Continuous (sometimes referred to as ongoing or online) migration is
a continuous flow of changes from a source PostgreSQL database to an AlloyDB for PostgreSQL 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 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, 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 2025-08-29 UTC."],[[["\u003cp\u003eContinuous migration involves an initial snapshot of the source PostgreSQL database, followed by a continuous stream of changes to the AlloyDB destination, allowing for minimal downtime during the final switch.\u003c/p\u003e\n"],["\u003cp\u003eContinuous migration includes taking an initial database snapshot, loading it into the destination, processing ongoing changes, and finally promoting the destination to primary status.\u003c/p\u003e\n"],["\u003cp\u003eOne-time migration, which is currently unavailable for AlloyDB, involves a single snapshot of the source database and applying it to the destination, requiring a halt in writes to the source during the migration.\u003c/p\u003e\n"],["\u003cp\u003eReplication delay is the time between when a write occurs on the source and the current time, which will account for the downtime during a continuous migration switch.\u003c/p\u003e\n"]]],[],null,["# Types of migration\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](/database-migration/docs/postgres/migration-types \"View this page for the PostgreSQL version of Database Migration Service.\") \\| PostgreSQL to AlloyDB\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 PostgreSQL database to an AlloyDB for PostgreSQL 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 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, 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\n4. 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\n5. 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\n| One-time migration is currently not available for AlloyDB.\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."]]
