Molt Replicator

molt replicator

Continuous change-data-capture (CDC) replication from source databases to CockroachDB. Run after molt fetch completes the initial bulk load.

Important: replicator is a separate binary from molt. It is not invoked by molt fetch. The data-load-and-replication mode in molt fetch is deprecated — use replicator directly instead.

Architecture

Source DB ──► [replicator] ──► Staging DB (_replicator schema) ──► Target CockroachDB
                  ▲
            Publication/
            Slot/BinLog/
            LogMiner

Replicator reads changes from the source, buffers them in a staging schema on the target CRDB cluster, and applies them to the target tables.

Subcommands by Source

Full Fetch → Replicator Workflow

Step 1: Initial bulk load with molt fetch

drop-on-target-and-recreate drops any existing target tables first; run this only against a fresh or empty target.

molt fetch \
  --source "postgresql://<user>:<password>@source:5432/db" \
  --target "postgresql://root@crdb:26257/db" \
  --bucket-path "s3://mybucket/migration" \
  --table-handling drop-on-target-and-recreate

Step 2: Create publication on source (PostgreSQL)

-- Run on source PostgreSQL:
CREATE PUBLICATION molt_fetch FOR ALL TABLES;
-- (molt fetch may have already created this; check first)

Step 3: Create staging database on target

-- Run on target CockroachDB:
CREATE DATABASE _replicator;

Step 4: Test connectivity

# preflight only takes --stagingConn and --targetConn (always required for the
# target; stagingConn required if the target is not CRDB)
replicator preflight \
  --stagingConn "postgresql://root@crdb:26257/_replicator" \
  --targetConn "postgresql://root@crdb:26257/db"

Step 5: Start replicator

replicator pglogical \
  --publicationName "molt_fetch" \
  --sourceConn "postgresql://<user>:<password>@source:5432/db" \
  --stagingConn "postgresql://root@crdb:26257/_replicator" \
  --stagingSchema "_replicator.public" \
  --targetConn "postgresql://root@crdb:26257/db" \
  --targetSchema "public" \
  --metricsAddr "0.0.0.0:8080"

Step 6: Monitor lag

curl http://localhost:8080/metrics | grep replicator_
# Watch for: mutations applied, unapplied mutations, lag

Step 7: Cutover

1. When lag reaches ~0, redirect app writes to CockroachDB
2. Let replicator drain remaining changes
3. Confirm no new writes on source
4. Stop replicator
5. Decommission the source only after molt verify passes and a source backup is confirmed

Source-Specific Setup

PostgreSQL (pglogical)

Source prerequisites:

• User with REPLICATION privilege
• Logical replication enabled (wal_level = logical)
• Publication exists (created by molt fetch or manually)

replicator pglogical \
  --publicationName "molt_fetch" \
  --slotName "replicator" \
  --sourceConn "postgresql://..." \
  --stagingConn "postgresql://root@crdb:26257/_replicator" \
  --stagingSchema "_replicator.public" \
  --targetConn "postgresql://root@crdb:26257/db" \
  --targetSchema "public"

MySQL (mylogical)

Source prerequisites:

• Binary logging enabled (binlog_format = ROW)
• GTID mode on (gtid_mode=ON, enforce_gtid_consistency=ON)
• User with REPLICATION CLIENT privilege

replicator mylogical \
  --sourceConn "mysql://<user>:<password>@source:3306/db" \
  --stagingConn "postgresql://root@crdb:26257/_replicator" \
  --stagingSchema "_replicator.public" \
  --targetConn "postgresql://root@crdb:26257/db" \
  --targetSchema "public"

Oracle (oraclelogminer)

Source prerequisites:

• Archive log mode enabled
• Supplemental logging enabled
• LogMiner permissions granted

replicator oraclelogminer \
  --sourceConn "oracle://<user>:<password>@oracle:1521/db" \
  --stagingConn "postgresql://root@crdb:26257/_replicator" \
  --stagingSchema "_replicator.public" \
  --targetConn "postgresql://root@crdb:26257/db" \
  --targetSchema "public"

Key Flags

# Performance
--parallelism 16          # concurrent DB transactions (default: 16)
--flushSize 1000          # rows per batch (default: 1000)
--flushPeriod 1s          # flush interval (default: 1s)

# Staging connection pool
--stagingMaxPoolSize 128
--stagingIdleTime 1m
--stagingMaxLifetime 5m

# Target connection pool
--targetMaxPoolSize 128
--targetStatementCacheSize 128

# Retry
--maxRetries 10
--retryInitialBackoff 25ms
--retryMaxBackoff 2s

# Monitoring
--metricsAddr "0.0.0.0:8080"    # Prometheus metrics endpoint
--schemaRefresh 1m               # refresh schema cache (0 = disabled)

# Dead letter queue (failed rows instead of stopping)
--dlqTableName "replicator_dlq"

# Logging
-v                               # debug
-vv                              # trace
--logFormat fluent                # for log aggregators
--logDestination "/var/log/replicator.log"

Gotchas

• Staging schema (_replicator.public) is auto-created by replicator, but the database (_replicator) must exist first
• --publicationName and --slotName must match what molt fetch created. molt fetch's --pglogical-publication-name defaults to molt_fetch and its --pglogical-replication-slot-name has no default; on the replicator side, --publicationName has no default and --slotName defaults to replicator. If the names don't line up, set both explicitly on both sides.
• DLQ table grows over time — monitor and purge failed rows periodically
• Replicator holds an open replication slot on the source — this blocks WAL cleanup; monitor source disk usage
• Graceful shutdown respects --gracePeriod (default: 30s); don't SIGKILL without it
• No built-in alerting — set up external alerts on the Prometheus metrics endpoint
• Long cutover windows increase replication lag — plan for a maintenance window if needed

See flags reference for the full flag list.