Seven Legacy Integration Patterns

If we have to integrate two (or more) systems nowadays, we know – we either use an API or, more rarely, some message queue.

Unfortunately, many systems in the world do not support API integration. And many more a being created as we speak, that don’t have APIs. So when you inevitably have to integrate with them, you are left with imperfect choices to make. Below are seven patterns to integrate with legacy systems (or not-so-legacy systems that are built in legacy ways).

Initially I wanted to use “bad integration patterns”. But if you don’t have other options, they are not bad – they are inevitable. What’s bad is that fact that so many systems continue to be built without integration in mind.

I’ve seen all of these, on more than one occasion. And I’ve heard many more stories about them. Unfortunately, they are not the exception (fortunately, they are also not the rule, at least not anymore).

  1. Files on FTP – one application uploads files (XML, CSV, other) to an FTP (or other shared resources) and the other ones reads them via a scheduled job, parses them and optionally spits a response – either in the same FTP, or via email. Sharing files like that is certainly not ideal in terms of integration – you don’t get real-time status of your request, and other aspects are trickier to get right – versioning, high availability, authentication, security, traceability (audit trail).
  2. Shared database – two applications sharing the same database may sound like a recipe for disaster, but it’s not uncommon to see it in the wild. If you are lucky, one application will be read-only. But breaking changes to the database structure and security concerns are major issues. You can only use this type of integration is you expose your database directly to another application, which you normally don’t want to do.
  3. Full daily dump – instead of sharing an active database, some organizations do a full dump of their data every day or week and provide to to the other party for import. Obvious data privacy issues exist with that, as it’s a bad idea to have full dumps of your data flying around (in some cases on DVDs or portable HDDs), in addition to everything mention below – versioning, authentication, etc.
  4. Scraping – when an app has no API, it’s still possible to extract data from it or push data to it – via the user interface. With web applications that’s easier, as they “speak” HTML and HTTP. With desktop apps, screen scraping has emerged as an option. The so-called RPA software (Robotic process automation) relies on all types of scraping to integrate legacy systems. It’s very fragile and requires complicated (and sometimes expensive) tooling to get right. Not to mention the security aspect, which requires storing credentials in non-hashed form somewhere in order to let the scraper login.
  5. Email – when the sending or receiving system don’t support other forms of integration, email comes as a last resort. If you can trigger something by connecting a mailbox or if an email is produced after some event happens in the sending system, this may be all you need to integrate. Obviously, email is a very bad means of integration – it’s unstructured, it can fail for many reasons, and it’s just not meant for software integration. You can attach structured data, if you want to get extra inventive, but if you can get both ends to support the same format, you can probably get them extended to support proper APIs.
  6. Adapters – you can develop a custom module that has access to the underlying database, but exposes a proper API. That’s an almost acceptable solution, as you can have a properly written (sort-of) microservice independent of the original application and other system won’t know they are integrating with a legacy piece of software. It’s tricky to get it right in some cases, however, as you have to understand well the state space of the database. Read-only is easy, writing is much harder or next to impossible.
  7. Paper – no, I’m not making this up. There are cases where one organizations prints some data and then the other organization (or department) receives the paper documents (by mail or otherwise) and inputs them in their system. Expensive projects exist out there that aim to remove the paper component and introduce actual integration, as paper-based input is error-prone and slow. The few legitimate scenarios for a paper-based step is when you need an extra security and the paper trail, combined with the fact that paper is effectively airgapped, may give you that. But even then it shouldn’t be the only transport layer.

If you need to do any of the above, it’s usually because at least one of the system is stuck and can’t be upgraded. It’s either too legacy to touch, or the vendor is gone, or adding an API is “not on their roadmap” and would be too expensive.

If you are building a system, always provide an API. Some other system will have to integrate with it, sooner or later. It’s not sustainable to build close systems and delay the integration question for when it’s needed. Assume it’s always needed.

Fancy ESBs may be able to patch things quickly with one of the approaches above and integrate the “unintegratable”, but heavy reliance on an ESB is an indicator of too many legacy or low-quality systems. And this should serve as an indicator that some systems needs replacing. This is much easier said than done, as migration is too often a huge headache, but count the lack of APIs in the list of reasons towards replacing some system (if it can’t be upgraded and and you are in a position to do so).

But simply having an API doesn’t cut it either. If you don’t support versioning and backward-compatible APIs, you’ll be in an even more fragile state, as you’ll be breaking existing integrations as you progress.

Enterprise integration is tricky. But, as with many things in software, it’s best handled in the applications that we build. If we build them right, things are much easier. Otherwise, organizations have to revert to the legacy approaches mentioned above and introduce complexity, fragility, security and privacy risks and a general feeling of low-quality that has to be supported by increasingly unhappy people.

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