Why double entry happens
Most businesses grow one tool at a time: accounting here, a CRM there, a spreadsheet in between. Each tool is useful on its own, but nothing connects them. People fill the gaps by copying and pasting, which is slow and introduces mistakes.
The cost is rarely visible on a single invoice. It shows up as delayed reports, customers with two different addresses, and managers who do not trust the numbers.
Step 1: decide which system owns which data
Before writing any integration, decide the system of record for each type of data. Customer contact details may belong to the CRM, while invoices and stock belong to the ERP. Integration then becomes a clear rule: data flows from the owner to the others.
Without this decision, two systems can overwrite each other and you end up with conflicts nobody can resolve.
Step 2: pick the right integration style
Real-time APIs suit events that need an immediate response, such as checking credit when an order is placed. Scheduled syncs suit bulk updates, such as nightly stock levels. Webhooks suit notifications, such as telling the CRM a payment has arrived.
Many projects use a mix. The goal is to match the style to how quickly the business actually needs the data.
Step 3: plan for failure
Networks fail and records are sometimes invalid. A reliable integration retries safely, logs every exchange and alerts a person when something cannot be fixed automatically. It should also be possible to replay a failed message without creating duplicates.
Step 4: start small and measure
Begin with the flow that wastes the most time, usually new customers or orders. Measure how long the manual process took before, then confirm the integration removes it. Add further flows once the first one is stable.