Erik McClure

Factorio Is The Best Technical Interview We Have


There's been a lot of hand-wringing over The Technical Interview lately. Many people realize that inverting a binary tree on a whiteboard has basically zero correlation to whether or not someone is actually a good software developer. The most effective programming test anyone's come up with is still Fizzbuzz. One consequence of this has been an increased emphasis on Open Source Contributions, but it turns out these aren't a very good metric either, because most people don't have that kind of time.

The most effective programming interview we have now is usually some kind of take-home project, where a candidate is asked to fix a bug or implement a small feature within a few days. This isn't great because it takes up a lot of time, and they could recieve outside help (or, if the feature is sufficiently common, google it). On the other hand, some large companies have instead doubled-down on whiteboard style interviews by subjecting prospective engineers to multiple hour-long online coding assessments, with varying levels of invasive surveillience.

All these interviewing methods pale in comparison to a very simple metric: playing Factorio with someone. Going through an entire run of Factorio is almost the best possible indication of how well someone deals with common technical problems. You can even tweak the playthrough based on the seniority of the position you're hiring for to get a better sense of how they'll function in that role.

Factorio?

Factorio is a game about automation. The best introduction is probably this trailer, but in essence, your job is to build an automated factory capable of launching a rocket into space.

You begin with nothing. You mine stone manually to craft a smelter that can smelt iron ore you mined into iron plates, which you then use to build a coal-driven automatic miner. You could grab the iron ore from the miner and put it in the smelter yourself, but it's more efficient to use an inserter to do the inserting for you. Then you can use the iron this gives you to make another miner, which automates coal mining. Then you can use belts to take the coal and use an inserter to put it in the iron miner. Then you use the iron plates this tiny factory produces to make a third miner to start gathering copper, which then lets you craft copper wire, which lets you craft a circuit, which lets you build a water pump. Combined with a boiler and a steam engine, you can then build produce power, and use this power to run a research facility to unlock new technology, like assembly machines. Once you've unlocked assembly machines, you can use your circuits to craft an assembly machine that can craft copper wire for you, and insert this into an assembly machine that crafts circuits for you.

Eventually you unlock trains and robots and logistic systems which help you deal with the increasing logistic complexity the game demands, until you finally manage to launch a rocket into space.

Self-Direction

The beginning of the game starts with no goals and barely any direction. A senior developer should be able to explore the UI and figure out a goal, then establish a plan for accomplishing that goal. A junior developer should be able to perform a task that a senior developer has provided for them. An intern is expected to require quite a bit of mentoring, but a junior developer should be able to troubleshoot basic problems with their own code before requiring assistance from the senior developer. An intermediate developer should be able to operate independently once given a task, but is not expected to do any architecture design.

In more concrete terms, you might expect the following:

  • An Intern is generally expected to be able to fill in a pre-placed blueprint, and use belts to hook up their blueprint with something else, like an ore patch.
  • A Junior Developer should be able to build a production line by themselves, although it probably won't be very optimal. They may need assistance from the senior developer on how to route the belts properly to all of the intermediate assembly machines.
  • An Intermediate Developer should be capable of designing a near-optimal production line (without beacons) once given direction, with minimal oversight.
  • The Senior Developer needs no direction, and is capable of determining what goals need to happen and designing a plan of action, then delegating these tasks to other coders.

Teamwork

A critical aspect of software development is the ability to work on a team. This means coordinating your efforts with other people, accomadating the needs of other people's designs and cooperating with the team, instead of simply running off on your own and refusing to adjust your design to help integrate it with someone else's work. This, naturally, arises all the time in Factorio, because base layout designs are limited by physical space. As a result, you need to carefully consider what other people are doing, and sometimes adjust your design to fit in size constraints or deal with someone else's design that took more room than anticipated.

Anyone who simply runs off and starts doing things themselves or fixing problems without telling people is going to quickly earn the ire of their fellow players, for the exact same reasons cowboy programmers do. Luckily, Factorio includes a built-in equivelent to git blame, by showing you the last player that modified any entity. Thus, when people duct tape temporary solutions and don't inform the team about the problem they were fixing, when their temporary solution finally blows up, people will find out. If people want to win they game, they'll have to learn to cooperate well with their teammates.

Debugging

One of the most important skills for any programmer is their ability to debug problems. This is perhaps the most obvious parallel between Factorio and real software engineering. Something can go wrong very far away from the actual source of the problem. Being able to rapidly hone in on the real problem is a critical skill, and the thinking process is almost identical to tracing the cause of a crash in an actual program. If an assembly machine has stopped working, first you have to see if there are multiple outputs that got backed up. Then you have to check what ingredient it's missing. Then you have to trace the ingredient back through your factory to find out where you're making it, and repeat ad nauseum.

Factorio's debugging gets fairly complicated quite quickly. As soon as you start working on oil processing you'll be dealing with cracking, where you're dealing with 3 different outputs and if any of them get backed up for any reason, the entire thing stops. There are cases where your entire factory can grind to a halt because you started researching something that doesn't require yellow science, which stopped using up robot frames, which stopped using up electric engines, which stopped using lubricant, which stopped consuming heavy oil, which backed up and stopped oil production, which made you run out of petroleum, which broke plastic, which broke red circuits, which broke the rest of the factory. Seasoned players will anticipate scenarios like this and use circuits to construct self-balancing oil cracking to ensure the system is balanced and will only back up if petroleum backs up. A new player who is a good programmer, when presented with a factory that has collapsed, will usually be able to trace the issue back to the source, realize what's happened, and promptly attempt to figure out a solution. On the other hand, if someone simply plops down a few storage tanks, unless they can provide a good reason (they are very confident we will never stop consuming lubricant in the future), then this is a red flag for how they approach problem solving in their programs.

Situations like these allow Factorio to closely mimic the complex interdependencies that programmers routinely deal with, and the complexity simply increases the more gameplay concepts are added. This closely follows the increased complexity that additional layers of abstraction introduce when attempting to debug a crash that could have potentially occured deep inside one of the frameworks you use.

Code Reviews

Often, initial designs need to be tweaked for performance or throughput. Good programmers will not only accept critique of their designs, but incorporate that feedback into their future work. If they disagree with a proposed change, they will provide a concrete reason for why they disagree so that the team can more accurately weigh the pros and cons of the proposed change.

Resisting feedback without providing good reasons is a well-known red flag, but what can also be problematic is a programmer who begrudgingly accepts proposed changes, but refuses to adjust future designs accordingly. They end up requiring constant reminders to adhere to some standard way of solving a problem while giving no reason for why they don't seem to like the way the team is doing things. These can be ticking time-bombs in organizations, because when left unsupervised they can rapidly accumulate technical debt for other team members. This kind of problem is almost impossible to catch in a traditional interview, unless it's an internship.

Code Style and Frameworks

Refusing to incorporate feedback is often just a slice of a much larger problem, where someone is unable to integrate properly into an existing framework being used. There are many ways to build a factory in Factorio, and each one requires standard methods of building pieces. Failing to adhere to standards can very quickly jam up an entire factory, often in subtle ways that aren't necessarily obvious to a careless developer.

In the Main Belt design, a set of 4-8 chunk of belts, divided by 2 spaces to allow for underground belts, are placed in the center of the factory, and all production happens perpendicular to the belt. This design relies on several rules that can wreck havoc if not followed correctly. One, players must always use a splitter to pull items off of a belt, never redirecting the entire belt, otherwise using the empty space for a different belt of items means you'll have permanently lost one entire belt of resources, even after upgrading belts. Two, all factories must be scalable in a direction perpendicular to the main belt. Failing to do this will rapidly result in either a massive waste of space, or a production line that cannot be scaled up because it's surrounded by other production lines.

There are also different ways of building logistic networks. The simplest method is with passive provider chests, but another method uses a storage chest with a filter, which is used to solve the trashed item problem. Both of these methods require properly setting limiters in the right location. Passive provider chests generally are limited by chest space. Storage chests require hooking the inserter for the chest up to the logistics network and ensuring that less than N of an item exists before inserting it. Forgetting to perform these limiting steps is a massive waste of resources. Consistently forgetting to put limiters on outputs is a red flag for someone who is careless about performance in real-world applications.

In other cases, the team may be using some pre-designed blueprints, like a nuclear reactor design, or a bot factory. These can be extremely complex, but as long as people are willing to learn how to use them, they can be huge time-savers. Beware of candidates who don't want to learn how to set up a new item in the bot factory simply because they can't debug the complex logic that drives it, or ones that get frustrated learning how to use a bot factory despite the clear and obvious benefits.

Multithreading

Trains in Factorio are a direct analogue to multithreading: one train is one thread of execution, and each train intersection or train stop is a place in memory where two threads could potentially write at the same time. Train signals are locks, or mutexes. All bugs in train networks manifest in exactly the same way software race conditions do, because they're literally physical race conditions. All of the tradeoffs apply here as well - if you make a lock too large, it slows down your throughput, because now the intersection is blocked for a longer period of time. Incorrectly signaled tracks routinely cause train deadlocks that are exactly the same as a software deadlock, because you end up with a circular lock dependency. The most common deadlock is when a train is too long and unexpectedly blocks a second intersection while waiting to enter one. This second intersection then prevents another train from leaving, preventing the first intersection from ever being unblocked.

The number of lanes of track in your network is equivilent to the number of cores available in your CPU. A single rail line is difficult to scale beyond a few threads because the entire system gets throughput limited very quickly, even with wait areas. The most common design is a two-lane design where each lane is a single direction, but this will eventually suffer from throughput issues when you need trains constantly being unloaded. Thus, large bases tend to have at least 4 lanes, with two outer lanes acting as bypasses to avoid the intersection whenever possible.

Missing signal problems in these systems can take a ridiculous amount of time to actually show up. A single missing signal in one rail network once caused a deadlock after functioning correctly for two weeks. This is remniscient of difficult to pin down race conditions in software that only occur once a month or so when under high contention.

Scaling

Just like in software, scaling up production in Factorio introduces new problems with initial designs, and often require complete redesigns that can pipe resources into factories as fast as possible, while taking advange of production modules and speed module beacons. Belt limits become problematic even at the fastest belt speed, forcing players to find ways to split designs up so that more belts can be put in later down the line, or split up their factories into modules.

Handling your logistics network itself becomes a logistics problem in the late game because of how problematic expansive bot networks are. You generally need to start segmenting the logistics network and either using trains to transport items between them, or build a requester chest/provider chest that propagates items across bounderies.

Managing trains in the late game necessitates switching to a pull architecture from a push architecture, because the push architecture can't function in high throughput. This inevitably requires taking advantage of the Train Limits feature and learning how circuit networks can be used to encode basic logic, such that a station only requests a train when it is actually ready to completely fill the train with resources, instead of the common early game tactic of simply telling a bunch of trains to go to stations named “Iron Pickup”. This minimizes the number of trains you need while making sure all stops are served on the network.

Often times, limitations in the number of possible inputs to an assembly machine and inserter speed require redesigning factories around them, just like how high-speed computing requires being aware of subtle bottlenecks in how your CPU works. These bottlenecks are almost never a problem until you reach a certain scale, at which point they begin to dominate your efficiency.

Microservices and Plugin Architectures

Eventually, factories get so enormous they must abandon a simple main belt or spaghetti design and use a more scalable framework instead. To reach Megabase-scale, factories generally either use a train system or a module system, which corresponds roughly to microservices or a plugin-architecture.

A train-based megabase is sometimes referred to a “city-block” design, where trains surrounded factory blocks and control all input and output. Thus, each individual city-block is isolated from all other city-blocks, since all their input is “pure” in that it comes from the train network. This is almost identical to a micro-services architecture (over HTTP) or a multi-process design (using IPC), and has similar potential issues with input and output latency, because results cannot be continually provided, they must be emitted in “packets”, or trains along the network.

The plugin architecture seeks to maintain some semblence of a main-belt, but instead splits belts off through the factory and uses modular blocks that take standard inputs and standard outputs. Sometimes this can be achieved entirely through bots, but materials usually need to be belted over long distances. This closely resembles a plugin system for a monolithic application, and has similar tradeoffs.

These megabases mark the extreme upper end of a vanilla Factorio server. However, there are plenty of mods to make things much more complex.

Distributed Systems

Space Exploration is an overhaul of Factorio that adds an entire space segment of the game, and makes planets have limited resources, requiring players to colonize other planets and use rockets to transfer resources between planets. Because of the enormous latency involved with shipping materials between planets, coordinating these different bases winds up having similar problems to a globally distributed database system. Even the circuit network has to contend with latency, because an automatic request system loses track of items that have been launched but haven't yet reached the target planet. Not accounting for this will result in a double-request for all the items you wanted, which is the exact same problem that distributed systems have when trying to ensure consistency.

Conclusion

Collectively, the software industry simply has no idea how to hire software developers. Factorio is probably the best technical interview we have right now, and that's embarassing. It is also wildly impractical, taking over 20 hours in an initial multiplayer playthrough, or 8 hours if you have a lot of people and know what you're doing. What's the takeaway from this? I don't know. We certainly can't switch to using Factorio as an interviewing method - you might as well just give a candidate a take-home assignment.

At the very least, we can do better than whiteboard interviews.


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