Visionaries hope for a modern metropolis modelled on Singapore, but pessimists fear the emergence of another dirt-poor city of slums. Dar es Salaam is one of the world's fastest growing cities, and it has reached its tipping point.In the dark basement of the cavernous Kariakoo market, dozens of traders gather at tiny makeshift stalls, arranging fruit and vegetables into neat piles. This part of the market has the least sought-after plots, and all of the stallholders have one thing in common: none of them was born in Dar es Salaam.Rolens Elias arrived seven years ago from a village near Morogoro, about 150km to the west. He had been a farmer but wanted to try his luck as a trader. He now makes about 3,000 shillings ($2; £1.50) each day selling tomatoes in the farthest corner of the basement."It has been hard to set up a life here," he says. "I came here by myself and had to wait until I had enough money to bring my wife and family. We all live in one room, but it's a better life than in the village."As he arranges his tomatoes, a group of his friends gather around and chip in with their own stories. They are all from Morogoro, and all came to Dar es Salaam in the hope of a better life. They all contrast the rural poverty they were born into with the lure of Dar es Salaam and its big-city opportunities.Their stories are repeated many thousands of times across the city.Every day new arrivals flood in, many of them setting up home in hastily erected shacks, many others sleeping on the streets. Those who cannot set up as greengrocers hawk goods, anything from baseball caps and mobile-phone chargers to bottled water and sweeping brushes.The dramatic influx has pushed the city's population up from roughly two million two decades ago to four million today. If the government does nothing, the population will hit eight million in 20 years, according to Nimrod Mushi, a lecturer at the city's Ardhi university.He is one of the experts commissioned by the government to produce a "master plan" to overhaul the city's infrastructure. Singapore is his role model, and he favours big projects to clear slums and build bridges, roads and out-of-town settlements."When we went to Singapore, we could see their satellite towns, their ring-roads, their skyscrapers and their decentralised services, and it's working very nicely there," he says.He points out that Dar es Salaam has gone 20 years without any guidance on planning, and now "badly needs a master plan".Superficially at least, his dream seems within reach. Tanzania's economy is booming, and cranes litter the skyline, putting the final touches to high-rise blocks. The country's super-rich, many of whom make their cash in the far-away mines of the north, are pouring money into the city.But Dar es Salaam is a long way from Singapore. The Asia city-state's economy was worth $260bn last year compared with $23bn for the whole of Tanzania, which remains one of the poorest countries in the world.