What Is an API Integration?

An API integration connects two software systems so they exchange data automatically: your app calls another service's API (Application Programming Interface) to read or write data instead of rebuilding that functionality yourself. The other service handles the hard part — processing payments, sending email, syncing calendars — and your app just makes requests and reacts to the responses.

How an API integration works

Most integrations today use REST over HTTPS. A single round trip looks like this:

1. Your app sends an HTTP request — GET to read, POST to create, PUT/PATCH to update, DELETE to remove — usually with an API key or token in the headers.
2. The external service authenticates the request, does the work, and returns a response (typically JSON) with a status code: 200 OK, 201 Created, 400 bad request, 401 unauthorized, 429 rate-limited, 500 server error.
3. Your app parses the response, updates its own database or UI, and handles any error path.

Polling vs. webhooks

There are two directions data can flow, and real integrations often use both:

A webhook is just an HTTP endpoint you expose so the external service can push updates to you the moment something changes — no constant polling required.

Common integrations in a SaaS build

• Stripe — payments, subscriptions, invoicing, refunds
• Resend / SendGrid — transactional email (receipts, password resets)
• Twilio — SMS and WhatsApp notifications
• Google Calendar / iCal — two-way calendar sync
• Supabase / Firebase — database, auth, and storage as an API

In practice these stack up. BookBed, a booking SaaS, combines Stripe for payments, Resend for email, and bidirectional iCal sync so a room booked on one channel instantly blocks the same dates everywhere — that calendar piece is several integrations cooperating, not one.

When to integrate vs. build it yourself

Integrate when the capability is solved, regulated, or risky to own: payments (PCI compliance), email deliverability, SMS carrier routing, mapping, identity. Reinventing these is slow and dangerous.

Build in-house when the logic is core to your product and differentiates it, when an off-the-shelf API can't model your data, or when per-call pricing would dominate your costs at scale. A booking engine's availability rules are usually worth owning; the payment rail almost never is.

Common mistakes

• No retry or idempotency. Networks fail mid-call. Without idempotency keys, a retried POST can charge a customer twice.
• Trusting webhooks blindly. Anyone can hit a public URL — verify the signature (e.g. Stripe's Stripe-Signature header) before acting on the payload.
• Ignoring rate limits. A 429 means back off and retry with exponential delay, not hammer the API harder.
• Leaking keys client-side. Secret API keys belong on the server. A key in front-end JavaScript is a key anyone can copy.
• Underscoping the work. The first successful call is the easy 20%. Error handling, retries, webhook verification, and edge cases are the other 80%.

What integrations cost in a build

Each non-trivial integration (Stripe, email, calendar sync) typically adds 1–3 days to an MVP scope — sometimes more once webhook handlers, retry logic, and failure states are accounted for. Simple read-only calls are quicker; bidirectional sync and money movement are the expensive end. Exact effort depends on the service and your data model, so for a specific build, contact for a quote.

Key takeaways

• An API integration lets your app borrow another service's functionality over HTTP instead of rebuilding it.
• Two flows matter: your app requesting data (polling) and the service pushing events to you (webhooks).
• The connection is fast; the reliability — auth, retries, idempotency, signature checks — is where the real engineering time goes.
• Integrate for solved/regulated capabilities (payments, email); build in-house for what makes your product unique.