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Version: 2.2.0

Frequently Asked Questions

This document answers some frequently asked questions about the MSR (Multi-Session Replay) module.

Change Data Capture Issues​

My database changes are not being captured by Debezium. What's wrong?​

The most common cause is forgetting to set REPLICA IDENTITY FULL on your database tables. Without this setting, PostgreSQL doesn't provide enough information for Debezium to capture all changes.

Solution:

ALTER TABLE <schema>.<table_name> REPLICA IDENTITY FULL;

Examples:

ALTER TABLE gis.geo_entity REPLICA IDENTITY FULL;
ALTER TABLE gis.bookmark REPLICA IDENTITY FULL;

I see "slot does not exist" errors in Kafka Connect logs​

This usually means the replication slot name is already in use or wasn't created properly. Each source connector needs a unique slot.name across the entire Kafka Connect cluster.

Solution:

  • Ensure slot.name is unique for each connector
  • Check PostgreSQL for existing slots: SELECT * FROM pg_replication_slots;
  • Drop unused slots if necessary: SELECT pg_drop_replication_slot('slot_name');

No data is appearing in the MSR CDC events table​

This could be due to several issues:

  1. Source connector not running: Check Kafka Connect status at http://localhost:8083/connectors
  2. Sink connector misconfigured: Verify the sink connector is consuming from the correct topics
  3. Transform errors: Check Kafka Connect logs for JSLT transformation errors
  4. Topic permissions: Ensure Kafka Connect has permission to read/write topics

I'm getting "permission denied" errors when setting up connectors​

Verify that:

  • Database user has replication permissions: ALTER USER admin REPLICATION;
  • Database user can access the required tables
  • Kafka Connect can reach both source and target databases

Performance Issues​

CDC processing is slow or lagging behind​

Consider these optimizations:

  • Increase batch.size in sink connector configuration
  • Adjust snapshot.fetch.size for source connectors
  • Monitor database work_mem settings: SET work_mem = '256MB';
  • Check network latency between components

High memory usage in Kafka Connect containers​

This is often caused by:

  • Large batch sizes processing too much data at once
  • JSLT transformations on very large JSON payloads
  • Insufficient memory allocation to Kafka Connect JVM

Solution: Tune JVM heap size and connector batch configurations.

Configuration Issues​

My JSLT transformation is failing with parsing errors​

Common JSLT issues include:

  • Incorrect JSON escaping in connector configuration
  • Missing fields in the source data structure
  • Type mismatches between expected and actual data types

Debug by:

  1. Testing JSLT expressions separately
  2. Checking source data structure in Kafka topics
  3. Validating JSON syntax in connector configuration

Connector keeps restarting or failing​

Check these common causes:

  • Database connection timeouts
  • Insufficient database connection limits
  • Missing required permissions
  • Network connectivity issues between services

Development and Debugging​

How do add logs to the MSR Web Worker​

The MSR Web Worker uses a custom logging system that sends all log messages to the main browser thread for proper console output. The worker includes a workerLog object with different log levels:

// Available log levels in the Web Worker
workerLog.debug("Debug message", { additionalData });
workerLog.info("Info message", { sessionData });
workerLog.warn("Warning message", { errorDetails });
workerLog.error("Error message", { errorContext });

To see Web Worker logs in your browser console:

  1. Open browser Developer Tools (F12)
  2. Go to the Console tab
  3. Worker logs will appear with [MSR Worker] prefix
  4. Use console filtering to show only MSR logs: [MSR Worker]

Common log messages to look for:

  • "Session created successfully" - Confirms session initialization
  • "Loaded entities into worker state" - Shows initial data loading
  • "Starting data polling" - Indicates background polling started
  • "Buffering changes" - Shows CDC data being processed
  • "Buffer size limit reached" - Warning about memory usage