Continuous (sometimes referred to as ongoing or online) migration is
a continuous flow of changes from a source to a destination that
follows an initial full dump and load. In the case of a migration, when the
time comes to switch to use the destination for reads and writes,
perform a promote operation. Promotion means that the destination Cloud SQL instance is disconnected from the source, and is promoted from a replica instance to a primary instance.
Continuous migration follows these steps:
Initially, a snapshot is taken of the source database.
This incurs a short (under 10 seconds) lockout on the database tables, one at
a time, as the dump is created. The source can continue accepting writes.
After the initial dump is taken, it's loaded into the destination.
After the load is completed, constraints such as primary keys, foreign keys,
and indexes are recreated on the instance.
After the constraints are recreated, the ongoing changes (also known as change data capture or CDC) are processed.
When the time comes to switch to using the destination, stop writing to the
source and initiate a promotion. This allows the application to read and
write against the destination database.
Dependent applications can experience downtime for at least the
duration of the replication delay at the time of the decision to promote.
One-time migration
This type of migration is a single point-in-time snapshot of the database,
taken from the source and applied to the destination. This is essentially a dump
and load, where the destination is ready to be
used when the load completes. Any applications that depend on the source database
can experience downtime during the migration process because there can be no new
writes to this database while the migration is in progress.
One-time migration follows these steps:
Stop writing to the source database.
Initiate a dump of the source database.
After the dump is complete, it's loaded into the destination. When the load
is completed, a promotion is initiated automatically. The
destination database now becomes the primary database, and dependent
applications should read and write to it.
[[["Easy to understand","easyToUnderstand","thumb-up"],["Solved my problem","solvedMyProblem","thumb-up"],["Other","otherUp","thumb-up"]],[["Hard to understand","hardToUnderstand","thumb-down"],["Incorrect information or sample code","incorrectInformationOrSampleCode","thumb-down"],["Missing the information/samples I need","missingTheInformationSamplesINeed","thumb-down"],["Other","otherDown","thumb-down"]],["Last updated 2025-08-25 UTC."],[[["\u003cp\u003eContinuous migration involves a continuous flow of changes from the source to the destination, following an initial full dump and load, and ends with a \u003ccode\u003epromote\u003c/code\u003e operation that disconnects the destination from the source.\u003c/p\u003e\n"],["\u003cp\u003eThe continuous migration process includes taking a snapshot of the source, loading it into the destination, recreating constraints, processing ongoing changes, and finally promoting the destination when ready.\u003c/p\u003e\n"],["\u003cp\u003eOne-time migration is a single, point-in-time snapshot of the database from the source applied to the destination, with the destination becoming ready after the load is complete, meaning no new writes can occur while the migration is in progress.\u003c/p\u003e\n"],["\u003cp\u003eOne-time migration involves stopping writes to the source, dumping the source, and loading the dump into the destination, after which the destination becomes the primary database, and a promotion is automatically initiated.\u003c/p\u003e\n"]]],[],null,["# Types of migration\n\n\u003cbr /\u003e\n\n\u003cbr /\u003e\n\n[MySQL](/database-migration/docs/mysql/migration-types \"View this page for the MySQL version of Database Migration Service.\") \\| PostgreSQL \\| [PostgreSQL to AlloyDB](/database-migration/docs/postgresql-to-alloydb/migration-types \"View this page for the PostgreSQL to AlloyDB version of Database Migration Service.\")\n\n\u003cbr /\u003e\n\n\u003cbr /\u003e\n\n\u003cbr /\u003e\n\nOverview\n--------\n\n### Continuous migration\n\nContinuous (sometimes referred to as ongoing or online) migration is a continuous flow of changes from a source to a destination that follows an initial full dump and load. In the case of a migration, when the time comes to switch to use the destination for reads and writes, perform a `promote` operation. Promotion means that the destination Cloud SQL instance is disconnected from the source, and is promoted from a replica instance to a primary instance.\n\nContinuous migration follows these steps:\n\n1. Initially, a snapshot is taken of the source database.\n This incurs a short (under 10 seconds) lockout on the database tables, one at\n a time, as the dump is created. The source can continue accepting writes.\n\n \u003cbr /\u003e\n\n2. After the initial dump is taken, it's loaded into the destination.\n\n3. After the load is completed, constraints such as primary keys, foreign keys,\n and indexes are recreated on the instance.\n\n4. After the constraints are recreated, the ongoing changes (also known as change data capture or CDC) are processed.\n\n | The delay between when a write occurs on the source and the current time is known as the `Replication delay`.\n\n \u003cbr /\u003e\n\n5. When the time comes to switch to using the destination, stop writing to the\n source and initiate a promotion. This allows the application to read and\n write against the destination database.\n\n6. Dependent applications can experience downtime for at least the\n duration of the replication delay at the time of the decision to promote.\n\n### One-time migration\n\nThis type of migration is a single point-in-time snapshot of the database,\ntaken from the source and applied to the destination. This is essentially a dump\nand load, where the destination is ready to be\nused when the load completes. Any applications that depend on the source database\ncan experience downtime during the migration process because there can be no new\nwrites to this database while the migration is in progress.\n\nOne-time migration follows these steps:\n\n1. Stop writing to the source database.\n\n2. Initiate a dump of the source database.\n\n3. After the dump is complete, it's loaded into the destination. When the load\n is completed, a promotion is initiated automatically. The\n destination database now becomes the primary database, and dependent\n applications should read and write to it."]]
