1. A system, comprising: a plurality of compute nodes, respectively comprising at least one processor and a memory, wherein the plurality of compute nodes implement a managed stream processing system comprising a control plane and a plurality of stream processing nodes, wherein the managed stream processing system provides customizable stream processing functions for one or more clients; the control plane, configured to: receive a request from a client of the one or more clients to apply a processing function to a data stream; identify one or more of the stream processing nodes to apply the processing function; configure the one or more stream processing nodes to apply the processing function; and initiate execution of the processing function at the one or more stream processing nodes such that the processing function is applied to data records of the data stream as the data records are received at the one or more stream processing nodes.