It is very common for devices to have constantly changing network conditions as a result of changing from a mobile data network to a wifi network, being in a tunnel for a short period, or perhaps due to intermittent network issues. We believe that developers shouldn't have to worry about service continuity when the connection is reestablished, and as such we ensure that the connection state for a client is retained on our servers whilst the client is disconnected.
When an Ably Realtime client is disconnected abruptly, it moves into the disconnected state and will reconnect automatically. By default, when in the disconnected state, our client libraries will keep trying to connect every 15 seconds and also whenever the OS notifies the client library that the internet connection has returned. The client library remains in the disconnected state for a maximum of 2 minutes as this is the amount of time our servers will retain a disconnected client's connection state for.
Whilst a client is in the disconnected state, it is safe to assume that when a connection is reestablished by that client, all channels will remain attached and any messages published whilst disconnected will be delivered upon reconnection. This is possible because Ably holds onto the connection state for 2 minutes following an abrupt disconnection, and thus queues all messages published on all the channels the client was subscribed to. Once the client is reconnected, the queued messages for each attached channel are delivered immediately.
However, once the client has been disconnected for more than 2 minutes, the client library moves into the suspended state indicating that all connection state is lost. As such, all channels are automatically detached at this point and it is then the responsibility of the developer to decide what action should be taken if continuity is required. It is therefore the responsibility of the developer to attach all channels again once the client enters the connected state again. If more than 2 minutes of message history is required, you can use the history API to retrieve all older messages.
Commonly asked questions
How many messages will you store whilst a client is disconnected?
There is no hard limit so we will in almost all circumstances be able to keep all messages for two minutes.
What happens if messages cannot be replayed or the connection cannot be recovered?
If the connection is not able to be recovered, Ably notifies the client and the client will detach the channels and emit an error. As a developer then, this gives you certainty and predictability you need to build a reliable application. If a channel is detached by the server, then you will need to decide what action you may take, including potentially retrieving the history of that channel via our history API.