A revolution can succeed in the narrow sense of winning its central demand and still fail to hold what it won. The Iranian Constitutional Revolution did exactly that: it forced a constitution and a parliament into existence, and then watched both erode within a few years under pressures it could not control.
A coalition of clergy, merchants, laborers and reformers, united more by opposition to the Qajar shah's arbitrary rule than by a single program, forced Iran's shah to grant a constitution in 1906 and convene the country's first parliament, the Majlis. The new constitutional order limited royal power and asserted rule of law in its place, one of the earliest such settlements anywhere in the Middle East. The revolutionary period continued through 1911, but its gains were eroded from multiple directions at once: renewed social and factional tension inside the coalition that had won the constitution, direct foreign intervention by Russia and Britain acting in their own strategic interests in Iran, and the onset of the First World War, which overtook the constitutional project before it could fully consolidate.
The 1906 constitution and its Majlis survived, at least on paper, into the Pahlavi era that followed, and constitutional monarchy remained Iran's nominal form of government until the 1979 revolution replaced it outright with a theocratic republic. Where Iran's constitutional movement had tried to limit a monarch's power under a constitution, the later revolution rejected the monarchy itself and rested Iran's new constitution instead on the doctrine of velayat-e faqih, the guardianship of a Shia jurist, a fundamentally different answer to the same original question of where ultimate authority resides.