When you connect to an OpenTok session on an app, you specify the session you want to connect to using an OpenTok session ID. Each session ID identifies a unique OpenTok session. You can think of a session as a room in which participants meet and chat.
The number of sessions you create and how clients connect to them depend upon the requirements of your app. If your app connects users with one another for a one-time , create a unique session for that meeting. However, if your app connects users over various days in the same "room," then you can create one session and reuse it. If one group of users meet with each other, while other groups meet independently, create unique sessions for each group.
OpenTok sessions do not expire. However, authentication tokens do expire. Also note that sessions cannot explicitly be destroyed.
When you create a session, you specify how clients in the session will send audio-video streams, known as the media mode. There are two options:
Relayed — In a relayed session, clients will attempt to send audio-video streams directly between each other (peer-to-peer). However, if clients cannot connect due to firewall restrictions, the session uses the OpenTok TURN server to relay audio-video streams. (Prior to version 2.2, the OpenTok server SDKs referred to these sessions as peer-to-peer sessions. However, even when using those SDKs, sessions will still use the OpenTok TURN server to relay streams if firewall restrictions block peer-to-peer streaming.)
The OpenTok Media Router can decrease bandwidth usage in multiparty sessions. (When the media mode property is set to relayed, each client publishing a stream must send it separately to each client subscribing to it. With the OpenTok Media Router, a publisher sends one stream once to the router and it forwards it to each subscribing client.)
The OpenTok Media Router can improve the quality of the user experience through audio fallback and video recovery. With these features, if a client's connectivity degrades to a degree that it does not support video for a stream it's subscribing to, the video is dropped on that client (without affecting other clients), and the client receives audio only. If the client's connectivity improves, the video returns.
The OpenTok Media Router supports the OpenTok archiving feature, which lets you record, save, and retrieve OpenTok sessions.
In sessions that use the OpenTok Media Router, lowering the frame rate for a published video proportionally reduces the bandwidth the stream uses.
The OpenTok Media Router supports the scalable video feature. Scalable video can greatly improve the quality of video in multi-party sessions. Using this feature, a client automatically publishes a multi-quality video stream, composed of multiple video resolutions and frame rates. This allows simultaneous subscribing endpoints to consume different video resolutions and frame rates.
The OpenTok Media Router dynamically switches between different video resolutions and frame rates for a subscriber's stream, as network conditions on the subscribing endpoint change.
You do not need to add any code to use the scalable video feature. The OpenTok Media Router handles the switching of subscriber stream resolutions and frame rates automatically. The OpenTok Media Router turns on this feature when it determines the session will benefit from it based on the topology of clients connected to the session.
Scalable video is only avalable in streams published by clients that use the OpenTok iOS SDK (on certain devices), the OpenTok Android SDK (on certain devices), or OpenTok.js in Chrome.
In clients using the OpenTok iOS and Android SDKs, relayed sessions support only two clients connected to the session. The OpenTok Media Router supports additional clients for multiparty sessions on mobile devices.
Note: In version 2.2 of the OpenTok server SDKs, the default media mode is relayed. (In earlier versions, the default was to use the OpenTok Media Router.)
When possible, do not reuse session IDs between different video chat conversations. Instead, generate new session IDs for each distinct video chat on your application.
This is important, especially when using OpenTok Inspector. In Inspector, session quality scores and data are indexed by session ID. A session ID that is reused for multiple conversations is more difficult to debug using Inspector, and sessions with re-used session IDs tend to report lower aggregate quality scores than the actual experienced call quality.
Use a relayed instead of a routed session, if you have only two participants (or maybe even three) and you are not using archiving. Using relayed sessions reduces the latency between participants, reduces the points of failure and you can get better quality video and audio in most cases.
Routed sessions are required if you want to archive your session. They are recommended if you have more than two or three participants in the session.
For more information, see The OpenTok Media Router and media modes.
While working on a test version of your app, you can obtain a test session ID using a Project Page of of your TokBox Account.
You can also use one of the the OpenTok server-side libraries or the OpenTok REST API to generate a session:
You can also use the OpenTok REST API to create a session.
If you need to dynamically generate multiple session IDs, use one of the OpenTok server-side libraries or the OpenTok REST API — not the Project Page.