  There exists a natural tension between encouraging a diverse ecosystem of
open-source search engines and supporting fair, replicable comparisons across
those systems. To balance these two goals, we examine two approaches to
providing interoperability between the inverted indexes of several systems. The
first takes advantage of internal abstractions around index structures and
building wrappers that allow one system to directly read the indexes of
another. The second involves sharing indexes across systems via a data exchange
specification that we have developed, called the Common Index File Format
(CIFF). We demonstrate the first approach with the Java systems Anserini and
Terrier, and the second approach with Anserini, JASSv2, OldDog, PISA, and
Terrier. Together, these systems provide a wide range of implementations and
features, with different research goals. Overall, we recommend CIFF as a
low-effort approach to support independent innovation while enabling the types
of fair evaluations that are critical for driving the field forward.
