OXenstored: an efficient hierarchical and transactional database using functional programming with reference cell comparisons

Thomas Gazagnaire and Vincent Hanquez

The 14th ACM SIGPLAN International Conference on Functional Programming (ICFP 2009)
Edinburgh, Scotland, 31st August - 2nd September 2009


We describe in this paper our implementation of the Xenstored service which is part of the Xen architecture. Xenstored maintains a hierarchical and transactional database, used for storing and managing configuration values.

We demonstrate in this paper that mixing functional data-structures together with reference cell comparison, which is a limited form of pointer comparison, is: (i) safe; and (ii) efficient. This demonstration is based, first, on an axiomatization of operations on the tree-like structure we used to represent the Xenstored database. From this axiomatization, we then derive an efficient algorithm for coalescing transactions which try to concurrently modify that structure. Finally, we experimentally compare the performance of our implementation, that we called OXenstored, and the C implementation of the Xenstored service distributed with the Xen hypervisor sources.

As a direct result of this work, OXenstored will be included in the future releases of Xenserver, the virtualization product distributed by Citrix Systems, where it will replace the current implementation of the Xenstored service.

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