Question 410 of 1,446
PCDE Migrate data solutions Practice Question
A company is migrating from PostgreSQL to AlloyDB using DMS. They need to set up logical replication on the source. Which THREE components are required for PostgreSQL logical replication?
Answer choices
Why each option matters
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
pglogical extension
Logical replication requires a publication on the source, a subscription on the destination, and the pglogical extension (or built-in logical replication). DMS is the migration service but not a component of logical replication itself. A replication slot is created automatically.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
DMS migration job
Why it's wrong here
DMS uses the replication, but it's not a component of logical replication itself.
- ✓
pglogical extension
Why this is correct
Provides the logical replication functionality (or built-in pgoutput).
- ✓
Publication on the source database
Why this is correct
Defines which tables to replicate.
- ✗
Replication slot
Why it's wrong here
A replication slot is created as part of the process, but it's not a separate component you configure manually in the same sense.
- ✓
Subscription on the destination database
Why this is correct
Connects to the publication and applies changes.
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Same concept, more angles
3 more ways this is tested on PCDE
These questions test the same concept from different angles. Work through them to make sure you can recognise it however the exam phrases it.
Variation 1. A team is migrating an on-premises PostgreSQL 13 database to AlloyDB using DMS continuous migration. During the CDC phase, the migration job shows an error: 'ERROR: could not start WAL streaming: ERROR: replication slots are not enabled on the source.' The source is running in Google Cloud Compute Engine. What is the most likely cause?
hard- A.The DMS connection profile uses the wrong public IP.
- B.The source database is in read-only mode.
- C.The source database does not have the pglogical extension installed.
- ✓ D.The source database parameter wal_level is set to 'replica' instead of 'logical'.
Why D: DMS continuous migration for PostgreSQL requires logical replication, which uses replication slots. The error indicates that the source database does not have the necessary configuration to support replication slots (wal_level=logical, max_replication_slots > 0).
Variation 2. A company is migrating an on-premises PostgreSQL 13 database to AlloyDB for PostgreSQL using DMS continuous migration. The source is configured with logical replication using the pglogical extension. During the initial sync, the migration job fails with the error 'could not open relation with OID xxxx'. What is the most likely cause?
hard- A.The source database has a firewall blocking the DMS IP address.
- B.The source database does not have the pglogical extension installed.
- C.The target AlloyDB cluster does not have the pglogical extension.
- ✓ D.A DDL operation on the source dropped or altered a table after the replication slot was created.
Why D: DMS uses logical replication slots when possible. If the source has DDL changes (e.g., table dropped) after the slot is created, the replication slot may become invalid.
Variation 3. A company is migrating a PostgreSQL database to Cloud SQL using DMS. The source database has logical replication enabled for the migration. What is the purpose of logical replication in this context?
easy- A.To enable point-in-time recovery on the source.
- B.To improve performance of the initial dump.
- ✓ C.To capture changes for continuous replication after the initial dump.
- D.To convert data types between source and destination.
Why C: DMS uses PostgreSQL logical replication to capture ongoing changes. The source database must have logical replication enabled (via WAL with logical decoding) to allow DMS to replicate changes continuously after the initial dump. Binary logging is a MySQL concept, not applicable to PostgreSQL.
Last reviewed: Jul 4, 2026
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