Local caching

Local caching enables Sendbird Chat SDK for JavaScript to locally cache and retrieve group channel and message data. Its benefits include reducing refresh time and allowing a client app to create a channel list or a chat view that can work online as well as offline, which can be used for offline messaging.

Note: You can enable local caching when initializing the Chat SDK by setting the optional parameter localCacheEnabled to true. See Initialize the Chat SDK and Connect to Sendbird server in the Authentication page to learn more.

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Local caching relies on the groupChannelCollection and messageCollection classes, which are used to build a channel list and a chat view, respectively.

Note: You can still use these collections when building your app without local caching.

Unlike SyncManager that listens to any data update, collections are designed to react to the events that can cause change in a view. An Event controller in the SDK oversees such events and passes them to a relevant collection. For example, if a new group channel is created while the current user is looking at a chat view, the current view won't be affected by this event.

Note: Understand the differences between local caching and SyncManager, and learn how the Migration should be done.

groupChannelCollection contains the following functions and variables.

var groupChannelFilter = new sb.GroupChannelFilter();
groupChannelFilter.includeEmpty = true;

var groupChannelCollection = sb.GroupChannel.createGroupChannelCollection()
    .setOrder(sb.GroupChannelCollection.GroupChannelOrder.LATEST_LAST_MESSAGE)
    .setFilter(groupChannelFilter)
    .build();

groupChannelCollection.setGroupChannelCollectionHandler({
    onChannelsAdded: function (context, channels) {
        // ...
    },
    onChannelsUpdated: function (context, channels) {
        // ...
    },
    onChannelsDeleted: function (context, channelUrls) {
        // ...
    },
});

// ...
try {
    // ...
        if (groupChannelCollection.hasMore) {
            groupChannelCollection.loadMore().then((channels) => {
                // ...
            });
        }
    // ...
} catch (e) {
    // ...
}
// ...

When the createGroupChannelCollection() method is called, the group channel data stored in the local cache and Sendbird server are fetched and sorted based on the value in setOrder() and setFilter().

setGroupChannelCollectionHandler() allows you to set the event listeners that can subscribe to channel-related events.

As for pagination, hasMore checks if there are more group channels to load whenever a user hits the bottom of the channel list. If true, loadMore() retrieves those channels from the local cache and the remote server.

To learn more about the collection and how to create a channel list view with it, see Group channel collection.

A messageCollection is composed of the following elements.

var messageFilter = new sb.MessageFilter();

var startingPoint = Date.now();
var messageCollectionFetchLimit = 100;
var messageCollection: MessageCollection = groupChannel.createMessageCollection()
    .setFilter(messageFilter)
    .setStartingPoint(startingPoint)
    .setLimit(messageCollectionFetchLimit)
    .build();

messageCollection.setMessageCollectionHandler({
    onMessagesAdded: function (context, channel, messages) {
        // ...
    },
    onMessagesUpdated: function (context, channel, messages) {
        // ...
    },
    onMessagesDeleted: function (context, channel, messages) {
        // ...
    },
    onChannelUpdated: function (context, channel) {
        // ...
    },
    onChannelDeleted: function (context, channelUrl) {
        // ...
    },
    onHugeGapDetected: function () {
        // ...
    }
});

messageCollection
    .initialize(sb.MessageCollection.MessageCollectionInitPolicy.CACHE_AND_REPLACE_BY_API)
    .onCacheResult(function (err, messages) {
        // Messages will be retrieved from the local cache.
    })
    .onApiResult(function (err, messages) {
        // Messages will be retrieved from Sendbird server through API.
        // According to the InitPolicy.CACHE_AND_REPLACE_BY_API,
        // the existing data source needs to be cleared
        // before adding retrieved messages to the local cache.
    });

// ...
try {
    // Load next messages.
    if (messageCollection.hasNext) {
    messageCollection
        .loadNext()
        .then(function (messages) {
            // ...
        })
        .catch(function (err) {
            // ...
        });
    }
    // ...
    // Load previous messages.
    if (messageCollection.hasPrevious) {
    messageCollection
        .loadPrevious()
        .then(function (messages) {
            // ...
        })
        .catch(function (err) {
            // ...
        });
    }
    // ...
} catch (e) {
    // ...
}
// ...

In a messageCollection, the initialization is dictated by MessageCollectionInitPolicy. This determines which data to use for the collection. Currently, we only support CACHE_AND_REPLACE_BY_API. According to this policy, the collection will load the messages stored in the local cache through onCachedResult(). Then onApiResult() will be called to replace them with the messages fetched from Sendbird server through an API request.

As for pagination, hasNext checks if there are more messages to load whenever a user hits the bottom of the chat view. If true, the loadNext() method retrieves those messages from the local cache. hasPrevious works the same way when the scroll reaches the oldest message in the view.

To learn more about the collection and how to create a chat view with it, see Message collection.

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A gap is created when messages or channels that exist in Sendbird server are missing from the local cache. Such discrepancy occurs when a client app isn't able to properly load new events due to issues with the connection status.

To prevent such a gap, the Chat SDK constantly communicates with Sendbird server and fetches data through background sync. The synchronization pulls the data from the latest to the oldest. This ensures that the local cache has all the data in order. However, because this synchronization takes place in the background, changes won't call collection event handlers or affect the view.

Despite background sync, a gap may still be created. When the client app is in the foreground, the SDK checks with Sendbird server to see if there has been a gap. If more than 300 messages are missing in the local cache compared to the remote server, Sendbird Chat SDK classifies this as a huge gap and onHugeGapDetected is called. In case of a huge gap, it is more effective to discard the existing message collection and create a new one. On the other hand, a relatively small gap can be filled in through changelog sync.

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While background sync makes sure that the local cache keeps its data up-to-date compared to Sendbird server, Changelog sync fetches any events that can affect the channel list view and the chat view.

Changelog sync pulls data when the client app is back online. When the client app is connected to the server, it fetches events in chronological order of the updated_at value, from first to last. Events fetched by changelog sync will be added to the collection, updating the view.

The events will be delivered to the collection through event handlers such as onMessageUpdated, onMessageDeleted, onChannelUpdated, onChannelDeleted, and onChannelAdded.

A message is normally sent through the WebSocket connection which means the connection must be secured and confirmed before sending any message. With local caching, you can temporarily keep an unsent message in local cache if the WebSocket connection is lost. The Chat SDK with local caching marks the failed message as pending, locally stores it, and automatically resends the pending message when the WebSocket is reconnected. This is called auto resend.

The following cases are eligible for auto resend when:

• A user message couldn't be sent because the WebSocket connection is lost even before it is established.

• A file message couldn't upload the attached file to Sendbird server.

• An attached file was uploaded to Sendbird server but the file message itself couldn't be sent because the WebSocket connection is closed.

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A user message is sent through the WebSocket. If a message couldn't be sent because the WebSocket connection is lost, the Chat SDK will receive an error through a callback and queue the pending message in the local cache for auto resend. When the client app is reconnected, the SDK will attempt to resend the message.

If the message is successfully sent, the client app will receive a response from Sendbird server. Then onMessageUpdated is called to change the pending message to a sent message in the data source and updates the chat view.

A file message can be sent through either the WebSocket connection or an API request.

When sending a file message, the attached file must be uploaded to Sendbird server as an HTTP request. To do so, the Chat SDK checks the status of the network connection. If the network is not connected, the file can't be uploaded to the server. In this case, the SDK will handle the file message as a pending message and put it in a queue for auto resend.

If the network is connected and the file is successfully uploaded to the server, its URL will be delivered in a response and the SDK will replace the file with its URL. At first, the SDK attempts to send the message through the WebSocket. If the WebSocket connection is lost, the client app checks the network connection once again to make another HTTP request for the message. If the SDK detects the network as disconnected, it will get an error code that marks the message as a pending message, allowing the message to be automatically resent when the network is reconnected.

On the other hand, if the network is connected but the HTTP request fails, the message isn't eligible for auto resend.

If the message is successfully sent, the client app will receive a response from Sendbird server. Then onMessageUpdated is called to change the pending message to a sent message in the data source and updates the chat view.

If a message couldn't be sent due to other error cases, onMessageUpdated will be called to re-label the pending message as a failed message. Such message can't be queued for auto resending. The following show some of those errors.

SendBirdError.ERR_NETWORK
SendBirdError.ERR_ACK_TIMEOUT

Note: A pending message can last in the queue only for three days. If the WebSocket connection is back online after three days, onMessageUpdated will be called to mark any 3-day-old pending message as a failed message.