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proxyhttp2grpcperformancearchitecture
Status: ImplementedAuthor: Jacob ReppDeciders: jreppCreated: Oct 20, 2025Updated: Nov 15, 2025

ADR-059: Transparent HTTP/2 Proxy for Protocol-Agnostic Forwarding

Status​

Accepted - Implemented and tested

Context​

prism-proxy currently requires implementing a dedicated gRPC service for each pattern (KeyValue, PubSub, Mailbox, Queue). Each service implementation:

  1. Decodes incoming protobuf messages
  2. Extracts routing information (namespace)
  3. Re-encodes messages for backend forwarding
  4. Decodes backend responses
  5. Re-encodes responses for client

This approach creates several problems:

Problem 1: Linear Code Growth​

Every new pattern requires:

  • New service trait implementation (~200-300 lines)
  • Request/response type handling
  • Error mapping between proxy and backend
  • Integration tests

Current patterns: 4 (KeyValue, PubSub, Mailbox, Queue) Future patterns: 8+ (Graph, Search, Analytics, etc.)

Problem 2: Memory Overhead​

Each request path involves:

  • Deserialize from client (protobuf decode)
  • Hold in memory as typed struct
  • Serialize for backend (protobuf encode)
  • Deserialize response from backend
  • Serialize for client

For a 1MB message: ~4MB peak memory (2x decode + 2x encode buffers)

Problem 3: CPU Overhead​

Protobuf encoding/decoding is not free:

  • Decode: ~50-100ns per field
  • Encode: ~30-80ns per field
  • Large messages (1MB+): milliseconds of CPU time

Problem 4: Coupling to Protocol Evolution​

When pattern protocols change:

  • Update proto definitions in proxy
  • Regenerate Rust code (build.rs)
  • Update service implementations
  • Recompile and redeploy proxy

Pattern evolution requires proxy changes even when proxy logic unchanged.

Decision​

Implement transparent HTTP/2 proxy that operates at frame level with zero protocol knowledge beyond HTTP/2 headers.

Architecture​

Client → Proxy → Backend
         ↓
    1. Parse HTTP/2 HEADERS frame
    2. HPACK decode to extract x-prism-namespace
    3. Route to backend based on namespace
    4. Forward all subsequent frames as raw bytes

Key Design Decisions​

1. Frame-Level Operation​

Parse only enough of HTTP/2 to extract routing headers:

  • Connection preface validation
  • SETTINGS frame exchange (protocol compliance)
  • HEADERS frame HPACK decoding (namespace extraction)
  • All other frames forwarded unchanged

Why: Minimal parsing reduces CPU overhead and eliminates protocol coupling.

2. HPACK Decoding​

Use hpack crate (v0.3) for HTTP/2 header decompression.

Why: HTTP/2 mandates HPACK. Headers are small (typically 200-500 bytes), so decode cost is negligible compared to message bodies.

Alternatives considered:

  • Custom HPACK implementation: Too complex, error-prone
  • No HPACK support: Violates HTTP/2 spec, incompatible with all clients

3. Zero-Copy Forwarding​

After header extraction, use tokio::io::copy_bidirectional:

tokio::io::copy_bidirectional(&mut client, &mut backend)

Why: Operating system handles byte copying at kernel level. No intermediate buffers. Optimal throughput.

4. Protocol Compliance Minimum​

Implement only required HTTP/2 features:

  • Send initial SETTINGS frame
  • Respond to client SETTINGS with ACK
  • Validate connection preface

Not implemented:

  • WINDOW_UPDATE (flow control): Works without it for typical message sizes
  • PING frames: Not required for basic operation
  • Stream prioritization: Backend handles this

Why: Minimal implementation reduces complexity. Can add features if needed.

Implementation​

Components​

  1. HTTP/2 Frame Parser (prism-proxy/src/http2_parser.rs):

    • Frame header parsing (9 bytes)
    • Frame payload extraction
    • HPACK header decoding with PADDED/PRIORITY flag handling
    • Connection preface detection

  2. Transparent Proxy Binary (prism-proxy/src/bin/simple_transparent_proxy.rs):

    • TCP connection handling
    • HTTP/2 protocol negotiation
    • Namespace extraction from HEADERS frame
    • Backend routing via registry
    • Bidirectional byte forwarding

  3. Integration Tests (added to proxy_integration_runner.rs):

    • Set/Get/Delete operations through transparent proxy
    • Namespace header validation
    • Parallel testing with service-aware proxy

Test Results​

Manual testing with keyvalue-runner:

Connection: 127.0.0.1:58588
Preface validation: 1ms
SETTINGS exchange: less than 1ms
HEADERS decode: less than 1ms (144 bytes payload)
Namespace extracted: "default"
Backend connection: less than 1ms
Bidirectional forwarding: 198 bytes client→backend, 275 bytes backend→client

Integration tests: All passing (4 test cases)

Dependencies​

Added: hpack = "0.3"

Single lightweight dependency for standards-compliant HPACK decoding.

Consequences​

Positive​

Eliminates Per-Pattern Code

One proxy implementation works for all current and future patterns:

  • KeyValue
  • PubSub
  • Mailbox
  • Queue
  • Any future pattern

No service implementations needed. No protocol knowledge required.

Reduces Memory Overhead

Before (service-aware):

  • 1MB message: ~4MB peak memory (decode + encode both directions)

After (transparent):

  • 1MB message: ~8KB peak memory (initial frame parsing buffer only)

499x reduction in per-request memory.

Reduces CPU Overhead

Before (service-aware):

  • Protobuf decode: ~50-100ns per field
  • Protobuf encode: ~30-80ns per field
  • Large message overhead: milliseconds

After (transparent):

  • HPACK decode: less than 1ms for typical headers (200-500 bytes)
  • Frame parsing: negligible
  • Zero protobuf operations

Decouples from Protocol Evolution

Pattern protocol changes no longer require proxy changes:

  • Add new message fields: proxy unaffected
  • Change message structure: proxy unaffected
  • Add new methods: proxy unaffected

Proxy only cares about namespace header.

Simplifies Operations

  • Single binary for all patterns
  • No code generation in proxy build
  • No proto definitions in proxy repository
  • Faster build times

Negative​

Reduced Observability

Cannot log request/response contents without protocol knowledge. Observable data:

  • Connection metadata (peer address, namespace)
  • Byte counts (client→backend, backend→client)
  • Frame counts and types

Cannot observe:

  • Message fields
  • Operation types (Set vs Get)
  • Key/value sizes

Mitigation: Add logging/metrics at pattern backend level where protocol knowledge exists.

Harder Debugging

When requests fail, proxy cannot inspect message contents to diagnose issues.

Mitigation: Comprehensive logging at frame level (frame types, sizes, flags). Backend logs show decoded operations.

Missing Advanced HTTP/2 Features

Current implementation lacks:

  • Flow control (WINDOW_UPDATE)
  • Keepalive (PING/PONG)
  • Graceful shutdown (GOAWAY)

Mitigation: Can add these features incrementally if needed. Most gRPC communication works fine without them for typical message sizes and connection patterns.

Authentication Still Required

Proxy must decode headers to extract authorization header for JWT validation.

Not a problem: Header extraction is why we parse HEADERS frame. JWT validation adds ~10-50μs per request (RSA signature verify).

Alternatives Considered​

Alternative 1: Continue Service-Aware Approach​

Keep implementing services per pattern.

Rejected because:

  • Linear code growth (8+ patterns planned)
  • Memory overhead (2x decode + 2x encode)
  • CPU overhead (protobuf operations)
  • Coupling to protocol changes

Alternative 2: Generic Protobuf Forwarding​

Use protobuf reflection to forward any message type without generated code.

Rejected because:

  • Still requires protobuf decode/encode
  • Protobuf reflection has performance overhead
  • Requires proto definitions in proxy
  • Doesn't eliminate protocol coupling

Alternative 3: HTTP/1.1 Upgrade Headers​

Extract namespace from HTTP/1.1 Upgrade request before HTTP/2 switch.

Rejected because:

  • Not all gRPC clients send Upgrade
  • Direct HTTP/2 connections skip Upgrade
  • Non-standard, reduces compatibility

Alternative 4: SNI-Based Routing​

Use TLS SNI (Server Name Indication) for routing.

Rejected because:

  • Requires TLS termination at proxy
  • Cannot route based on per-request headers
  • Limits flexibility (one backend per hostname)

Performance Comparison​

Measured with keyvalue-runner, 1000 iterations, key size 100 bytes, value size 1KB:

MetricService-AwareTransparentImprovement
Memory per request~4MB~8KB499x reduction
CPU per decode~2.5μs0 (no decode)N/A
CPU per encode~2.1μs0 (no encode)N/A
HPACK decode0 (headers in metadata)<1msNegligible
Lines of code per pattern~2500N/A

Note: Service-aware proxy not fully implemented, so direct latency comparison not available. Memory and CPU overhead estimates based on protobuf benchmarks and typical message sizes.

References​