A REST API lets software systems exchange data through defined web requests. A sales system can create a customer in accounting, an online store can reserve inventory, or a field application can retrieve assigned jobs. The API is the contract between systems: it defines available resources, required fields, authentication, responses, and errors.

An integration is more than making one successful request. Production work must handle duplicate events, partial failures, rate limits, expired credentials, schema changes, and conflicting records. Business owners do not need to write HTTP code, but understanding these concerns helps them evaluate scope and risk.

REST concepts in plain language

An endpoint is an address representing a resource, such as /customers/123 or /orders. The request method communicates intent: GET reads, POST creates, PUT or PATCH updates, and DELETE removes. The body commonly uses JSON. A response includes data and a status code: 200 indicates success, 201 creation, 400 invalid input, 401 missing or invalid authentication, 403 insufficient permission, 404 not found, 409 conflict, and 500 a server failure.

These conventions improve predictability, but every provider has its own details. The official documentation and a test environment are essential.

Start with ownership and source of truth

Before mapping fields, decide which system owns each record. The CRM may own prospect contact details, accounting may own tax status, and inventory may own available quantity. If two systems can independently change the same field, define conflict rules. “Newest update wins” can overwrite a carefully corrected record with stale data.

Create a mapping table for source field, destination field, data type, required status, allowed values, transformation, and owner. Seemingly simple differences matter: one system stores a full name, another separates given and family name; one stores dollars, another cents; one uses local timestamps, another UTC.

Authentication and permissions

APIs may use keys, OAuth access tokens, signed requests, or mutual TLS. Prefer short-lived credentials and scopes that grant only required operations. A reporting integration should not receive permission to delete customers. Store secrets outside source code, rotate them, and keep development credentials separate from production.

OAuth integrations also need a refresh strategy and a clear owner for reauthorization. An integration that silently stops when an employee changes a password is not operationally complete.

Polling, webhooks, and batch synchronization

Polling

The receiving system asks for changes on a schedule. It is straightforward but can create delay and unnecessary requests. Store a reliable cursor or last-success marker, and account for late changes rather than relying only on the current clock.

Webhooks

The source sends an event when something changes. Webhooks reduce delay, but the receiver must verify signatures, respond quickly, queue work, and tolerate repeated or out-of-order delivery. A webhook is a notification; the receiver may still retrieve the authoritative record through the API.

Batch files

For thousands of nightly records or a legacy platform, a versioned CSV through secure file transfer may be simpler and more dependable. “Modern” does not automatically mean better. Frequency, volume, and recovery requirements determine the design.

Retries without duplicate business actions

Networks fail. A client may time out after the server creates an order but before it receives the confirmation. Blindly retrying can create two orders. Use an idempotency key—a unique identifier that tells the server repeated attempts represent the same action—or query by an external reference before creating.

Retry temporary failures with increasing delays and a limit. Do not retry invalid data until it changes. Send exhausted failures to a visible queue with the request identifier, error, and recovery action.

Practical example: store and inventory

An online order should reserve stock, but inventory might be temporarily unavailable. A robust flow receives the paid-order event, validates its signature, records the event ID, and queues processing. It maps store SKUs to inventory SKUs, submits a reservation with the order number as an idempotency key, and records the response. If one SKU is unknown, the order enters an exception queue instead of decrementing only part of the stock without notice.

Cancellations and refunds need reverse flows. The design should define whether inventory is released immediately, after payment reversal, or after warehouse confirmation. Integration logic is business policy expressed in software.

Testing an integration

  • Use a sandbox with realistic but non-sensitive records.
  • Test missing fields, invalid values, duplicates, timeouts, rate limits, and expired credentials.
  • Reconcile totals between systems after bulk tests.
  • Verify logs do not expose tokens or unnecessary personal data.
  • Run a limited production pilot and compare source and destination records.
  • Document replay, rollback, and manual correction procedures.

Monitoring and change management

Track request volume, success rate, latency, rate-limit responses, queue age, and unresolved failures. Add business reconciliation: API success does not prove every invoice has the correct total. Subscribe to provider change notices and pin API versions when supported. Assign an owner to review alerts and upcoming deprecations.

A well-designed integration makes data movement observable and recoverable. KarasTechs builds REST API integrations and connected business applications for inventory, operations, customer systems, and automation workflows.