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Message M saying ‘Starting at moment T_u, schema is S’ is sent with timestamp T_m (that moves safeTime forward). A schema storage client that wants to obtain a schema active at moment T_op waits for Meta-storage’s safeTime>=T_op. T_u must be ≥ T_m (because we’ll construct messages in this way), so, if T_m<T_op, after waiting, the client sees M (as T_m<T_op<=safeTime, so T_m<safeTime, hence M is visible to the client’s node) and knows that S has been activated at T_u before it obtains schema for moment T_op.

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