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 distinct channels. The messages are ordered and stored in a plurality of buffers, with each channel having its own respective buffer. After a message has been written to a writable portion of the buffer for a channel, a pointer demarking a boundary between a readable portion of the buffer and the writeable portion of the buffer is advanced in an atomic operation. Following the atomic operation, the message resides in the readable portion and may be accessed by PubSub system components and/or processes. In general, one or more subscribers, components, or processes may read messages from the readable portion, in parallel.