MongoDB Oplog Setup

Want faster, more efficient real-time updates in your Meteor app? Oplog tailing is the answer. Instead of polling your database for changes, your app listens directly to MongoDB's operations log. The result? Instant reactivity with less database load.

What is the Oplog?

The oplog (operations log) is a special capped collection in MongoDB that records every write operation. It's how replica sets stay in sync, but it's also incredibly useful for applications that need to react to data changes in real-time.

For Meteor apps, oplog tailing supercharges reactivity. Your observeChanges callbacks fire instantly when data changes, rather than relying on polling. This means snappier UIs and happier users.

Why Set Up Oplog Tailing?

There are a few good reasons to enable oplog tailing:

• Better Meteor reactivity: Your app sees database changes instantly. No more waiting for the next poll interval.
• Reduced database load: Polling means constant queries asking "did anything change?" Oplog tailing eliminates that overhead.
• Monti APM insights: With oplog configured, Monti APM can show you detailed metrics around observeChanges and oplog notifications. Without it, those graphs stay empty.
• Third-party integrations: Many tools that sync with MongoDB rely on oplog tailing to detect changes in real-time.

Creating an Oplog User

Your oplog user needs read access to the local database (where the oplog lives). Here's how to set it up.

mongosh "mongodb://admin:password@your-host:27017/admin"

use admin;

db.createUser({
  user: "oploguser",
  pwd: "YourSecurePassword",
  roles: [{ role: "read", db: "local" }]
});

mongosh "mongodb://oploguser:YourSecurePassword@your-host:27017/local?authSource=admin"

Building the Oplog Connection String

The oplog connection string looks similar to your main MONGO_URL, but it points to the local database where the oplog resides.

Here's the format:

mongodb://oploguser:password@host1:27017,host2:27017,host3:27017/local?authSource=admin&replicaSet=yourReplicaSetName

Let's break that down:

Configuring Your Meteor App

Add the MONGO_OPLOG_URL environment variable to your Meteor settings. Place it in the galaxy.meteor.com.env section, right below your MONGO_URL:

{
  "galaxy.meteor.com": {
    "env": {
      "ROOT_URL": "https://your-app.meteorapp.com",
      "MONGO_URL": "mongodb+srv://user:password@cluster.mongodb.net/yourdb",
      "MONGO_OPLOG_URL": "mongodb://oploguser:password@cluster.mongodb.net/local?authSource=admin&replicaSet=rs0"
    }
  }
}

Notice the parameters at the end of MONGO_OPLOG_URL. Each one serves a specific purpose:

Save your settings and deploy. Meteor automatically detects MONGO_OPLOG_URL and enables oplog tailing.

Verifying Oplog Tailing

How do you know it's working? A few ways:

• Check your app logs: When Meteor starts with oplog tailing enabled, you'll see messages about connecting to the oplog.
• Test reactivity: Open your app in two browser windows. Make a change in one. If oplog tailing is working, the other window updates almost instantly.
• Monti APM: If you're on Galaxy's Professional plan, check the Monti APM dashboard. The observeChanges and Oplog graphs should now show data. If these were empty before, that's your confirmation.

Common Issues

Quick Reference

Here's everything in one place:

Create the user:

use admin;
db.createUser({
  user: "oploguser",
  pwd: "YourSecurePassword",
  roles: [{ role: "read", db: "local" }]
});

Connection string format:

mongodb://oploguser:password@hosts/local?authSource=admin&replicaSet=rsName

Meteor settings:

{
  "galaxy.meteor.com": {
    "env": {
      "MONGO_URL": "your-main-connection-string",
      "MONGO_OPLOG_URL": "your-oplog-connection-string"
    }
  }
}

That's it. Deploy, and your Meteor app will use oplog tailing for lightning-fast reactivity.

What's Next?