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Hi all,

since it's a high impact change in the migration tree, I wanted to
drop an email to everyone affected (basically, anyone who wants to
introduce a new migration from now on).

So there was a proposal to split migration rules into independent
branches, with one 'expand' branch containing only those rules that
are safe to apply while neutron-server is running. Proposal is at:

http://specs.openstack.org/openstack/neutron-specs/specs/liberty/online-
schema-migrations.html

And the first patch to implement it just landed in neutron:

http://git.openstack.org/cgit/openstack/neutron/commit/?id=c7acfbabdc13e
d2a73bdbc6275af8063b8c1eb2f

- From now on,

- - there are multiple alembic heads for any database state;
- - there is a new file structure under
neutron/db/migration/versions/alembic_versions/{cycle}_{branch};
- - you may need to split your migrations into pieces (for expand and
contract branches, respectively, depending on the character of schema
changes; more details in the spec);
- - 'neutron-db-manage upgrade head' still applies all heads;
- - I'd like to rearrange migration trees for *aas repos in the same
way, though neutron-db-manage still supports the old file layout.

To get an example of how the split would look like for existing
migration rules in review, I took Kevin's patch for RBAC:

https://review.openstack.org/191707

And transformed it into something that adopts the new file layout:

https://review.openstack.org/202013

Changes I made:
- - split migration script into two pieces;
- - updated HEADS file;
- - made the contract phase script depends_on the expand one;

Note that 'neutron-db-manage revision --autogenerate' command does not
yet filter operations into corresponding branches, though we would
like to have it in L once new alembic is released.

Ihar
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