Need to migrate from one database to another without downtime?

Dual writes are one approach that deserves more attention.

Most database migrations fall into one of a few buckets:

• Export and import
• Replication between two databases
• Services specifically built to synchronize data

All of those approaches can work well. But sometimes you need both databases active while gradually migrating traffic from one to the other.

That’s where dual writes become interesting.

What Are Dual Writes?

The idea is exactly what it sounds like. Every write is sent to both databases.

Insert a row? Write it twice.

Update a record? Update it twice.

Delete something? Yup, delete it twice.

The goal is to keep the old and new databases synchronized while both are active.

How It Works

The most common approach is implementing dual writes directly in the application.

Instead of maintaining one database connection pool, the application maintains two and executes write operations against both systems.

In some cases, infrastructure can help as well.

For example, Envoy supports request mirroring patterns that can be useful when migrating certain technologies, such as Redis.

The implementation will vary, but the concept remains the same: each write is performed twice.

The Hard Part: Failure Handling

The hard part is not writing twice, but rather handling partial success.

What happens when Database A succeeds, but Database B fails?

Now the two databases disagree.

Do you retry? Can the operation be safely retried?

Can you roll back the successful write?

Do you have locking or reconciliation mechanisms?

This is where dual writes become significantly more complex than they initially sound.

Writing twice is easy. Maintaining correctness is the hard part.

Where Dual Writes Work Best

Dual writes tend to work best when:

• Updates occur frequently
• Eventual consistency is acceptable
• Reconciliation processes exist
• Future updates naturally correct drift

They become much harder in systems that require strict consistency guarantees for every operation.

Why Teams Use Them

Despite the complexity, dual writes enable a powerful migration approach.

You can:

• Introduce a new database platform or table
• Keep it synchronized with the old platform
• Gradually migrate read traffic
• Eventually retire the old database

All without requiring a large downtime window.

Final Thoughts

Like most migration strategies, dual writes are a tradeoff.

They add complexity.

But they also enable something extremely valuable: migrating between database platforms while the system remains live.

When downtime is not an option, dual writes can be one of the most powerful migration techniques available.