# FIX Adapter
## Module Structure
```
wingfoil/src/adapters/fix/
mod.rs # All types, codec, session logic, node impls, public API
integration_tests.rs # LMAX London Demo integration tests (requires credentials)
```
The adapter is a single `mod.rs` rather than the `read.rs`/`write.rs` split used by async
adapters. This is intentional: FIX sessions are stateful, bidirectional TCP connections where
read and write share a `FixSession` state machine (sequence numbers, heartbeat tracking).
Splitting would require threading the session state across files with no benefit.
## Design Decisions
### Synchronous poll-based architecture
Unlike async adapters (etcd, ZMQ) that use `produce_async`/`consume_async`, the FIX adapter
uses custom `MutableNode` implementations polled by the graph engine. Two poll modes:
- **`AlwaysSpin`** — graph spin loop drives non-blocking socket reads (~1–5 µs latency)
- **`Threaded`** — background thread with channel bridge (~10–100 µs latency)
This avoids the overhead of an async runtime for a protocol where microsecond latency matters.
### FixConnection and fix_sub
`fix_connect_tls` returns a `FixConnection` bundling the data/status streams and session handle.
`FixConnection::fix_sub(&["4001"])` creates a graph node that watches the status stream and
automatically sends `MarketDataRequest` messages once the session reaches `LoggedIn`.
`FixConnection::inject(msg)` and `FixConnection::injector()` provide raw access for advanced
use cases (e.g. throttled sweeps, custom message types). `FixSenderNode` is a separate sink for
cases where a dedicated outbound connection is needed (e.g. order routing).
### No testcontainers
There is no standard Docker image for a FIX engine. Integration tests connect to the LMAX
London Demo (free account required) and are gated behind environment variables.
### Custom FIX codec
The adapter includes a hand-written FIX 4.4 tag-value codec (`encode_message`, `decode_fields`,
`build_message`, `find_message`) rather than using the `fefix` crate's codec. The `fefix`
dependency is declared but currently only used for potential future dictionary-driven validation.
## Pre-commit Requirements
```bash
cargo fmt --all
cargo clippy --workspace --all-targets --exclude wingfoil-python -- -D warnings
cargo test -p wingfoil -- fix::tests # unit tests (no network)
```
Integration tests (requires LMAX credentials):
```bash
LMAX_USERNAME=xxx LMAX_PASSWORD=yyy \
cargo test --features fix-integration-test -p wingfoil \
-- fix::integration_tests --nocapture --test-threads=1
```