molt-fetch

molt fetch

Bulk data migration from source databases (PostgreSQL, MySQL, Oracle, MSSQL) to CockroachDB.

Basic Structure

molt fetch \
  --source "<source-conn>" \
  --target "<crdb-conn>" \
  --bucket-path "s3://bucket/prefix"   # or --direct-copy or --local-path
  [options]

Storage Backends (pick one)

Option	When to use
--bucket-path "s3://..."	AWS S3 (also gs:// for GCS, azure:// for Azure)
--direct-copy	No intermediate storage; fastest for accessible networks
--local-path "/tmp/molt" + --local-path-listen-addr "0.0.0.0:9005"	CRDB must reach the listen addr

Cloud auth: pass --use-implicit-auth for IAM/ADC/managed identity, or set AWS_ACCESS_KEY_ID/GOOGLE_APPLICATION_CREDENTIALS env vars.

Table Handling (--table-handling)

Value	Behavior
none (default)	Append to existing tables
drop-on-target-and-recreate	Drop + recreate from source schema; enables auto schema creation
truncate-if-exists	Truncate before loading; errors if table missing

Import Mode

IMPORT INTO (default): Table goes OFFLINE during load. Highest throughput.

COPY FROM (--use-copy): Table stays ONLINE. Use with --direct-copy. Cannot use compression.

# Zero-downtime load
molt fetch --source "..." --target "..." --direct-copy --use-copy

Key Flags

# Filtering
--table-filter "customers|orders"      # POSIX regex for tables to include
--table-exclusion-filter "temp_.*"     # exclude pattern
--schema-filter "public"               # PostgreSQL only

# Performance
--table-concurrency 4                  # parallel tables (default: 4)
--export-concurrency 4                 # export threads (default: 4)
--row-batch-size 100000                # rows per SELECT (default: 100k)

# Schema
--type-map-file "types.json"           # custom type mappings
--transformations-file "transforms.json"  # column exclusions, table aliases

# Logging
--log-file "migration.log"             # or "stdout"
--logging debug                        # info (default), debug, trace
--metrics-listen-addr "0.0.0.0:3030"  # Prometheus scrape endpoint

Source-Specific Prerequisites

MySQL: GTID mode required (gtid_mode=ON, enforce_gtid_consistency=ON). ONLY_FULL_GROUP_BY must be off. Or use --ignore-replication-check.

Oracle: Binary must be built with CGO_ENABLED=1 -tags="cgo source_all". Oracle Instant Client in LD_LIBRARY_PATH.

PostgreSQL: Replication privileges needed, or --ignore-replication-check.

Common Workflows

1. Validate before migrating

molt fetch --dry-run --source "..." --target "..." --bucket-path "s3://..."
# Exports 1 row, imports, verifies, cleans up. Returns immediately.

2. Full migration with schema creation

molt fetch \
  --source "postgresql://user:pass@pg:5432/db" \
  --target "postgresql://root@crdb:26257/db" \
  --bucket-path "s3://mybucket/migration" \
  --table-handling drop-on-target-and-recreate \
  --table-filter "customers|orders|payments" \
  --log-file migration.log

3. Resume after failure

# List available continuation tokens
molt fetch tokens --fetch-id "abc-123" --target "postgresql://root@crdb:26257/db"

# Resume all failed tables
molt fetch \
  --source "..." --target "..." \
  --bucket-path "s3://mybucket/migration" \
  --fetch-id "abc-123" \
  --non-interactive

4. Validate flag syntax without connecting

molt fetch --compile-only --source "..." --target "..." --bucket-path "..."
# Returns JSON: {"status":"ok","message":"arguments parsed successfully"}

Error Recovery

Error	Cause	Fix
"GTID-based replication not enabled"	MySQL missing GTID	Enable gtid_mode=ON or add --ignore-replication-check
"Column mismatch"	Schema diverged	Fix target schema manually or use --type-map-file
Silent IMPORT INTO	CockroachDB import running	SHOW JOBS on CRDB to check progress
"timestamp in the future"	Docker/Mac clock drift	Sync clocks between hosts

Gotchas

• COPY mode: cannot use --compression gzip; must use --compression none (or omit, default is none with copy)
• Table is offline during IMPORT INTO — use --use-copy for zero downtime
• Schema changes between runs require starting from scratch
• --fetch-id continuation tokens live in the target's exceptions table
• For MySQL, --ignore-replication-check skips GTID validation but replication-dependent features won't work
• After fetch, run molt verify to confirm data integrity

See flags reference for the full flag list.