I’m a big fan of UUIDs. They make life better, wherever I need to organise things - correlating objects between vastly different data sources, storage formats, structures, non-structures, databases, caches, etc. I already use them as PKs in Postgres, filenames in S3, and many more; today I wanted to spread their usage to accounting.

At Intio, we have an internal rule, that all objects (wherever reasonable) should be addressed with a UUID, which then must be kept consistent across all systems / services. So, whenever an object crosses a service boundary (e.g. a row in a database, directly related to an asset in S3), there is no confusion about the relationship - this is the video file, and this is its metadata: encoding parameters, number of user impressions, edit history, etc. The benefits are not always immediately obvious; for example: when we started tackling business intelligence, having these same UUIDs stored in our access logs saved us from an extra layer of indirection in our analytical queries.

This is great, as long as other parties you communicate and exchange data with, are willing to accept this schema. And perhaps the most important kind of party a company interacts with, are their customers. We already label each partner with a UUID - so can we take it a step further, and (ab)use UUIDs as invoice numbers?

An invoice number, to be practical, should be readable and meaningful to humans. I’d like it to sort chronologically, and perhaps also to include some metadata, such as the customer ID, currency, whether it corrects an existing invoice, etc; and, ideally, to have the most important metadata exposed to the naked eye.

The structure

Let’s look at a UUID:


This pattern of hyphens already suggests, that some of these sections could be abused to squeeze in our own metadata! Maybe we could do something like this, to encode date and time?


So I’ve started my research. While a UUID looks completely random, there is a certain structure to the string of hex digits. If you’re interested, you should read RFC 4122 and the Wikipedia article, but the TL;DR is there are four variants, with the common (RFC 4122) subdivided into versions, like so:


Here, N takes the value of 8, 9, a or b to indicate the RFC 4122 variant. Then, we look at M; versions 1 through 5 are currently in use (we’d rather not use a version reserved for future use). So, at least theoretically, all of the x’s are up for grabs, no?

Digging further, versions 1 and 2 already encode the timestamp, but there is nothing human readable about that:


These can be actually decoded, using software (or pen and paper, for the patient), in which case the x’s have a defined meaning: a timestamp, and a MAC address. We don’t want our abuse to clash with someone else’s (like our future selves) intended use, somewhere down the line.

Versions 3 and 5 are explicitly defined to take the result of a hash function as an input while being generated; theoretically, this could allow us to make up the concrete values of our x’s as we wish. This usage however, would again, clash with the intended purpose: UUID 3/5 can be used to derive IDs, which sounds like a very useful property: things such as domain names, URLs, or other kinds of “foreign” identifiers can be brought to the “UUID space” in a deterministic manner.

This leaves us with version 4, which is defined as: just random data. It sounds like the perfect target.

import datetime, random, uuid

def randch(n: int, a: str = "0123456789abcdef") -> str:
    return "".join(random.choice(a) for _i in range(n))

def invoice_uuid(now=None) -> uuid.UUID:
    now = now or datetime.datetime.now()
    return uuid.UUID(
            r2=randch(1, "89ab"),

This generates a UUID that looks like this: 20200115-1245-473a-85ca-8845cc45ac6c.

Can we cram in more metadata? Say we have a customer, with number ae0e9a50-d2f5-593c-81a7-0931bb11a33a (in case you were wondering, this is the UUID5 derived from intio.tv in the DNS namespace). Maybe we can take one of the sections, and copy them directly into our invoice UUID? This way, if we see several invoice numbers (without any other context), all ending in 0931bb11a33a, we know they were issued to a particular customer. It takes one copy-paste into the search box in the invoicing software to filter and find the customer. We can still have 281474976710656 customers before there’s a collision.

Performance considerations

Since we’re considering using these UUIDs as primary keys, we must consider the indexing performance of this scheme. When you look at UUID1, this seems like the exact reason behind the timestamp encoding; run this Python code and take note of the output:

import time, uuid
for _i in range(10):

The prefix is definitely related to current time - it changes regularly, and more often than the rest of the identifier. I don’t have a source to quote here, but whoever came up with this idea, must’ve had indexes in mind.

(But what are indexes (or indices)?)

It’s the secret sauce that allows a search query to be faster. Imagine a library full of books; they may lay on the shelves, physically divided e.g. into categories, But what if you needed to find the 10 oldest books in the collection?

If the librarian kept an index of all books, sorted by their publication date, now it becomes a matter of looking up the titles there, instead of looking at each and every single book in succession. By building an index, we traded a tiny bit of extra up-front work, and got much faster search in return.

This makes a lot of sense, whenever you have a collection that is read/searched way more often than it is updated. Books are read much more often than they’re written.

There is approximately an infinity of methods to implement an index, and many of these will rely on looking at a prefix of some value.

For example, to implement revision history, git keeps a hash of the historical contents of each file it tracks; these hashes are then used as file names; these files then store the concrete contents (this is called content-addressable storage).

Since looking at a very long list of file names in a directory can be slow (it has to be scanned one-by-one), git groups them into subdirectories, using a two-digit prefix. Looking at that short list of prefixes is a very fast operation; then we need one more fast lookup to find the actual object. Now what would happen, if all of these hashes had an identical two-digit prefix? The entire scheme would be defeated, and the performance of many operations would suck horribly.

Many other systems can be implemented this way; for example, in a SQL database, an index on a character string field can be made to take into account only the first N characters - which makes sense in some cases: the index is smaller, faster to build, still achieves acceptable performance, etc.

It is imaginable, that relying on the prefix of a typical UUID value being “sufficiently random”, someone might come to a conclusion, that it might be beneficial to keep a more compact index.

My gut feeling though is, your biggest user-visible performance bottleneck is still going to be the loading time of that 6MB blob of JavaScript.