GDPR – A Practical Guide For Developers

You’ve probably heard about GDPR. The new European data protection regulation that applies practically to everyone. Especially if you are working in a big company, it’s most likely that there’s already a process for getting your systems in compliance with the regulation.

The regulation is basically a law that must be followed in all European countries (but also applies to non-EU companies that have users in the EU). In this particular case, it applies to companies that are not registered in Europe, but are having European customers. So that’s most companies. I will not go into yet another “12 facts about GDPR” or “7 myths about GDPR” posts/whitepapers, as they are often aimed at managers or legal people. Instead, I’ll focus on what GDPR means for developers.

Why am I qualified to do that? A few reasons – I was advisor to the deputy prime minister of a EU country, and because of that I’ve been both exposed and myself wrote some legislation. I’m familiar with the “legalese” and how the regulatory framework operates in general. I’m also a privacy advocate and I’ve been writing about GDPR-related stuff in the past, i.e. “before it was cool” (protecting sensitive data, the right to be forgotten). And finally, I’m currently working on a project that (among other things) aims to help with covering some GDPR aspects.

I’ll try to be a bit more comprehensive this time and cover as many aspects of the regulation that concern developers as I can. And while developers will mostly be concerned about how the systems they are working on have to change, it’s not unlikely that a less informed manager storms in in late spring, realizing GDPR is going to be in force tomorrow, asking “what should we do to get our system/website compliant”.

The rights of the user/client (referred to as “data subject” in the regulation) that I think are relevant for developers are: the right to erasure (the right to be forgotten/deleted from the system), right to restriction of processing (you still keep the data, but mark it as “restricted” and don’t touch it without further consent by the user), the right to data portability (the ability to export one’s data in a machine-readable format), the right to rectification (the ability to get personal data fixed), the right to be informed (getting human-readable information, rather than long terms and conditions), the right of access (the user should be able to see all the data you have about them).

Additionally, the relevant basic principles are: data minimization (one should not collect more data than necessary), integrity and confidentiality (all security measures to protect data that you can think of + measures to guarantee that the data has not been inappropriately modified).

Even further, the regulation requires certain processes to be in place within an organization (of more than 250 employees or if a significant amount of data is processed), and those include keeping a record of all types of processing activities carried out, including transfers to processors (3rd parties), which includes cloud service providers. None of the other requirements of the regulation have an exception depending on the organization size, so “I’m small, GDPR does not concern me” is a myth.

It is important to know what “personal data” is. Basically, it’s every piece of data that can be used to uniquely identify a person or data that is about an already identified person. It’s data that the user has explicitly provided, but also data that you have collected about them from either 3rd parties or based on their activities on the site (what they’ve been looking at, what they’ve purchased, etc.)

Having said that, I’ll list a number of features that will have to be implemented and some hints on how to do that, followed by some do’s and don’t’s.

  • “Forget me” – you should have a method that takes a userId and deletes all personal data about that user (in case they have been collected on the basis of consent, and not due to contract enforcement or legal obligation). It is actually useful for integration tests to have that feature (to cleanup after the test), but it may be hard to implement depending on the data model. In a regular data model, deleting a record may be easy, but some foreign keys may be violated. That means you have two options – either make sure you allow nullable foreign keys (for example an order usually has a reference to the user that made it, but when the user requests his data be deleted, you can set the userId to null), or make sure you delete all related data (e.g. via cascades). This may not be desirable, e.g. if the order is used to track available quantities or for accounting purposes. It’s a bit trickier for event-sourcing data models, or in extreme cases, ones that include some sort of blockchain/hash chain/tamper-evident data structure. With event sourcing you should be able to remove a past event and re-generate intermediate snapshots. For blockchain-like structures – be careful what you put in there and avoid putting personal data of users. There is an option to use a chameleon hash function, but that’s suboptimal. Overall, you must constantly think of how you can delete the personal data. And “our data model doesn’t allow it” isn’t an excuse. What about backups? Ideally, you should keep a separate table of forgotten user IDs, so that each time you restore a backup, you re-forget the forgotten users. This means the table should be in a separate database or have a separate backup/restore process.
  • Notify 3rd parties for erasure – deleting things from your system may be one thing, but you are also obligated to inform all third parties that you have pushed that data to. So if you have sent personal data to, say, Salesforce, Hubspot, twitter, or any cloud service provider, you should call an API of theirs that allows for the deletion of personal data. If you are such a provider, obviously, your “forget me” endpoint should be exposed. Calling the 3rd party APIs to remove data is not the full story, though. You also have to make sure the information does not appear in search results. Now, that’s tricky, as Google doesn’t have an API for removal, only a manual process. Fortunately, it’s only about public profile pages that are crawlable by Google (and other search engines, okay…), but you still have to take measures. Ideally, you should make the personal data page return a 404 HTTP status, so that it can be removed.
  • Restrict processing – in your admin panel where there’s a list of users, there should be a button “restrict processing”. The user settings page should also have that button. When clicked (after reading the appropriate information), it should mark the profile as restricted. That means it should no longer be visible to the backoffice staff, or publicly. You can implement that with a simple “restricted” flag in the users table and a few if-clasues here and there.
  • Export data – there should be another button – “export data”. When clicked, the user should receive all the data that you hold about them. What exactly is that data – depends on the particular usecase. Usually it’s at least the data that you delete with the “forget me” functionality, but may include additional data (e.g. the orders the user has made may not be delete, but should be included in the dump). The structure of the dump is not strictly defined, but my recommendation would be to reuse definitions as much as possible, for either JSON or XML. If the data is simple enough, a CSV/XLS export would also be fine. Sometimes data export can take a long time, so the button can trigger a background process, which would then notify the user via email when his data is ready (twitter, for example, does that already – you can request all your tweets and you get them after a while). You don’t need to implement an automated export, although it would be nice. It’s sufficient to have a process in place to allow users to request their data, which can be a manual database-querying process.
  • Allow users to edit their profile – this seems an obvious rule, but it isn’t always followed. Users must be able to fix all data about them, including data that you have collected from other sources (e.g. using a “login with facebook” you may have fetched their name and address). Rule of thumb – all the fields in your “users” table should be editable via the UI. Technically, rectification can be done via a manual support process, but that’s normally more expensive for a business than just having the form to do it. There is one other scenario, however, when you’ve obtained the data from other sources (i.e. the user hasn’t provided their details to you directly). In that case there should still be a page where they can identify somehow (via email and/or sms confirmation) and get access to the data about them.
  • Consent checkboxes – this is in my opinion the biggest change that the regulation brings. “I accept the terms and conditions” would no longer be sufficient to claim that the user has given their consent for processing their data. So, for each particular processing activity there should be a separate checkbox on the registration (or user profile) screen. You should keep these consent checkboxes in separate columns in the database, and let the users withdraw their consent (by unchecking these checkboxes from their profile page – see the previous point). Ideally, these checkboxes should come directly from the register of processing activities (if you keep one). Note that the checkboxes should not be preselected, as this does not count as “consent”. Another important thing here is machine learning/AI. If you are going to use the user’s data to train your ML models, you should get consent for that as well (unless it’s for scientific purposes, which have special treatment in the regulation)
  • Re-request consent – if the consent users have given was not clear (e.g. if they simply agreed to terms & conditions), you’d have to re-obtain that consent. So prepare a functionality for mass-emailing your users to ask them to go to their profile page and check all the checkboxes for the personal data processing activities that you have.
  • “See all my data” – this is very similar to the “Export” button, except data should be displayed in the regular UI of the application rather than an XML/JSON format. I wouldn’t say this is mandatory, and you can leave it as a “desirable” feature – for example, Google Maps shows you your location history – all the places that you’ve been to. It is a good implementation of the right to access. (Though Google is very far from perfect when privacy is concerned). This is not all about the right to access – you have to let unregistered users ask whether you have data about them, but that would be a more manual process. The ideal minimum would be to have a feature “check by email”, where you check if you have data about a particular email. You also need to tell the user in what ways you are processing their data. You can simply print all the records in your data process register for which the user has consented to.
  • Age checks – you should ask for the user’s age, and if the user is a child (below 16), you should ask for parent permission. There’s no clear way how to do that, but my suggestion is to introduce a flow, where the child should specify the email of a parent, who can then confirm. Obviously, children will just cheat with their birthdate, or provide a fake parent email, but you will most likely have done your job according to the regulation (this is one of the “wishful thinking” aspects of the regulation).
  • Keeping data for no longer than necessary – if you’ve collected the data for a specific purpose (e.g. shipping a product), you have to delete it/anonymize it as soon as you don’t need it. Many e-commerce sites offer “purchase without registration”, in which case the consent goes only for the particular order. So you need a scheduled job/cron to periodically go through the data and anonymize it (delete names and addresses), but only after a certain condition is met – e.g. the product is confirmed as delivered. You can have a database field for storing the deadline after which the data should be gone, and that deadline can be extended in case of a delivery problem.

Now some “do’s”, which are mostly about the technical measures needed to protect personal data (outlined in article 32). They may be more “ops” than “dev”, but often the application also has to be extended to support them. I’ve listed most of what I could think of in a previous post.

  • Encrypt the data in transit. That means that communication between your application layer and your database (or your message queue, or whatever component you have) should be over TLS. The certificates could be self-signed (and possibly pinned), or you could have an internal CA. Different databases have different configurations, just google “X encrypted connections. Some databases need gossiping among the nodes – that should also be configured to use encryption
  • Encrypt the data at rest – this again depends on the database (some offer table-level encryption), but can also be done on machine-level. E.g. using LUKS. The private key can be stored in your infrastructure, or in some cloud service like AWS KMS.
  • Encrypt your backups – kind of obvious
  • Implement pseudonymisation – the most obvious use-case is when you want to use production data for the test/staging servers. You should change the personal data to some “pseudonym”, so that the people cannot be identified. When you push data for machine learning purposes (to third parties or not), you can also do that. Technically, that could mean that your User object can have a “pseudonymize” method which applies hash+salt/bcrypt/PBKDF2 for some of the data that can be used to identify a person
  • Protect data integrity – this is a very broad thing, and could simply mean “have authentication mechanisms for modifying data”. But you can do something more, even as simple as a checksum, or a more complicated solution (like the one I’m working on). It depends on the stakes, on the way data is accessed, on the particular system, etc. The checksum can be in the form of a hash of all the data in a given database record, which should be updated each time the record is updated through the application. It isn’t a strong guarantee, but it is at least something.
  • Have your GDPR register of processing activities in something other than ExcelArticle 30 says that you should keep a record of all the types of activities that you use personal data for. That sounds like bureaucracy, but it may be useful – you will be able to link certain aspects of your application with that register (e.g. the consent checkboxes, or your audit trail records). It wouldn’t take much time to implement a simple register, but the business requirements for that should come from whoever is responsible for the GDPR compliance. But you can advise them that having it in Excel won’t make it easy for you as a developer (imagine having to fetch the excel file internally, so that you can parse it and implement a feature). Such a register could be a microservice/small application deployed separately in your infrastructure.
  • Log access to personal data – every read operation on a personal data record should be logged, so that you know who accessed what and for what purpose. This does not follow directly from the provisions of the regulation, but it is kinda implied from the accountability principles. What about search results (or lists) that contain personal data about multiple subjects? My hunch is that simply logging “user X did a search for criteria Y” would suffice. But don’t display too many personal data in lists – for example see how facebook makes you go through some hoops to get a person’s birthday. Note: some have treated article 30 as a requirement to keep an audit log. I don’t think it is saying that – instead it requires 250+ companies (or companies processing data regularly) to keep a register of the types of processing activities (i.e. what you use the data for). There are other articles in the regulation that imply that keeping an audit log is a best practice (for protecting the integrity of the data as well as to make sure it hasn’t been processed without a valid reason)
  • Register all API consumers – you shouldn’t allow anonymous API access to personal data. I’d say you should request the organization name and contact person for each API user upon registration, and add those to the data processing register.

Finally, some “don’t’s”.

  • Don’t use data for purposes that the user hasn’t agreed with – that’s supposed to be the spirit of the regulation. If you want to expose a new API to a new type of clients, or you want to use the data for some machine learning, or you decide to add ads to your site based on users’ behaviour, or sell your database to a 3rd party – think twice. I would imagine your register of processing activities could have a button to send notification emails to users to ask them for permission when a new processing activity is added (or if you use a 3rd party register, it should probably give you an API). So upon adding a new processing activity (and adding that to your register), mass email all users from whom you’d like consent.
  • Don’t log personal data – getting rid of the personal data from log files (especially if they are shipped to a 3rd party service) can be tedious or even impossible. So log just identifiers if needed. And make sure old logs files are cleaned up, just in case
  • Don’t put fields on the registration/profile form that you don’t need – it’s always tempting to just throw as many fields as the usability person/designer agrees on, but unless you absolutely need the data for delivering your service, you shouldn’t collect it. Names you should probably always collect, but unless you are delivering something, a home address or phone is unnecessary.
  • Don’t assume 3rd parties are compliant – you are responsible if there’s a data breach in one of the 3rd parties (e.g. “processors”) to which you send personal data. So before you send data via an API to another service, make sure they have at least a basic level of data protection. If they don’t, raise a flag with management.
  • Don’t assume having ISO XXX makes you compliant – information security standards and even personal data standards are a good start and they will probably 70% of what the regulation requires, but they are not sufficient – most of the things listed above are not covered in any of those standards

Overall, the purpose of the regulation is to make you take conscious decisions when processing personal data. It imposes best practices in a legal way. If you follow the above advice and design your data model, storage, data flow , API calls with data protection in mind, then you shouldn’t worry about the huge fines that the regulation prescribes – they are for extreme cases, like Equifax for example. Regulators (data protection authorities) will most likely have some checklists into which you’d have to somehow fit, but if you follow best practices, that shouldn’t be an issue.

I think all of the above features can be implemented in a few weeks by a small team. Be suspicious when a big vendor offers you a generic plug-and-play “GDPR compliance” solution. GDPR is not just about the technical aspects listed above – it does have organizational/process implications. But also be suspicious if a consultant claims GDPR is complicated. It’s not – it relies on a few basic principles that are in fact best practices anyway. Just don’t ignore them.

16 thoughts on “GDPR – A Practical Guide For Developers”

  1. Hi Bozho,
    Excellent article. I was wondering what your thought were on how to handle historical backups when implementing “Forget me”. Would every backup containing data on the subject need to be restored in order to delete the relevant data and then subsequently backed up again? This could be a nightmare scenario for a large company with a lot of data and a lot of forget me requests
    Kind Regards,

  2. I really like your article! I have just one comment which I think is worth to mention, you do not have to implement everything, if you could with high probability assume that for example right to portability will be used very rarely you could define manual process for extracting personal data from database and use it when it will be needed. I think GDPR put a requirement on data Controller to provide possibility to do so, the way is up to the controller.

    Kind regards,

  3. Further to Darren’s comment/question about right-to-erasure and backups: A similar problem also occurs when using the event sourcing architecture, if personal data is stored in an immutable event log. One option for these scenarios is to use cryptographic erasure: encrypt personal data field upfront, with a key specific to the data subject, and deleting the key when needed to enforce deletion of the data. This is something we’ve implemented for Java. More info here:

  4. Excellent article ! Is it possible to have backlinks references to the officiel GDPR pdf ? Like article/paragraph for each recommandation ?

    Thanks a lot.

  5. @Darren I added a little more about backups. Basically, you keep a list of forgotten user IDs and re-delete them on restore.

    @Frans yes, that’s a good approach. In some cases events (in event sourcing) can be deleted or modified/anonymized without affecting anything else, so that’s also an option (slightly easier, but potentially breaking)

    @Albert – that’s right. It better be automated, but it doesn’t have to be. I’ll add a clarification
    @Dawn – yup

  6. Nice to read article, but I don’t think it is as easy as this.

    Basically you assume, that you already have perfect data quality and have identified all persons with some account id. But the regulation never mentions some id, it requires to identify natural persons, not accounts.

    Some example from my real live experience with data we have seen at almost every customer companies. You have an contract with an ISP for your internet and another contract with the same ISP for your mobile phone. What we have seen is, that most of the companies create TWO seperate accounts for this and don’t get the data connected. Especially, if there are some company fusions or just different departments.
    The result is currently, that you might get two ad mails for a new product of the ISP.

    For the GDRP it would NOT be sufficent, to make some buttons after the account login, if the natural person has two accounts. You have to find ALL data regarding this one natural person. So the buttons are good, but if you don’t control your data quality you could get into trouble.

    So, you are right, GDPR is not THAT complicated, but it isn’t THAT easy as you say. The basic implementation for some features might only require some weeks, but only if you already have solved some very hard problems. Maybe it is quite easy for small or “new” companies, which only have ONE (at most two) database, but for most companies we talk to, this is not the reality.

    Just my thoughts (I work at a company with heavy experience with data(-quality))
    Greetings Marcel

  7. Well, yes, data deduplication is something I do expect to have happened already. But I can add it to the list of best practices.

  8. Hello Bozho,
    first of thank you for a comprehensive and an exhaustive article.
    I have a bit of an obscure question.
    How about third parties who generate user interaction data which is used for ROI, conversion and such measurements?
    Especially where they don’t explicitly or implicitly know the user ?
    do those 3rd parties need to provide data export for the specific user?
    I am asking because in order to offer an export, they’d need to be able to bind the actual app user to their user agnostic tracking system.

  9. If they can’t deduce the user, they cannot do any of the above.
    However, they should follow the e-privacy directive and the upcoming e-privacy regulation which defines how cookies and other tracking mechanisms are used

  10. Thanks for this very nice article!
    I have only one question : Is it an exhaustive list of gdpr development or do you think there are more to do?

  11. I think it covers the most common use cases . There will certainly be edge cases depending on the business needs that are not covered above, though. The other day we got such a question – “what to do in case we get the data of the user and their consent over the phone”. Seems like the proper thing is to just mark the consent in a CRM on behalf of the user, but it is not yet clear – maybe some call archiving will be needed in case of sensitive data? Can’t say at this point without consulting with legal experts.

  12. Thanks for sharing your analysis.
    I’ve spent some days in 2017 to scan official ressources, including the original GDPR text, and for some points, I came to a slightly different conclusion.
    Basically, almost every time you write “must” (encrypt data base content; provide a data download button; allow direct personal data editing; etc…), on my side, went to the conclusion that this is an option, not a requirement.
    What is required is to grant each individual access to their personal data; the how (is it automated or manually) is not enforced. Thus, a snail mail process would meet the requirement.
    Regarding encryption, the text states “shall implement appropriate technical and organisational measures to ensure a level of security appropriate to the risk”.
    The notion of “level of security appropriate to the risk” is key here : whether data are usual ecommerce data (postal address) or personal insurance data (history of failures, …) does matter, and measures are to be adapted.
    By the way, it is not only a technical point, but also a process point : what about the developer who would code the encryption of the data : how do you ensure that he will not be able to access/decypher all data ?

  13. Hi Bozho

    Thanks for a great article.

    I was curious if GDPR only applies to client data or if it also applies to employee/admin user data as well.

    For example with event sourcing or access logs, would have something like “Employee X changed Customer Y’s address on 01/01/2018”. Can the employee/admin ask for their data to be forgotten? (eg when employee leaves company)

    What would you recommend for people who are both customers and employees?


  14. It applies to employee/admin data as well, yes, BUT it is based on contract, rather than consent. So the employee can’t ask to be forgotten. You just have to define a data retention period for that kind of audit data (it shouldn’t be “forever”)

  15. You are correct. It is more fuzzy than “must” vs “must not”. I’ve listed the general good practices that would make you safer, but whether a compliance audit will absolutely require them – depends on many factors.

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