Maintaining persistence of a messaging system

1. A method comprising:

performing by one or more computers:

receiving, from a first client device, a request for messages of a first channel of a plurality of channels starting at a position in the first channel, wherein each channel comprises a respective ordered plurality of messages stored in one or more respective buffers according to the order, each buffer having a respective time-to-live;

identifying a first message in the first channel corresponding to the position in the order, wherein the first message is stored in a buffer having a time-to-live that has expired;

sending to one or more second client devices that subscribe to the first channel a request for messages beginning at the position in the order;

receiving, from one of the second client devices, a response message comprising one or more messages of the first channel that are at or after the position in the order;

retrieving messages for the first channel from one or more buffers of the first channel having time-to-lives that have not expired and according to the order;

combining the retrieved messages and the messages from the response message according to the order; and

sending the combined messages to the first client device.

2. The method of claim 1 , further comprising:

determining, for one or more of the second client devices subscribing to the first channel, respective connection latency to the first channel; and

identifying one of the second client devices that has a shortest connection latency.

3. The method of claim 1 , further comprising:

determining, for one or more second client devices subscribing to the first channel, time durations of respective current connection sessions to the first channel; and

identifying one of the second client devices having the longest current connection session among the one or more second client devices.

4. The method of claim 1 , wherein combining the retrieved message and the messages from the response message according to the order further comprises:

removing from the combined messages one or more duplicate messages.

5. A system comprising:

one or more computers programmed to perform operations comprising:

receiving, from a first client device, a request for messages of a first channel of a plurality of channels starting at a position in the first channel, wherein each channel comprises a respective ordered plurality of messages stored in one or more respective buffers according to the order, each buffer having a respective time-to-live;

identifying a first message in the first channel corresponding to the position in the order, wherein the first message is stored in a buffer having a time-to-live that has expired;

sending to one or more second client devices that subscribe to the first channel a request for messages beginning at the position in the order;

receiving, from one of the second client devices, a response message comprising one or more messages of the first channel that are at or after the position in the order;

retrieving messages for the first channel from one or more buffers of the first channel having time-to-lives that have not expired and according to the order;

combining the retrieved messages and the messages from the response message according to the order; and

sending the combined messages to the first client device.

6. The system of claim 5 wherein the operations further comprise:

determining, for one or more of the second client devices subscribing to the first channel, respective connection latency to the first channel; and

identifying one of the second client devices that has a shortest connection latency.

7. The system of claim 5 wherein the operations further comprise:

determining, for one or more second client devices subscribing to the first channel, time durations of respective current connection sessions to the first channel; and

identifying one of the second client devices having the longest current connection session among the one or more second client devices.

8. The system of claim 5 , wherein combining the retrieved message and the messages from the response message according to the order further comprises:

removing from the combined messages one or more duplicate messages.

9. A manufacture comprising a non-transitory computer-readable medium having instructions stored thereon that when executed by one or more computers causes the computers to perform operations comprising:

receiving, from a first client device, a request for messages of a first channel of a plurality of channels starting at a position in the first channel, wherein each channel comprises a respective ordered plurality of messages stored in one or more respective buffers according to the order, each buffer having a respective time-to-live;

identifying a first message in the first channel corresponding to the position in the order, wherein the first message is stored in a buffer having a time-to-live that has expired;

sending to one or more second client devices that subscribe to the first channel a request for messages beginning at the position in the order;

receiving, from one of the second client devices, a response message comprising one or more messages of the first channel that are at or after the position in the order;

retrieving messages for the first channel from one or more buffers of the first channel having time-to-lives that have not expired and according to the order;

combining the retrieved messages and the messages from the response message according to the order; and

sending the combined messages to the first client device.

10. The manufacture of claim 9 wherein the operations further comprise:

determining, for one or more of the second client devices subscribing to the first channel, respective connection latency to the first channel; and

identifying one of the second client devices that has a shortest connection latency.

11. The manufacture of claim 9 wherein the operations further comprise:

determining, for one or more second client devices subscribing to the first channel, time durations of respective current connection sessions to the first channel; and

identifying one of the second client devices having the longest current connection session among the one or more second client devices.

12. The manufacture of claim 9 , wherein combining the retrieved message and the messages from the response message according to the order further comprises:

removing from the combined messages one or more duplicate messages.