BigBlueButton is built upon many amazing software components – nginx, red5, FreeSWITCH, tomcat7, redis, and others. This page describes the overall architecture of BigBlueButton and how the components work together.
High-level Architecture Overview for BigBlueButton
The following diagram provides a high-level overview of the BigBlueButton architecture.
We’ll break down each component below.
The Client is a Flash application which runs inside the browser. The client connects to Red5 using RTMP (port 1935) or RTMPT (port 80) if needs to tunnel. When it needs to connect using RTMPT, it connects through Nginx which proxies the connection to Red5.
The Client also uploads presentations to Web API.
HTML5 Client and Server
The HTML5 client and server is built using Meteor and communicates with the other components of the system through redis pubsub.
See the HTML5 Overview document for more info.
The Web API provides the integration endpoint for third-party applications – such as Moodle, Wordpress, Canvas, Sakai, etc. – to control the BigBlueButton server.
Every access to BigBlueButton comes through a front-end portal (we refer to as a third-party application). BigBlueButton integrates Moodle, Wordpress, Canvas, Sakai, and others (see third-party integrations). BigBlueButton comes with its own front-end called GreenLight. When using a learning management system (LMS) such as Moodle, teachers can setup BigBlueButton rooms within their course and students can access the rooms and their recordings.
Uploaded presentations undergoes conversion process in order to be displayed in the Flash client. If the uploaded file is an Office document, it gets converted into PDF using LibreOffice and then converted to SWF using SWFTools. The conversion process is described later in this page.
Redis PubSub provides a communication channel between different applications running on the BigBlueButton server.
When a meeting is recorded, all events are stored in Redis DB. When the meeting ends, the Recording Processor will take all the recorded events as well as the different raw (PDF, WAV, FLV) files for processing.
Red5 Apps (Screenshare, Apps, Voice, Video)
We think Red5 rocks! (We just had to get that upfront).
Red5 Apps are different applications that provide media streaming in the meeting and forwards messages between clients and Apps Akka.
The Apps is the main BigBlueButton application that handles users, chat, whiteboard, presentation information shared by all users in a meeting. The Screenshare application allows the presenter to share their screen. The Voice application allows the user to call into the voice conference using a headset or join listen-only. The Video application provides a user to share his/her webcam to the users in the meeting.
BigBlueButton Apps is the main application that pulls together the different applications to provide real-time collaboration in the meeting. It provides the list of users, chat, whiteboard, presentations in a meeting.
Below is a diagram of the different components of Apps Akka.
The meeting business logic is in the MeetingActor. This is where information about the meeting is stored and where all messages for a meeting is processed.
We have extracted out the component that integrates with FreeSWITCH into it’s own application. This allows others who are using voice conference systems other than FreeSWITCH to easily create their own integration. Communication between apps and FreeSWITCH Event Socket Layer (fsels) uses messages through redis pubsub.
We think FreeSWITCH rocks too!
FreeSWITCH provides the voice conferencing capability in BigBlueButton. Users are able to join the voice conference through the headset. Users joining through Google Chrome or Mozilla Firefox are able to take advantage of higher quality audio by connecting using WebRTC. FreeSWITCH can also be integrated with VOIP providers so that users who are not able to join using the headset will be able to call in using their phone.
Joining a Voice Conference
A user can join the voice conference (running in FreeSWITCH) in several ways. Users can join using Flash, WebRTC, or phone. When joining through Flash, the user can choose to join listen-only or listen-and-talk. Users joined with Chrome and Firefox are able to join using WebRTC. Thanks to the browser’s ability to send opus encoded audio packets via UDP, WebRTC provides users higher-quality audio with lower delay. If FreeSWITCH is integrated with a VOIP provider, users are able to call in using their phone by dialing a number and pressing the conference number on their keypad.
Uploading a Presentation
Uploaded presentations go through a conversion process in order to be displayed inside the Flash client. When the uploaded presentation is an Office document, it needs to be converted into PDF using LibreOffice. The PDF document is then converted in SWF using SWFTools. There are times when a PDF page fails to convert to SWF. In this case, an image snapshot of the page (as PNG) is taken using pdftocairo the resulting image is converted SWF.
The conversion process sends progress messages to the client through the Redis pubsub.
Presentation conversion flow
The diagram below describes the flow of the presentation conversion. We take in consideration the configuration for enabling and disabling SWF, SVG and PNG conversion.
Then below the SVG conversion flow. It covers the conversion fallback. Sometimes we detect that the generated SVG file is heavy to load by the browser, we use the fallback to put a rasterized image inside the SVG file and make its loading light for the browser.
And finally, the SWF conversion flow. We cover it too with its fallback conversion.