Case studies: platforms and apps interview questions

Because clocks disagree and a counter cannot. A phone's clock can be minutes off, so sorting by the sender's time can put a reply before the question it answers, and two servers whose clocks differ by a few milliseconds can do the same with a quick reply. Arrival order differs on every phone, because pushes take different paths. A sequence number is given by one owner, the conversation, when it stores the message, so every phone sorts the same way and can also see a gap: a phone holding 40 and 42 knows that 41 is missing and asks for it. The time is still stored for display next to the message, but never used to order it. One counter per conversation is enough, because no phone needs an order across conversations.

So that an ID names exactly one object of one kind. If only the content were hashed, a file whose bytes happened to equal a tree's encoding would get the tree's ID, and the store could hand back a tree where a file was expected. Hashing a header with the type keeps a blob and a tree with equal bytes apart. The size makes the encoding self-delimiting: a reader knows where the header ends and how many bytes of content follow, so the bytes can be checked against the ID when they are read back. Git hashes the same header before the content: the type, a space, the length and a zero byte. One consequence surprises people: a blob's ID is not the plain SHA-256 of the file, so a checksum tool shows another number.

It stops the classes from multiplying. Kinds of message (sign-in codes, order updates, promotions) and channels (SMS, email, push) change for different reasons: a new campaign adds a kind, a new provider adds a channel. With a class for every pair, three kinds and three channels need nine classes, and every new channel adds one per kind. The Bridge splits the two: a notification type decides what to say, how urgent it is and how long it is worth delivering; a channel delivers a short or a long text. The service asks the type for content in the channel's form and hands it to the channel. Three and three then need six classes, and a new channel is one class that every kind can use at once.

Because every candidate must get the same time, and only the server's clock is the same for all of them. A browser's clock can be minutes wrong, and a browser can be changed by the person using it, so a deadline checked there can be moved. Fixing the deadline once, when the section opens, means that nothing the browser does later can move it: a disconnect costs the candidate only the minutes they were away, and a reconnect does not restart the section. The browser still shows a countdown, but each answer from the server corrects it with the seconds left. Code that needs the time asks an injected clock, so tests can move time by hand instead of waiting.