Methods, systems, and apparatus, including computer programs for identifying a first message in a first channel of a plurality of channels corresponding to a position in the first channel, wherein the first message is stored in a buffer having a time-to-live that has expired. The methods may also include sending, by one or more computer processors, a request for messages beginning at the position to one or more client devices that subscribe to the first channel and receiving, from at least one client device, a response message comprising one or more messages of the first channel that are at or after the position. The methods may further include retrieving messages for the first channel from one or more buffers of the first channel having time-to-lives that have not expired, and combining, by the one or more computer processors, the retrieved messages and the messages from the response message.