Why not just fix the C++ bugs instead of rewriting?

The root cause was C++'s inherent memory unsafety. Fixing individual bugs didn't prevent new ones, and audits kept finding more each year.

Was rewriting worth the 8-month investment?

Yes, the exchange saved 3x the rewrite cost in prevented outages and reduced audit fees within the first year.

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Improving Memory Safety in Financial Platforms with Rust

Executive Summary

A cryptocurrency exchange had experienced three critical outages in 18 months due to memory corruption bugs in their C++ order matching engine. After rewriting the engine in Rust, they eliminated entire classes of vulnerabilities and improved system reliability to 99.999% uptime.

Key Outcomes

▹ Zero memory safety incidents in 12 months
▹ 99.999% uptime achieved after migration
▹ Audit costs reduced by 60%

Client Situation

The exchange processed $2B daily volume. Their C++ codebase had accumulated technical debt over 6 years, including buffer overflows and use-after-free bugs discovered during security audits.

Key Challenges

⚠ Three critical outages caused by memory corruption
⚠ Annual security audit identifying 15+ memory safety issues
⚠ Difficulty recruiting C++ engineers with security focus

Existing Architecture

A monolithic C++ application handling order matching, risk management, and trade settlement. The codebase used raw pointers and custom memory pools.

Buffer overflows in network parsing logic
Use-after-free in order book maintenance
Race conditions in concurrent order processing

Solution Design

We rewrote the order matching engine in Rust, preserving C++ for non-critical peripheral systems during the transition.

Key Decisions

✓ Model order book state using Rust's ownership system to prevent double booking
✓ Use Rust's type system to encode trading rules at compile time
✓ Implement WebAssembly for customer strategy sandboxing

RustWASMgRPCTokioScyllaDB

Implementation

A complete rewrite over 8 months with parallel testing against production C++ engine for 3 months before cutover.

Phase 1: Phase 1: Order Book Core
Implemented price-time priority order book with Rust's BTreeMap for O(log n) operations.
Phase 2: Phase 2: Matching Engine
Added fill logic and trade generation with property-based testing.
Phase 3: Phase 3: Integration
Integrated with existing risk and settlement systems via gRPC.

Technical Challenges

Maintaining C++ ABI compatibility

Impact: Risk during phase-out of C++ components

Resolution: Used CXX for safe bidirectional bindings with C++

Auditing Rust unsafe code blocks

Impact: Small amount of unsafe needed for FFI and performance

Resolution: Formalized review process for every unsafe block with safety comments

Results

Memory safety vulnerabilities: Before12-15 per audit
After0
Improvement100% elimination
System uptime: Before99.95%
After99.999%
Improvement5x reduction in downtime
Order matching latency (P99): Before850 microseconds
After320 microseconds
Improvement62% reduction

Lessons Learned

📘 Rust's ownership model directly maps to financial invariants like 'an order cannot be double-filled'
📘 Property-based testing with proptest caught edge cases missed by unit tests
📘 The borrow checker prevented real bugs that existed in C++ version

What We Would Do Differently

💡 Integrate differential testing earlier to find behavioral mismatches
💡 Use criterion benchmarks to ensure no performance regression from day 1

Role Relevance

Rust engineers were essential to safely rewrite a mission-critical financial system, understanding both low-level memory safety and domain-specific trading invariants.

Critical Skills Demonstrated

Memory safety principlesDomain modeling with type systemsConcurrent programmingFFI with C/C++Property-based testing

Related Roles

Rust Engineer Quant Engineer

Frequently Asked Questions

Why not just fix the C++ bugs instead of rewriting?: The root cause was C++'s inherent memory unsafety. Fixing individual bugs didn't prevent new ones, and audits kept finding more each year.
Was rewriting worth the 8-month investment?: Yes, the exchange saved 3x the rewrite cost in prevented outages and reduced audit fees within the first year.