So, you’ve planned your database modernization journey. You’ve set up Google Cloud’s Database Migration Service (DMS), configured replication, and successfully synchronized your application databases from your on-premises or cloud systems to a fully managed Cloud SQL for SQL Server instance. The replication is complete, the data is up to date, and you’re ready for cutover. But when your application attempts to connect to the newly migrated database, you’re hit with a frustrating roadblock: Msg 18456, Level 14, State 1, Line 1: Login failed for user 'app_user.
The culprit is simple: your SQL Server logins didn't migrate with your database. In this post, we’ll look at why this gap exists, why it actually protects your organization's security posture, and how easy it is to bridge using standard, time-tested SQL Server tools. Database Migration Service (DMS) is highly efficient at replicating database-level schemas and transactional data. However, it purposefully doesn’t migrate instance-level objects, such as the system master database or server logins and permissions.
While this might feel like a missing feature, it is actually a deliberate design choice built around three core pillars: Security Isolation and Privilege Boundaries: The source environment and the destination Cloud SQL environment operate under different security paradigms. Replicating the master system database directly could lead to unauthorized privilege escalation. For example, an on-premises login with sysadmin privileges shouldn’t have unrestricted sysadmin access to a fully managed Google Cloud database.
When the cloud provider manages physical backups, patching, and security, it needs to limit underlying operating system access to ensure correct operation. Compliance and Audit Governance: Automated migration of encrypted password hashes and server-level security credentials without explicit administrator oversight frequently violates enterprise compliance frameworks such as PCI-DSS or SOC 2. By keeping security object migration as a deliberate, administrator-driven step, organizations can guarantee that only approved identities are provisioned in the cloud landing zone.