1) Connectivity topology (how brokers actually connect)
Brokers rarely "connect to FIX" in a vacuum. They connect through a topology:
- Trading platform: MT5/MT4/cTrader/custom UI
- Bridge / aggregator (optional): routing, aggregation, symbol mapping, markups
- Execution FIX: order flow to LP/PoP/PB
- Market data FIX (optional): quotes from venues
- Drop copy FIX (optional): mirrored events for monitoring and audit
If you're designing broader liquidity architecture, see: Liquidity provider integration guide
2) Session management: sequence numbers, heartbeats, resends
Most FIX outages are session outages. A production broker needs deterministic behavior for: disconnects, resends, logon storms, and sequence resets.
Sequence numbers: treat them like money
- Persist incoming/outgoing sequence numbers reliably
- Design a safe reset procedure (and document it)
- Prevent "double-send" logic during reconnect
Resend handling
Resends are where naive implementations melt down. You must handle:
- Resend requests with large gaps
- Out-of-order fills during recovery
- Duplicate messages (idempotency)
Heartbeat discipline
- Detect half-open connections
- Trigger safe reconnect strategies
- Avoid reconnect storms (backoff + jitter)
3) Order lifecycle and state model
FIX order flow must map to an internal state machine. Don't rely on "what we think happened"—rebuild state from events.
Internal order identity
Use an internal immutable order ID and map it to FIX IDs. Your biggest safety tool is idempotency: if the same event arrives twice, state must not double-apply.
Partial fills
Partial fills are normal in institutional execution. Your platform must handle:
- Multiple fills per order
- Fees and commissions per fill
- Correct final state transition on completion/cancel
4) Market data vs execution streams
Many teams confuse market data connectivity with execution quality. Your UI may stream via WebSockets while execution routes via FIX. Price construction and quote integrity become part of your dispute and risk story.
- UI streaming: WebSockets with snapshot + incremental updates
- Execution pricing: LP quotes, last look, rejects
- Audit: what price was shown vs what price executed
5) Drop copy and reconciliation
Drop copy is the practical way to build confidence. It enables monitoring, post-trade reconciliation, and state recovery after incidents.
Why brokers add drop copy
- Rebuild positions after outages
- Detect missing fills or mismatched reports
- Support evidence logs for disputes
6) Pre-trade risk and throttling
Your FIX gateway is not only a transport. It is a control point. Implement pre-trade checks and throttles to prevent runaway exposure and accidental storms.
- Max order rate per account/IP
- Max order size per symbol group
- Kill switch for volatility events
7) Latency budgets and performance
Low latency is not just code. It's topology, hosting location, and operational discipline. Measure p95 and p99 latency, not only averages.
8) Observability: logs, metrics, tracing
Production FIX needs observability. Without it, you'll debug by screenshots during incidents.
- Correlation IDs across UI → OMS → FIX gateway
- Metrics: reject rate, resend volume, disconnect count
- Structured logs: message types + seq ranges (avoid storing sensitive data)
9) Production go-live checklist
- Document reset procedure and rollback plan
- Simulate disconnect storms and resend scenarios
- Verify idempotency under duplicate messages
- Test partial fills and reconciliation paths
- Set alerts on rejects, latency spikes, and session churn
Where Brokeret fits
Brokeret helps brokers implement institutional connectivity and operational tooling: routing rules, monitoring, evidence logs, and scalable back office workflows. If you want FIX connectivity that survives volatility, we can help.