How do you ensure data consistency between two separate databases when performing transactions?

To ensure data consistency between two separate databases during transactions, several approaches can be used:

1. Two-Phase Commit (2PC) — a classic protocol that guarantees atomicity of transactions across multiple resources. In Node.js, libraries or external transaction managers supporting 2PC can be used.

2. Sagas — a pattern that breaks a long transaction into a series of local transactions with compensating actions in case of errors. This is a more flexible and scalable approach, especially in distributed systems.

3. Using message queues and events — for asynchronous data synchronization between databases, where each operation publishes an event, and the other database reacts to it.

Example using Saga:

// Pseudocode for Saga
async function transferData() {
  try {
    await db1.insert(data1);
    await db2.insert(data2);
  } catch (error) {
    // compensating actions
    await db1.delete(data1);
    throw error;
  }
}

This approach allows controlling consistency without strict resource locking.