that is negligible in practice?

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