Really sorry about the title that probably doesn't make much sense. Hopefully I can explain myself better here as it's something that's kinda bugged me for ages, and is now becoming a pressing concern as I write a bit of software with configuration.

Most software comes with default configuration options stored in the app itself, and then there's a configuration file (let's say) that a user can edit. Once created/edited for the first time, subsequent updates to the application can not (easily) modify this configuration file for fear of clobbering the user's own changes to the default configuration. So my question is, if my application adds a new configurable parameter, what's the best way to aid discoverability of the setting and allow the user (developer) to override it as nicely as possible given the following constraints:

  • I actually don't have a canonical default config in the application per se, it's more of a 'cascading filesystem'-like affair - the config template is stored in default/config.json and when the user wishes to edit the configuration, it's copied to user/config.json. If a user config is found it is used - there is no automatic overriding of a subset of keys, the whole new file is used and that's that. If there's no user config the default config is used. When a user wishes to edit the config they run a command to 'generate' it for them (which simply copies the config.json file from the default to the user directory).

  • There is no UI for the configuration options as it's not appropriate to the userbase (think of my software as a library or something, the users are developers, the config is done in the user/config.json file).

  • Due to my software being library-like there's no simple way to, on updating of the software, run some tasks automatically (so any ideas of look at the current config, compare to template config, add ing missing keys) aren't appropriate.

The only solution I can think of right now is to say "there's a new config setting X" in release notes, but this doesn't seem ideal to me.

If you want any more information let me know. The above specifics are not actually 100% true to my situation, but they represent the problem equally well with lower complexity. If you do want specifics, however, I can explain the exact setup.

Further clarification of the type of configuration I mean: think of the Atom code editor. There appears to be a default 'template' config file somewhere, but as soon as a configuration option is edited ~/.atom/config.cson is generated and the setting goes in there. From now on is Atom is updated and gets a new configuration key, this file cannot be overwritten by Atom without a lot of effort to ensure that the addition/modification of the key does not clobber. In Atom's case, because there is a GUI for editing settings, they can get away with just adding the UI for the new setting into the UI to aid 'discoverability' of the new setting. I don't have that luxury.

Clarification of my constraints and what I'm actually looking for:

The software I'm writing is actually a package for a larger system. This larger system is what provides the configuration, and the way it works is kinda fixed - I just do a config('some.key') kinda call and it knows to look to see if the user has a config clone and if so use it, otherwise use the default config which is part of my package.

Now, while I could make my application edit the user's configuration files (there is a convention about where they're stored), it's generally not done, so I'd like to live with the constraints of the system I'm using if possible.

And it's not just about discoverability either, one large concern is that the addition of a configuration key won't actually work as soon as the user has their own copy of the original template. Adding the key to the template won't make a difference as that file is never read. As such, I think this is actually quite a big flaw in the design of the configuration cascading system and thus needs to be taken up with my upstream.

So, thinking about it, based on my constraints, I don't think there's going to be a good solution save for either editing the user's configuration or using a new config file every time there are updates to the default configuration. Even the release notes idea from above isn't doable as, if the user does not follow the advice, suddenly I have a config key with no value (user-defined or default).

So the new question is this: what is the general way to solve the problem of having a default configuration in template config files and allowing a user to make user-specific version of these in order to override the defaults? A per-key cascade (rather than per-file cascade) where the user only specifies their overrides?

In this case, what happens if a configuration value is an array - do we replace or append to the default (or, more realistically, how does the user specify whether they wish to replace or append to)?

It seems like configuration is kinda hard, so how is it solved in the wild?

4 Answers 4


For new question: you can read the default configuration inside an array/associative mapping, so you have key->default values. Then you read the user configuration, overwriting (in RAM) only the options you encounter (new options will be probably missing), so you have the default values, eventually customized. The files remain the same.

  • Yup this seems like the correct solution to the larger problem. Unfortunately the system I work in doesn't have this behaviour. I think I'll take the issue upstream and see what comes out of it and for now ignore my (currently only hypothetical) problem. Commented Jun 13, 2014 at 8:35

I'd do the following. When the updated software runs and reads the configuration (already cloned), it can see it knows new configuration X but that's not inside the configuration file.

So you can show a message to the user and, if you like, add the option inside one or both the configuration files, maybe with a default value (usually a value that doesn't change previous behavour of the software). I don't know if this violates your 3rd condition.

Duplication gives always some kind of problems. Maybe you can do a clone of the configuration file just in case the edited version cause your application to break (and should be used only by developer, and not your software, to check the last working (backup) version and fix the error in the edited version).

  • +1, exactly my thoughts. I did not understand as well why the OP excludes the possibility of adding the missing keys automatically. There is no need to have an automatic installation or update process for that, the only thing one needs is write access to the user config at the time the software is started.
    – Doc Brown
    Commented Jun 11, 2014 at 13:06
  • @ both - yeah while writing up a clarification of my situation I realised that the problem is the way the configuration cascading works (or doesn't work, I should say). And while I could edit the 'published' configuration file, I'm not really supposed to in the spirit of the system I am working in. So yeah, I think I need to take the issue up with upstream. Commented Jun 11, 2014 at 13:53

Since your format is json, it should be relatively safe to do this:

  • Read and parse the whole config file. If parsing fails, probably the user has made a fatal mistake and this should be corrected before any attempts to automatically append it are made

  • Check for the existence of your new options. If you don't find them, append them (with the default value) in the object model that results from parsing the config file

  • If any such changes have been made, rename the old config.json to config.json.sav or similar and write the changed object model to config.json

The only likely problem is that the user has added whitespace to the file, or rearranged the some itmes, to make the config file more readable by his personal standards. Those changes would be gone. To mitigate this problem, the JSON file should be written in a readable manner by default (intention and linebreaks where appropriate).

Unfortunately, JSON doesn't allow for comments, so marking the new options in the file with comments is not an option.

  • While this seems like a good solution, it breaks constraint #3. I'll update my question to make it a little clearer what I'm actually looking for and what constraints I am being held to by other code I'm relying on. Commented Jun 11, 2014 at 13:23
  • Ooops, I missed that... though you could do that whenever you notice a missing option in the config file
    – user281377
    Commented Jun 11, 2014 at 14:15

If there is a "larger system" responsible for providing the configuration, and your program is something like a "plugin" to that system, the problem has to be solved at the level of that "larger system". And the general solution still remains exactly as described in the other answers: the "larger system" needs to update the user-config file, adding missing keys around the time when the config is read.

If the "larger system" does not know how, since it does not know the entries your program needs, you need a mechanic where the "larger system" asks your plugin to check the available entries in the config, and if some entries are missing, your plugin has to provide them to the system (which afterwards writes the user config for you).

A "per-key cascade", where the user only specifies overrides in the user config, is theoretically possible (for example: that's the way the Firefox user config works), and to make this work also the larger system should provide the mechanic. On the other hand, this solution has the drawback that when you look into the user config file with a text editor, you don't see all the available options - and since you don't want to provide a UI for the config, this is not very user-friendly.

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