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DiceDB

Reactivity and Query Subscriptions

DiceDB is a reactive database that allows you to create query subscriptions. When you WATCH a particular key or command, DiceDB proactively executes the query and sends you the result whenever the data changes.

This eliminates the need to poll the database for changes or work with change events. To demonstrate this, here’s a quick “Hello, World!” example.

But, before we start, make sure you have complete the Hello, World! example.

Watching a key

To see what reactivity is, let’s watch a key.

You need to open three terminal sessions, connect all of them with the same DiceDB server using the DiceDB CLI.

In the first two terminal sessions, run the following command to watch the key k1.

Terminal window
localhost:7379> GET.WATCH k1

In the third terminal session, run the following command to set the value of k1 to v1.

Terminal window
localhost:7379> SET k1 v1
OK OK

In the first two terminal sessions, you will see the following output.

Terminal window
localhost:7379> GET.WATCH k1
entered the watch mode for GET.WATCH k1
OK [fingerprint=2356444921] v1

Now, if you set the value of k1 to v2, you will see the following output in the first two terminal sessions.

Terminal window
localhost:7379> GET.WATCH k1
entered the watch mode for GET.WATCH k1
OK [fingerprint=2356444921] v1
OK [fingerprint=2356444921] v2

This is the reactivity in action. Here you are watching the key k1 and getting notified whenever the value of k1 changes, but it is not just a change event, but the actual result set. This makes it really powerful and flexible to build real-time applications.

Note: In CLI, you can watch one command at a time, but programmatically you can watch multiple commands at the same time and handle them in parallel. You can take a look at our Chatroom to see how to do that.

What else can I watch?

Any readonly command will have a .WATCH version that allows you to watch the result of the command. This is a simple convention that we follow to make it easier to understand what is being watched.

we are in process of implementing the .WATCH commands for all query commands in DiceDB, but today we support GET.WATCH, etc. You can find them in the commands section. With subsequent releases, we will keep shipping .WATCH commands for all the query commands in DiceDB.