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API Integration Checklist: How to Integrate Systems Without Breaking Data

Businesses integrate systems to save time—ERP with e-commerce, CRM with support, accounting with sales, dashboards with everything.
But integrations can also create the worst problems: duplicate customers, missing invoices, wrong stock, broken reports, and “we don’t know which system is correct.”

This guide is a practical checklist to integrate systems safely—whether you’re using:

  • WordPress/WooCommerce + ERP

  • CRM + WhatsApp/Email tools

  • POS + Accounting

  • Custom systems + third-party APIs

The goal is simple: no duplicates, no missing records, and full traceability.

The 5-Step Safe Integration Framework

  1. Define the data contract (what is sent, how it’s formatted, who owns each field)

  2. Map + validate data (reject bad payloads early)

  3. Deliver reliably (retries + idempotency + queues)

  4. Observe everything (logs, metrics, trace IDs)

  5. Reconcile + recover (backfill, replays, audits)

1) Define the Data Contract (before coding)

A strong contract prevents 80% of integration bugs.

Contract checklist

  • Schema + example payloads for each endpoint/event

  • Required vs optional fields (and defaults)

  • Data types (string vs number, date formats, currency decimals)

  • IDs strategy

    • Internal ID (system A)

    • Internal ID (system B)

    • External ID (stable cross-system key)

  • Ownership

    • Which system is “source of truth” for price, status, customer details, etc.

  • Versioning

    • /v1, /v2 or schema version headers

  • Error codes + retry rules

    • What’s retryable vs not retryable

  • Rate limits + pagination (important for backfills)

Business rule: If you don’t define ownership, systems will “fight” and data will drift.

2) Map + Validate Data (stop corruption at the boundary)

Mapping is not just “rename fields.” It’s business logic.

Mapping rules you must document

  • Date/time formats and time zones

  • Currency, rounding, and unit conversions

  • Enum mapping (status codes)

  • Null/empty handling

  • Dedup rules (unique keys)

  • Upsert vs create-only

  • PII handling (masking, encryption, allowed fields)

Validation checklist

  • Validate schema (types + required fields)

  • Validate business rules

    • quantity ≥ 0

    • total = sum(items) − discounts + tax

  • Reject early with clear errors

  • Store the rejected payload (for debugging)

Tip: Validation should happen before data reaches your database.

3) Deliver Reliably (assume the network will fail)

Most “broken data” happens because integrations fail halfway:

  • a request times out but actually succeeded

  • the sender retries and creates duplicates

  • a system is down and data is lost

Reliability checklist

✅ Idempotency

  • Use an Idempotency-Key per request, or

  • Use upserts with a stable external ID

  • Prevent duplicates on retries

✅ Retries + backoff

  • Retry only transient errors (timeouts, 5xx, network)

  • Use exponential backoff + jitter

  • Respect rate limits (429)

✅ Queue where needed
If data must never be lost, add a queue:

  • integration consumes messages

  • failed messages go to DLQ (dead-letter queue)

✅ Exactly-once mindset
Exactly-once delivery is hard.
Instead, build “at-least-once delivery” with idempotent processing.

4) Observability (so you can prove what happened)

If you can’t answer “where is this order?” your integration will cause support tickets.

Observability checklist

  • Log every transaction with:

    • timestamp

    • source system

    • target system

    • endpoint/event name

    • status (success/fail)

    • error message (if any)

  • Add a Trace ID / Correlation ID

  • Track metrics:

    • success rate

    • failure rate

    • latency p95/p99

    • retry count

    • DLQ size

Tip: Trace IDs turn “it disappeared” into a fast investigation.

5) Reconciliation + Recovery (the “safety net”)

Even good integrations need reconciliation, especially when:

  • doing bulk imports/backfills

  • syncing orders, invoices, inventory

  • systems are updated

Reconciliation checklist (daily/weekly)

  • Record counts (source vs target)

  • Totals checksum (total revenue, quantities)

  • Missing IDs report

  • Duplicate detection report

  • Backfill process (replay window)

  • Audit trail by date range

  • SLA report (uptime + success %)

Recovery checklist

  • Replay failed messages from DLQ

  • Re-run backfill safely (idempotent)

  • Document a runbook: “how to replay orders safely”

Common Mistakes That Break Data

  • No external IDs → duplicates everywhere

  • No idempotency → retries create double records

  • Direct system-to-system calls → no control, no logs

  • No validation → corrupted data enters DB

  • No reconciliation → issues discovered too late

  • No staging/testing → production becomes test environment

Quick API Integration Checklist (printable)

  • ✅ Contract: schema, IDs, ownership, versioning

  • ✅ Mapping: formats, enums, rounding, null rules

  • ✅ Validation: schema + business rules + reject bad payloads

  • ✅ Reliability: idempotency + retries + queue/DLQ

  • ✅ Observability: logs + metrics + trace IDs

  • ✅ Reconciliation: counts + totals + missing/duplicates + backfill

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