For years a CloudFormation stack operation has stayed IN_PROGRESS until every resource in it stabilized. On July 2 AWS shipped Express mode, a deployment configuration that has CloudFormation report the stack complete as soon as resource configuration is applied, with resources continuing to become available in the background. Rollback is off by default in the new mode. The trade-off is explicit in the announcement on the AWS DevOps blog: you get a faster signal, and you accept a weaker one.
The completion signal, moved forward
Express mode does not change how CloudFormation talks to service APIs. It changes when it reports done. In the default mode the stack sits IN_PROGRESS while each resource stabilizes, the period between the configuration call succeeding and the resource being ready to serve traffic. Under Express mode CloudFormation flips the resource to a completion status as soon as the configuration call returns. Per the announcement, the status message is now literal about what that means: "It may continue becoming available in the background."
Dependencies inside the stack are still respected. Transient failures still retry. The mode changes the finish line, not the race.
Where the time comes from
