# Using elastic channels to host live events in Amazon Chime SDK meetings
Elastic channels support large-scale chat experiences with up to 1-million members. Typical uses include watch parties for sporting or political events. You can use elastic channels only in the US East (N. Virginia) region.
An elastic channel consists of a single channel with a common configuration, plus a varying—or *elastic*—number of sub-channels. The configuration also includes minimum and maximum thresholds for the members in the sub-channels.
For example, say you create an elastic channel with 100 sub-channels, and for the sub-channels you set a low threshold of 500 members and a high threshold of 10,000 members. When users join this example channel, the system automatically assigns them to a single sub-channel until the member count exceeds 10,000. At that point, the system creates a new sub-channel and adds any new members there. As users leave, the system deletes sub-channels and distributes members across the remaining sub-channels.
Splitting the audience across sub-channels makes conversations easier for participants to follow. Moderators also have reduced workloads, because they only need to watch some of the sub-channels. In addition, moderators can use the built-in tools that elastic channels provide. For example, moderators can [ban users](https://docs.aws.amazon.com/chime-sdk/latest/APIReference/API_messaging-chime_CreateChannelBan.html) from a channel, [create moderators](https://docs.aws.amazon.com/chime-sdk/latest/APIReference/API_messaging-chime_CreateChannelModerator.html), and use [channel flows](https://docs.aws.amazon.com/chime-sdk/latest/dg/using-channel-flows.html) to automatically moderate all the messages in the channel.
For more information about Amazon Chime SDK Messaging quotas, refer to [Messaging Quotas](https://docs.aws.amazon.com/general/latest/gr/chime-sdk.html) in the *Amazon Chime SDK General Reference*.
**Topics**
+ [Prerequisites](#elastic-prereqs)
+ [Elastic channel concepts](#elastic-concepts)
+ [Additional supported features](#additional-features)
+ [Creating elastic channels for Amazon Chime SDK meetings](create-elastic-channel.md)
+ [Managing elastic channel members for Amazon Chime SDK meetings](manage-elastic-members.md)
+ [Sending elastic channel messages in Amazon Chime SDK meetings](send-messages-elastic.md)
+ [Understanding WebSocket system messages in elastic channels for Amazon Chime SDK meetings](websocket-messages-elastic.md)
+ [Using Kinesis streams to receive system messages for Amazon Chime SDK meetings](elastic-onboard-streams.md)
+ [Testing elastic channels for Amazon Chime SDK meetings in our demo app](elastic-testing.md)
## Prerequisites
You must have the following to use elastic channels.
+ Knowledge of Amazon Chime SDK Messaging functionality, such as managing channels, and sending and receiving messages.
+ The ability to invoke the Amazon Chime SDK Messaging APIs.
## Elastic channel concepts
To use elastic channels effectively, you must understand these concepts.
**Sub-channels**
Elastic channels divide their members into logical containers called sub-channels. When you add an `AppInstanceUser` to an elastic channel, the user becomes a member of a sub-channel. That user can send and receive messages, but only with other members of that sub-channel. The system never allows messages from one sub-channel to appear in other sub-channels.
**Scaling**
To support user engagement, every sub-channel must meet a minimum membership requirement. You provide that value when you create an elastic channel. As users join or leave an event, the system transfers members to different sub-channels, which makes the overall channel "elastic." Sub-channels run the following scaling actions.
+ **SCALE\_OUT** – When a new elastic channel membership request comes in and all sub-channels are full, the system scales out by creating a new sub-channel, and then transferring memberships from existing sub-channels to the new sub-channel.
+ **SCALE\_IN** – When a sub-channel membership count goes below the minimum requirement, and another sub-channel has the capacity to accommodate all the members of the first sub-channel, a `SCALE_IN` event transfers those memberships, and then deletes the sub-channel and all messages.
If you need to access messages from channels that have been deleted, you must first turn on message streaming. For more information, refer to [Streaming messaging data in Amazon Chime SDK messaging](streaming-export.md).
**Member transfer**
This occurs when membership balancing moves an `AppInstanceUser` from one sub-channel to another. The `AppInstanceUser` still belongs to the elastic channel after the transfer. However, the new sub-channel contains different memberships and messages, so the messages sent by the `AppInstanceUser` after the transfer go to those different members. Membership balancing does not affect moderator memberships.
**Note**
Elastic channels don't support hidden memberships, membership preferences, and read message timestamps.
## Additional supported features
Elastic channels also support these messaging features.
+ [Prefetch](websockets.md#prefetch)
+ [Channel flows](using-channel-flows.md)