Systems practice questions.

On 19 July 2024 a faulty update to a widely installed endpoint security agent caused Windows computers to crash at airlines, hospitals and broadcasters in many countries within the same hours. Which property of the wider system best explains why organisations with nothing in common failed together?

• The faulty code copied itself from one affected organisation to the next
• The same supplier's component was running inside all of the affected systems
• The affected organisations were all connected through one physical network backbone
• The affected organisations had switched off encryption on the servers that crashed

Answer: The same supplier's component was running inside all of the affected systems

Shared dependency, not contagion: each organisation ran the same vendor component, so one bad update reproduced the identical fault everywhere at once, and recovery needed hands on every machine. A worm spreads between victims, whereas a monoculture simply fails with them.

A police department directs patrols using a place-based crime prediction tool, of the kind the Los Angeles Police Department ran before ending its programmes in 2019 and 2020. Officers record more incidents in the flagged areas, and those records are then used to retrain the tool. Which systems feature does this describe?

• A reinforcing feedback loop, because the output becomes the next input
• A system boundary, because the tool covers only one police district
• Redundancy, because two independent sources have to confirm every incident
• An interoperability standard, because records move between agencies

Answer: A reinforcing feedback loop, because the output becomes the next input

The extra patrols generate the very records that justify more patrols, so the system measures police attention rather than crime. Nothing in the code is faulty: the loop is a property of how the tool was deployed, which is why an audit of the model alone would find nothing.

The EU Digital Markets Act, applicable since 2023, requires designated gatekeepers to open their messaging services to interoperability requests from other providers. Which system-level problem is this obligation aimed at?

• The cost of keeping years of message archives on the provider's own servers
• The delay added when a message crosses a national network border
• Users being locked in because the value lies in where their contacts are
• The energy that end-to-end encryption adds to every delivered message

Answer: Users being locked in because the value lies in where their contacts are

Network effects make leaving costly even when a rival product is better, because a user would be leaving the contacts rather than the software. The obligation attacks the switching cost itself, which is a structural feature of the system rather than a fault of any one firm.

India's Unified Payments Interface, operated by the National Payments Corporation of India since 2016, routes payments between banks through shared central infrastructure. Which measure most directly reduces the risk that a single fault stops payments nationwide?

• Encrypting every transaction message that passes between a bank and the switch
• Requiring a two-factor authentication step from every paying customer
• Publishing the specification so that any bank may connect to the switch
• Running the switch in more than one data centre so another can take over

Answer: Running the switch in more than one data centre so another can take over

Encryption and authentication protect confidentiality and identity, not availability, and only duplication removes a single point of failure. Merchants and customers feel an outage as lost trade, so availability is a stakeholder question rather than a purely technical one.

A school exports its timetable from one supplier's system and loads it into a different supplier's system, where the lessons, rooms and teachers arrive as timetable entries that staff can edit straight away. No converter was written for that pair of products. Which property does the exchange demonstrate?

• Interoperability, because the receiving system acts on the meaning of the fields
• Compatibility, because the transferred file happens to open without an error
• Portability, because a copy of the timetable was moved from one device onto another one
• Redundancy, because a second usable copy of the timetable now exists elsewhere

Answer: Interoperability, because the receiving system acts on the meaning of the fields

Compatibility only promises that a file opens; interoperability means the second system understands the fields well enough to work with them, which is why it is a property of agreed formats rather than of one product. That difference decides whether the school can change supplier without paying for a converter each time.

In April 2018 the UK bank TSB moved all customer records from its former parent's platform to a new system in a single weekend; on reopening, up to 1.9 million customers were locked out of online banking and disruption lasted weeks. Which systems lesson does the episode best support?

• cloud hosting is unsuitable for holding a bank's customer records
• customers should be moved only after they have given individual consent
• a system that passes its security audit cannot fail for other reasons
• migrating a live system in one step concentrates risk that staging spreads out

Answer: migrating a live system in one step concentrates risk that staging spreads out

A big-bang migration means every defect appears at once, under full load, with no old system to fall back on; phased cutovers surface faults while the blast radius is small. The error is judging a migration by the quality of the new system alone rather than by the transition path.