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Define the data contract
A market data API exposes agreed pricing or instrument information to consuming applications. Specify whether consumers need current quotes, a stream of updates, historical observations or instrument metadata. Each use case has different expectations for freshness, ordering, persistence and recovery. The interface should communicate those expectations explicitly.
Fields and units must be unambiguous
Document instrument identifiers, bid and ask fields, timestamp units, timezone, precision and any sequence identifiers. Indicate whether a timestamp describes the source event or the point at which your system received it. Consumers should not have to infer these details from a few example payloads.
Snapshot and streaming behaviour
A new consumer may need a snapshot before processing updates. Define how it detects gaps, reconnects and recovers after downtime. If polling is used, agree the request limits and caching behaviour. If streaming is used, determine how a slow client is isolated and which update guarantees the service can actually provide.
Access, distribution and entitlements
Identify the data sources and the permitted use of their content. An API endpoint does not automatically grant redistribution rights. Authentication, consumer permissions and distribution rules should be agreed for the project, including whether data may be displayed publicly or used only within selected operational systems.
Test the consumer integration
Use examples for missing fields, invalid symbols, unavailable instruments, out-of-order events and disconnected clients. Confirm that stale values can be distinguished from current ones. Request an interface assessment with your consumer type, instrument list, required delivery pattern and relevant source arrangements.