glance/doc/source/admin/zero-downtime-db-upgrade.rst

Zero-Downtime Database Upgrades

Warning

This feature is EXPERIMENTAL in the Ocata and Pike releases. We encourage operators to try it out, but its use in production environments is currently NOT SUPPORTED.

A zero-downtime database upgrade enables true rolling upgrades of the Glance nodes in your cloud's control plane. At the appropriate point in the upgrade, you can have a mixed deployment of release n (for example, Ocata) and release n-1 (for example, Newton) Glance nodes, take the n-1 release nodes out of rotation, allow them to drain, and then take them out of service permanently, leaving all Glance nodes in your cloud at release n.

That's a rough sketch of how a rolling upgrade would work. For full details, see rolling-upgrades.

Note

Downgrading a database is not supported. See downgrades for more information.

The Expand-Migrate-Contract Cycle

For Glance, a zero-downtime database upgrade has three phases:

1. Expand: in this phase, new columns, tables, indexes, or triggers are added to the database.
2. Migrate: in this phase, data is migrated to the new columns or tables.
3. Contract: in this phase, the "old" tables or columns (and any database triggers used during the migration), which are no longer in use, are removed from the database.

The above phases are abbreviated as an E-M-C database upgrade.

New Database Version Identifiers

In order to perform zero-downtime upgrades, the version identifier of a database becomes more complicated since it must reflect knowledge of what point in the E-M-C cycle the upgrade has reached. To make this evident, the identifier explicitly contains 'expand' or 'contract' as part of its name.

Thus the ocata01 migration (that is, the migration that's currently used in the fully supported upgrade path) has two identifiers associated with it for zero-downtime upgrades: ocata_expand01 and ocata_contract01.

During the upgrade process, the database is initially marked with ocata_expand01. Eventually, after completing the full upgrade process, the database will be marked with ocata_contract01. So, instead of one database version, an operator will see a composite database version that will have both expand and contract versions. A database will be considered at Ocata version only when both expand and contract revisions are at the latest revisions. For a successful Ocata zero-downtime upgrade, for example, the database will be marked with both ocata_expand01, ocata_contract01.

In the case in which there are multiple changes in a cycle, the database version record would go through the following progression:

stage	database identifier	comment
E	bexar_expand01	upgrade begins
E	bexar_expand02
E	bexar_expand03
M	bexar_expand03	bexar_migrate01 occurs
M	bexar_expand03	bexar_migrate02 occurs
M	bexar_expand03	bexar_migrate03 occurs
C	bexar_expand03, bexar_contract01
C	bexar_expand03, bexar_contract02
C	bexar_expand03, bexar_contract03	upgrade completed

Database Upgrade

In order to enable the E-M-C database upgrade cycle, and to enable Glance rolling upgrades, the glance-manage tool has been augmented to include the following operations.

Expanding the Database

glance-manage db expand

This will run the expansion phase of a rolling upgrade process. Database expansion should be run as the first step in the rolling upgrade process before any new services are started.

Migrating the Data

glance-manage db migrate

This will run the data migrate phase of a rolling upgrade process. Database migration should be run after database expansion but before any new services are started.

Contracting the Database

glance-manage db contract

This will run the contraction phase of a rolling upgrade process. Database contraction should be run as the last step of the rolling upgrade process after all old services are upgraded to new ones.