Large infrastructures of Internet companies, such as Facebook and Twitter, are composed of several layers of microservices. While this modularity provides scalability to the system, the I/O associated with each service request strongly impacts its performance. In this context, writing concise programs which execute I/O eciently is especially challenging. In this paper, we introduce Ÿauhau, a novel compile-time solution. Ÿauhau reduces the number of I/O calls through rewrites on a simple expression language. To execute I/O concurrently, it lowers the expression language to a dataow representation. Our approach can be used alongside an existing programming language, permitting the use of legacy code. We describe an implementation in the JVM and use it to evaluate our approach. Experiments show that Ÿauhau can signicantly improve I/O, both in terms of the number of I/O calls and concurrent execution. Ÿauhau outperforms state-of-the-art approaches with similar goals.