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docs: Add notification inband mode for LiveObjects #3158

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155 changes: 146 additions & 9 deletions src/pages/docs/liveobjects/inband-objects.mdx
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Expand Up @@ -37,16 +37,27 @@ If you're using LiveObjects from one of the the following languages, then use th

Inband objects works by delivering changes to channel objects as regular channel messages, similar to [inband occupancy](/docs/channels/options#occupancy).

There are two inband modes available:

| Mode | Description |
| --- | --- |
| `objects` | Sends the full state of all objects on the channel. Delivers an initial state snapshot on attach and sends updates whenever an object operation is applied. |
| `notification` | Sends a notification when an object operation has been received, without the full state data. Clients can then fetch the current state via the REST API. Does not send an initial state on attach. |

## Enable Inband Objects <a id="inband-objects-enable"/>

To enable inband objects, use the `objects` [channel parameter](/docs/channels/options#objects) when getting a channel:
To enable inband objects, use the `objects` [channel parameter](/docs/channels/options#objects) when getting a channel. Set the value to either `objects` or `notification` depending on which mode you want to use:

<Code>
```javascript
// When getting a channel instance
// Enable full state sync mode
const channelOpts = { params: { objects: 'objects' } };
const channel = realtime.channels.get('my-channel', channelOpts);

// Or enable notification mode
const channelOpts = { params: { objects: 'notification' } };
const channel = realtime.channels.get('my-channel', channelOpts);

// Or using setOptions on an existing channel
await channel.setOptions({ params: { objects: 'objects' } });
```
Expand All @@ -58,10 +69,10 @@ Clients require the `channel-metadata` [capability](/docs/auth/capabilities) to

## Subscribe to updates <a id="inband-objects-subscribe"/>

When using inband objects, the client will receive special `[meta]objects` messages whenever the objects on the channel are updated. These messages provide a snapshot of the current set of objects on the channel.
When using inband objects, the client will receive special `[meta]objects` messages whenever the objects on the channel are updated.

<Aside data-type='note'>
If there is a high rate of updates to the channel objects the inband messages are throttled. However, the client is guaranteed to receive a sequence of inband messages after the last change occurs so that the latest data is always available.
If there is a high rate of updates to the channel objects the inband messages are throttled. However in the case of `inband:objects`, the client is guaranteed to receive a sequence of inband messages after the last change occurs so that the latest data is always available.
</Aside>

[Subscribe](/docs/api/realtime-sdk/channels#subscribe) to `[meta]objects` messages like you would any other message on the channel. For convenience, use a message name filter to only receive messages with the name `[meta]objects` in your listener:
Expand All @@ -70,15 +81,18 @@ If there is a high rate of updates to the channel objects the inband messages ar
```javascript
// Subscribe to [meta]objects messages
channel.subscribe('[meta]objects', (message) => {
const { syncId, nextCursor, object } = message.data;
console.log("Received inband objects message:", syncId, nextCursor, JSON.stringify(message.data));
console.log("Received inband objects message:", JSON.stringify(message.data));
});
```
</Code>

## Message Format <a id="inband-objects-message-format"/>
## Objects mode <a id="inband-objects-mode"/>

The `objects` mode sends the full state of all objects on the channel. When a client attaches to the channel, it immediately receives the current state. Subsequent updates are sent whenever the objects change.

Inband objects messages are sent as a sequence of messages, where each message contains a snapshot of a single object on the channel. Taken together, a set of messages belonging to the same sequence describes the complete set of objects on the channel.
### Message format

Inband objects messages in `objects` mode are sent as a sequence of messages, where each message contains a snapshot of a single object on the channel. Taken together, a set of messages belonging to the same sequence describes the complete set of objects on the channel.

Each inband objects message has a message `name` of `[meta]objects`.

Expand All @@ -90,4 +104,127 @@ The message `data` is a JSON object with the following top-level properties:
| `nextCursor` | A cursor for the next message in the sequence, or `undefined` if this is the last message in the sequence. |
| `object` | A JSON representation of the object included in the message. |

The shape of the `object` is the same as the response format of an object when listing them via the [REST API](/docs/liveobjects/rest-api-usage#fetching-objects-list-values).
The shape of the `object` is the following format:

**Inband Counter Message**
<Code>
```javascript
{
"name": "[meta]objects",
"data": {
"syncId": "1148d241",
"nextCursor": "root",
"object": {
"objectId": "counter:APjeaY7i_IQmrlxV5sEcR7NLTualutgS1QF3wNVJNYI@1769502401136",
"counter": {
"count": 12
},
"siteTimeserials": {
"cbf": "01769502451073-000@cbfpt5aQQByRKW13548557:000"
},
"tombstone": false
}
}
}
```
</Code>

**Inband Map Message**

<Code>
```javascript
{
"name": "[meta]objects",
"data": {
"syncId": "95ad3a9f",
"object": {
"objectId": "root",
"map": {
"semantics": 0,
"entries": {
"hello": {
"timeserial": "01769446125853-000@cbfpt5aQQByRKW61375059:000",
"data": {
"string": "world"
},
"tombstone": false
},
"votes": {
"timeserial": "01769502401144-000@cbfpt5aQQByRKW13548557:001",
"data": {
"objectId": "counter:APjeaY7i_IQmrlxV5sEcR7NLTualutgS1QF3wNVJNYI@1769502401136"
},
"tombstone": false
}
}
},
"siteTimeserials": {
"cbf": "01769446125853-000@cbfpt5aQQByRKW61375059:000"
},
"tombstone": false
}
}
}
```
</Code>


### Example

<Code>
```javascript
// Subscribe to [meta]objects messages in objects mode
channel.subscribe('[meta]objects', (message) => {
const { syncId, nextCursor, object } = message.data;
console.log("Received object:", syncId, nextCursor, JSON.stringify(object));
});
```
</Code>

## Notification mode <a id="inband-notification-mode"/>

The `notification` mode notifies clients when an objects operation is received, without sending the full state data. This reduces bandwidth for use cases where clients only need to know that an operation occurred and can fetch the state on demand.

Unlike `objects` mode, `notification` mode does not send an initial state when the client attaches to the channel. The first notification is sent immediately when the first object operation is received.

### Message format

Each notification message has a message `name` of `[meta]objects`.

The message `data` is a JSON object with the following property:

| Property | Description |
| --- | --- |
| `link` | A relative URL path to fetch the current state of the channel object on the channel via the [REST API](/docs/liveobjects/rest-api-usage#fetch-channel-object). This can be used directly with the Ably REST client's [request](/docs/api/rest-sdk#request) method. |

### Example

<Code>
```javascript
// Subscribe to [meta]objects messages in notification mode
channel.subscribe('[meta]objects', async (message) => {
const { link } = message.data;
console.log("Objects operation received, fetching state from:", link);

// Fetch the current state using the REST API
const response = await ablyREST.request('GET', link);
if (response.items.length > 0 ) {
console.log("Current state:", response.items[0]);
}
});
```
</Code>

### When to use notification mode

Use `notification` mode when:

* You want to minimize bandwidth usage and only fetch state when needed.
* Your application can tolerate fetching state on demand rather than receiving it immediately.
* You have infrequent operations and don't need continuous state updates.

Use `objects` mode when:

* You need the full state immediately on attach.
* You want to receive state updates in realtime without additional REST API calls.
* Your application requires continuous synchronization of state.