About data flow for Oracle to Cloud SQL for PostgreSQL
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There are two important concepts you need to understand to know how data flows
from your source to the destination instance: migration types and migration
phases. Database Migration Service supports two types of migration: continuous and one-time.
Only continuous migrations are available for heterogeneous Oracle to
Cloud SQL for PostgreSQL migrations.
With continuous migrations, your data is migrated in two phases: full dump, and
Change Data Capture (CDC).
Full dump phase
The full dump phase is the first part of the migration process. During
full dump, Database Migration Service connects to your source instance, reads the content
of the databases you selected for migration, and then loads the data to the Cloud SQL for PostgreSQL
destination instance. In this phase, Database Migration Service captures actual contents
of your database.
During the full dump phase, Database Migration Service can open multiple connections to
your source databases for improved performance. You can configure the number of
concurrent connections when you
create a migration job.
For granular control over your data movement, you can also perform the full dump
and load the data to the destination instance on your own, outside of
Database Migration Service. If you choose to perform this phase manually, your migration
job will start with the CDC phase instead and Database Migration Service
will expect that you provide the System Change Number (SCN) of your redo log
files as the starting point.
Change Data Capture (CDC)
When the full dump phase is over (or if you choose to perform the full
dump yourself and start with the CDC phase), Database Migration Service switches to the
CDC phase where it keeps monitoring your source databases for changes, and then
continuously replicates them on the destination instance. In this phase,
Database Migration Service doesn't copy actual data from your source databases: instead
it reads log files to replicate changes in the destination.
Database Migration Service uses the Oracle LogMiner API to access and parse archived Oracle
redo log files in your source database. Before you migrate your Oracle databases
to Cloud SQL for PostgreSQL with Database Migration Service, we recommend that you adjust your
Oracle redo log files settings for optimal performance and data fidelity. For
more information, see
Work with Oracle redo log files.
[[["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\u003eDatabase Migration Service utilizes two migration types, continuous and one-time, with only continuous migrations available for Oracle to Cloud SQL for PostgreSQL.\u003c/p\u003e\n"],["\u003cp\u003eContinuous migrations are divided into two phases: the full dump phase, which transfers the initial data to the destination, and the Change Data Capture (CDC) phase, which replicates subsequent changes.\u003c/p\u003e\n"],["\u003cp\u003eDuring the full dump phase, Database Migration Service copies data directly from the source database to the destination, with the option to adjust concurrent connection numbers for optimal performance.\u003c/p\u003e\n"],["\u003cp\u003eAlternatively, users can manually perform the full dump and then start the migration job with the CDC phase, providing the System Change Number (SCN) as the starting point.\u003c/p\u003e\n"],["\u003cp\u003eThe CDC phase relies on monitoring log files to replicate changes, using the Oracle LogMiner API to read and parse archived Oracle redo log files for continuous data synchronization.\u003c/p\u003e\n"]]],[],null,["# About data flow for Oracle to Cloud SQL for PostgreSQL\n\nThere are two important concepts you need to understand to know how data flows\nfrom your source to the destination instance: migration types and migration\nphases. Database Migration Service supports two types of migration: continuous and one-time.\nOnly continuous migrations are available for heterogeneous Oracle to\nCloud SQL for PostgreSQL migrations.\n\nWith continuous migrations, your data is migrated in two phases: full dump, and\nChange Data Capture (CDC).\n\nFull dump phase\n---------------\n\nThe **full dump phase** is the first part of the migration process. During\nfull dump, Database Migration Service connects to your source instance, reads the content\nof the databases you selected for migration, and then loads the data to the Cloud SQL for PostgreSQL\ndestination instance. In this phase, Database Migration Service captures actual contents\nof your database.\n\nDuring the full dump phase, Database Migration Service can open multiple connections to\nyour source databases for improved performance. You can configure the number of\nconcurrent connections when you\n[create a migration job](/database-migration/docs/oracle-to-postgresql/create-migration-job).\n\nFor granular control over your data movement, you can also perform the full dump\nand load the data to the destination instance on your own, outside of\nDatabase Migration Service. If you choose to perform this phase manually, your migration\njob will start with the CDC phase instead and Database Migration Service\nwill expect that you provide the System Change Number (SCN) of your redo log\nfiles as the starting point.\n\nChange Data Capture (CDC)\n-------------------------\n\nWhen the **full dump** phase is over (or if you choose to perform the full\ndump yourself and start with the CDC phase), Database Migration Service switches to the\nCDC phase where it keeps monitoring your source databases for changes, and then\ncontinuously replicates them on the destination instance. In this phase,\nDatabase Migration Service doesn't copy actual data from your source databases: instead\nit reads log files to replicate changes in the destination.\n\nDatabase Migration Service uses the Oracle LogMiner API to access and parse archived Oracle\nredo log files in your source database. Before you migrate your Oracle databases\nto Cloud SQL for PostgreSQL with Database Migration Service, we recommend that you adjust your\nOracle redo log files settings for optimal performance and data fidelity. For\nmore information, see\n[Work with Oracle redo log files](/database-migration/docs/oracle-to-postgresql/work-with-oracle-database-redo-log-files)."]]
