add comments to explain the interface
add comments to explain the interface

raintank/raintank-metric,replay/metrictank,raintank/raintank-metric,raintank/metrictank,replay/metrictank,raintank/metrictank,replay/metrictank,replay/metrictank,raintank/raintank-metric,raintank/metrictank,raintank/metrictank