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    <p>First, I would have the independent invoicing systems in example.com, example2.net and anotherexample.com all have their own internal primary keys for the invoices generated from within each of these systems. Each system should have its own independent copy of the invoicing logic because you don't want an outage on one server knocking out invoicing on every server.</p> <p>Whenever you have a distributed system where local copies are creating records for something that will be amalgamated later, it's a good idea to use a GUID as the local primary key, or if you have a philosophical objection to GUID as PKs, create a GUID as a <em>candidate</em> key. This way, you can bring together invoices from all of your systems (and any future ones) without worrying about key collisions and you'll be able to track the combined records back to the source records, should you ever have to do that.</p> <p>Next, you'll need an integrated invoice system where all of the invoice details are collected periodically. To facilitate this, you need processes on each local invoicing system pumping their own records up to the centralized system. Keep a flag on the invoices in the local systems to identify which invoices have been successfully uploaded - or if you have very high volumes, use a work-list table containing the invoice keys that still need transmitting instead of a flag right on the local invoice table.</p> <p>The centralized invoice system will also want to have a source code on the combined invoice table so that you can easily tell which website created the invoice originally.</p> <p>As to invoice numbers, I'm assuming from your question that the customer is a bit fussy about having proper sequencing of your invoice numbers. You can have the centralized system generate these numbers for you using a web service to pick up the next invoice ID. If the centralized service is down for any reason, you can still give the customer an "order reference" i.e. the GUID and just hold back on the invoice number until it can be generated through the central server. This should satisfy your customer's need for tight sequential invoice numbers while preserving the ability to operate multiple sites on multiple servers.</p> <p>If your customer actually doesn't care about tight sequencing of the invoice numbers, then another alternative is to have the central system generate blocks of reserved invoice numbers and allot them to each website. When the website is getting low on its allotment, it asks the central server for another block. This gives you breathing room in the sequences in case there are communication difficulties.</p>
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    1. USJoel Brown
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    1. COThanks for amazing answer. The main issue is that I feel that I cannot have independent invoicing system for every single application or I don't have any idea how to solve the main issue: invoice numbering. It's not about customers. It's about tax office in my country. Basically, a company needs to have a strict invoices numbering, starting from 1 every year. This is the biggest and the only problem I ran into while thinking about the system.
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    2. COWhen I say have an independent invoicing system for each application, I mean code one invoicing system and copy it into each application so that each website will operate on its own server without having to depend on other servers. Then you can run your master invoice number generating web service / master invoice list from any one of your servers - or a different one altogether.
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    3. COThat's why I'm thinking about too. Independent invoicing system in all applications. Just one table merging them - by giving invoicing numbers.
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