8.2 KiB
+++ title = "A One-Part Serialized Mystery" slug = "one-part-serialized-mystery" date = "2023-06-22" updated = "2023-06-22" [taxonomies] tags = ["software", "rnd", "proclamation", "upscm"] [extra] toc = false +++
Mise en Scene
I recently spent a couple days moving from one type of universally unique identifier to a different one, for an in-progress database-backed web-app. The initial work didn't take very long, but debugging the serialization and deserialization of the new IDs took another day and a half, and in the end, the alleged mystery of why it wasn't working was a red herring due to my own stupidity. So come with me on an exciting voyage of discovery, and once again, learn from my folly!
Keys, primarily
Most large distributed programs that people interact with daily via HTTP are, in essence, a fancy facade for some kind of database. Facebook? That's a database. Gmail? That's a database.
In most databases, each entry ("row") has a field that acts as a primary key, used to uniquely identify that row inside the table it's in. Since databases typically contain multiple tables, and primary keys have to be unique only within their own table, you could just use a simple integer that's automatically incremented every time you add a new record, and in many databases, if you create a table without specifying a primary key, they will automatically and implicitly use a mechanism like that. You may also recognize the idea of "serial numbers", which is what these sorts of IDs are.
This is often totally fine! If you only ever have one copy of the database, and never have to worry about inserting rows from a different instance of the database, then you can just use those simple values and move on your merry way.
However, if you ever think you might want to have multiple instances of your database running, and want to make sure they're eventually consistent with each other, then you might want to use a fancier identifier for your primary keys, to avoid collisions between primary keys.
UUIDs
A popular type for these is called a
v4 UUIDs. These are 128-bit random
numbers1, and when turned into a string, usually look something like
1c20104f-e04f-409e-9ad3-94455e5f4fea; this is called the "hyphenated" form, for fairly obvious
reasons. Although sometimes they're stored in a DB in that form directly, that's using 36 bytes to
store 16 bytes' worth of data, which is more than twice as many bytes as necessary. And if you're
a programmer, this sort of conspicous waste is unconscionsable.
You can cut that to 32 bytes by just dropping the dashes, but then that's still twice as many bytes as the actual data requires. If you never have to actually display the ID inside the database, then the simplest thing to do is just store it as a blob of 16 bytes2. Finally, optimal representation and efficiency!
Indexes?
And at first, that's what I did. The external library I'm using to interface with my database automatically writes UUIDs as a sequence of sixteen bytes, if you specified the type in the database3 as "blob", which I did.
But then I saw a blog post where the following tidbit was mentioned:
We prefer using an Universally Unique Lexicographically Sortable Identifier (ULID) for these idempotency keys instead of a random version 4 UUID. ULIDs contain a 48-bit timestamp followed by 80 bits of random data. The timestamp allows ULIDs to be sorted, which works much better with the b-tree data structure databases use for indexing. In one high-throughput system at Shopify we’ve seen a 50 percent decrease in INSERT statement duration by switching from UUIDv4 to ULID for idempotency keys.
Whoa, that sounds great! But this youtube video tempered my expectations a bit, by describing the implementation-dependent reasons for that dramatic improvement. Still, switching from UUIDs to ULIDs couldn't hurt4, right? Plus, by encoding the time of creation (at least to the nearest millisecond), I could remove a "created at" field from every table that used them as primary keys. Which, in my case, would be all of them, and I'm worried less about the speed of inserts than I am about keeping total on-disk size down anyway.
Plus, I was familiar with the idea of using sortable IDs, from KSUIDs. It's an attractive concept to me, and I'd considered using KSUIDs from the get-go, but discarded that for two main reasons:
- they're FOUR WHOLE BYTES!!! larger than UUIDs
- I'd have to manually implement serialization/deserialization for them anyway, since SQLx didn't have native support for them
In reality, neither of those are real show-stoppers; 20 vs. 16 bytes is probably not that significant, and I'd have to do the manual serialization stuff anyway.
I was ready to do this thing.
Serial problems
"Deserilization" is the act of converting a static, non-native representation of some kind of datatype into a dynamic, native computer programming object, so that you can do the right computer programming stuff to it. It can be as simple as when a program reads in a string of digit characters and parses it into a real number, but of course the ceiling on complexity is limitless.
In my case, it was about getting those sixteen bytes out of the database and turning them into ULIDs. Technically, I could have let Rust handle that for me by automatically deriving that functionality. There were a couple snags with that course, though:
- the default serialized representation of a ULID in the library I was using to provide them is as 26-character strings
- you could tell it to serialize as a 128-bit number, but that only kicked the problem one step down the road since SQLite can only handle up to 64-bit numbers, as previously discussed, so I'd still have to manually do something for them
This meant going all-in on fully custom serialization and deserialization, something I'd never done before, but how hard could it be? (actually not that hard!)
Great coders steal
steal the uuid serde impls from sqlx
A puzzling failure
When in trouble, be sure to change many things at once
Death to the littlendians, obviously
- endianness
- profit
-
Technically, most v4 UUIDs have only 122 random bits, as six out of 128 are reserved for version metadata. ↩︎
-
Some databases have direct support for 128-bit primitive values (numbers). The database I'm using, SQLite, only supports up to 64-bit primitive values, but it does support arbitrary-length sequences of bytes called "blobs". ↩︎
-
I'm using SQLite for reasons that I plan to dive into in a different post, but "blob" is specific to SQLite. In general, you'll probably want to take advantage of implementation-specific features of whatever database you're using, which means that your table definitions won't be fully portable to a different database. This is fine and good, actually! ↩︎
-
You may wonder: have I benchmarked this system with UUIDs vs. ULIDs? Ha ha, you must have never met a programmer before! So, no, obviously not. But that's coming in a follow-up. ↩︎