44 lines
2.9 KiB
Markdown
44 lines
2.9 KiB
Markdown
 | # ADR-003: Behavior Injection via Function Pointers for Dynamic Mocking
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## Status: Accepted
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## Context
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In high-reliability institutional algorithmic trading engines, execution modules must be validated against unexpected terminal and broker behaviors (Chaos Engineering):
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- Successive requotes during low liquidity
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- Order rejections and price slippages
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- Gateway/network disconnects (`TRADE_RETCODE_CONNECTION`)
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- Custom Depth of Market (DOM) snapshots
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In languages like C++ or C#, mocking frameworks (e.g., GoogleMock, Moq) use dynamic proxies, closures/lambdas, or template metaprogramming to configure mock responses on the fly.
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MQL5 has strict language constraints:
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1. No native lambda functions or true closure captures.
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2. No reflection or runtime type introspection.
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3. No compile-time mock generation libraries.
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Previous iterations of the test harness relied solely on static tick arrays (`MockTick.injected_error`), which required constructing rigid multi-tick datasets to test dynamic scenarios like retry loops.
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## Decision
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Use **C-style function pointers (`typedef`)** to implement **Behavior Injection** directly into `CSyntheticEngine`:
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1. Define delegate signatures in `BehaviorDelegates.mqh` mirroring the `IMarketEnvironment` interface methods.
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2. Provide setter methods in `CSyntheticEngine` (e.g., `MockOrderSend()`, `MockMarketBookGet()`) to inject delegate function pointers at runtime.
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3. Use a **Dual-Mode Priority Resolution**:
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- **Behavior Injection Hook (Highest Priority)**: If a function pointer is non-NULL, execute it immediately.
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- **Tick-Driven Simulation (Fallback)**: If no delegate is assigned, fall back to sequential `MockTick` replay.
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- **Default Safe State**: If no ticks are loaded and no behavior is set, return safe defaults.
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4. For stateful behaviors across successive calls (such as counting requote retries), use **local static variables** inside standalone behavior functions (`BehaviorLibrary.mqh`).
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## Consequences
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### Positive
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- **Dynamic Chaos Engineering**: Behaviors can be hot-swapped mid-flight in unit tests without recreating tick arrays or restarting the test environment.
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- **Zero Overhead in Production**: `CLiveEnvironment` continues to delegate directly to global MT5 functions via the scope resolution operator (`::`), unaffected by function pointer indirection.
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- **Clean Architecture & Decoupling**: Separation of behavior definitions (`BehaviorDelegates.mqh`), behavior implementations (`BehaviorLibrary.mqh`), and the mock engine (`CSyntheticEngine.mqh`).
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- **Idiomatic MQL5**: Full compatibility with strict MQL5 compiler without requiring external tooling.
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### Negative
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- **Manual State Reset**: Stateful behavior functions relying on internal `static` variables require careful sequencing or automatic reset after completion cycles.
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- **Function Signature Constraints**: Injected functions must strictly adhere to the `typedef` delegate signature.
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