  The interaction of spatially small dipoles with nucleons, nuclei is
calculated in the DGLAP approximation at the top of HERA energies and found to
be close to the $S$-channel unitarity limit in the case of the color octet
dipoles. The DGLAP analyses of the current diffractive data appear to support
this conclusion as they indicate a $\sim 30-40%$ probability of the gluon
induced diffraction for $Q^2\sim 4 $GeV$^2$. The need for the high-precision
measurements of the $t$-dependence of inclusive and exclusive diffraction for
pinpointing higher twist effects in the gluon sector is emphasized.The $eA$
collisions at HERA would provide a strong amplification of the gluon densities
allowing to reach deep into the regime of nonlinear QCD evolution. Connection
between the leading twist nuclear shadowing and leading twist diffraction in
$ep$ scattering is explained. The presented model independent results for the
nuclear shadowing for light nuclei indicate much larger shadowing for the gluon
sector than for the sea quark sector.It is argued that HERA in $eA$ mode would
be able to discover a number of new phenomena including large gluon shadowing,
large nonlinearities in parton evolution, small $x$ color transparency in the
vector meson production followed by color opacity at $x\le .005$, large
probability of inclusive diffraction. Implications for the nucleus-nucleus
collisions at LHC are discussed as well.
