Thread by @tdpauw

#ContinuousIntegration Pro-tip (of course I mean single branch):
Start adopting the expand-contract pattern.

Expand-contract is what blue-green deployments do.
You expand by installing a new version of the app, and then you contract by removing the old version.

Expand-contract is also heavily used for zero-down time database schema changes.
Let’s say you need to rename a table column.
You expand by adding a new column with the new name.
Feed the new column with data.
Once the app uses the new column, contract by removing the old one.

In software, expand-contract is mostly used for method signature changes.

You expand by duplicating the method and change the signature.
Modify all consumers to use the new signature.
Contract once all consumers are using the new signature by removing the old method.

Expand-contract can also be applied to infrastructure definitions like for instance renaming AWS security groups.

Expand by duplicating the security group with the new name.
Use the new security group for existing EC2 instances.
Contract by removing the old security group.

When introducing a new remote API version by adding a new URL-endpoint expand-contract is being applied.
When evolving the API on the same endpoint Postel’s Law should be followed which is another form of expand-contract.

Of course, to be effective, expand-contract requires the ability to perform multiple incremental deployments in very short iterations.
#ContinuousDelivery

You might move slower and have to think harder when applying expand-contract. I give you that.
But, oh dear, it vastly pays back in delivery flexibility and never blocking the flow of delivery as at any time you’ll have a releasable working software system.