Cassandra to MongoDB
real-time replication
TwinCommit replicates Apache Cassandra to MongoDB in real time by reading the Cassandra transaction log and applying every committed change exactly once and in commit order. Use it to serve Cassandra data from MongoDB at scale without dual writes, with no triggers or capture tables on the source.
Cassandra
log reader
commit-ordered · exactly-once
MongoDB
native writer
How capture works on Cassandra
TwinCommit tails the CDC commit-log segments on every node, deduplicates replica writes, and orders mutations by write timestamp per partition.
Prerequisites
- ✓cdc_enabled: true in cassandra.yaml
- ✓WITH cdc = true on replicated tables
- ✓Agent sidecar or shared volume access to cdc_raw on each node
How apply works on MongoDB
Relational rows become documents keyed by primary key and are written with bulkWrite upserts inside MongoDB transactions, together with the checkpoint.
Requirements
- ✓Replica set or sharded cluster (change streams need an oplog)
- ✓changeStream and find privileges on watched databases
- ✓Pre- and post-images enabled for full before/after values (optional)
Setup
Set up Cassandra → MongoDB in four steps
- 1
Prepare Cassandra
Run twincommit check. It verifies cdc_enabled: true in cassandra.yaml and the other prerequisites, and prints the exact fix for anything missing.
- 2
Point at MongoDB
Add a target block for MongoDB. TwinCommit can generate target DDL with twincommit ddl -apply.
- 3
Snapshot
twincommit snapshot copies each table under a brief share lock and records the exact log position.
- 4
Stream
twincommit run streams every committed change from that position, exactly once and in commit order.
source:
type: cassandra
contact_points: [cass-1, cass-2, cass-3]
cdc_dir: /var/lib/cassandra/cdc_raw
target:
type: mongodb
uri: ${MONGODB_URI}
database: replica
document_id: primary_key
tables:
- source: sales.orders
- source: sales.customersCassandra to MongoDB replication FAQ
How does TwinCommit replicate Cassandra to MongoDB?
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TwinCommit reads changes from the Cassandra transaction log, buffers them per transaction, and applies each committed transaction to MongoDB in commit order. Relational rows become documents keyed by primary key and are written with bulkWrite upserts inside MongoDB transactions, together with the checkpoint.
Does Cassandra to MongoDB replication need triggers or capture tables?
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No. TwinCommit reads the Cassandra log directly and creates no objects in the source database.
Is Cassandra to MongoDB replication exactly-once?
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Yes. TwinCommit commits its checkpoint in the same MongoDB transaction as the data, so a crash or restart never loses or duplicates a transaction.
What Cassandra versions are supported?
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Apache Cassandra 4.0, 4.1, 5.0 · DataStax Enterprise 6.8+ · Astra DB (target) · ScyllaDB (target).
Stream Cassandra to MongoDB today.
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