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

Zero-Downtime Database Upgrades

Warning

This feature is EXPERIMENTAL in the Ocata, Pike and Queens 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

When we speak of a "database upgrade", we are simply talking about changing the database schema and its data from the version used in OpenStack release n (say, Pike) to the version used in OpenStack release n+1 (say, Queens). We are not talking about upgrading the database management software.

Note

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

The Expand-Migrate-Contract Cycle

It's possible to characterize three phases of a database upgrade:

1. Expand: in this phase, new columns, tables, indexes, 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 (which are no longer in use) are removed from the database.

The "legacy" Glance database migrations performed these phases as part of a single monolithic upgrade script. Currently, the Glance project creates a separate script for each the three parts of the cycle. We call such an upgrade an E-M-C database migration.

Zero-Downtime Database Upgrade

The E-M-C strategy can be performed offline when Glance is not using the database. With some adjustments, however, the E-M-C strategy can be applied online when the database is in use, making true rolling upgrades possible.

Note

Don't forget that zero-downtime database upgrades are currently considered experimental and their use in production environments is NOT SUPPORTED.

A zero-downtime database upgrade takes place as part of a rolling upgrade strategy <rolling-upgrades> for upgrading your entire Glance installation. In such a situation, you want to upgrade to release n of Glance (say, Queens) while your release n-1 API nodes are still running Pike. To make this possible, in the Expand phase, database triggers can be added to the database to keep the data in "old" and "new" columns synchronized. Likewise, after all data has been migrated and all Glance nodes have been updated to release n code, these triggers are deleted in the Contract phase.

Note

Unlike the E-M-C scripts, database triggers are particular to each database technology. That's why the Glance project currently provides experimental support only for MySQL.

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 Ocata cycle migration has two identifiers associated with it: 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

For offline database upgrades, the glance-manage tool still has the glance-manage db sync command. This command will execute the expand, migrate, and contract scripts for you, just as if they were contained in a single script.

In order to enable zero-downtime database upgrades, the glance-manage tool has been augmented to include the following operations so that you can explicitly manage the upgrade.

Warning

For MySQL, using the glance-manage db expand or glance-manage db contract command requires that you either grant your glance user SUPER privileges, or run set global log_bin_trust_function_creators=1; in mysql beforehand.

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.