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Scaling applications to support large calls

Video call UI displays many participants on the call

Daily defines large meetings as calls with fifty or more participants. The user interface and experience on each call varies, but they all need to route hundreds to thousands of participant media tracks at a time. This guide covers the Daily recommended settings for few-to-many broadcast sessions and interactive large meetings, and shares best practices to scale large calls.

Few-to-many broadcast sessions

Keynotes, panels, and webinars are examples of few-to-many broadcast sessions. A speaker, or several presenters, addresses many attendees who are only viewing and listening, not sharing video or audio, on the call.

These kinds of sessions can be built on Daily in two ways: through Daily live streaming, or on a Daily call with up to 1,000 participants.

With either of these implementations, Daily APIs support adding features like:

  • A "green room" for speakers to test media devices and talk to event production staff
  • Interfaces for event production staff to control the visual layout, visible speakers, and external media playback
  • Interactive features such as participant hand raising, microphone passing so that a viewer can briefly participate in the broadcast (e.g. ask a question), text chat Q&A, live polling, and transcription
  • Recording options, including for individual video and audio tracks for high quality post-production

Daily sessions can be broadcast to multiple live streaming platforms, like AWS IVS or YouTube Live. The layout of the stream can be configured and include up to nine video and 50 audio tracks. Live streaming could be the best option for a large call if hundreds of thousands of people or more could be tuning in, or if providing a viewing link to attendees is preferred to managing silent participants on a call. For more details on Daily live streaming and how to implement it, head to the guide.

1,000 participant calls

Daily supports up to 1,000 participants in a call, with every client receiving real-time (100-200ms of latency) video and audio. At most six cameras and microphones can "broadcast" their audio and video (have their camera and microphone on) in addition to one screen share. All other participants must have cameras and microphones off. This applies to both Daily Prebuilt, the ready-to-use, embeddable video chat interface, or building a completely custom application with the Daily call object.

Daily Prebuilt 1,000 participant calls

To set up a few-to-many broadcast session with Daily Prebuilt, set the owner_only_broadcast room property to true. This specifies that only a participant who joins the call with a meeting token that identifies them as an owner will be able to share audio or video.

Custom 1,000 participant calls

To control who starts a call as a presenter and who starts as a viewer, use the start_audio_off and start_video_off meeting token properties. false, the default, identifies presenters. Setting these properties to true designates viewers.

Interactive large sessions

Daily classifies calls with up to fifty participants who all need to engage with each other, keeping cameras and microphones on, as interactive large sessions. Conference breakout rooms or workshops, corporate trainings, and other virtual events all fall under this category.

These kinds of sessions are seeing a lot of evolution and experimentation, especially in social and spatial video. For example, two-dimensional maps with video avatars for each participant, three-dimensional WebGL worlds, and virtual event spaces with topic-specific social "tables" that participants can hop between.

Platforms to support interactive large sessions can be built using either Daily Prebuilt or the Daily call object. Whether embedding the ready-to-use interface or creating a custom implementation, core Daily features like recording, output to live streaming platforms, and transcription are all available.

Daily Prebuilt interactive large sessions

In interactive large sessions that use Daily Prebuilt, each participant can view either an Active Speaker layout, with the person currently speaking taking up the majority of the screen, or Grid View, a paginated grid layout with a subset of participants visible on screen.

Daily Prebuilt includes built-in features like text chat, a participant list, recording and live streaming.

Custom interactive large sessions

Interactive sessions built on the Daily call object not only offer deeper control over branding, design, and experience, but also open up potential for direct track subscription management.

These calls can support up to 50 participants with all cameras on, with each participant viewing at most six other participants at a time. The next section of the guide covers how to implement those limits with track subscriptions and pagination.

Best practices to scale large calls

As the number of call participants increases, the number of audio, video, and screen media tracks that need to be routed and then handled by each web client multiplies. This can drain CPU, strain networks to their limits (risking undelivered media streams), and degrade user experience quickly, especially on older or mobile devices.

To optimize performance on calls with hundreds of participants, pagination, track subscriptions, and simulcast layer control can all be implemented using Daily’s APIs. Analyzing call logs can also help improve call quality.

Pagination

Pagination limits the number of participants displayed on a screen at a time, so a call participant has to click to rotate through all other attendees. This reduces the load on any individual participant’s CPU and bandwidth.

Demo app displays colored tiles that rotate when the right and left arrow buttons are clicked

The gif highlights a demo app built on React, but pagination can be implemented regardless of framework, some assembly and state management required.

Use the Daily participants() method (to reference an object detailing the current meeting participants) and the following pseudocode to add pagination:

  1. Establish constant UI elements like the minimum participant video tile width, default aspect ratio, and the maximum number of tiles per page.

  2. Use the UI constants and the number of call participants to calculate the total number of pages. The number of pages, and which participants are on each page, should update as the number of call participants changes, so set up the state management of your choice.

  3. Add click handlers to the pagination buttons that update the app state to reflect the current page being viewed.

  4. Set up a handler to determine the visible participants on a given page, using the current page, number of participants, and maximum number of tiles per page to make a copy of the participants object that only includes visible participants.

  5. Iterate over the visible participants object to render each participant in the UI.

Track subscriptions

Daily calls operate on a publish-subscribe model: participants publish audio, video, and screen MediaStreamTracks, and are subscribed to other participant’s tracks.

By default, Daily routes a participant’s distinct set of tracks to all the other participants on a call. In large calls with hundreds and thousands of participants, decoding all that video can demand a lot of network bandwidth and processing power.

Turning off Daily’s default track handling in favor of manual track subscriptions can deliver better call quality during sessions with many participants. Track subscriptions can also enable features like breakout groups, and improve features like pagination.

There are three steps to set up direct track subscriptions:

  1. Set the subscribeToTracksAutomatically call object property to false.

This turns off Daily’s default track management. This property can be passed on join(), at createCallObject(), or via the setSubscribeToTracksAutomatically() method. The latter is a good option if you want to wait to turn on track subscriptions until a certain number of participants have joined the call.

  1. Call updateParticipant() or updateParticipants() to change the subscribed value of a participant's tracks property.

Each individual participant’s tracks property can be found on the Daily participants object.

The tracks for a participant include audio (microphone), video (camera), and screenAudio and screenVideo (screenshare) properties. Each track type contains: a raw media stream track object available on both track and persistentTrack, the state of the track, and its subscribed value.

state indicates if the track can be played (full possible states in the participants() documentation).

subscribed tells us whether or not the local participant is receiving the track information from the participant who the track belongs to. Its value is true if the local participant is receiving, false if they are not, or "staged".

A subscribed status of "staged" keeps the connection for that track open, but stops any bytes from flowing across. Compared to a complete connection teardown through an unsubscribe, staging a track speeds up the process of showing or hiding participants' video and audio. Staging also cuts out the processing and bandwidth required for that track.

On calls with many participants, setting the subscribed value to staged or false depending on the interface can minimize the load on participants’ connections, improving the user experience. Paginated grid apps (including Daily Prebuilt!) often subscribe to the tracks of participants on a current page, stage those on the previous and next pages, and unsubscribe from the rest.

Graphic with subscribed participant tiles in green, staged in the previous and next tiles in yellow, unsubscribed in red

Use the updateParticipant() and updateParticipants() Daily methods to update participants’ subscribed states.

updateParticipant() receives an object with the participant’s id as the key, and the value a setSubscribedTracks object indicating the new subscribed status of each track type.

If using updateParticipants(), combine all participants’ updates into one object.

  1. Listen for participant events to reflect updated participant state in the app interface.

The updateParticipant() method fires a corresponding "participant-updated" event. Add a listener to this event to update the app interface as participants’ subscribed statuses change.

Simulcast layer control

To take advantage of simulcast layers, the call must be happening over an SFU connection.

To test over SFU in development, use the setNetworkTopology() method. This is only for testing. In production, once a fifth participant joins a call, an SFU connection will be automatically established.

Once a fifth participant joins a Daily call, instead of direct peer-to-peer (P2P) connections, tracks are first sent to a Selective Forwarding Unit (SFU). From there, the SFU processes, re-encrypts, and routes media tracks to participants, allowing tracks to be "selectively" forwarded.

SFU square in center of graphic forwards arrows out from other squares representing tracks forwarded to other participants

With WebRTC simulcast, instead of sending a single track to the SFU, a publishing participant’s web client sends the same source track at a few different resolutions, bitrates, and framerates. Each of these groups of settings is known as a simulcast layer. The default layers sent on Daily desktop calls are:

Layer 0Layer 1Layer 2
Bitrate (bps)80,000200,000680,000
Frame rate (fps)101530
Resolution (width height)320x180640x3601280x720

The publishing participant sends all three layers to the SFU. The SFU decides which layer to forward to the subscribed participants depending on the subscriber’s network quality. If the network is very strained, it sends the lowest quality layer 0.

Receive-side video quality

The Daily receiveSettings API can be used to override the SFU estimated forwarding and request a specific simulcast layer instead.

If many participant videos will be displayed or if they will be rendered on small mobile screens, for example, programmatically requesting a lower bitrate layer can improve call performance. Lower quality videos not only save downstream bandwidth, but also can dramatically improve CPU performance since they require fewer resources to render.

Graphic shows four participants at highest quality layer, nine at medium quality, twelve at lowest quality

To use Daily receiveSettings to request a specific simulcast layer:

  1. Request the updated simulcast layer, either through updateReceiveSettings() or by passing a receiveSettings object on join().

  2. Listen for the "receive-settings-updated" event to update the app interface in response.

Send-side video quality

Daily recommends aiming for a maximum bandwidth usage around 3 Mbps for most participants to ensure the best experience for everyone.

It is possible to adjust the quality of video that participants send to the server using Daily.

This could be useful in few-to-many style broadcasts featuring one presenter in a well-equipped studio. In this case, increasing their quality settings (especially if only the speaker will be on the screen) can improve the user experience.

To configure the input quality of the video that the camera sends to the browser, use the setBandwidth() method and include the trackConstraints property. This sets the maximum resolution and frame rate.

The setBandwidth() call needs to happen before a video track is requested from the participant. Disable video on join() for participants whose bandwidth constraints need to be set manually, then call setBandwidth, then enable the camera.

Analyzing large call performance

See the logs and metrics guide for details on analyzing call quality.

Tutorials about scaling large calls