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Don’t Delete – Just Don’t

Tuesday, September 1st, 2009.


After reading Ayende’s post advocating against “soft deletes” I felt that I should add a bit more to the topic as there were some important business semantics missing. As developers discuss the pertinence of using an IsDeleted column in the database to mark deletion, and the way this relates to reporting and auditing concerns is weighed, the core domain concepts rarely get a mention. Let’s first understand the business scenarios we’re modeling, the why behind them, before delving into the how of implementation.

The real world doesn’t cascade

Let’s say our marketing department decides to delete an item from the catalog. Should all previous orders containing that item just disappear? And cascading farther, should all invoices for those orders be deleted as well? Going on, would we have to redo the company’s profit and loss statements?

Heaven forbid.

So, is Ayende wrong? Do we really need soft deletes after all?

On the one hand, we don’t want to leave our database in an inconsistent state with invoices pointing to non-existent orders, but on the other hand, our users did ask us to delete an entity.

Or did they?

When all you have is a hammer…

We’ve been exposing users to entity-based interfaces with “create, read, update, delete” semantics in them for so long that they have started presenting us requirements using that same language, even though it’s an extremely poor fit.

Instead of accepting “delete” as a normal user action, let’s go into why users “delete” stuff, and what they actually intend to do.

The guys in marketing can’t actually make all physical instances of a product disappear – nor would they want to. In talking with these users, we might discover that their intent is quite different:

“What I mean by ‘delete’ is that the product should be discontinued. We don’t want to sell this line of product anymore. We want to get rid of the inventory we have, but not order any more from our supplier. The product shouldn’t appear any more when customers do a product search or category listing, but the guys in the warehouse will still need to manage these items in the interim. It’s much shorter to just say ‘delete’ though.”

There seem to be quite a few interesting business rules and processes there, but nothing that looks like it could be solved by a single database column.

Model the task, not the data

Looking back at the story our friend from marketing told us, his intent is to discontinue the product – not to delete it in any technical sense of the word. As such, we probably should provide a more explicit representation of this task in the user interface than just selecting a row in some grid and clicking the ‘delete’ button (and “Are you sure?” isn’t it).

As we broaden our perspective to more parts of the system, we see this same pattern repeating:

Orders aren’t deleted – they’re cancelled. There may also be fees incurred if the order is canceled too late.

Employees aren’t deleted – they’re fired (or possibly retired). A compensation package often needs to be handled.

Jobs aren’t deleted – they’re filled (or their requisition is revoked).

In all cases, the thing we should focus on is the task the user wishes to perform, rather than on the technical action to be performed on one entity or another. In almost all cases, more than one entity needs to be considered.

Statuses

In all the examples above, what we see is a replacement of the technical action ‘delete’ with a relevant business action. At the entity level, instead of having a (hidden) technical WasDeleted status, we see an explicit business status that users need to be aware of.

The manager of the warehouse needs to know that a product is discontinued so that they don’t order any more stock from the supplier. In today’s world of retail with Vendor Managed Inventory, this often happens together with a modification to an agreement with the vendor, or possibly a cancellation of that agreement.

This isn’t just a case of transactional or reporting boundaries – users in different contexts need to see different things at different times as the status changes to reflect the entity’s place in the business lifecycle. Customers shouldn’t see discontinued products at all. Warehouse workers should, that is, until the corresponding Stock Keeping Unit (SKU) has been revoked (another status) after we’ve sold all the inventory we wanted (and maybe returned the rest back to the supplier).

Rules and Validation

When looking at the world through over-simplified-delete-glasses, we may consider the logic dictating when we can delete to be quite simple: do some role-based-security checks, check that the entity exists, delete. Piece of cake.

The real world is a bigger, more complicated cake.

Let’s consider deleting an order, or rather, canceling it. On top of the regular security checks, we’ve got some rules to consider:

If the order has already been delivered, check if the customer isn’t happy with what they got, and go about returning the order.

If the order contained products “made to order”, charge the customer for a portion (or all) of the order (based on other rules).

And more…

Deciding what the next status should be may very well depend on the current business status of the entity. Deciding if that change of state is allowed is context and time specific – at one point in time the task may have been allowed, but later not. The logic here is not necessarily entirely related to the entity being “deleted” – there may be other entities which need to be checked, and whose status may also need to be changed as well.

Summary

I know that some of you are thinking, “my system isn’t that complex – we can just delete and be done with it”.

My question to you would be, have you asked your users why they’re deleting things? Have you asked them about additional statuses and rules dictating how entities move as groups between them? You don’t want the success of your project to be undermined by that kind of unfounded assumption, do you?

The reason we’re given budgets to build business applications is because of the richness in business rules and statuses that ultimately provide value to users and a competitive advantage to the business. If that value wasn’t there, wouldn’t we be serving our users better by just giving them Microsoft Access?

In closing, given that you’re not giving your users MS Access, don’t think about deleting entities. Look for the reason why. Understand the different statuses that entities move between. Ask which users need to care about which status. I know it doesn’t show up as nicely on your resume as “3 years WXF”, but “saved the company $4 million in wasted inventory” does speak volumes.

One last sentence: Don’t delete. Just don’t.

  
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68 Comments

  1. Gilligan Says:

    Wow, perfect timing! I just received a “bug report” from a staff member that they could not delete a company from our website. I told them it is because we need that company’s information for ordering and that this is a design issue we need to discuss. I am happy to know my mind is on the right track with these things. But sometimes I wish it were as simple as a delete. Idealism vs. Reality, eh?


  2. Eyston Says:

    My question would be how much would this impact the rest of the system?

    You now have to go through every query and evaluate if it should care about the status?

    But also behavior is changed. Certain methods on Customer make no sense if they are inactive. So do you now have two interface/classes: ActiveCustomer and InActiveCustomer? Does NHibernate need to know? Do you use the status as discriminator? Or ‘it depends’?

    The concept makes sense, I just would be very worried if I had to implement this status on an already established app :)


  3. Andy Says:

    I agree with everything you’ve said, but what about when a user makes a mistake? Maybe he creates an entity incorrectly and just wants to delete it. Similar to when you create a file, maybe in the wrong format, and just want to delete it and start over.

    Thoughts?


  4. Deisbel Says:

    [Udi's note - translated with Google, original text below]

    Pardon the language but I take more than writing in English and I am short of time. A friend sent me this article, with which it agrees completely refactored recently a project under this philosophy ELIMINR NO.

    However, I believe that the concepts are mixing a little. One thing is the concept of eliminating and over: the various states that may or may not Entity.

    I’ve been doing Data Management Systems for many customers and have found all kinds of requirements. In the vast majority of cases is very true that we should not remove the records, but in another large percent, depending on the institution, is very, very necessary. Once a registry X a datawarehouse will be removed. We’re not talking the same system, although connected.

    ** Original Text **

    Perdonen por el idioma pero me demoraria mas escribir en Ingles y estoy corto de tiempo. Un amigo me ha enviado este articulo, con el cual esta de acuerdo totalmente pues recientemente refactorizamos un proyecto bajo esta filosofia de NO ELIMINR.

    NO obstante creo que los conceptos se estan mezclando un poco. Una cosa es el concepto de ELIMINAR y otra: los varios estados que puede o no alcanzar una Entidad.

    Llevo años haciendo Sistemas de Gestion de Datos para muchos clientes y he encontrado de todo tipo de requerimientos. En la gran mayoria de los casos es muy cierto que no debemos eliminar los registros, pero en otro gran porciento, dependiendo de la entidad, es muy, pero muy necesario. Una vez que un Registro X vaya a un DatawareHouse HAY QUE ELIMINARLO. No estamos hablando del mismo sistema, aunque esten relacionados.

    Que seria de las BD si nunca se Eliminara un registro?? Ni con 1TB de HDD nos alcanzaria, ni con 100.

    No creo que tampoco debamos mezclar siempre el Estado de Eliminado y de CANCELADO, para no hablar de “SIN INVENTARIAR”, “EN INVENTARIO”, “PENDIENTE”, etc, etc, por solo hablar de la entidad FACTURA DE INVENTARIO. En este caso especifico, jamas se debe Eliminar, claro esta que seria una regla de negocio totalmente independiente, pero no deberiamos mezclarla con resto de los Estados.

    Ahora, una cosa es cierta, casi nunca el Cliente conoce estos detalles tecnicos: somos nosotros quienes debemos ofrecerle un sistema flexible y potente, implentando, de ser necesario, todas estas filosofias por el bien de todos.

    Asi que yo terminaria el articulo modificando la frase:
    NO ELIMINE, A MENOS QUE ESTE BIEN JUSTIFICADO.


  5. Ray Jezek Says:

    What you’re saying here makes a lot of sense and I completely agree. One scenario that is more difficult though is the one where someone makes a mistake and puts something into the system they shouldn’t have. In those cases you need to cascade the delete since erroneously adding the item will cause problems downstream as other systems take action in response.

    For instance, an employee may add new product to the catalog by mistake (perhaps they thought they were in a test system, weren’t trained properly, it was already added by someone else, etc..). In this case you may want to remove the product as if it never existed to avoid confusion.


  6. Jonathan Oliver Says:

    I hear a blog battle brewing. My guess is that Ayende will respond to this within the hour.


  7. Joseph Gutierrez Says:

    I’ve seen this before. Where that item is obsoleted and replaced with a new item. You still need to maintain the information so that when a customer calls in with an order for the obsoleted part number you can give them the replacement part number.


  8. Jim Sally Says:

    Yet another reason why temporal database design (http://en.wikipedia.org/wiki/Temporal_database) needs to become a first-class citizen of the business information systems developer toolbox. Too few developers know about this topic that really should be a commonly known subject. See this post in response to Ayende’s post…

    http://bigjimindc.blogspot.com/2009/08/temporal-database-design-soft-deletes.html


  9. Tim Scott Says:

    Very well explained. A couple of observations.

    1. There are cases that hard “delete” is a valid business operation. Let’s say I uploaded a photo of my mom instead of a scan of the contract. We might want to allow the user to delete within a certain context. Or maybe not. It’s a business decision. But to be clear, “delete” as a business operation should be understood to wipe something out with no auditing trail left behind.

    2. These concepts apply just as much to Access as anything else. Ages ago when I wrote some Access apps I always followed these same concepts.


  10. udidahan Says:

    Gilligan,

    Reality bites :-)


  11. udidahan Says:

    Eyston,

    Trying to shoe-horn this approach into an already structure CRUD application probably won’t be easy – agreed.

    That being said, why build anything at all unless you’re quite confident that it’s what your users actually need?


  12. udidahan Says:

    Andy,

    When someone creates an entity the first time, give it a status, say ‘tentative’, which requires some kind of authorization to make it visible to everyone else.

    We might only allow the ‘delete’ task to be performed on entities in the ‘tentative’ status.

    Mistakes can happen in entity creation, but those are of relatively minor concern. The bigger concern is mistakenly deleting an entity. In this model, since you don’t delete, you can more easily revert back to a previous status (uncanceling an order).

    Hope that answers your questions.


  13. udidahan Says:

    Deisbel,

    Apologies in advance for possibly misunderstanding your comments, and thanks for sending them in.

    It looks like I wasn’t thorough enough in my post – I should have said that there may be a need to delete *data*, often in the context of data cleansing and those kinds of scenarios. These activities, though, are often quite different from the default interaction model most users are presented with in today’s business applications.


  14. udidahan Says:

    Ray,

    See my comments to Andy above. Thanks.


  15. udidahan Says:

    Jonathon,

    You’d be surprised at the kinds of things me and Ayende agree (and disagree) about.


  16. udidahan Says:

    Joseph,

    Yup – that’s exactly right.


  17. udidahan Says:

    Jim,

    I did read your comments and blog post before writing this post. I’d agree that more developers should be aware of temporal DBs and have them as a part of their toolbox.

    I would say, though, that having the context I’m talking about in this post is quite critical in knowing how to design your temporal relationships – so the two very much can and should work together.

    Thanks.


  18. udidahan Says:

    Tim,

    I totally agree – the business context is what shows us the way.

    When uploading a file, often the intent is to “overwrite” the previous file, but that doesn’t necessarily mean that we lose the audit trail. And apologies for disparaging Access that way – it was just a way to make the alternatives concrete.


  19. Think Before Coding Says:

    You also have to write the follow up post : Don’t Update, Just Don’t !

    You do not update Employee’s adress without a reason :
    The Employee moves, or there was a mistake and you correct the address (compensating action..)


  20. udidahan Says:

    > You also have to write the follow up post : Don’t Update, Just Don’t !

    I think I wrote that post over 2 years ago:

    http://www.udidahan.com/2007/01/22/realistic-concurrency/


  21. Eyston Says:

    No, I agree, these distinctions provide business value. The ‘Make Roles Explicit’ methods you introduce in your NDC session also lend themselves to this much better than my current infrastructure :)


  22. udidahan Says:

    Eyston,

    Glad you were able to reconcile these points of view.


  23. Szymon Kulec Says:

    Hello Udi,

    it seems to be quite funny that although I work with pretty CRUDy application, all the ideas about not-deleting have been incorporated into it automatically, for instance dismissable employees which are sources of foreign keys (speaking in terms of DB). The delete operations can be applied only to an entity which haven’t started its business life. You can treat this kind of deletes as a simple erasing sth with a rubber before sending it forward. Once the entity started its business life, all you can do is to change its statuses.


  24. Reflective Perspective - Chris Alcock » The Morning Brew #425 Says:

    [...] Don’t Delete – Just Don’t – Udi Dahan continues the discussion about soft deletes looking at why deletes often shouldn’t be a part of our design, despite users often giving requirements in CRUD terms when there is actually a more correct business process/term involved in these operations (such as discontinued in the example of ecommerce) [...]


  25. udidahan Says:

    Szymon,

    Sounds like you and I are very much in agreement.


  26. What about Soft Deletes? « Any fool can make things bigger and more complex Says:

    [...] Udi Dahan – Just don’t delete [...]


  27. Deisbel Says:

    Hi udidahan:

    My apologies for the language. From now on at least translate my post and keep a copy of the original Spanish.

    The completed post, translated with google is here
    **

    Pardon the language but I would take more write in English and I am short of time. A friend sent me this article, with which it agrees completely refactored recently a project under this philosophy of NOT DELETE.

    However, I believe that the concepts are mixing a little. One thing is the concept of eliminating and over: the various states that may or may not Entity.

    I’ve been doing Data Management Systems for many customers and have found all kinds of requirements. In the vast majority of cases is very true that we should not remove the records, but in another large percent, depending on the institution, is very, very necessary. Once a registry X a datawarehouse will be removed. We’re not talking the same system, although connected.

    That would be the BD if you never delete a record? With 1TB HDD or catch us, or with 100.

    I do not think we should not always mix Eliminated the state and to cancel, not to speak of “NO STOCK”, “INVENTORY”, “Pending”, etc, etc, by just talking about the entity INVENTORY INVOICE. In this specific case, will never be suppressed, clear that it would be a totally independent business rule, but we should not mix with other States.

    Now, one thing is certain, almost never the Client is aware of these technical details: we who must offer a flexible and powerful implentando, if necessary, all these philosophies for the good of all.

    So I would conclude the article by changing the phrase:
    DO NOT REMOVE UNLESS THIS well justified.

    ** Original Text in Spanish **

    Perdonen por el idioma pero me demoraria mas escribir en Ingles y estoy corto de tiempo. Un amigo me ha enviado este articulo, con el cual esta de acuerdo totalmente pues recientemente refactorizamos un proyecto bajo esta filosofia de NO ELIMINR.

    NO obstante creo que los conceptos se estan mezclando un poco. Una cosa es el concepto de ELIMINAR y otra: los varios estados que puede o no alcanzar una Entidad.

    Llevo años haciendo Sistemas de Gestion de Datos para muchos clientes y he encontrado de todo tipo de requerimientos. En la gran mayoria de los casos es muy cierto que no debemos eliminar los registros, pero en otro gran porciento, dependiendo de la entidad, es muy, pero muy necesario. Una vez que un Registro X vaya a un DatawareHouse HAY QUE ELIMINARLO. No estamos hablando del mismo sistema, aunque esten relacionados.

    Que seria de las BD si nunca se Eliminara un registro?? Ni con 1TB de HDD nos alcanzaria, ni con 100.

    No creo que tampoco debamos mezclar siempre el Estado de Eliminado y de CANCELADO, para no hablar de “SIN INVENTARIAR”, “EN INVENTARIO”, “PENDIENTE”, etc, etc, por solo hablar de la entidad FACTURA DE INVENTARIO. En este caso especifico, jamas se debe Eliminar, claro esta que seria una regla de negocio totalmente independiente, pero no deberiamos mezclarla con resto de los Estados.

    Ahora, una cosa es cierta, casi nunca el Cliente conoce estos detalles tecnicos: somos nosotros quienes debemos ofrecerle un sistema flexible y potente, implentando, de ser necesario, todas estas filosofias por el bien de todos.

    Asi que yo terminaria el articulo modificando la frase:
    NO ELIMINE, A MENOS QUE ESTE BIEN JUSTIFICADO.


  28. Andrig Miller Says:

    I agree with the basic premise of this. I have actually never worked on a system where deleting anything was actually the business requirement. I have worked on many systems where entities come in and out of affect. Pricing options, discounts, special promotions, products that are services that are discontinued, etc. Having said that, databases also cannot perpetually grow with entities that are no longer in affect. Performance and storage issues tend to mean at some point you have to delete stuff.

    Our solution was to have a running duration of time that would be in the database (18 months that eventually moved to 12), and delete (with all the business rules for consistency in tack) the stuff older than the duration in a batch process separate of the main OLTP application.


  29. Ian Cooper Says:

    Hi Udi,

    Agreed, I think that the tendency to jump into classifying the logic as CRUD elides what the business task is.

    I’m seeing a similar thing with Audit. Instead of jumping into some automatic audit trail feature via NH interceptors or triggers, it’s important to find out what the intent is with regard to the audit information.


  30. Lex Says:

    Interesting and for the most part I’d agree, referential integrity often makes this something of a necessity unless you want large cascading deletes that has a far greater impact than the original user envisioned.

    However there are two major use cases that this doesn’t play nice with; firstly as mentioned above mistakes – this is especially costly when the record ties up some unique resource – say a GUID, code or even a reserved name that then requires some data fix to unbind.

    Secondly data archiving; not a problem for lots of systems that never reach the volumes that cause performance degradation that the hardware cannot offset but still definitely a consideration when running into millions of records. Is that information still needed? Can it be curtailed? If so what can be and what is the impact of it going? One last thing to keep in mind is that sometimes this can be enforced by law; i.e. the UK Data Protection Act enforces some fairly strict procedures of the data that a company keeps about its customers and sometimes the easiest way to comply is to simply not hold that data anymore.

    Lex


  31. udidahan Says:

    Deisbel,

    Your statement “DO NOT REMOVE UNLESS THIS well justified” is a good one, but the challenge is always teaching people what “well justified” is and, more importantly, isn’t.

    Thanks for taking the time to translate your comments


  32. udidahan Says:

    Andrig,

    You bring up a valid point that when a given graph of entities is in a certain state for a certain period of time, it can be in the business interest to archive it away from the main database for performance reasons.


  33. udidahan Says:

    Ian,

    It is a general organizational pattern with developers to accept “requirements” at face value and then go develop them exactly as specified. We need to invest more in human-to-human interaction in order to solve/prevent the software problems we’ve been seeing as an industry.


  34. udidahan Says:

    Lex,

    Taking up a GUID in the case of a mistake is not such a big deal. However, since we can set up a preliminary state for data as its being typed in (where most mistakes happen), data in that state doesn’t actually count towards reserving a given name/code/resource. When entities in that state are “published” or “authorized” is when we check for uniqueness and “reserve” that name/code/resource.

    As for the archival, I acceded that point above in my comments to Andrig.

    Thanks for your comments


  35. Dhananjay Goyani Says:

    Udi,
    The status thing that you are suggesting is sort of incremental where in the user will create something first and then will act upon – cancellation, retirement, marking it as inactive, etc. I follow your point in saying that such scenario should be handled by marking the instance with appropriate status rather physically removing from the system.

    Well, the concern here is with something you want to revert. Revert an action completely which was by mistake (different than status increments). And there comes physical and its counterpart ‘IsDeleted’ can play a big role. Such entries are useless by any meanse, and can create mess in the system – so the best option is to throw them out – obviously with care. ;-)

    Your thoughts?


  36. Thomas Says:

    Flexibility equates to complexity. Hard deletes are easier to implement but can have far more profound effects. Soft deletes are more flexible, but require a lot more work to implement properly. As someone mentioned, every report and every screen has to account for the state of the record. In some cases, they have to allow for ignoring the state (i.e., don’t filter on anything). Thus, it is not just a matter of gleaning from the user what they are really trying to accomplish, it is a matter of presenting a cost/benefit proposal of development time vs flexibility.


  37. udidahan Says:

    Dhananjay,

    See my answer (12) to Andy. If the user mistakenly added an entity, then we could call entity.AddedByMistake(); which may result in a delete.

    Other actions are just as easy to revert when you have the original context.

    Hope that answers your questions.


  38. udidahan Says:

    Thomas,

    You’re absolutely right – sometimes MS Access really is the right tool for the job .


  39. Ray Says:

    Agree so much!


  40. Thomas Says:

    Interesting article!

    The way I see it, if you (as suggested) chose to replace physical deletes
    with modelling the business rules underlying this operation, you will still
    be faced with the two main problems associated with soft deletes.

    1. You bypass the referential integrity of the RDMS. (delete is replaced with status)
    2. You end up with more complicated and error-prone queries.

    It’s a tradeoff, but if you can’t come up with an argument of why you should model
    the business rules underlying the delete operation – maybe avoiding the two
    problems above and keeping the model simple is the best choice. In my experience,
    when you introduce new entity states, you quickly end up in a situation where users cannot explain the difference between two states.


  41. Thomas 2 Says:

    To be fair to Thomas, posting 40 is written by another Thomas than the Thomas of posting 36 ;)


  42. udidahan Says:

    Thomas (40),

    The question of business status is more important than the answer. What I mean by that is that as long as the business people are making the tradeoffs between complexity/cost and value in the statuses, then we as developers have done our job.

    It’s when we assume that deleting (whether soft or not) is a good enough technical solution – without vetting that with the business, that we overstep our responsibility.

    To your point #1, we don’t bypass RI – we actually model the business life cycle of important entities. And to point #2, correspondingly, we show users what they need to see – adding a single where clause might make it a *tiny* bit more complex, but I’d hardly go so far as to say “error-prone”.

    Thanks for your comments.


  43. Jørn Wildt Says:

    Udi wrote: adding a single where clause might make it a *tiny* bit more complex, but I’d hardly go so far as to say “error-prone”.

    Unfortunately, “error-prone” is no so far off. It probably depends on how you structure your code, but adding an extra status to some central widely used entity can ripple changes to many places: you can have a bunch of reports that need to be changes, querying in the UI needs to be changed, the new status has to be added as an extra parameter to UI search windows, you may need to add extra language translation for that status and so on.

    In short, adding a simple status can have wide spread consequences.

    The SQL etc. might be rather trivial to fix, but the biggest problem is to locate all the places that need fixing – and *that* is error prone.

    Neither is a big test suite going to help you: all your test will run fine since none of them are aware of the new status. Adding new tests helps of course, but figuring out which to add is also a problem.

    This is *not* to say that I prefer a technical delete over true bussiness logic. I just want to illustrate that it might be a bit more expensive than exptected.

    If you have a good solution then please share it :-)


  44. Thomas (36) Says:

    #42
    There is no question that the onus is on the developer to vet the real requirement about deletions from the stakeholders and IMO, it must be a deliberate decision to allow soft deletes as it is much costlier to implement. It can definitely create a RI/schema issue depending on how it is implemented. If you go the “state” column route, it can create a series of issues:

    First, you are compelled to use surrogate keys as natural keys are out of the question because of the need to add a “state” column. Second, the only way to protect against referencing an inactive value at the database level is to use triggers. Third, we lose our database level referential integrity that tells us that an about to be deleted value has been referenced. If we simply change the state, it does nothing to compel us to correct the child references to use an active parent value or to change the state of those child records. Lastly, you have to code against phantom values:

    For example, suppose we have a status table with a name column and a series of attributes. We cannot put our unique constraint on name alone; it must be on the state and the name. For each addition, we must check whether we have ever had the given name and if so, “reactivate” it. If we simply reactivate, the new status will contain the attributes of the previously deleted instance unless we guard against that.

    It is more error prone because it is more work to implement correctly especially in reporting as all the report designers have to know to account for the state column.

    If we go the route of a mirror table contained deleted values, that has its own set of issues. None of these problems are insurmountable, but they do add work, which means cost which means a cost/benefit problem for the stakeholder.

    IMO, soft deletes must be a conscious, well thought out decision as it adds cost to the project which can be substantial depending where the soft deletes are needed. It is not just a “tiny” bit more complex. Every query has to account for the state and typically those errors happen in reporting.


  45. Thomas (36) Says:

    As I think about the natural key, surrogate key choice, I suppose it’s no different than handling the unique constraint.


  46. Le “D” de CRUD est-il utile? Says:

    [...] suis d’accord avec Udi Dahan (Don’t Delete – Just Don’t) et Ayende sur le fait de ne pas détruire d’entitée. Dans les Enterprise Application, les [...]


  47. udidahan Says:

    Thomas(44 and 36 – I think),

    That sums it up very well.


  48. David Nelson Says:

    “It is not just a ‘tiny’ bit more complex.”

    I have to agree with this, as I see this problem occur frequently. In particular, I have seen it in relation to a data source that is central to most of the LOB applications my company writes (the security master for a trading firm). Because the data is so central, we cannot have one person or a group of people in charge of all access to that data; they would simply be overwhelmed. So everyone must have access to that data. But not everyone has the time to become an expert in the security master. And because the designer of the security master subscribes to the “never delete” policy, the number and combinations of statuses has exploded, and is continually increasing. It is very difficult to accurately write a query that will actually return the data that you think you are asking for.

    Even something as simple as finding the ticker for a stock is non-trivial, because all of the previous tickers for the stock are still in the table (with various statuses depending on the reason for the change), AND the current ticker might have a status that indicates that it has been scheduled to be changed (but has not been changed yet). While there are some corner cases where seeing the entire ticker history for a stock might be useful, the vast majority of our operational systems don’t care; they just want to know what the ticker is right now. But these systems must deal with the increased complexity for the sake of maintaining a history that is rarely if ever useful.


  49. udidahan Says:

    David,

    I absolutely agree that trying to solve this problem at the database level is very complex – especially when that database is used by many different applications.

    The recommendation of not deleting is for a database that belongs in its entirety to a single application – what Martin Fowler calls an “application database”, as opposed to an “integration database”. In this kind of case, it is less likely to see that same kind of explosion that you’ve experienced.

    Hope that makes sense.


  50. To Delete or Not To Delete – Read Bits Says:

    [...] not the same as deleting. "Delete" is just the simplest term to use. Software developer Udi Dahan elaborates: "We’ve been exposing users to entity-based interfaces with “create, read, update, [...]


  51. Dare Obasanjo aka Carnage4Life - Building Scalable Databases: Perspectives on the War on Soft Deletes Says:

    [...] above that there are situations where one wants to hard delete data from the database in his post Don’t Delete – Just Don’t where he [...]


  52. Leo Says:

    Thanks pal!!! I was wondering if someone was concern about the domain model.. I’m trying to apply some DDD and it makes sense to me that deletes have a reason behind it.. I will say no to deletes from now on!!!

    Saved my day!!!!

    Thanks,
    Leo!


  53. Service-Oriented, Event-Driven Part 1: Events « christopherDeweese.com Says:

    [...] Dahan posted an article titled “Don’t Delete – Just Don’t” a few weeks back.  In that article Udi explores soft deletes versus hard deletes, but he [...]


  54. GBruenetti Says:

    Of course there is the need to delete things physically. Temporary data, old data, erroneous data.

    Before the computers took over we had paper baskets and shredders in our businesses. Since the basic business requirements are mainly unchanged we also need the digital equivalent of them today. I don’t want to filter my valuable information out of a trash mountain.

    This observation does not mute your main point: Make sure you understand what your user wants/needs and *then* implement the correct logic for this. However, sometimes that can be a plain DELETE.


  55. SJB Says:

    How does this tie in with Data Protection laws, where data can only be held for x-amount of time?


  56. udidahan Says:

    SJB,

    At that point, we create rollups of the data which can be stored for longer periods of time.


  57. ricky lively Says:

    When I have had this kind of issue, I found that using a date range indicating when the record is active helpful. After the date range is over, the record still exists (for joins to records that used it), but is not used in anything going forward.


  58. udidahan Says:

    Ricky,

    What you’re describing is a kind of soft-delete.


  59. Why you should be using CQRS almost everywhere… Says:

    [...] means you don’t ever delete data (see my previous blog post on the subject), and you definitely don’t overwrite it with an update – even if you think [...]


  60. Thomas (new one) Says:

    Interesting ideas but they are based on data which is directly based on real life objects which basically exist forever.

    Do we really want to store everything from now to eternity?

    If we don’t, we have to delete something. Like orders older than 10 years or customers who haven’t bought anything in 2 years.

    Or stock data from 5 years ago. Storing personal data of ex-employees is probably illegal in many countries, they have to be deleted as soon as employment ends.

    So: As a blanket idea ‘Do not delete’ is very bad idea: It will cause massive problems sooner or later.


  61. udidahan Says:

    Thomas,

    When it comes to personally identifying information, often we can comply with the privacy laws by just scrambling the data.


  62. Travis J Says:

    A nice article about not deleting. However, not really much attention is paid to what to do with the data that wasn’t deleted. In fact, it is glossed over.

    In my opinion this is because you are focusing on the wrong issue. Not just “what does deleting mean”, but what IS being deleted. When a record is to be removed, what is really being removed is a node in a graph – not just a single record. That whole graph integriy is the reason for people to bandaid over the issue with “soft deletes” as advocated here. These bandaid solutions tend to hide the gangrene underneath – a festering problem which only gets worse with time.

    What’s worse is that in order to accompany the soft delete logic must be included all over (many times breaking various conventions and implementing anti-patterns) to account for the possible breaks in the object graph. Moreover, what kind of business logic is “isDeleted”?!

    I believe a very strong solution to this problem, the problem of removing an object while retaining the referential integrity of the object graph, is to use an archival pattern. On delete of an object, the object is archived then deleted. The archive database, a mirror database with meta data (temporal database design can be used and is very relevant here), would then receive the object to be archived and restored if necessary.

    This makes it very direct to avoid listing or including a deleted object as the relevant database will no longer hold it. Now, the same logic which was applied looking for “isDeleted” “isActive” or “DeletedDate” can be applied in the correct place (Not all over the place) to foreign keys of retrieved objects. When a foreign key is present, but the object is not, then there is now a logical explanation and a logical set of options. Display that the containing object was deleted and some course of action: “Restore, Delete Current Containing Object, View Deleted”. These options can be either chosen by the user, or explicitly defined in code in a logical manner. Depending on how advanced the archival database is, perhaps more options exist such as who deleted it, when, why, etc. etc.


  63. Lelala Says:

    Wow,
    thanks for that impressive article (i came to you from Ayende’s site). Your example with the marketing department is right, absolutely – while i can, absolutely, imagine a case in which it is not that relevant that you keep all stuff: You could save the final invoice, e.g., instead of storing all products that you have sold 10 years ago…
    It depends completely on the use case, without considering this there is no true-vs-false.


  64. Luiyo Says:

    well, at some stages the DELETE is nothing but required. for instance in buffer tables, for some continuous flow processing in large scale systems as the ones I work with.

    trxs flow in a table through different async processes by changing status, and finally have a “logical deleted” status. But to keep +15 process in the flow with 120 trxs / second throughput, this table has also to be purged.

    there are different strategies for the purging activities, and the processes and table structure design some times makes it quite tricky.

    of course for usual business applications (even for large companies) the logical deletion is quite fine, but when dealing with massive data and near real time processing capacity things get a cool twist :)

    cheers,


  65. Lelala Says:

    The thing is, in smaller webprojects one could delete data, it depends on the context:
    If its not your bookkeeping or any critical stuff, i’d say there is really no issue in deleting those rows from their tables. Sure -this should not be applied to your taxing-table ;-)
    Hence it depends on the situation/problem one wants to solve.
    Regards


  66. udidahan Says:

    Lelala,

    I wouldn’t say it’s the issue of small vs big, but rather (as you said) the context.

    When the data is (what I call) private – meaning only seen/used by a single user, then that user can delete that data completely without any concern.

    However, when the data becomes “public” – meaning that there is data from other users that gets connected to it, one should avoid deleting and think of more meaningful lifecycle-type changes to it.

    Hope that clears things up.

    Thanks.


  67. Oscar Says:

    Hi Udidahan,

    I have been facing this topic on several systems and I do believe in soft delete. But I have seen there is a problem when the data is highly connected. A very simple example: let’s say that we are managing stores & employees. When we decide to deactivate a store, we can not think on deactivate the employees too (cause that will be thinking in the cascade way that we are trying to avoid), so, we can mark them as “free” employees, without an assigned store. Seeing this from reporting side, what should we do when we decide to do a report of the stores existing on the months before delete?, should we show the store and the link with all its employees at that time?. And a second report after delete time, should the store not be shown? (of course thinking also on an employee report to show them as free after delete but show them linked to the store on a report before delete).

    The thing here is, should all be managed as really historic records? or once we decide to deactivate something the relations should be forgotten by the system?. In my example, would be to never show the store even in reports previous to its deactivation and show the employees as free persons since the start of the time.

    It could be confusing on making a difference when deactivation means something (like the closing of a store) or it is just deleting an input error (user wrongly enters a store that never existed).

    I know that this could make the topic more complicated, but I believe that in some systems could be need to take in consideration and could have an impact on deciding if we should use soft or physical delete.

    From my experience I support the idea soft delete but the final user needs to be part of the process on designing the logic behind it. We as analyst and designers can have a lot of good points and ideas, but the goal of a system is that it can make sense to our clients (even if it looks kind of gross to our logic)

    Thanks in advance, regards


  68. udidahan Says:

    Oscar,

    > the final user needs to be part of the process on designing the logic behind it (the delete)

    I definitely agree with that statement.


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Udi Dahan is the real deal.

We brought him on site to give our development staff the 5-day “Advanced Distributed System Design” training. The course profoundly changed our understanding and approach to SOA and distributed systems.

Consider some of the evidence: 1. Months later, developers still make allusions to concepts learned in the course nearly every day 2. One of our developers went home and made her husband (a developer at another company) sign up for the course at a subsequent date/venue 3. Based on what we learned, we’ve made constant improvements to our architecture that have helped us to adapt to our ever changing business domain at scale and speed If you have the opportunity to receive the training, you will make a substantial paradigm shift.

If I were to do the whole thing over again, I’d start the week by playing the clip from the Matrix where Morpheus offers Neo the choice between the red and blue pills. Once you make the intellectual leap, you’ll never look at distributed systems the same way.

Beyond the training, we were able to spend some time with Udi discussing issues unique to our business domain. Because Udi is a rare combination of a big picture thinker and a low level doer, he can quickly hone in on various issues and quickly make good (if not startling) recommendations to help solve tough technical issues.” November 11, 2010

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It was evident through that discussion that Udi is one of the most knowledgeable people in the SOA space. It was also clear why – Udi does not settle for mediocrity, and seeks to fully understand (or define) the logic and principles behind things.
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“I attended both of Udi's "UI Composition Key to SOA Success" and "DDD in Enterprise Apps" sessions and they were exceptionally good. I will definitely participate in his sessions again. Udi is a great presenter and has the ability to explain complex issues in a manner that everyone understands.”

Eran Sagi, Software Architect at HP
“So far, I heard about Service Oriented architecture all over. Everyone mentions it – the big buzz word. But, when I actually asked someone for what does it really mean, no one managed to give me a complete satisfied answer. Finally in his excellent course “Advanced Distributed Systems”, I got the answers I was looking for. Udi went over the different motivations (principles) of Services Oriented, explained them well one by one, and showed how each one could be technically addressed using NService bus. In his course, Udi also explain the way of thinking when coming to design a Service Oriented system. What are the questions you need to ask yourself in order to shape your system, place the logic in the right places for best Service Oriented system.

I would recommend this course for any architect or developer who deals with distributed system, but not only. In my work we do not have a real distributed system, but one PC which host both the UI application and the different services inside, all communicating via WCF. I found that many of the architecture principles and motivations of SOA apply for our system as well. Enough that you have SW partitioned into components and most of the principles becomes relevant to you as well. Bottom line – an excellent course recommended to any SW Architect, or any developer dealing with distributed system.”

Consult with Udi

Guest Authored Books
Chapter: Introduction to SOA    Article: The Enterprise Service Bus and Your SOA

97 Things Every Software Architect Should Know



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