IT and software

22 drawings in IT and software, each generated from a single YAML file.

• An on-premise server layout rack by rack, with the redundant power feeds and network links drawn as separate paths
• A cloud architecture showing the VPC and its subnets, and where the load balancer, application, database and object store sit
• Where data goes from collection through transformation, storage and aggregation to display, and what kind of container holds it at each step
• A network layout of the links and equipment between sites, with the address range carried by each segment
• An order management ER diagram with primary and foreign keys, and the one-to-many relationships drawn as crow feet
• A server room where racks face each other to make hot and cold aisles, with the clearance of each aisle written in
• A UML class diagram where inheritance, realisation, association and aggregation are told apart by line type and end symbol
• A UML state machine with transitions, the events and guards that fire them, and the pseudostates that begin and end it
• A UML use case diagram with the actors, the use cases, and the include and extend relationships between them
• A UML activity diagram from start to end, including decisions, merges and the fork and join of parallel work
• Protocol headers are drawn 32 bits wide because that is the unit they are read in: the width of a field is its bit count, and nothing that does not divide into 32 ever appears
• Rack holes are deliberately unevenly spaced — 15.875 / 15.875 / 12.7 — so that the middle of the 12.7 gap marks where one U ends and the next begins
• A PoE standard quotes two wattages, not one: what the switch sends and what the device receives — the difference is what the 100m of copper turns into heat
• Adding layers is not only about routing: it brings the ground plane close, and the same 50 ohms goes from a 2.85mm trace to a 0.34mm one
• 2N does not mean doubling: it means two paths that never join before the equipment, because one shared point is a single point of failure — and the UPS is not a power source, just the bridge over the 10 seconds the generator needs
• A QR code can be 21 modules across or 177, but the three finder squares stay 7 by 7 whatever the version — the thing you have to find first is always the same size, and its 1:1:3:1:1 run of black and white reads the same scanned sideways or diagonally, so orientation never has to be known in advance
• Opening a TCP connection takes three segments and closing it takes four, and the extra one is the whole point: "I have nothing more to send" and "I will not receive any more" are separate statements, so between the two FINs one side is still allowed to send. The eleven RFC 793 states are listed beside the ladder, along with why the last side to send an ACK has to wait before it disappears.
• There are two wiring standards for an eight-pin Ethernet plug and they differ in only four wires: T568A and T568B swap the orange pair with the green pair, while pins 4, 5, 7 and 8 are identical. Pair 3 is split across pins 3 and 6 with pair 1 sitting between them, a layout inherited from telephone wiring, and gigabit uses all four pairs rather than two.
• Wire the same four machines three ways, send one frame from A to B, and the difference is how far it travels: a repeater hub copies it to every other port, a switch sends it only to the port it has learned B is on, and over the air everyone in range receives it. The dashed outlines are collision domains, and only the switch cuts them apart.
• It looks like one line of work, but the three steps in the middle — deposit a film, print a pattern with light, etch away what is not wanted — run once per layer, and there are many layers. Everything up to the wafer test is the front end; after that the wafer is cut into chips, mounted, sealed against shock and moisture, and tested again.
• If every rack faces the same way, each row breathes in the exhaust of the row in front. Turning alternate rows around creates a cold aisle between the fronts and a hot aisle between the backs, so cold air rises through floor grilles into the fronts and hot air leaves the backs for the ceiling. Air cooling tops out around 15kW a rack, and PUE of 1.0 is the theoretical floor.
• Nothing about the shape of a dot or a dash is fixed; what is fixed is the ratio of durations — dash three, gap inside a letter one, between letters three, between words seven. That is why the same code carries over sound, light or a key. Laid out at true length, SOS comes to exactly 27 units.