docs: pattern purity and codebase evolution analysis

How pure are these codebases? When do they break their own conventions and why? Key finding: purity inversely correlates with size. Ecto has 0 TODOs; CockroachDB has 1,048. Neither is wrong — they're different kinds of systems.
2026-04-30 10:56:15 -07:00
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+# Pattern Purity & Evolution: How Codebases Live With Themselves
+
+No codebase is pure. The interesting question is: HOW
+do they manage impurity? When do they break their own
+conventions, and what does that reveal about real
+engineering versus ideal engineering?
+
+---
+
+## CockroachDB: 1,048 TODOs and Counting
+
+### Convention Adherence (measured)
+
+**Stopper usage (goroutine lifecycle):**
+- 91 calls via RunAsyncTask/RunTask (old API)
+- 295 via Handle API (new API, still tracked)
+- 3 truly untracked `go func()` calls
+
+Purity: **99.6%** — and one of those 3 has a literal
+`// TODO(irfansharif): Use a stopper here instead.`
+
+**Error wrapping:**
+- 194 uses of `errors.Wrap` (cockroachdb/errors library)
+- 13 uses of `fmt.Errorf("%w")` (stdlib pattern)
+- 15 uses of `fmt.Errorf` without wrapping
+
+The 13 stdlib-style uses are all in `asimview` (a CLI
+tool). The core KV layer uses cockroachdb/errors
+exclusively. **Pattern breaks happen at the periphery,
+not the core.**
+
+### How New Patterns Are Introduced
+
+**The Handle pattern replaced RunAsyncTask:**
+- RunAsyncTask: 63 remaining usages
+- Handle (newer): 295 usages
+
+The newer API coexists with the old. There's no big-bang
+migration. New code uses Handle; old code stays on
+RunAsyncTask until someone touches it. The convention
+evolved *gradually*.
+
+**Lesson:** CockroachDB never does a codebase-wide
+rewrite. They introduce the new pattern, new code uses
+it, old code migrates opportunistically when touched for
+other reasons.
+
+### Stale TODOs (Technical Debt That Survived)
+
+```go
+// TODO(irfansharif): Remove this in 21.2
+// TODO(ajwerner): Remove in 22.2.
+// TODO(nvanbenschoten): remove this entirely in v22.1
+```
+
+Six TODOs referencing releases from 3+ years ago. These
+are promises that were never kept. The code still works
+— the TODO became dead documentation.
+
+**Why they survive:** Nobody is breaking anything. The
+old code path works. The TODO is aspirational, not
+critical. In a 2M-line codebase, someone would have to
+care enough to touch this file for another reason.
+
+**Lesson:** Purity is not binary. CockroachDB is 99.6%
+pure on goroutine tracking but carries years-old dead
+TODOs. They invest enforcement energy where it matters
+(leak detection fails tests) and accept impurity where
+it doesn't (cosmetic cleanup).
+
+---
+
+## Prometheus: The Completed Migration
+
+### Convention Adherence (measured)
+
+**Logging migration (go-kit/log → slog/promslog):**
+- promslog/slog usages: 373
+- go-kit/log usages: **0**
+
+They completed a full logging framework migration. Zero
+remnants of the old pattern in production code. Only 3
+references remain in comments explaining the hack for
+klog compatibility.
+
+**This is rare.** Most codebases abandon migrations
+halfway. Prometheus finished it.
+
+### How Patterns Coexist During Migration
+
+**AppenderV2 alongside V1:**
+```go
+type scrapeLoopAppendAdapter interface {
+    append(b []byte, contentType string, ts time.Time) (...)
+}
+
+// Old path
+type scrapeLoopAppender struct { storage.Appender }
+
+// New path  
+type scrapeLoopAppenderV2 struct { storage.AppenderV2 }
+```
+
+An interface (`scrapeLoopAppendAdapter`) abstracts over
+both. The caller doesn't know which version it's using.
+The selection happens once at construction:
+
+```go
+if sl.appendableV2 != nil {
+    return &scrapeLoopAppenderV2{...}
+}
+return &scrapeLoopAppender{...}
+```
+
+**Pattern:** When evolving an interface, introduce an
+adapter layer that unifies old and new. The caller
+changes once. The implementations coexist until the old
+one can be removed.
+
+### Where They Break Conventions
+
+**Global vars in scrape:**
+```go
+var ScrapeTimestampTolerance = 2 * time.Millisecond
+var AlignScrapeTimestamps = true
+var UserAgent = version.PrometheusUserAgent()
+```
+
+These are configurable behavior via package-level vars.
+This breaks the "config struct" convention used elsewhere.
+They survive because:
+- They're set once at startup
+- Tests can override them directly
+- Moving them to a config struct would break the API
+
+**Functional options in leaf packages:**
+```go
+type WriterOption func(*writerOptions)
+func WithUncachedIO(enabled bool) WriterOption { ... }
+```
+
+The main tsdb config is a struct. But chunk writer
+(added later) uses functional options. The newer code
+adopted a different convention than the older code.
+
+**Why:** Chunk writer has 2 options. A config struct for
+2 fields is ceremony. Functional options are simpler at
+that scale. The convention adapts to the context.
+
+---
+
+## Ecto: Zero TODOs, Maximum Discipline
+
+### Convention Adherence (measured)
+
+- TODOs in lib/: **0**
+- FIXMEs: **0**
+- HACKs: **0**
+
+This is remarkable for a 12-year, 6000-commit codebase.
+Either they never leave TODOs (discipline) or they clean
+them up before merging (process).
+
+**Adapter abstraction purity:**
+- References to specific adapters (Postgrex, MyXQL) in
+  lib/: **1** (in a documentation example)
+- All DB interactions go through the adapter interface
+
+### How Ecto Introduces New Patterns
+
+**Schemaless changesets** (later addition):
+- Same module (`Ecto.Changeset`)
+- Same API (`cast/3`, `validate_required/3`)
+- Just works without a schema by passing a types map
+
+They didn't create `Ecto.SchemalessChangeset`. They made
+the existing API more general. The new pattern is
+invisible — it's the same function with a broader input.
+
+**Dynamic repos** (later addition):
+- `put_dynamic_repo/1` / `get_dynamic_repo/0`
+- Process dictionary for repo resolution
+- Existing code doesn't need to change
+
+Again: new capability added without breaking or
+splitting the existing pattern. The extension point was
+designed in from the start (`@default_dynamic_repo`).
+
+### When Ecto Breaks Its Own Rules
+
+Ecto's Queryable protocol says "anything can become a
+query." But internally, the planner directly pattern-
+matches on `%Ecto.Query{}` structs:
+
+```elixir
+%Ecto.Query{from: %Ecto.Query.FromExpr{source: ...}}
+```
+
+This is pragmatic. The protocol gives consumers
+extensibility. But internally, Ecto knows the concrete
+types and uses them directly for performance.
+
+**Lesson:** Abstractions are for consumers. Internals
+can cheat when performance or simplicity demands it.
+
+---
+
+## Oban: Structural Consistency Across Engines
+
+### Convention Adherence (measured)
+
+- TODOs: **0**
+- All Repo access: through `Oban.Repo` (never bypasses)
+- Engine implementations all follow the same callback
+  structure
+
+**Engine size comparison:**
+- Basic: 553 lines
+- Dolphin: 426 lines
+- Lite: 402 lines
+- Inline: 140 lines (testing, deliberately minimal)
+
+The three production engines are within 30% of each
+other in size. They implement the same callbacks in
+roughly the same way. This is structural consistency.
+
+### Where The Pattern Stretches
+
+The Dolphin engine (MySQL) has different query patterns
+than Basic (Postgres) due to MySQL limitations:
+- No `FOR UPDATE SKIP LOCKED`
+- No `LISTEN/NOTIFY`
+- Different JSON handling
+
+Same interface, different implementation *shape*. The
+engine behaviour forces structural consistency but can't
+force implementational consistency.
+
+---
+
+## Cross-Cutting: When and Why Patterns Break
+
+### Pattern Breaks Have Three Causes:
+
+**1. Age (gradual drift)**
+- CockroachDB: RunAsyncTask vs Handle
+- Prometheus: Config structs vs functional options in
+  newer code
+
+Old code establishes a pattern. New code (sometimes by
+different authors) uses a different approach. Neither is
+wrong — the codebase evolved.
+
+**2. Context (different scale/needs)**
+- Prometheus: global vars for 2-3 scrape settings vs
+  full config structs for complex subsystems
+- CockroachDB: fmt.Errorf in CLI tools vs errors.Wrap
+  in core
+
+The pattern that's right for the core isn't always right
+for peripheral code. The convention adapts to context.
+
+**3. Migration (deliberate coexistence)**
+- Prometheus: AppenderV1 + V2 with adapter interface
+- CockroachDB: RunAsyncTask + Handle coexisting
+
+New pattern introduced but old one isn't removed yet.
+This is intentional — forcing migration on all existing
+code would be high risk for no immediate benefit.
+
+### The "Purity Spectrum"
+
+| Codebase     | TODOs | Convention breaks | Migration discipline |
+|-------------|-------|-------------------|---------------------|
+| Ecto        | 0     | Minimal (internal) | Extremely high      |
+| Oban        | 0     | Structural only   | High                |
+| Prometheus  | Low   | Completed migration| High (once decided) |
+| CockroachDB | 1,048 | Gradual drift     | Opportunistic       |
+
+**Insight:** Purity correlates with codebase size
+*inversely*. Ecto (3.8M, 1 maintainer with very high
+standards) is pure. CockroachDB (845M, 700 contributors)
+has 1,048 TODOs. This isn't failure — it's the natural
+state of a large living system.
+
+### What This Means for Code Review
+
+1. **Don't enforce purity mechanically.** A pattern break
+   might be intentional (different context, migration in
+   progress, peripheral code).
+
+2. **Ask WHY it breaks.** Is this:
+   - Gradual drift (worth fixing opportunistically)?
+   - Context-appropriate deviation (leave it)?
+   - Migration in progress (track completion)?
+
+3. **Enforce at the boundary.** CockroachDB's leak tests
+   are 99.6% effective because they enforce at test-time,
+   not code-review-time. Ecto's zero TODOs come from
+   merge-time discipline, not review comments.
+
+4. **New patterns need an adoption path.** Prometheus's
+   adapter interface and CockroachDB's gradual migration
+   both work. Big-bang rewrites don't.
+
+5. **Peripheral code doesn't need core conventions.**
+   CLI tools can use `fmt.Errorf`. Test helpers can break
+   rules. Enforce conventions where they matter (the core
+   that carries the system's invariants).
+
+<!-- PATTERN_COMPLETE -->