Subscriptions to show online users

We cruised through our GraphQL queries and mutations. We queried for todos, added a new todo, updated an existing todo, removed an existing todo.

Now let's get to the exciting part.

GraphQL Subscriptions

We have a section of UI which displays the list of online users. So far we have made queries to fetch data and display them on the UI. But typically online users data is dynamic.

We can make use of GraphQL Subscription API to get realtime data from the graphql server to efficiently handle this.

But but but...

We need to tell the server that the user who is logged in is online. We have to poll our server to do a mutation which updates the last_seen timestamp value of the user.

We have to make this change to see yourself online first. Remember that you are already logged in, registered your data in the server, but not updated your last_seen value?

The goal is to update every few seconds from the client that you are online. Ideally you should do this after you have successfully authenticated with Auth0. So let's update some code to handle this.

Open src/components/OnlineUsers/OnlineUsersWrapper.svelte and add the following imports

src/components/OnlineUsers/OnlineUsersWrapper.svelte

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+ import { onMount, onDestroy } from "svelte";
+ import { gql } from "@apollo/client/core";
+ import { subscribe, mutation } from "svelte-apollo";

we will create a setInterval to update the last_seen of the user every 30 seconds in onMount life cycle method and we will clear interval in onDestroy life cycle method.

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+  const updateLastSeen = async () => {
+    // Use the apollo client to run a mutation to update the last_seen value
+    await updateLastSeenMutation({
+      variables: { now: new Date().toISOString() },
+    });
+  };
+
+  onMount(async () => {
+    // Every 20s, run a mutation to tell the backend that you're online
+    await updateLastSeen();
+    onlineIndicator = setInterval(async () => await updateLastSeen(), 20000);
+  });
+
+  onDestroy(() => {
+    clearInterval(onlineIndicator);
+  });

Now let's write the definition of the updateLastSeen.

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+ const UPDATE_LASTSEEN_MUTATION = gql`
+   mutation updateLastSeen($now: timestamptz!) {
+     update_users(where: {}, _set: { last_seen: $now }) {
+       affected_rows
+     }
+   }
+ `;
+ const updateLastSeenMutation = mutation(UPDATE_LASTSEEN_MUTATION);

  const updateLastSeen = async () => {
    // Use the apollo client to run a mutation to update the last_seen value
    await updateLastSeenMutation({
      variables: { now: new Date().toISOString() },
    });
  };

We are calling mutation function with mutation query to create updateLastSeenMutation mutate function which is called with variables inside updateLastSeen.

Great! Now the metadata about whether the user is online will be available in the backend. Let's now do the integration to display realtime data of online users.