Source-aware field guide · 20 answers
Trading APIs and Market Data
Technical references for FIX, REST, WebSocket, message lifecycles, authentication, rate limits, testing and production monitoring.
Published · reviewed for scope, source visibility and answer ownership
How to use this guide
Technical references for FIX, REST, WebSocket, message lifecycles, authentication, rate limits, testing and production monitoring. The guide is written for broker developers, quant engineers, integration teams and technical product owners. The collection explains integration contracts and operational evidence. It does not claim protocol support for an endpoint that has not been demonstrated.
Use every answer to resolve one operational question: Which interface, message, identifier and state transition is authoritative for each trading or data workflow? Protocol standards allow implementation choices. Counterparty rules, message dictionaries, certificates and tested behavior define the real integration.
The supporting set is FIX Trading Community — Standards; FIXimate — FIX message reference; OpenAPI Initiative — Specification. These links provide standards, regulator material or official product documentation for the subject; they do not imply endorsement, worldwide applicability or a verified feature in a particular deployment. Check the current source, contract, configuration and qualified local advice before a production, trading or compliance decision.
What Is the FIX Protocol?
FIX is a family of open messaging standards used for electronic trading communications, with agreed session, application and counterparty-specific rules.
- Use it to
- Select a version, extension set, data dictionary and bilateral rules before coding.
- Check the boundary
- Using FIX tags does not guarantee compatibility without certification.
REST API vs FIX API
REST commonly exposes resource-oriented request-response operations over HTTP, while FIX provides session-oriented trading messages with established workflows and state semantics.
- Use it to
- Choose by workflow, latency, state, recovery, security and counterparty support.
- Check the boundary
- Protocol labels do not determine performance or feature scope by themselves.
WebSocket vs REST for Market Data
WebSocket can maintain a bidirectional connection for streaming updates, while REST commonly retrieves snapshots or submits discrete requests.
- Use it to
- Design snapshot, subscription, sequencing, reconnect and backfill together.
- Check the boundary
- Receiving a live stream without recovery can leave silent gaps.
What Is a FIX Session?
A FIX session is a stateful communication relationship using CompIDs, sequence numbers, logon settings, heartbeats, resend behavior and agreed schedules.
- Use it to
- Certify startup, steady state, disconnect, restart and recovery under the counterparty rules.
- Check the boundary
- TCP connectivity alone does not establish a valid FIX session.
What Is a FIX CompID?
A FIX CompID identifies the sending or receiving party at the session level in configured message headers and counterparty agreements.
- Use it to
- Protect credentials and map CompIDs to environment, session and permissions.
- Check the boundary
- Reusing identifiers across test and production can route or reject messages incorrectly.
How Do FIX Sequence Numbers Work?
FIX sequence numbers provide ordered message accounting within a session so missing, repeated or replayed traffic can be detected and handled.
- Use it to
- Record reset policy, persisted state and resend or gap-fill behavior on both sides.
- Check the boundary
- Resetting numbers to solve a mismatch can hide unprocessed application messages.
How Do FIX Heartbeats Work?
FIX heartbeats and test requests monitor session liveness according to the negotiated interval and current traffic.
- Use it to
- Test delayed responses, blocked application processing and disconnect thresholds.
- Check the boundary
- Heartbeat success does not prove downstream order or market-data systems are healthy.
Which FIX Messages Submit Orders?
FIX order workflows commonly use messages such as NewOrderSingle, OrderCancelRequest and OrderCancelReplaceRequest with required fields and bilateral restrictions.
- Use it to
- Map every supported order instruction to the certified dictionary and response states.
- Check the boundary
- Standard message names do not mean every order type or field is accepted.
How Do FIX Execution Reports Work?
Execution reports communicate order and fill state through identifiers, execution types, order statuses, quantities, prices and rejection details under the implementation rules.
- Use it to
- Build an order state machine and deduplicate by stable message identifiers.
- Check the boundary
- Messages can be repeated or corrected and should not be booked solely by arrival count.
How Does FIX Market Data Work?
FIX market-data workflows request snapshots or subscriptions and deliver entries representing quotes, trades or book changes according to the counterparty's model.
- Use it to
- Define instruments, depth, entry types, update action, sequence and recovery.
- Check the boundary
- Incremental updates without a valid starting snapshot can create a corrupt book.
How Should API Rejects Be Classified?
API rejects should be classified by authentication, permission, validation, business rule, rate, market state, provider and temporary-system cause with retained raw responses.
- Use it to
- Map each class to a safe retry or human action.
- Check the boundary
- Retrying every error can duplicate activity or amplify an outage.
What Is a Drop Copy?
A drop-copy feed provides a secondary stream of order or trade events for monitoring, risk or reconciliation under defined scope and timing.
- Use it to
- Reconcile it with the primary path and document omissions and corrections.
- Check the boundary
- Drop copy is not necessarily an authoritative settlement ledger.
What Is Trading Reference Data?
Reference data describes instruments, identifiers, currencies, sessions, contract terms and status needed to interpret orders and market data.
- Use it to
- Version it and apply changes atomically across connected systems.
- Check the boundary
- Stale contract or tick information can corrupt validation and valuation.
What Is Tick Data?
Tick data records individual quote or trade updates with timestamps and fields defined by the feed, venue and storage process.
- Use it to
- Preserve provenance, ordering, corrections, precision and gap indicators.
- Check the boundary
- Tick does not always mean an executed trade and feeds can be sampled or aggregated.
How Are OHLC Bars Built?
OHLC bars summarize the open, high, low and close from selected observations within a defined interval, timezone and inclusion rule.
- Use it to
- Rebuild sample bars from raw input and test boundary timestamps.
- Check the boundary
- Quote bars, trade bars and midpoint bars can differ legitimately.
How Should Corporate Actions Be Applied?
Corporate actions such as splits, dividends and symbol changes can alter price series, quantities and identifiers under exchange and data-vendor rules.
- Use it to
- Store raw and adjusted series with event provenance and effective times.
- Check the boundary
- Mixing adjusted history with unadjusted live data creates false jumps and signals.
How to Secure Trading API Authentication
Trading API authentication verifies a client through keys, signatures, tokens, certificates or sessions and must be paired with authorization and credential lifecycle controls.
- Use it to
- Test issuance, least privilege, expiry, rotation, revocation and replay resistance.
- Check the boundary
- Keeping a secret in client-side code or logs defeats strong protocol design.
How to Handle API Rate Limits
Rate limits constrain requests, messages, subscriptions or work over a defined key, account and time window with specified responses.
- Use it to
- Budget normal and burst traffic and implement backoff without losing state.
- Check the boundary
- Parallel retries can worsen throttling and reorder dependent operations.
What Is an API Sandbox?
An API sandbox is a non-production environment for learning and testing supported workflows with simulated or isolated data and credentials.
- Use it to
- Document which production behaviors, limits, instruments and providers are not represented.
- Check the boundary
- Sandbox success does not certify production permissions or execution.
How to Monitor a Trading API
Trading API monitoring observes availability, authentication, latency, errors, state divergence, message gaps and business outcomes across the full integration.
- Use it to
- Combine synthetic checks, telemetry and reconciliation with actionable ownership.
- Check the boundary
- An HTTP success rate can remain green while orders or balances are wrong.
Primary and official references
These sources establish definitions, standards or official product behavior used across this guide. Follow the exact source and check its current version before a live implementation.