Methods, systems, and apparatus, including computer programs encoded on computer storage media, for delivering event content. One of the methods includes initiating, for one or more client devices, a subscription to a topic; receiving an event relevant to a topic; determining a number of channel servers that correspond to the topic, wherein each channel server is associated with one or more client devices; determining that the number of channel servers that correspond to the topic is greater than a threshold number of channel servers; writing the received event from the event processor to an event fan-out queue, in response to the determination that the number of channel servers that correspond to the topic is greater than the threshold number of channel servers; reading the received event from the event fan-out queue; and sending the received event to the one or more client devices associated with each respective channel server.