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    Why PATCH endpoints matter

    Here's a painful lesson from production that brought several engineers almost to tears: Please add PATCH endpoints to your public APIs.

    This is true for all APIs: REST, GraphQL, RPC, or sockets. You need PATCH, a way for users to update an object.

    Here's why:

    Months ago we integrated with a vendor, who will go un-named, to handle calendar events for us. They have good integrations with gCal and such, a nice API, and solve fun algorithmic problems that we don't have time to tackle.

    Optimal packing of a user schedule is akin to the knapsack problem, a famous NP-complete (-ish, it's complicated) problem, and ain't nobody got time to solve unsolvable problems at a startup. But this unnamed vendor has a Good Enoughβ„’ solution! Perfect.

    What the heck??

    We build this integration and save for a few hiccups with daylight savings everything is fine.

    Until spooky things started to happen. Events un-cancelling long after they were cancelled. Timezones shifting for no reason. Attendees going missing ...

    🀨

    What could it be!? A distributed systems problem with updates being dropped? Wonky eventual consistency? Silent errors we don't notice? Accidentally unresolved promises causing issues?

    Nope it was nothing fun like that. A massive amount of logging revealed we were overwriting our own data. Changing events to drop fields πŸ€¦β€β™€οΈ

    No PATCH equals pain

    You see this vendor has no PATCH. There is only a POST (insert) and it acts like a PUT (upsert), but that's fine, we can handle that. The lack of PATCH hurts, though.

    Here's what happens:

    Say you create an event on the user's calendar. Friday 1pm for an hour. You POST an object like this:

    {
    calendar_id: 1234
    attendees: [
    { user_id: 1234 }
    ],
    start: '2022-10-28T20:00:00.000Z'
    end: '2022-10-28T21:00:00.000Z'
    }

    This adds an event to calendar 1234, adds the user as an attendee, and sets start/end times.

    How would you update this event to set a cancelled status?

    That's right, you send the whole object again.

    {
    calendar_id: 1234
    attendees: [
    { user_id: 1234 }
    ],
    start: '2022-10-28T20:00:00.000Z'
    end: '2022-10-28T21:00:00.000Z'
    status: 'cancelled'
    }

    What about adding another attendee? Multiple people tend to join a meeting.

    That's right, you send the whole object again!

    {
    calendar_id: 1234
    attendees: [
    { user_id: 1234 },
    { user_id: 1235 }
    ],
    start: '2022-10-28T20:00:00.000Z'
    end: '2022-10-28T21:00:00.000Z'
    }

    Oh no look what we did! The event is no longer cancelled 😱

    User 1235 didn't know the event was cancelled. Client hasn't synced yet, for example. When they joined the meeting, they didn't send the status: 'cancelled' field and the unnamed vendor dutifully overwrote the data.

    Now user 1234 has two meetings at the same time because someone else saw a hole on their schedule and quickly booked a time to chat. Awkward ...

    How PATCH helps

    Now imagine that same sequence, but with a PATCH endpoint.

    First we create the event:

    {
    calendar_id: 1234
    attendees: [
    { user_id: 1234 }
    ],
    start: '2022-10-28T20:00:00.000Z'
    end: '2022-10-28T21:00:00.000Z'
    }

    Then we send a PATCH to cancel:

    {
    event_id: ...,
    status: 'cancelled'
    }

    And a PATCH to add an attendee:

    {
    event_id: ...,
    attendees: [
    { user_id: 1235 }
    ]
    }

    The system merges all this data into the final object:

    {
    event_id: ...
    calendar_id: 1234
    attendees: [
    { user_id: 1234 },
    { user_id: 1235 }
    ],
    start: '2022-10-28T20:00:00.000Z'
    end: '2022-10-28T21:00:00.000Z',
    status: 'cancelled'
    }

    PATCH, or its equivalents in other API genres, lets you update an object without knowing the full state. Super useful when you want the API to be the source of truth, not every client.

    Without PATCH, it is easy to create a race condition where multiple clients send updates, each missing parts of state the other clients are sending. Whomever comes last, that's the new object. Not great.

    Now you might ask "Okay smartass, what about removing an attendee? How do you do that with PATCH?

    And you'd be right, that's where REST patterns get wonky. You'd need an explicit DELETE endpoint of sorts. A function that lets you say "Remove X from event Y".

    Cheers,
    ~Swizec

    Did you enjoy this article?

    Published on October 28th, 2022 in Fullstack, Backend, Lessons,

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