Complex sentence: Minkus was contracted to compose ballet music on demand-he was obliged to score a new ballet every season, along with the constant revision of the music of already existing works for Petipa's numerous revivals.
Split into simpler sentences: Minkus was contracted to compose ballet music on demand. He was obliged to score a new ballet every season, along with the constant revision of the music of already existing works for Petipa's numerous revivals.