Very few business applications live alone. Your billing system needs to take payments, your CRM needs to send emails, your order system needs to tell the courier what to collect, and your accountant wants everything in the accounting package. Connecting these systems is called API integration, and done well it removes hours of copying data between screens. Done badly, it creates silent errors that surface weeks later. This guide explains how integrations work and what separates robust ones from fragile ones.
What an API is
An API (application programming interface) is a defined way for one program to ask another for data or to perform an action. Think of it as a service counter: you make a request in a set format, and you get a response in a set format. You do not need to know how the other system works internally.
Most modern web APIs follow a style called REST. Your application sends an HTTP request to a URL (an endpoint) and receives data back, usually as JSON, a lightweight text format for structured data. A request to create a customer in another system might look like this:
POST https://api.example.com/v1/customers
Authorization: Bearer sk_live_xxxxxxxx
Content-Type: application/json
{"name": "Acme Traders", "email": "accounts@acme.example"}The response would include the new customer's ID in the other system, which your application stores so the two records stay linked.
Common API integration patterns
Request and response
Your application calls the other system when it needs something: fetch an exchange rate, create an invoice, check stock. Simple and predictable, but your application has to ask each time.
Webhooks
A webhook reverses the direction. The other system calls a URL on your application when something happens: a payment succeeded, a parcel was delivered, a form was submitted. Webhooks avoid constant polling and deliver near real-time updates. Payment gateways rely heavily on them, because a payment may be confirmed some time after the customer leaves your checkout page.
Scheduled sync
Some systems, especially older ones, are best integrated with a periodic batch job: every night, export new orders and import them into the warehouse system. Less immediate, but often the most reliable option for legacy software.
Middleware and integration platforms
When many systems must talk to each other, a central integration layer (your own service or a commercial integration platform) can route and transform data so each system connects once rather than to every other system.
Authentication: proving who you are
- API keys. A secret string sent with each request. Simple, but the key must be stored securely on the server, never in browser code or a mobile app where users could extract it.
- Basic authentication. A username and password sent in a header, encoded with Base64. Encoding is not encryption; anyone who sees the header can decode it, as you can try with our Base64 encoder and decoder. It is only acceptable over HTTPS.
- OAuth 2.0. A standard that lets a user grant your application limited access to their account on another service without sharing their password. Your application receives tokens that expire and must be refreshed, which needs careful handling.
- Webhook signatures. Incoming webhooks should be verified, usually by checking a signature header calculated with a shared secret, so attackers cannot fake a "payment succeeded" message.
Building integrations that fail safely
Every external system will at some point be slow, unavailable or return something unexpected. Robust integrations plan for this:
- Timeouts. Never wait indefinitely for a response. Set sensible limits so one slow partner does not freeze your application.
- Background queues. Make non-urgent calls from a background job rather than while the user waits. If the call fails, the job can retry.
- Retries with backoff. Retry failed requests after increasing delays, rather than hammering a struggling service.
- Idempotency. An operation is idempotent if performing it twice has the same effect as once. Many payment and order APIs accept an idempotency key so a retried request does not create a duplicate charge or order. Your own webhook handlers should also ignore events they have already processed, because providers may deliver the same event more than once.
- Rate limits. Most APIs limit how many requests you can make per minute. Respect the limits and read the response headers that report them.
- Logging and alerts. Record each request and response (without secrets or sensitive personal data) and alert someone when failures persist. Silent failures are the most expensive kind.
- Reconciliation. For money and stock, run periodic checks comparing both systems, and flag mismatches for a human.
Data mapping: the hidden work
The technical connection is often the easy part. The harder part is agreeing how concepts map between systems. Is a "customer" in your CRM the same as a "contact" or an "account" in the accounting package? How are tax codes, currencies, units and statuses represented? What happens when a record is deleted in one system but referenced in another? Write these mappings down and have someone from the business confirm them.
Practical checklist before you start
- Read the provider's official API documentation and check that the operations you need actually exist.
- Confirm there is a sandbox (test) environment, so development does not touch real money or customers.
- Check pricing: some APIs charge per call or require a higher subscription tier.
- Note the API version and the provider's deprecation policy, so you are not surprised by breaking changes.
- Store credentials in environment configuration or a secrets manager, not in source code.
- Decide who owns monitoring once the integration is live.
Exposing your own API
Sometimes the integration runs the other way: partners, a mobile app or your own other systems need to access your application. Then you need to design endpoints, authentication, versioning, rate limiting and documentation yourself. Treat your API as a product with its own users. Our web application development team builds both sides of these connections, and a quick inspection of response headers with the HTTP header checker is a useful first step when diagnosing integration problems.
Key takeaways
- API integration connects systems through defined requests and responses, webhooks or scheduled syncs.
- Keep credentials on the server, verify webhook signatures and always use HTTPS.
- Design for failure: timeouts, queues, retries, idempotency, logging and reconciliation.
- Agree data mappings with the business early; they cause more problems than the code.