Postgres Watermarks for Parallel CDC & Backfill Sync

Solution Approaches for CDC Coordination

Solution A: Serialized Capture Processes

The simplest approach eliminates coordination complexity by processing streams sequentially:

• 01Create replication slot before table capture begins
• 02Execute complete table state capture
• 03Apply accumulated slot changes after table capture completion

-- Create slot for change accumulation
SELECT pg_create_logical_replication_slot('cdc_slot', 'pgoutput');

-- Execute table capture
COPY (SELECT * FROM target_table) TO output_stream;

-- Process accumulated changes
SELECT * FROM pg_logical_slot_get_changes('cdc_slot', NULL, NULL);

Technical limitations: Change capture halts during table operations, creating inflexibility for operational systems requiring continuous synchronization. Large table captures can stall change processing for extended periods.

Operational impact: Unsuitable for frequent replay scenarios where downstream systems need rapid data recovery without interrupting live synchronization.

Solution B: WAL-Unified Processing

This approach forces all operations through the Write-Ahead Log for strict ordering guarantees:

-- Generate WAL entries for all rows
UPDATE target_table SET id = id WHERE condition;

By channeling everything through WAL, the system processes only one stream, eliminating coordination requirements.

Critical drawbacks: Massive database load from artificial updates, performance degradation, and unsuitable resource consumption for production environments.

Solution C: Complete Memory Buffering

Buffer the entire table capture in memory with WAL coordination:

• 01Start slot processor for change tracking
• 02Table reader buffers complete dataset in memory
• 03Slot processor tracks modified primary keys
• 04Emit buffered data after filtering against WAL-seen records

-- Emit coordination message
SELECT pg_logical_emit_message(true, 'cdc',
 '{"backfill_id": 1, "marker": "end"}');

Scalability constraints: Requires sufficient memory for complete datasets, impractical for large tables, risk of memory exhaustion on enterprise-scale data.

Solution D: Chunked Watermark Coordination

The most sophisticated approach processes tables in memory-efficient chunks with watermark synchronization:

• 01Low watermark emission: Signals chunk processing start
• 02Parallel processing: Table reader accumulates chunk while slot processor tracks primary keys
• 03High watermark coordination: Triggers filtered output emission
• 04Memory management: Automatic cleanup prevents accumulation

-- Low watermark
SELECT pg_logical_emit_message(true, 'cdc:low-watermark',
 '{"table_oid": 1234, "batch_id": "batch_001"}');

-- High watermark  
SELECT pg_logical_emit_message(true, 'cdc:high-watermark',
 '{"table_oid": 1234, "batch_id": "batch_001"}');

This approach delivers enterprise-scale performance while maintaining operational flexibility and memory efficiency.

Implementation Architecture

Watermark Protocol Design

Effective watermark coordination requires structured messaging between processes:

Low Watermark Structure:

{
 "table_oid": 1234,
 "batch_id": "unique_identifier",
 "backfill_id": "session_id"
}

Process Coordination:

• Table reader emits low watermark before chunk selection
• Slot processor begins primary key accumulation for specified table
• Table reader completes chunk selection and emits high watermark
• Slot processor retrieves chunk, applies PK filtering, emits filtered messages

Error Handling and Recovery

Orphaned Watermark Management: Slot processor validates batch existence with table reader, discarding invalid accumulations when processes restart unexpectedly.

Cursor Persistence: Table reader maintains checkpoint positions, enabling recovery from exact locations after system failures.

Memory Pressure Handling: Dynamic batch size adjustment based on available resources and processing capacity.

Production Implementation Example

Modern data integration platforms implement this coordination through process orchestration:

# Process coordination pattern
class TableReader:
   def emit_low_watermark(self, table_oid, batch_id):
       self.emit_message("low-watermark", {
           "table_oid": table_oid,
           "batch_id": batch_id
       })
   
   def process_chunk(self, chunk_size=100000):
       batch_id = self.generate_batch_id()
       self.emit_low_watermark(self.table_oid, batch_id)
       
       # Buffer chunk in memory
       chunk = self.select_chunk(chunk_size)
       
       self.emit_high_watermark(self.table_oid, batch_id)
       return chunk

class SlotProcessor:
   def handle_low_watermark(self, message):
       table_oid = message["table_oid"]
       self.tracked_pks[table_oid] = set()
   
   def handle_high_watermark(self, message):
       batch_id = message["batch_id"]
       chunk = self.table_reader.get_chunk(batch_id)
       
       if chunk:
           filtered_chunk = self.filter_chunk(chunk)
           self.emit_filtered_messages(filtered_chunk)
           self.cleanup_tracked_pks(message["table_oid"])

Performance Characteristics

Processing Rates: Chunked coordination enables processing rates exceeding 1M records per minute for standard table captures.

Memory Efficiency: 99%+ reduction in memory requirements compared to complete buffering approaches.

Latency Impact: Sub-second coordination message processing with minimal overhead on source databases.

Scalability: Linear scaling with chunk size optimization based on system resources and network capacity.

Enterprise Integration Patterns

Multi-System Synchronization

Watermark coordination extends across multiple database types and business systems. Production deployments coordinate CDC across PostgreSQL, MySQL, and cloud databases while maintaining consistency with CRM and ERP systems.

Database Integration:

• PostgreSQL: Native logical replication with pg_logical_emit_message
• MySQL: Binlog coordination with custom watermark tables
• Oracle: LogMiner integration with watermark persistence

Business System Integration:

• CRM synchronization: Real-time coordination across API boundaries
• ERP integration: Transactional consistency with financial systems
• Data warehouse loading: Exactly-once delivery semantics

Operational Reliability Features

Automatic Error Recovery: Built-in retry logic, rollback capabilities, and failure isolation eliminate custom error handling implementation.

Conflict Resolution: Primary key-based filtering with timestamp ordering prevents duplicate processing and maintains referential integrity.

Monitoring Integration: Real-time visibility into watermark processing, batch status, and coordination health through operational dashboards.

Production Deployment Considerations

Resource Requirements

Memory Allocation: Base requirement of 100MB plus 1KB per tracked primary key for optimal performance.

CPU Impact: Less than 5% overhead on source database systems during normal operations.

Network Overhead: Minimal - only coordination messages and filtered data transfer required.

Security and Compliance

Data Protection: Watermark messages contain only metadata, never sensitive data content.

Access Control: Replication slot management requires appropriate PostgreSQL privileges.

Audit Requirements: Complete message flow logging for compliance and troubleshooting.

Conclusion

Watermark-based coordination represents the definitive technical solution for parallel CDC and backfill operations. The chunked approach delivers enterprise performance while maintaining operational simplicity and memory efficiency.

This coordination strategy eliminates traditional trade-offs between operational flexibility and data consistency, enabling reliable CDC implementation without extensive custom development. The technical architecture scales from thousands to millions of records while providing the reliability guarantees required for mission-critical operational systems.

For organizations requiring robust CDC coordination, evaluate platforms that implement advanced watermark synchronization with built-in error handling and enterprise security. The combination of technical sophistication and operational simplicity represents the current state-of-the-art for production CDC systems.