Autumn trees on a dusty road in Magoebaskloof, South Africa. Photo by Garren Smith, iPhone 13 Pro.

Tigris now supports object notifications! Object notifications are how you receive events every time something changes in a bucket. Think of it as your bucket's way of saying "Hey, something happened! Come check it out!", much like the inotify subsystem in Linux. These notifications can be helpful for keeping track of what's going on in your application.

Use Case: Automatic Image Processing​

Imagine you're building a photo-sharing app. Every time a user uploads a new picture, you want to automatically generate a thumbnail and maybe even run it through an AI to detect any inappropriate content. With object notifications, this becomes a breeze!

1. User uploads an image to your Tigris bucket.
2. Tigris sends a notification to your webhook.
3. Your server receives the notification and springs into action.
4. It downloads the new image, creates a thumbnail, and runs it through an AI check.
5. The processed image and its metadata are saved back to Tigris.

All of this happens automatically, triggered by that initial upload.

Behind the Scenes: Building Object Notifications​

Now, let's pull back the curtain and see how we built this feature and a few tricky situations we had to handle. Grab your hard hat, because we're going on a little tour of Tigris's inner workings!

Tigris isn't just any object store – it's a global object store. This means that objects can be changed in multiple regions around the world. This makes them available in multiple regions, always ready when you need them. But means we need a way of keeping track of all the changes for the same object. This is where replication comes in.

Replication: Keeping Everyone in the Loop​

To make sure everything stays in sync, we replicate changes to multiple regions. This ensures high availability and improved redundancy of our objects.

The caveat to this is that replication is a background task, and the speed at which an object is replicated from one region to another can be affected by many external factors.

To solve this, when a change is received at a region it looks at the Last Modified timestamp of the metadata to determine if the change is new and needs to be applied or if the region has already seen a newer change. It will discard the change if it is old.

The Object Notification Hub​

When object notifications are enabled for a bucket, we assign one region to be the object notification hub for that bucket. This region gets the important job of keeping track of all the changes. We create a special index which is very similar to a secondary index in that region's FoundationDB. We order the changes by FoundationDB Versionstamp, when the change is added to the index, and Last Modified timestamp of object metadata.

The Versionstamp helps the worker keep track of which events it has seen and processed.

Why one region you may ask? If we didn't do this, we end up with multiple regions sending the same events to the webhook, hello friendly DDos attack, or having to build a complex system to try and co-ordinate the regions so they don't send duplicate events.

The Background Task: Our Diligent Messenger​

In our object notification region, we have a background task running. Think of it as a tireless worker that's always on the lookout for changes. Every so often, it checks the special index we mentioned earlier, collects all the latest changes, and sends them off to the webhook.

The worker will also keep track of the last processed change and will retry a few times if the request failed. Finally it will remove old changes from the index that have already been processed.

Why We Can't Guarantee Ordered Events​

We talked about how object changes replicated from many regions can take different times. The problem arises when the worker is ready to send the latest events for an object. It has no way of knowing if all changes for an object have been replicated to its region. It could in theory contact every region and check, but this would be prohibitively expensive. And still not a complete guarantee.

This forces us to make the trade off of sending events out of order. The worker will read the latest list of changes that have been replicated to the region and send them to the webhook.

Wrapping Up​

That's how we built object notifications in Tigris. We took a global system, added some global replication, threw in a change index, topped it off with a hardworking background task.

The result? A system that keeps you in the loop about what's happening in your buckets, no matter where in the world those changes occur. Whether you're building the next big photo-sharing app or just want to keep tabs on your storage, object notifications have got your back!

We hope this peek behind the scenes was fun and informative. Happy coding!