1
Scanner
Evaluate a defined universe on a schedule or event stream and produce ranked candidates.
Lesson · Algorithmic and Automated Trading
Scanners, Alerts, and Human Approval
Screen instruments, rank candidates, deduplicate alerts, communicate the exact thesis, and expire decisions that are no longer current.
Core concepts
What to understand
Keep the boundaries, state, and authority explicit.
2
Alert contract
Include symbol, direction, setup, evidence, entry condition, invalidation, timestamp, and expiration.
3
Approval gate
Require a person to accept, reject, or modify a proposed order within fixed permissions.
Interactive lab
Scanner and Approval Lab
Change scan interval, score threshold, alert expiration, and approval delay. Observe duplicate and stale decisions.
Scanner and Approval Lab
Deterministic synthetic educational data · no broker or live-market connection
SYSTEM MODEL
Process
Keep alerts actionable
The recipient should not need to infer the trade rules from a generic message.
Scan
Evaluate eligible symbols with the same versioned rules.
Rank
Apply quality, liquidity, risk, and portfolio filters.
Communicate
Send one deduplicated alert with an explicit expiration and approval action.
Knowledge check
Check your understanding
Select one answer for each question. Feedback appears immediately, and a wrong choice is answered on its own terms before the rule is restated.
1. Why should an alert expire?
The original setup may no longer satisfy entry or risk conditions.
2. A useful alert includes:
The decision should be reproducible and time-bounded.
3. Approval should occur before:
The gate controls whether the proposed trade reaches execution.
4. A setup stays true for 20 minutes; the scanner runs every 5 minutes and finds 3 candidates. How many alerts are sent when alerting on the condition, and how many when alerting on the transition?
Four scans see the condition (0, 5, 10, 15 min) → 4 × 3 = 12 alerts on the condition; one transition per candidate → 3 alerts on the transition.
Practical rules
- Deduplicate alerts by setup and state transition.
- Include explicit expiration and current price context.
- Re-run risk checks after approval delay.
Common mistakes
- Sending repeated alerts while a condition remains true.
- Using confidence without evidence or invalidation.
- Submitting the original quantity after a long approval delay.
References
Official reading
These sources cover algorithmic controls, market access, broker APIs, system operations, or security.
FINRA · Regulatory Notice 15-09
Official guidance on supervision, testing, controls, documentation, and monitoring for algorithmic trading strategies.
Alpaca · WebSocket Market Data
Official example of receiving real-time stock, option, crypto, and news updates over a streaming API.