How Debezium reads Oracle changes

The Debezium Oracle connector uses the LogMiner utility to read changes from the redo stream. LogMiner needs access to both online redo logs (for recent changes) and archived redo logs (for historical data and restarts). This dependency on raw archive log files on disk is the key operational constraint.

1. Oracle Database

DML changes are written to the active online redo log by the LGWR process.

2. Archive Log Destinations

On each log switch, ARCn copies the full redo log to one or more archive destinations on disk.

3. Debezium via LogMiner

Debezium calls DBMS_LOGMNR.ADD_LOGFILE and queries V$LOGMNR_CONTENTS to stream changes.

The conflict: If RMAN deletes an archived log (step 2) before Debezium has processed all changes in it (step 3), the connector fails with ORA-00308: cannot open archived log and shuts down immediately.

Oracle prerequisites

These settings must be in place before starting the connector. Missing any of them causes a startup failure or, worse, silent data loss with no immediate error.

Required

ARCHIVELOG mode

Without ARCHIVELOG mode, Oracle overwrites online redo logs on each log switch. There are no persistent archive files for LogMiner to read.

ALTER DATABASE ARCHIVELOG;

Required

FORCE LOGGING

Without this, direct-path inserts (INSERT /*+ APPEND */) bypass the redo log entirely. Bulk loads disappear from the CDC stream with no error or warning.

ALTER DATABASE FORCE LOGGING;

Required

Supplemental logging

Database-level minimal logging is required for LogMiner to parse redo. Table-level logging determines the completeness of Debezium's before field.

ALTER DATABASE ADD SUPPLEMENTAL LOG DATA;

ALTER TABLE t ADD SUPPLEMENTAL LOG DATA (ALL) COLUMNS; -- recommended

ALTER TABLE t ADD SUPPLEMENTAL LOG DATA (PRIMARY KEY) COLUMNS; -- minimum

(ALL) COLUMNS populates the full row pre-image in the before field. (PRIMARY KEY) leaves before incomplete for UPDATE events. (ALL) COLUMNS increases redo volume significantly — size archive storage accordingly.

Required

Connector user privileges

The connector user needs: LOGMINING, CREATE SESSION, EXECUTE_CATALOG_ROLE, SELECT_CATALOG_ROLE, EXECUTE ON DBMS_LOGMNR, and SELECT on the key V$ views: V_$DATABASE, V_$ARCHIVED_LOG, V_$ARCHIVE_DEST_STATUS, V_$LOGMNR_CONTENTS.

A missing grant on a V$ discovery view is a common failure mode. The connector starts, then fails when querying V$ARCHIVED_LOG to locate the log files for its starting SCN.

Dual-destination archive strategy

The Flash Recovery Area (FRA) is RMAN's default destination, managed aggressively to stay within a size limit. Do not let Debezium share the FRA. Configure a dedicated second destination on separate, cost-effective storage:

-- Set up a dedicated CDC archive destination (e.g., NFS mount)
ALTER SYSTEM SET LOG_ARCHIVE_DEST_2 = 'LOCATION=/path/to/cdc_archive_mount';

-- LOG_ARCHIVE_DEST_1: USE_DB_RECOVERY_FILE_DEST (FRA, RMAN-managed, short retention)
-- LOG_ARCHIVE_DEST_2: dedicated CDC destination (7+ days, SCN-aware cleanup only)

Tell Debezium which destination to read. Without this property, the connector reads from the FRA by default:

{
  "connector.class": "io.debezium.connector.oracle.OracleConnector",
  "log.mining.archive.destination.name": "LOG_ARCHIVE_DEST_2"
}

Do

+ Enable ARCHIVELOG mode and FORCE LOGGING before starting the connector.
+ Use ADD SUPPLEMENTAL LOG DATA (ALL) COLUMNS on all captured tables.
+ Configure a dedicated LOG_ARCHIVE_DEST_2 for CDC on non-FRA storage and set log.mining.archive.destination.name.
+ Set CONFIGURE ARCHIVELOG DELETION POLICY TO NONE; in RMAN for the CDC destination.
+ Monitor CommitScn JMX metric as the cleanup safe watermark. Automate SCN-aware cleanup.
+ Run CROSSCHECK ARCHIVELOG ALL before any RMAN delete operation.

Don't

- Use BACKUP ... DELETE INPUT on any destination Debezium reads from. This deletes logs immediately after backup, before Debezium can read them.
- Delete archive log files with OS commands (rm, del). This corrupts V$ARCHIVED_LOG and breaks LogMiner.
- Use RMAN RETENTION POLICY or DELETE OBSOLETE to manage the CDC archive destination.
- Assume Debezium or LogMiner can read from RMAN backup sets. LogMiner requires raw archive log files on disk.
- Calculate retention based only on connector downtime. Long-running transactions can extend the requirement by 24 hours or more.

Key connector properties

Property	Default	Recommendation
log.mining.strategy	redo_in_catalog	Keep default. Handles DDL safely. Size storage to match extra archive volume.
log.mining.archive.destination.name	(first valid local)	Set to `LOG_ARCHIVE_DEST_2` — your dedicated CDC destination.
log.mining.archive.log.only.mode	false	Leave false unless increased latency is acceptable.

Oracle Archive Log Management for Debezium

How Debezium reads Oracle changes

Oracle prerequisites

ARCHIVELOG mode

FORCE LOGGING

Supplemental logging

Connector user privileges

Retention vs. downtime conflict

The long-running transaction trap

Dual-destination archive strategy

Do

Don't

Key connector properties

FRA usage visualizer

Safe RMAN configuration for the CDC destination

FRA capacity and archive destination health

Step 1: get the safe watermark SCN from Debezium JMX

Step 2: find the last safe log sequence

Step 3: apply a safety buffer and delete (example: result - 5 = 17)