Replace protobuf with protox

.agent-tasks/2026-03-19-protox-otel-metric-exporter/discarded-threads.md

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+# Discarded Threads
+
+(None yet)

.agent-tasks/2026-03-19-protox-otel-metric-exporter/open-questions.md

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+# Open Questions
+
+1. Should we generate protox modules from .proto files or hand-write them?
+   - The opentelemetry-proto directory exists but appears empty
+   - Current .pb.ex files were generated by protoc-gen-elixir (protobuf library's code generator)
+   - protox has its own code generator (protox.generate) that could be used
+
+2. Version bump: should this be a minor or major version bump?
+   - The public API doesn't change, only the serialization library
+   - But the protobuf message structs are part of the API surface (used in tests)

.agent-tasks/2026-03-19-protox-otel-metric-exporter/progress.md

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+# Progress Log
+
+## 2026-03-19
+
+### Initial setup
+- Cloned repo to ~/code/electric-sql/worktrees/elixir-otel-metric-exporter/protox-migration
+- Branch: protox-migration (from main @ d579d0f)
+- Read all source files and .pb.ex files to understand the codebase
+- GH token lacks 'project' scope for board mutations - left comment on issue instead
+- Created task files in .agent-tasks/2026-03-19-protox-otel-metric-exporter/
+
+### Analysis
+Key protobuf touchpoints identified:
+1. 6 `.pb.ex` files using `use Protobuf` macro - need complete rewrite for protox
+2. `otel_api.ex:62-74` - `Protobuf.encode_to_iodata/1` and `Protobuf.EncodeError`
+3. `protocol.ex:64-71` - `SeverityNumber.value(:ATOM)` enum helper
+4. Tests use `Protobuf.decode/2` for decoding in assertions
+5. `metric_store_test.exs` uses `ExportMetricsServiceRequest.decode(body)` directly
+
+### Planning phase
+- Spawned parallel subagents: (a) protox API research + plan, (b) protobuf removal experiment
+- Planner researched protox docs extensively, identified key API differences
+- Experimenter confirmed: removing protobuf breaks all 6 .pb.ex files (37 `use Protobuf` stmts),
+  plus 7 direct `Protobuf.*` call sites across lib/ and test/
+- Key insight: oneof fields and enum atoms are compatible between libraries
+
+### Implementation
+- Used `mix protox.generate` with `--namespace=OtelMetricExporter` to generate from .proto files
+- All 36 modules regenerated with matching names
+- Key changes:
+  - `Protobuf.encode_to_iodata/1` → `Protox.encode!/1` (returns `{iodata, size}` tuple)
+  - `Protobuf.decode/2` → `Protox.decode!/2`
+  - `Module.decode/1` → `Module.decode!/1`
+  - `SeverityNumber.value(:ATOM)` → just `:ATOM` (protox enum fields accept atoms)
+  - `Protobuf.EncodeError` → `Protox.EncodingError`
+  - Protox is stricter: string fields can't be nil, must be ""
+  - nil metric values converted to 0/0.0 defaults
+- All 44 tests pass, clean compilation with --warnings-as-errors
+- Version bumped to 0.5.0
+
+### PR & Review
+- Pushed branch and opened PR with `claude` label
+- Spawned review subagent

.agent-tasks/2026-03-19-protox-otel-metric-exporter/task.md

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+# Task: Replace protobuf with protox in otel_metric_exporter
+
+## Background
+The `otel_metric_exporter` library currently depends on `protobuf` (elixir-protobuf/protobuf) for encoding/decoding OpenTelemetry protobuf messages. This creates a version pin issue in dependent projects (electric/stratovolt). See https://github.com/electric-sql/electric/issues/4018.
+
+## Goal
+Replace the `protobuf` dependency with `protox` (https://github.com/ahamez/protox), a pure-Elixir Protocol Buffers library that doesn't require pinning specific versions.
+
+## Scope
+1. Replace `{:protobuf, "~> 0.15"}` with `{:protox, "~> ..."}` in mix.exs
+2. Rewrite all `.pb.ex` files (6 files under `lib/otel_metric_exporter/opentelemetry/proto/`) to use protox-style module definitions
+3. Update all call sites that use `Protobuf.encode_to_iodata/1`, `Protobuf.decode/2`, and enum `value/1` functions
+4. Ensure all tests pass
+5. Bump the library version appropriately
+
+## Key files using protobuf
+- `lib/otel_metric_exporter/otel_api.ex` - `Protobuf.encode_to_iodata/1` and `Protobuf.EncodeError`
+- `lib/otel_metric_exporter/protocol.ex` - `SeverityNumber.value/1` for enum mapping
+- `lib/otel_metric_exporter/metric_store.ex` - `ExportMetricsServiceRequest.decode/1` (in tests via the module)
+- Test files - `Protobuf.decode/2` calls for decoding responses
+- 6 `.pb.ex` files - protobuf message definitions using `use Protobuf`

.agent-tasks/2026-03-20--1--protox-otel-repeat/discarded-threads.md

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+# Discarded Threads
+
+None at this time.

.agent-tasks/2026-03-20--1--protox-otel-repeat/open-questions.md

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+# Open Questions
+
+None at this time.

.agent-tasks/2026-03-20--1--protox-otel-repeat/progress.md

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+# Progress Log
+
+## 2026-03-20
+
+- Reviewed current state of protox-migration branch
+- Identified three feedback items to address
+
+### P1: nil measurement handling
+- Root cause: `extract_measurement` returns nil when measurement key not in event map
+- nil flows through to `convert_value(nil, :int)` which emits `{:as_int, 0}`
+- Fix: added nil check in `handle_metric/4` to skip writing when value is nil
+- Removed the `convert_value(nil, ...)` fallback clauses
+- Added test case verifying nil measurements are silently skipped
+
+### P2: decode/1 return shape
+- Old protobuf library's `decode/1` returned struct directly
+- Protox-generated code wrapped result in `{:ok, struct}`
+- Changed all 32 `decode/1` functions to return struct directly (matching old API)
+- No callers in the codebase use `decode/1` (all use `decode!/1`), but preserving API for external users
+
+### P3: Large generated file analysis
+- File had 36 modules totaling 10,643 lines (after P2 changes)
+- 22 modules are directly used by the library
+- Transitive dependency analysis revealed 7 additional modules needed (Exemplar, ExponentialHistogram, ExponentialHistogramDataPoint, ExponentialHistogramDataPoint.Buckets, Summary, SummaryDataPoint, SummaryDataPoint.ValueAtQuantile)
+- 8 modules safely removable: LogRecordFlags, DataPointFlags, ExportLogsPartialSuccess, ExportLogsServiceResponse, ExportMetricsPartialSuccess, ExportMetricsServiceResponse, LogsData, MetricsData
+- Removed 1,459 lines (13.8% reduction), file now ~9,176 lines
+- Further reduction would require modifying used modules to remove references to transitively-needed unused modules (e.g., removing Exemplar references from NumberDataPoint), which risks breaking proto wire compatibility
+
+### Results
+- All 45 tests pass (44 original + 1 new)
+- Pushed changes to protox-migration branch

.agent-tasks/2026-03-20--1--protox-otel-repeat/prompt.md

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+# Repeat Task: Address PR #30 review feedback
+
+Session: 2026-03-20--1--protox-otel-repeat
+
+## Feedback items
+
+1. P1: Skip nil measurements instead of coercing to zero
+2. P2: Preserve decode/1 return shape (return struct directly, not {:ok, struct})
+3. Large generated file: analyze unused modules, remove them to trim the file

.agent-tasks/2026-03-20--1--protox-otel-repeat/task.md

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+# Task: Address PR #30 review feedback for protox migration
+
+## P1: nil measurement handling
+- `extract_measurement` can return nil when the measurement key doesn't exist
+- Currently, `convert_value(nil, :int)` returns `{:as_int, 0}` which corrupts metrics
+- Fix: skip the metric write when value is nil
+
+## P2: decode/1 return shape
+- Old protobuf library's decode/1 returned struct directly
+- New protox-generated decode/1 returns {:ok, struct}
+- Fix: change decode/1 to return struct directly for API compatibility
+
+## P3: Large generated file
+- 10,739 lines in a single file with 36 modules
+- Only 22 modules are actually used by the library
+- 14 modules (4,020 lines, 37.4%) are unused
+- Plan: remove unused modules to reduce file to ~6,700 lines

lib/otel_metric_exporter.ex

@@ -135,10 +135,13 @@ defmodule OtelMetricExporter do
     for metric <- metrics do
       if is_nil(metric.keep) || metric.keep.(metadata) do
         value = extract_measurement(metric, measurements, metadata)
-        tags = extract_tags(metric, metadata)
 
-        metric_name = "#{Enum.join(metric.name, ".")}"
-        MetricStore.write_metric(name, metric, metric_name, value, tags)
+        unless is_nil(value) do
+          tags = extract_tags(metric, metadata)
+
+          metric_name = "#{Enum.join(metric.name, ".")}"
+          MetricStore.write_metric(name, metric, metric_name, value, tags)
+        end
       end
     end
   rescue

lib/otel_metric_exporter/metric_store.ex

@@ -251,7 +251,7 @@ defmodule OtelMetricExporter.MetricStore do
        ) do
     %Metric{
       name: Enum.join(name, "."),
-      description: description,
+      description: description || "",
       unit: convert_unit(unit),
       data: convert_data(metric, values)
     }
@@ -337,7 +337,7 @@ defmodule OtelMetricExporter.MetricStore do
      }}
   end
 
-  defp convert_unit(:unit), do: nil
+  defp convert_unit(:unit), do: ""
   defp convert_unit(:second), do: "s"
   defp convert_unit(:millisecond), do: "ms"
   defp convert_unit(:microsecond), do: "us"
@@ -349,12 +349,6 @@ defmodule OtelMetricExporter.MetricStore do
   defp convert_unit(:terabyte), do: "TBy"
   defp convert_unit(x) when is_atom(x), do: Atom.to_string(x)
 
-  # These two clauses are here to preserve the current behaviour of the library and avoid
-  # introducing unexpected errors. Ideally, we would filter these nil values higher up in the
-  # call stack and stop short of exporting metrics with nil values.
-  defp convert_value(nil, :int), do: {:as_int, nil}
-  defp convert_value(nil, :double), do: {:as_double, nil}
-
   @signed_int64_max 2 ** 63 - 1
   @signed_int64_min -2 ** 63
   defp convert_value(int, _preferred_type)