Logical Data Replication - CockroachDB - DBA

1. What is LDR?

LDR = Logical Data Replication.

One CockroachDB cluster sends its data changes to another CockroachDB cluster.

In our example:

Mumbai Cluster ↔ Singapore Cluster

Both sides can be active.

Mumbai Cluster                    Singapore Cluster
   2 Nodes                              2 Nodes
      │                                    │
      └──────────── LDR ───────────────────┘
                 ↕
          Data Changes

So, when data changes in Mumbai, the change can be replicated to Singapore.

And when data changes in Singapore, the change can be replicated back to Mumbai.

That is why we call it Bidirectional LDR.

2. Step 1 — Prepare both clusters

First, we prepare Mumbai and Singapore.

We do things like:

Mumbai
  │
  ├── Enable rangefeed
  ├── Create replication user
  └── Grant required replication privilege

Same thing on Singapore:

Singapore
  │
  ├── Enable rangefeed
  ├── Create replication user
  └── Grant required replication privilege

Why rangefeed?

Very simple:

Rangefeed helps CockroachDB continuously watch for changes happening to the data.

For example:

UPDATE customer
SET city = 'Hyderabad'
WHERE id = 101;

CockroachDB needs to know:

“Something changed.”

LDR uses this change information for replication.

3. Step 2 — External Connections

Now Mumbai needs to know how to connect to Singapore.

And Singapore needs to know how to connect to Mumbai.

Think about it like saving another server’s connection details.

Mumbai
   │
   │  "How can I reach Singapore?"
   ▼
Singapore URI


Singapore
   │
   │  "How can I reach Mumbai?"
   ▼
Mumbai URI

So each cluster stores the other cluster’s connection information.

This is why we create External Connections.

4. Step 3 — Start LDR

Now we actually start replication.

Important point:

The LDR stream is started from the destination cluster.

For example:

Mumbai ─────────────────────► Singapore
       Source                  Destination

Here:

Mumbai = Source
Singapore = Destination

The destination Singapore starts/accepts the LDR setup.

5. First LDR stream

We create the first replication direction:

Mumbai
  │
  │  Data Changes
  │  ───────────────►
  │
Singapore

Example:

Mumbai:

INSERT INTO customer VALUES (101, 'Ravi');

LDR captures the logical change and sends it toward Singapore.

Conceptually:

Mumbai
  │
  │ INSERT customer 101
  ▼
LDR Stream
  │
  ▼
Singapore
  │
  ▼
customer 101

6. Second LDR stream

For bidirectional replication, we need another stream in the opposite direction.

Mumbai ◄──────────────────── Singapore
       Reverse LDR Stream

Now Singapore can send its changes to Mumbai.

So finally:

              LDR
       ┌─────────────────┐
       │                 │
       ▼                 │
   Mumbai ◄──────────► Singapore
       │                 │
       └─────────────────┘

This is Bidirectional LDR.

7. Very important: LDR is not physical replication

This is a very important interview point.

Physical replication

Usually:

Primary
   │
   │ Physical/WAL-level changes
   ▼
Replica

The replica is maintaining a physical copy of the database state.

LDR

LDR works at the logical data-change level.

Conceptually:

Mumbai
   │
   │ Logical changes
   │ INSERT
   │ UPDATE
   │ DELETE
   ▼
LDR
   │
   ▼
Singapore

So don’t think:

“Mumbai disk blocks are copied to Singapore.”

Instead think:

“Changes to the data are logically replicated to the other cluster.”

8. Step 4 — Monitor

After starting LDR, we should not simply assume:

“Replication is working.”

We need to monitor it.

We check from the DB Console and relevant SQL/replication status.

We want to confirm:

Mumbai → Singapore
        HEALTHY

Singapore → Mumbai
        HEALTHY

We should look for things such as:

Replication status
Stream health
Replication lag
Errors
Connection problems
Data replication progress

9. Final architecture

The complete flow is:

                LDR SETUP
                    │
          ┌─────────┴─────────┐
          │                   │
       MUMBAI             SINGAPORE
       2 Nodes              2 Nodes
          │                   │
          │  Step 1           │
          │  Prepare          │
          │                   │
          └─────────┬─────────┘
                    │
             Step 2: External
                Connections
                    │
                    ▼
             Step 3: Start LDR
                    │
          ┌─────────┴─────────┐
          │                   │
          ▼                   ▼
       Mumbai ───────────► Singapore
          ▲                   │
          │                   │
          └───────────────────┘
              Reverse LDR

                    │
                    ▼
             Step 4: Monitor

                    │
                    ▼

       MUMBAI ◄────────────► SINGAPORE
                ACTIVE

10. Important Points

Just remember these 4 steps:

Step	What we do	Easy meaning
1. Prepare	Rangefeed + user + privilege	Make both clusters ready
2. Connect	External Connections	Tell each cluster how to reach the other
3. Start LDR	Create/start replication streams	Start sending changes
4. Monitor	DB Console/status	Check whether replication is healthy

Logical Data Replication – CockroachDB