DevOps tech: Database change management

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Database changes are often a major source of risk and delay when performing deployments. DevOps Research and Assessment (DORA) investigated which database-related practices help during the process of implementing continuous delivery, improving both software delivery performance and availability.

DORA's research found that integrating database work into the software delivery process positively contributes to continuous delivery. But how can your teams improve your database delivery as part of implementing continuous delivery? A few practices predict performance outcomes.

DORA discovered that good communication and comprehensive configuration management that includes the database matter. Teams that do well at continuous delivery store their database changes as scripts in version control and manage these changes in the same way they manage production application changes. Furthermore, when changes to the application require database changes, these teams discuss them with the people responsible for the production database, and ensure the engineering team has visibility into the progress of pending database changes.

When teams follow these practices, database changes don't slow them down or cause problems when they perform code deployments.

How to implement database change management

There are two aspects to implementing effective database change management: cultural and technical. This section discusses both.

Establish effective communication of database changes

Research shows that teams do best when they discuss changes with the people responsible for managing the production database, and when everyone has visibility into the progress of pending database changes.

Discussing proposed changes with production database administrators (DBAs) is important for a few reasons. First, these experts can advise on how best to achieve results, and point out potential issues such as performance problems. (Many operations have very different performance characteristics in production systems when compared to developer workstations). This discussion also gives DBAs insight into what is happening upstream, which helps them better prepare for the impact of upcoming changes.

Making sure everybody has visibility into the progress of changes is also crucial so that teams, including DBAs, can understand which changes are coming up, their testing status, and which schema changes have made it to the various production and non-production shared databases. You can facilitate visibility by:

  • Keeping all database schema changes in version control, together with the application code the schema belongs to;
  • Using a tool that records which changes have been run against which environments, and what the results were.

These practices also ensure that there is a canonical source of truth for all changes, and makes the history of changes easy to access for auditing purposes.

Treat all database schema changes as migrations

A widely used pattern for versioning database changes is to capture every change as a migration script which is kept in version control, as shown in the following diagram. Each migration script has a unique sequence number, so that you know in which order to apply migrations.

Diagram shows capturing every change to your database as a migration script with a unique sequence number.

You then ensure that every database instance has a table that records which migrations have been run against that particular instance. In this way you version-control the database schema, so you can use a tool to apply the migration scripts to take the database to the schema version you want. Examples of tools include:

You can also use migrations to create empty database schemas for development and testing.

As shown in the following digram, every database instance has a table that records which migrations you have run against that instance. Then you can perform updates automatically using a tool or script which executes migrations that have not already been applied against the database instance, updating the migrations table after each one successfully completes.

Example of a table that records migrations against an instance.

You can manage database changes in the same way you manage application changes: through an automated process that uses version control as its source of truth.

Zero-downtime database changes

Many organizations schedule downtime for their services when making database schema changes due to the need to coordinate them with application deployments, or due to database table locking during the execution of such changes. Continuous delivery aims to eliminate downtime for deployments, so here are some strategies to make database schema changes without downtime:

  • Use an online schema migration framework such as gh-ost or pt-online-schema-change. These tools create a "ghost" copy of each table you want to change, migrate the empty copy, and then incrementally copy data from the original table including any updates that happen during the migration. After this process is complete, they replace the original table with the ghost. Some databases, for example Cloud Spanner, can perform schema updates with zero downtime.
  • Decouple database changes and application changes with the parallel change pattern. In this pattern, you never mutate existing database objects. Instead, you add new structures alongside old ones. For example, consider changing a column for "address" to two columns: address_1 and address_2.

    Instead of deleting the old column and adding the new one, and rolling out a new version of the application at the same time, you add the new columns but keep the old ones. You can do this before the application deployment occurs. Then the new version of the application can look for the new columns, reading from them if they are present and not null, otherwise reading from the old column. The application can then write to both old and new columns, lazily migrating the data, and also allowing for application rollback without requiring a database rollback.

    In this way the application deployment is decoupled from the database change, which can typically be made without incurring downtime since it doesn't involve migrating data. We have traded off some additional complexity in the application in order to reduce deployment pain. Alternatively, database triggers can be used to keep data in new and old columns synchronized.

  • Design and implement a data partitioning and archiving strategy. A major cause of long migrations is database tables with a large number of rows. Make sure your applications are designed to allow for partitioning and archiving of data to avoid tables growing too large. One example of this would be to create multiple instances of a table for each quarter, for example, instead of a survey_answers table, you might have survey_answers_2020Q1, survey_answers_2020Q2 and so forth. Make sure application design and architecture reviews include validating the application's data partitioning / archiving strategy.

  • Use an event sourcing architecture. In an event sourcing architecture, rather than having the database storing the current state of the application, we have it store changes to its state instead, in the form of a log of events known as commands. So when your customer changes their address, rather than updating a table with the customer details, the application issues an address change command which is stored in the database. This is how database transaction logs and version control work, and is a common pattern in distributed systems. In an event sourced architecture, events can be queued allowing database migrations to happen while events queue up. Events in the queue can then be flushed to the database once the migration is complete. Some databases are able to queue queries while schema migrations run, which can be effective if migrations complete quickly enough.

  • Use a NoSQL solution. Some NoSQL databases, such as Firestore and Cloud BigTable, don't suffer from the issue of downtime created by schema changes. Document databases like Firestore have an implicit schema, which means that the schema is managed at the application layer rather than the database layer. However there are trade-offs associated with using NoSQL databases: they are not optimal for every application.

As well as eliminating scheduled downtime, you also want to avoid unscheduled downtime. Make sure you test every schema change against a production-like data set (with any personal or confidential information scrubbed, of course) to make sure your application behaves the way you expect during and after migration. Some organizations create a scrubbed version of their production database on a daily schedule to use for this purpose. If you are using a database management system with more than one node in production, make sure you're testing against an instance with at least two nodes to help catch distributed system issues.

Common pitfalls of implementing database change management

There are a few common pitfalls to be aware of when implementing the key practices described here.

First, many organizations are heavily siloed. Often DBAs work on their own separate team which uses its own process to manage changes. When software delivery teams implement a new process for managing database changes without consulting the DBA team they are likely to face resistance to using their new process to make changes to databases managed by the DBA team. This can substantially reduce the benefits of moving to a new process.

The first step is to get together with them to discuss how to achieve the objectives presented in this article. It is important to get their buy-in to any proposed process and technology changes. The best way to do this is to ask them what problems they are facing, see how the ideas presented in this article can help them address these problems, and offer to help out. Ideally delivery teams and DBAs can find a mutually acceptable solution.

This obstacle is often exacerbated by another pitfall: the common situation where multiple applications share the same database schema. This means that teams working on one application cannot alter the schema without potentially impacting other applications. This requires that a single solution be put in place to manage database changes for all applications sharing the database schema. It is certainly possible, and indeed even more beneficial, to implement a version-controlled, self-service mechanism to deploy database changes in this situation. However, it involves careful planning and rollout.

Finally, implementing both migration-based database change management and zero-downtime deployments can involve significant architectural change. This should be taken into consideration when estimating the effort required to implement these practices.

How to measure database change management

The goal of an effective database change management system is that database changes don't slow down deployments or cause problems. It's worth measuring the percentage of failed changes in which database changes were a contributing factor, and the extent to which work related to database changes contributes towards overall lead time from version control to release.

If database changes require scheduled downtime, this is also an important consideration. To measure the economic impact of scheduled downtime, consider both the potential lost revenue resulting from downtime and the salary costs of paying people to work out of regular hours in order to perform deployments. Deploying outside of business hours can also contribute to team burnout. These impacts can be used to justify the work required to implement the solutions for zero-downtime deployments discussed in this document.

In terms of measuring the level of automation, consider the proportion of database changes that are made in a push-button way using a fully automated process. The goal should be that 100% of database changes are made in this way.

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