Single source of truth

A single source of truth means that each piece of information, such as a client’s tax ID, is edited in one place, and other systems take it from there.

Also called: SSOT, single point of truth

Filip Kostecki

Published 14 August 2026

When a client’s tax ID, address or case deadline is recorded in 3 places, sooner or later one copy falls out of step, and the error shows up on an invoice, in a letter or in a call with the client. The principle settles in advance where each piece of information gets corrected, so nobody has to guess which version is current.

It matters most for the details of clients, suppliers and employees, and for your list of services, because they end up in almost every invoice, contract, letter and deadline reminder.

How does it work?

Inventory. List where client data, deadlines and documents are kept today and who changes them: accounting software, spreadsheets, mailboxes, calendars, the case system.
One place to edit. For each piece of information, name one place where it is entered and corrected: client details in the client list, deadlines in the case register. Invoices come from one system, which takes the buyer’s details from that client list.
Connecting the rest. The other systems fetch data from that place or receive a read-only copy that is updated automatically, for example by a webhook when a record changes. If a copy sometimes has to be edited, decide which change wins when two of them conflict.
Clean-up and ownership. Merge duplicates into one record and give each type of data an owner who approves changes and makes sure nothing is missing.

Example from practice

At a service firm we work with, more than half the client records hung off a single account, and in 8 out of 10 records the basic details were missing. In that situation the clean-up starts with the records: first one place to edit them and complete basic details, then the documents created from them.

At another service firm, a 477-cell official form fills itself from data given once, and the documents that follow take the same details from that one record. Both projects are described in our case studies.

When does it make sense, and when not?

The principle makes sense when:

the same client details are typed into several systems or spreadsheets;
documents are generated from data: invoices, contracts, letters, official forms;
several people change the details of the same clients;
the firm connects systems, for example accounting software with an e-invoicing platform, or a case register with a client portal.

For a one-person business with one system and no integrations, a rule such as ‘this information is corrected only here’ will do. Nor do you need one system that does everything. Decide which of your current systems has the final say on which information. When we design web systems, we settle this at the start, because a new system without that decision becomes one more copy of the data.

Tell us in the form which systems hold your client details today. We usually reply within 1–2 working days with a suggestion of where to start.

Describe your systems

What to watch out for

A mistake on an invoice is hard to undo. In KSeF, the Polish national e-invoicing system, an invoice issued to the wrong buyer tax ID cannot be cancelled. According to the Ministry of Finance manual (in Polish), you issue a corrective invoice that brings it to zero and a new invoice with the correct details, and the wrong one is assigned to whoever holds that tax ID. One verified client record makes such a mistake less likely.

Every copy is one more place to correct. When a client asks you to correct their details, you have to do it everywhere they are stored.

An error in the source spreads everywhere. If a client record holds a wrong address, it will appear in every document created from it. Before moving the data, merge duplicates and fill gaps. Merging duplicates alone can take more effort than the move itself.

A single point of failure. When data lives in one place, losing it stops all work at once. Back the data up from day one, and check every so often that you can restore it from the backup.

The cost of connections. Every connection between systems has to be built and maintained, and checked again after one of the systems changes.

Working copies. An export to a spreadsheet, an email attachment or a file on someone’s desktop soon goes out of date, because nobody updates it along with the source.

Questions and answers

Does a single source of truth mean one system for everything?

It does not have to. The principle concerns where data is edited: accounting, the case system and email can run separately, as long as the client’s tax ID is corrected in only one of them and the others fetch it automatically or keep a read-only copy.

Where do you start tidying up client data?

With an inventory: where client data lives today and who changes it. Then choose one place to edit each piece of information, merge duplicates and name the person who approves changes. Finally, connect the other systems so that they read the data from that place.