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, the method includes storing messages of each of a plurality of channels in a writable portion of a respective buffer for the channel. The method may also include moving a pointer delineating a boundary between the writable portion and a readable portion of the buffer such that the messages are in the readable portion after the pointer has moved. The method may also include allowing one or more subscribers to read from the readable portion of one or more of the buffers during the storing.