Reduce maintenance window
Achieve zero downtime deployments
"Code is easy, state is hard"
Changes in a database schema from one version to another are called database migrations
Tools: Flyweight Liquibase
Migrations require back and forward compatibility
Baby steps = Smallest Possible Batch Size
Too many rows = Long Locks
Shard your updates (not updating the entire table in one go)
Renaming a column
ALTER TABLE customers RENAME COLUMN wrong TO correct;
becomes:
ALTER TABLE customers ADD COLUMN correct VARCHAR(20);
UPDATE customers SET correct = wrong WHERE id < 100;
UPDATE customers SET correct = wrong WHERE id >= 100 AND id < 200;
...
(later) ALTER TABLE customers DELETE COLUMN wrong;
Adding a column
ADD COLUMN, setting NULL/DEFAULT value/computed value
Next release: Use Column
Renaming / Changeing Type / Format of a Column:
Next version: ADD COLUMN, Copy data using small shards
Next release: Code reads from old column and writes to both
Next release: Code reads from new column and writes to both
Next release: Code reads and writes from new column
Next release: Delete old column
Deleting a column
Next version: Stop using the column but keep updating the column
Next version: Delete the column
For migrating from a monolithic application with a monolithic database to many microservices with own database each:
Using Event Sourcing
tool: debezium.io
You tell it which tables you want to monitor, and from then on it monitors them
and generates an event for each DDL/DML statement you issue.
The event is propagated to as many event consumers as you want.
So, microservices can receive these events and update their own databases.
"HTTP and REST are incredibly slow"
