chore(ADR): Create fractional-non-string-rand-units.md (#1783)

## This PR
- Proposes `Support Non-String Inputs for Fractional Bucketing` ADR

### Related Issues
#1737

---------

Signed-off-by: cupofcat <[email protected]>
Signed-off-by: Maks Osowski <[email protected]>

Author: cupofcat

docs/architecture-decisions/fractional-non-string-rand-units.md

@@ -0,0 +1,169 @@
+---
+# Valid statuses: draft | proposed | rejected | accepted | superseded
+status: draft
+author: Maks Osowski (@cupofcat)
+created: 2025-08-21
+updated: 2025-09-02
+---
+
+# Harden Hashing Consistency And Add Support For Non-string Attributes in Fractional Evaluation
+
+This proposal aims to enhance the `fractional` operator to:
+
+1. Explicitly ensure hashes are consistent across all providers and platforms.
+2. Support non-string values as the hashing input (i.e., the randomization unit).
+
+Currently, all inputs are coerced to strings before hashing, which, in some rare cases, can lead to inconsistent bucketing across different provider implementations (e.g. Java provider running on a non UTF-8 platform). With this change, the targeting attributes of various types will be supported and will always be explicitly encoded in a consistent, language- and platform-independent manner in every provider
+
+This change will be backward-compatible in terms of flags schema but will be a breaking behavioral change for 100% of the users due to rebucketing.
+
+```json
+"fractional": [
+  {
+     // This will now work for non-string types
+     "var": "my-non-string-var"
+  },
+  ["a", 50],
+  ["b", 50]
+]
+```
+
+## Background
+
+The `fractional` operator in flagd determines bucket allocation (e.g., for percentage rollouts) by hashing an input value. Currently, there are two primary methods for providing this input:
+
+1. **Implicitly:** Providing a string-type `targetingKey` in the evaluation context, which is used if the `fractional` block only contains the variant distribution.
+2. **Explicitly:** Providing an expression as the first element of the `fractional` array. Today, this expression *must* evaluate to a string (standard recommendation is to use the `"cat"` operator with `$flagd.flagKey` and `"var"`); that string will be used as hashing input, usually via murmur's `StringSum32` method.
+
+The requirement that the input evaluates to a string has two main drawbacks:
+
+* **Inconsistent Hashing:** Different providers (Go, PHP, Java) may encode the same string into bytes differently (e.g., UTF-8 vs UTF-16). Since hashing functions like MurmurHash3 operate on bytes, this leads to different hash results and thus different bucket assignments for the same logical input across platforms.
+* **Unnecessary Coercion:** If a user wishes to bucket based on a numeric ID (e.g., `userId: 12345`), they must first explicitly cast it to a string (`"12345"`) within the flag definition using an operator like `"cat"`.
+
+This proposal seeks to resolve these issues by allowing `fractional` to operate directly on the byte representation of non-string inputs and to explicitly encode values to bytes with deterministic encoders.
+
+## Requirements
+
+### 1. Users must be able to use both string and non-string variables (e.g., integers, booleans) as the primary input for `fractional` evaluation
+
+### 2. Same "value" (e.g. 57.2, "some text", true, etc) should result in the same bucket assignment no matter the language of the provider and platform used
+
+Please note:
+
+* some languages (e.g. Python) don't necessarily have standard types by default (e.g. int32 vs int64).
+* [OpenFeature spec 312](https://openfeature.dev/specification/sections/evaluation-context/#requirement-312) dictates that evaluation context needs to support `boolean` | `string` | `number` | `structure` | `datetime` types.
+* JSON supports 6 fundamental types: `boolean` | `string` | `number` | `object` | `array` | `null`
+
+As such, the encodings for the following types as first argument (either as literals or results of evaluation) will be standardized:
+
+1. boolean
+2. string
+3. integer (any integer number, Python style)
+4. float (any floating point number, Python style)
+5. object (structure / map)
+6. datetime
+7. null
+
+**array / sequence** will be explicitly not supported as the first argument in fractional so it's possible to distinguish between hashing input and variant bucket. Nevertheless, it can be a part of object type and its encoding needs to be standardized as well.
+
+## Non-requirements
+
+* This change does not need to be backward-compatible.
+* Support advanced features like salting non-string types in JSON directly (that will be a separate ADR).
+* Bucketing improvements (that will be a separate ADR).
+
+## Considered Options
+
+1. **Proposed:** *Type-Aware Hashing:* Extend the current behavior to support non-string types as first arguments to `fractional`.
+2. *New Operator:* Introduce a new operator, such as `"bytesVar"`, to explicitly signal that the variable's raw bytes should be hashed.
+3. *Operator Overloading:* Reuse an existing operator (e.g., `"merge"`) or structure (e.g., providing a list) to imply byte-based hashing.
+
+Option 1 was chosen for its ergonomics and zero-impact on existing schemas. Option 2 adds unnecessary complexity to the flag definition language, and Option 3 creates confusing and non-obvious semantics.
+
+## Proposal
+
+We will modify the evaluation logic for the `fractional` operator.
+
+When inspecting the first element of the `fractional` array:
+
+1. If the first element in `fractional` evaluates to a non-array type then deterministically encode it to a well defined byte array and hash the bytes.
+2. Otherwise, if `targetingKey` is a string, build a 2-elements array of `flagKey` and `targetingKey`, deterministically encode that and hash (**NOTE:** This is different than string concatenation used today).
+3. Otherwise, if `targetingKey` is non-string, report an error and return nil (as this breaks the [OpenFeature spec](https://openfeature.dev/specification/glossary/#targeting-key)).
+4. Otherwise, if `targetingKey` is missing, report an error and return nil
+
+```json
+// Will use the new logic
+"fractional": [
+  {
+    "var": "my-non-string-var"
+  },
+  ["a", 50], ...
+]
+
+// Will use new logic
+"fractional": [
+  {
+    "cat": [{"var" : "$flagd.flagKey"}, {"var" : "some-var"}]
+  },
+  ["a", 50], ...
+]
+
+// Will use targetingKey
+"fractional": [
+  ["a", 50], ...
+]
+
+// Will use targetingKey
+"fractional": [
+  {
+    "merge": [{"var" : "evaluates-to-some-variant-name"}, {"var" : "evaluates-to-some-int"}]
+  },
+  ["a", 50], ...
+]
+```
+
+### Deterministic and consistent byte encodings
+
+To meet requirement (2) [RFC 8949 Concise Binary Object Representation (CBOR)](https://www.rfc-editor.org/rfc/rfc8949.html) will be used to decide on byte encodings.
+
+* `boolean` is major type 7
+* `null` is major type 7
+* `string` is major type 3
+* `integer`:
+    * `unsigned integer` is major type 0
+    * `negative integer` is major type 1
+* `float` is major type 7
+* `map` (object, structure, dict) is major type 5
+* `array` (list, sequence) is major type 4
+* `datetime` is converted to POSIX epoch time (including fractional seconds for sub-second precision) and CBOR Tag 1 is used
+
+**ATTENTION: When encoding strings, CBOR appends the size of the encoding in first bytes. As such, even though the actual encoding of the string is still UTF-8, the resulting byte array will differ from raw UTF-8 encoding. As such, after this change, all hashes will change, which will result in rebucketing.**
+
+Additionally, it is required to use [4.2.1. Core Deterministic Encoding Requirements](https://www.rfc-editor.org/rfc/rfc8949.html#section-4.2.1) (which includes Preferred Serizalization), to ensure:
+
+1. **Map Key Ordering**: Implementations must strictly adhere to the requirement that keys in maps (objects/structures) must be sorted using bytewise lexicographic order of their deterministic encodings.
+2. **Preferred Serialization (Numbers)**: CBOR mandates using the shortest possible encoding. Providers must ensure consistency, especially between integer and float representations, and across different precisions. For example, if a value fits within a 32-bit float, it must be used instead of a 64-bit float, regardless of the native type in the provider's language.
+
+### API changes
+
+There are **no** changes to the flagd JSON schema. The change is purely semantic, affecting the evaluation logic within providers.
+
+### Consequences
+
+* Good, because any variable can be used for hashing.
+* Good, because it avoids unnecessary casting.
+* Bad, because all of the users will experience rebucketing.
+
+### Timeline
+
+Prior to flagd 1.0 launch.
+
+## More Information
+
+Today, flagd recommends salting the variable with flagKey directly in the `fractional` logic, using the `"cat"` operator. This will not be possible for non-string types. Advanced features like that will be considered in a separate ADR.
+
+Salting of the string types will continue to be possible using the `"cat"` operator as it is built directly into JSON Logic.
+
+### Testing considerations
+
+As part of implementation of this ADR, the current Gherkin suite will need to be updated to ensure more in-depth testing of consistency (e.g. by looking at the distribution of buckets for many samples), as well as support for many new types.