this post was submitted on 07 Jul 2025
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So, I get that 256 is a base 2 number. But we're not running 8-bit servers or whatever here (and yes, I understand that's not what 8-bit generally refers to). Is there some kind of technical limitation I'm not thinking of where 257 would be any more difficult to implement, or really is it just that 256 has a special place in someone's heart because it's a base 2 number?
Because 256 is exactly one byte. If you want to add a 257th member, you need a whole second byte just for that one person. That's a waste of memory, unless you want to go to the 64k barrier of users per chat.
Except that they're almost certainly just using
int, which is almost certainly at least 32 bits.256 is chosen because the people writing the code are programmers. And just like regular people like multiples of 10, programmers like powers of 2. They feel like nice round numbers.
Well, no. They are not certainly using
int, they might be using a more efficient data type.This might be for legacy reasons or it might be intentional because it might actually matter a lot. If I make up an example,
chat_participant_idis definitely stored with each message and probably also in some index, so you can search the messages. Multiply this over all chats on WhatsApp, even the ones with only two people in, and the difference betweenu8andu16might matter a lot.But I understand how a TypeScript or Java dev could think that the difference between 1 and 4 bytes is negligible.
Shots fired.
All these tough guys think you can’t bit shift in Java, never worked on a project with more than two people. Many such cases.
Fair point, but still better than wasting a nuclear power plant worth of electricity to solve math homework with an LLM
Probably why I said "almost certainly". And I stand by that. We're not talking about
chat_participant_id, we're talking aboutGROUP_CHAT_LIMIT, probably a constant somewhere. And we're talking about a value that would require a 9-bit unsigned int to store it, at a minimum (and therefore at least a 16-bit integer in sizes that actually exist for types). Unless it's 8-bit and interprets a 0 as 256, which is highly unorthodox and would require bespoke coding basically all over instead of a basicnum <= GROUP_CHAT_LIMIT.Orrrr they have a u8 chat_participant_id of some kind and a binary data format for message passing. The GROUP_CHAT_LIMIT const may have a bigger data type, but they may very well be trying to conserve 3 bytes per message. Ids can easily start at 0.
150 gigs of bandwidth saved per day doesn't seem like a whole lot at their scale, but if they archive all the metadata, that's over 50 terabytes a year saved on storage - multiplied by how many copies they have of their data. Still not a lot tbh, but if they also conserve data in every other place they can, they could be saving petabytes per year in storage.
Still weird because then they'd have to reuse ids when people leave, otherwise you could join and leave 255 times to disable a group lol
I think you're just very confused friend, or misunderstanding how binary counting works, because why in the 9 hells would they be using 9 bits (512 possible values) to store 8 bits (256 possible members) of data?
I think you're confusing indexing (0-255) with counting (0-256), and mistakenly including a negation state (counting 0, which would be a null state for the variable) in your conception of the process. Because yes, index 255 is in fact count 256.
I'm imagining something like this:
If
GROUP_CHAT_LIMITis 8 bits, this does not work.So add a +1 like you would for any index to count comparison?
I guess I'm failing to see how this doesn't work as long as you properly handle the comparison logic. Maybe you can explain how this doesn't work...
It'll have to do with packet headers, 8 bits is a lot for an instant message packet header.
For high volume wire formats using
uint8instead ofuint32can make a huge difference when considering the big picture. Not everyone is working on bootcamp level software.It’s not that they “like it”. It’s ultimately a hardware limitation. Of course we can have 64 bit integers, or however many bits. It’s an appealing optimization.
If each user is assigned a number as to where they're placed in the group, I guess. But what happens when people are added and removed? If #145 leaves a full group, does #146 and beyond get decremented to make room for the new #256? (or #255 if zero-indexed). It just doesn't seem like something you'd actually see in code not designed by a first semester CS student.
Also, more importantly, memory is cheap AF now 🤷♂️
While I completely agree with the sentiment, snorting too much "memory is cheap AF" could lead to terminal cases of Electron.
There would be no need to decrement later people because they're definitely referred to using pointers. You'd just need to update the previous person's pointer to the new next person.
Memory and network stop being cheap AF when you multiply it by a billion users. And Whatsapp is a mobile app that's expected to work on the crappiest of networks and connections.
when writing somewhat low-level code, you always make assumptions about things. in this case, they chose to manage 256 entries in some array; the bound used to be lower.
but implicitly there's a tradeoff, probably memory / CPU utilisation in the server.
it's always about the tradeoff between what the users want, what is easier for you to maintain, what your infrastructure can provide, etc.
There's often a lot of fun cheats you can use - bitwise operators, etc - if your numbers are small powers of two.
Also it's easier to organize memory, if you're doing funky memory management tricks, if the memory you're allocating fits nicely into the blocks available to you which are always in powers of two.
They're not necessarily great reasons if you're using a language with sufficient abstraction, but it's still easier in most instances to use powers of two anyways if you're getting into the guts of things.
Maybe each user has a user id in a group, and that group user id is stored as 8bit int? Idk
The issue isn't storing each individual ID, it's all of the networking operations that are done and total things that are stored/cached per user in each chat. All of those things are handled and stored as efficiently as possible. Sure they could set it to any number, but 256 is a nice round one when considering everything that is happening and the use cases involved. They have user research data and probably see that 128 is too close to a group size that happens with some regularity, but group sizes very rarely get close to 256, and 512 is right out.