We study the motion of a random string in a convex domain O in Rd,
namely the solution of a vector-valued stochastic heat equation, confined in
the closure of O and reflected at the boundary of O. We study the structure
of the reflection measure by computing its Revuz measure in terms of an
infinite-dimensional integration by parts formula. Our method exploits recent
results on weak convergence of Markov processes with log-concave invariant
measures