Every smart-city journey arrives, sooner or later, at the words "data lake."
Pleasant words: they suggest depth, calm, abundance. The brochures show a clean blue diagram. Nobody mentions the part where it turns green.
A data lake is genuinely excellent for an organisation with three things: enormous volumes of data, great variety in it, and a team whose job is to fish. Amazon has all three. A national tax authority has all three. A town of twenty thousand has a parking system, two air-quality stations, a fleet of buses reporting their position, and an enthusiastic mayor. That is not a lake. It is a tidy puddle with excellent intentions.
Here is the failure mode, so common it deserves a name.
The town buys the lake. Data flows in, raw, unlabelled, in whatever shape each vendor felt like sending. A year later someone asks a simple question: how reliable were the buses last winter? The answer is in there, technically, in the way a specific grain of sand is technically on a beach. The lake has become a swamp: everything went in, nothing comes out, and the consultant who recommended it has moved on to recommend it to someone else.
What the town needed first was boring and cheap.
A common language, a shared way of describing a bus, a parking bay, an air-quality reading, so that data from different systems and vendors actually fits together and can be queried, today, by a normal person. Get that right and you have a single source of truth delivering value immediately, at a fraction of the cost. Get it right and (the part the lake-sellers will not volunteer) a data lake later becomes useful, because the water arriving in it is already clean.
The sequence is not "lake, then figure it out."
It is "common language now, reservoir later, if and when you genuinely have a flood." Keep the data lake on the wall as the five-year vision. Build the foundation this year. You will spend less, get value sooner, and never have to explain to a council why the expensive blue thing smells faintly of regret.
Disclosure
I sell foundations, not lakes, so weigh this accordingly, though you will notice I am not telling you to buy anything, only to buy it in the right order.