[_gabe_]

I'm just curious, I know it's a long running joke about how we're so stupid to think that we would never run out of unique digits with 2^32 possible values, but is this also the case with 64 bit values? Every new bit doubles the amount of information, so if 32 bits lasted reddit 10 years, presumably 33 bits would last them 20 years, 34 would last 40 and so on. Eventually, 64 bits would last them 10×2^32 years, which seems like a safe bet.

So am I being naive when I use 64 bit values for unique IDs? Or is it actually plausible that 64 bits is plenty of information for centuries to come?

Edit: Also, technically reddit was using signed int32s. So they really only had 2^16 unique digits. If they used unsigned int32s, then that would have bought them a lot of time.

[jxf]

> Edit: Also, technically reddit was using signed int32s. So they really only had 2^16 unique digits. If they used unsigned int32s, then that would have bought them a lot of time.

Small correction: Signed int32s means you have 31 bits for the integer value (2^31 values), not 16 bits (2^16 values). There are 32 bits; 1 bit is used up for the sign, and 31 remain for the number.

[_gabe_]

Yep, I made the same mistake I was trying to call out here haha. Thanks for the correction!

[KMag]

As others have pointed out, there are 2^31 non-negative values an Int32 can take on, and linear growth is probably a bad assumption.

I should also probably point out some ambiguity in your analysis. To be clear, under the faulty assumption of linear consumption, if Int32s get exhausted in 10 years, switching to UInt32s will last 20 years of which 10 are already spent. So, under the faulty assumption of linearity, switching to UInt32s buys you an extra 10 years.

64-bit identifiers will last quite a while. Another defensible choice would be to switch to 128-bit unguessable identifiers (such as assigning them via AES-256 in counter mode) in order to provide defence in depth. If something goes wrong with your authentication, but the attacker is likely to spend millennia hammering your REST API with invalid IDs before exploiting an actual asset, then the consequences aren't so bad.

[rezonant]

> Edit: Also, technically reddit was using signed int32s. So they really only had 2^16 unique digits. If they used unsigned int32s, then that would have bought them a lot of time.

Signed integers have 2^31 positive values (well, just about). It doesn't take 16 bits to store the negative sign :-)

[HideousKojima]

Not for most implementations, a signed 32 bit int goes from ~-2 billion to ~+2 billion, an unsigned from 0 to ~4 billion

[taeric]

That isn't disagreeing? It is half the numbers. But that is just minus one exponent.

Right?

[HideousKojima]

The GP said it would have "bought Reddit a lot of time," the post I responded to said it was merely 2^31 implying that it wouldn't have bought them a lot of time, that's the part I was replying to (though I didn't word it clearly). 2 billion more values would have bought Reddit a lot of time.

I should have taken more time to read over both posts and respond more clearly though

[taeric]

Ah, I read the GP as a correction that it wasn't 2^16, but 2^31 values. The "doubling" was correctly stated, but the edit said they only had 2^16 values to work with. Almost certainly just a mistake in the post.

[rezonant]

Yeah I think we're all in agreement here.

[bagels]

For most implementations 2^31 is ~2 billion.

[lilyball]

If 32 bits lasted 10 years, 33 bits would not last 20, as engagement is not a constant. If we look at the last 10 years, presumably there were a lot more posts in the second 5 years than in the first 5 years. I don't know what Reddit's actual user/engagement metrics show though. Certainly if they got to a stable point where they had approximately the same activity going forward as they have going back then it would, but that's not usually how social sites work.

That said, it would still take quite a while to get to 64 bits.

[bagels]

This is just basic engineering: What rate do you anticipate creating these things? What is the upper bound rate you will create these things? Consider if every person were a customer, how many of these things would/could they create or cause to become created? How do those assumptions change over time?

[IIAOPSW]

"we assigned everyone 9 digit phone numbers, but that ran out after 50 years of population growth led to over a billion people. Now we assign everyone an 18 digit phone number. Personally, I'm worried we might run out in another 50 years."

[ThunderSizzle]

We should've dropped phone numbers and just make it like email - dial person-name@verizon

Infinite options at that point.

[hbn]

I mean as long as you can still set your email-number to whatever you want and update it if you want to drop the old one.

I feel like there's also implications for spam, like you want to target X demographic, all you have to do is try numbers with common name combinations from various cultures. Like in Nathan For You when he made a targeted billboard for a psychic that said "Maria Garcia, I had a dream about you" so all the Maria Garcias who saw the sign would become potential customers.

[loteck]

presumably

Isn't the answer to your question buried in your assumptions? For example, you seem to assume the rate of comsumption of unique IDs is static.

[_gabe_]

It would have to increase A LOT to significantly change the outcome. For example, according to this website[0] (no idea how accurate it is) Facebook users upload more than 300 million photos a day. At that rate, 2^32/300,000,000 = 14.3. So 32 bits would give Facebook 14 days worth of unique ids for their photos. Whereas 2^64/300,000,000 = 61,489,147,000 days, which is around 168 million years worth of ids.

All I'm saying is the jump from 2^32 to 2^64 is astronomical. I don't see using 64 bit integers for uids in my hobby code as something to be concerned about. In production code for a company I would use something more significant, but even then I feel like 64 bits will last a very long time.

[0]: https://bernardmarr.com/how-much-data-do-we-create-every-day...

[brrrrrm]

if everyone on earth shared a memory space and every person had 4 gigabytes of byte addressable storage, we'd be at 65 bits.

[chrismcb]

I think it is a horrible assumption to think that Reddit won't grow and will take exactly the same length of time to double the number of unique ids required. But still 64 bits should be plenty for a while. And presumably if it comes closer to using that many they'll have enough time to switch to 128

[aidenn0]

2^63 (assuming signed 64 bit ints) is enough for 10 billion people to post 900 million posts each. Short of faster-than-light travel or the singularity happening, it seems pretty safe.

[masklinn]

Doubling the number of bits does not double the number of values, but squares it.

2^31 seconds is 68 years, a middling human lifetime.

2^63 seconds is 290 billion years.

[masklinn]

(Just so we’re clear, that’s “billion” with a “b”, so 20 times the age of the universe)

[eCa]

> have enough time to switch to 128

It appears that the original developer thought that for 32 bits :)

I say that as someone who inherited a system that allowed for 2^32 session references stored in the db. Lets just say that sessions were created a lot faster than anticipated for the amount of traffic we had.

So one fine Sunday morning in a November a long time ago we ran out.