High-Frequency Trading: Architecture, Execution and Controls

Establish access before designing speed

1.1 Permissions and market structure

An automated strategy still operates through a permitted market-access arrangement. Identify the legal entity, broker responsibility, market-data licence, order-entry interface and applicable exchange rules before implementation. A public data website does not imply a licensed low-latency feed or permission to send automated orders. Obtain written confirmation of the actual service, eligible instruments, testing requirements and limits. The SECP algorithmic-trading concept paper is a policy discussion source; it must not be treated as an enacted rule or a retail entitlement.

1.2 Co-location and the latency budget

Co-location places equipment near venue infrastructure to reduce some network distance, where an approved service exists. It does not remove feed distribution delay, matching-engine queues or competitors. Divide latency into feed receipt, parsing, decision, risk checks, transmission and acknowledgement. Measure distributions and tail delays, not just the fastest observation. Hardware acceleration cannot fix an incorrect strategy or unknown queue state. This curriculum does not verify that a particular PSX co-location or API service is available to the reader.

Step-by-step method

1. List required permissions and authoritative service documents.
2. Measure each feed-to-acknowledgement stage separately.
3. Record unverified access assumptions as deployment blockers.

Build an event-driven state machine

2.1 Feeds, sequence and order lifecycle

Maintain separate market state, desired orders and confirmed exchange state. Sequence numbers and timestamps help detect missing or out-of-order messages, but clocks must have known synchronization quality. An order sent is not necessarily accepted, and a cancellation request does not mean the order is already cancelled. Partial fills, rejections, disconnects and duplicate messages require explicit state transitions. Rebuilding state after a gap is a correctness problem before it is a performance problem.

2.2 Hardware acceleration and correctness

A typical architecture separates ingestion, normalized book state, strategy, independent risk gate, order gateway and durable audit log. Field-programmable gate arrays can implement selected deterministic tasks with low latency, but introduce development, verification and change-control complexity. Avoid sacrificing risk checks for headline speed. Use bounded queues, backpressure policies and deterministic recovery; silently dropping a risk event is more dangerous than a slower response. Benchmark the whole system under stress rather than a single isolated calculation.

Step-by-step method

1. Define valid order states and transitions.
2. Detect gaps and stop decisions based on uncertain state.
3. Replay failures and reconcile local state with authoritative records.

Understand market-making economics and toxicity

3.1 Spread capture versus inventory

A market maker may quote buy and sell prices to earn a spread, but fills arrive selectively. Inventory accumulates when one side trades more than the other. A quoting model balances expected spread, fees, inventory risk, adverse selection and permitted quoting obligations. A displayed two-sided spread is not a realized profit; the unfilled leg, subsequent price change and hedging costs matter. Do not assume short selling or a hedge instrument is available under the reader’s access arrangement.

3.2 Adverse selection and order-flow signals

Order-flow toxicity refers broadly to the risk that incoming trades occur when counterparties possess information or urgency that makes the quote stale. Measures based on imbalance, trade direction or post-fill price movement are proxies, not direct observations of intent. Estimate outcomes conditional on fills rather than all quoted opportunities. A faster cancellation can reduce some stale exposure but does not guarantee avoiding an informed trade. Excessive messages may encounter limits, charges or controls that must be verified.

Step-by-step method

1. Track inventory and mark positions consistently.
2. Separate spread revenue from fees and adverse price movement.
3. Test toxicity proxies against subsequent outcomes without assuming causality.

Simulate what could actually have executed

4.1 Queue position and latency

An event replay must respect information arrival and decision latency. Seeing a historical trade at your limit price does not prove your hypothetical order would have filled: orders ahead may consume the quantity. Market-by-price data often cannot identify precise queue position. Model uncertainty explicitly using conservative and alternative fill assumptions. Include partial fills, cancellations racing with trades, rejected orders and outages rather than granting every favourable opportunity.

4.2 Data and backtest bias

Use training, validation and untouched test periods with documented parameter selection. Include fees, taxes where applicable, spread costs, hedge feasibility and capacity. Timestamp granularity and feed filtering may make an HFT claim untestable with the dataset available. Shadow mode can compare expected decisions with live observations without sending orders, but it does not establish real fill probabilities. A profitable replay is evidence about assumptions, not proof of executable profit or future resilience.

Step-by-step method

1. Align event, receive and decision timestamps.
2. Model queue uncertainty, rejects and delayed cancellations.
3. Report results across costs and pessimistic execution assumptions.

Design independent controls and deployment gates

5.1 Pre-trade limits and kill switch

Risk controls should limit order size, price deviation, position, aggregate exposure and message rate independently of the strategy. Use authoritative account state where available and define what happens when it becomes stale. A kill switch must stop new orders and initiate the permitted cancel procedure, but cancellation cannot erase fills already executed or guarantee zero remaining exposure. Test its behaviour during disconnects, duplicate acknowledgements and simultaneous fills.

5.2 Monitoring, recovery and review

Deployment requires current permission, controlled testing, versioned release approval and staffed monitoring appropriate to the arrangement. Define alerts for stale feeds, clock drift, unexpected inventory and divergence between local and broker records. Keep tamper-evident records sufficient to reconstruct decisions. Recovery begins with reconciliation, not blindly restarting the strategy. Post-incident review should identify whether model assumptions, software or operations failed and require evidence before resuming. A faster system magnifies both sound decisions and errors.

Step-by-step method

1. Set independent exposure and message limits.
2. Test stop, cancel and reconciliation under failure.
3. Require documented access and operational readiness before activation.

A controlled event-to-order path

Reference table

Glossary of key terms

Adverse selection

A fill occurs when subsequent price movement is unfavourable to the quote.

Co-location

Approved placement near venue infrastructure to reduce some network distance.

FPGA

Reconfigurable hardware for selected deterministic processing tasks.

Kill switch

Control stopping new activity and initiating defined cancellation actions.

Latency

Time between specified events, measured across a defined path.

Queue position

An order’s priority relative to other resting orders.

Frequently asked questions

Common pitfalls