Methods, systems, and apparatus, including computer programs encoded on a computer storage medium, are described for storing message data in a PubSub system. In certain examples, messages are received from a plurality of publishers for a plurality of channels. The messages are stored in a writable portion of a respective buffer for the channel according to an order, wherein messages in the writable portion of the buffer are inaccessible to subscribers. The method may also include advancing a pointer demarcating a boundary between the writable portion and a readable portion of the buffer such that the message is in the readable portion after the pointer has advanced.