Using Consul for dynamic configuration management

Question

I am working on designing a little project where I need to use Consul to  manage application configuration in a dynamic way so that all my app machines can get the configuration at the same time without any inconsistency issue. We are using Consul already for service discovery purpose so I was reading more about it and it looks like they have a Key/Value store which I can use to manage my configurations.
All our configurations are json file so we make a zip file with all our json config files in it and store the reference from where you can download this zip file in a particular key in Consul Key/Value store. And all our app machines need to download this zip file from that reference (mentioned in a key in Consul) and store it on disk on each app machine. Now I need all app machines to switch to this new config at the same time approximately to avoid any inconsistency issue.
Let's say I have 10 app machines and all these 10 machines needs to download zip file which has all my configs and then switch to new configs at the same time atomically to avoid any inconsistency (since they are taking traffic). Below are the steps I came up with but I am confuse on how loading new files in memory along with switch to new configs will work:

All 10 machines are already up and running with default config files as of now which is also there on the disk.
Some outside process will update the key in my consul key/value store with latest zip file reference.
All the 10 machines have a watch on that key so once someone updates the value of the key, watch will be triggered and then all those 10 machines will download the zip file onto the disk and uncompress it to get all the config files.
(..)
(..)
(..)

Now this is where I am confuse on how remaining steps should work.

How apps should load these config files in memory and then switch all at same time?
Do I need to use leadership election with consul or anything else to achieve any of these things?
What will be the logic around this since all 10 apps are already running with default configs in memory (which is also stored on disk). Do we need two separate directories one with default and other for new configs and then work with these two directories?

Let's say if this is the node I have in Consul just a random design (could be wrong here) -
{"path":"path-to-new-config", "machines":"ip1:ip2:ip3:ip4:ip5:ip6:ip7:ip8:ip9:ip10", ...}

where path will have new zip file reference and machines could be a key here where I can have list of all machines so now I can put each machine ip address as soon as they have downloaded the file successfully in that key? And once machines key list has size of 10 then I can say we are ready to switch? If yes, then how can I atomically update machines key in that node? Maybe this logic is wrong here but I just wanted to throw out something. And also need to clean up all those machines list after switch since for the next config update I need to do similar exercise.
Can someone outline the logic on how can I efficiently manage configuration on all my app machines dynamically and also avoid inconsistency issue at the same time? Maybe I need one more node as status which can have details about each machine config, when it downloaded, when it switched and other details?

tzachs · Answer

I can think of several possible solutions, depending on your scenario.
The simplest solution is not to store your config in memory and files at all, just store the config directly in the consul kv store. And I'm not talking about a single key that maps to the entire json (I'm assuming your json is big, otherwise you wouldn't zip it), but extracting smaller key/value sets from the json (this way you won't need to pull the whole thing every time you make a query to consul).
If you get the config directly from consul, your consistency guarantees match consul consistency guarantees. I'm guessing you're worried about performance if you lose your in-memory config, that's something you need to measure. If you can tolerate the performance loss, though, this will save you a lot of pain.
If performance is a problem here, a variation on this might be to use fsconsul. With this, you'll still extract your json into multiple key/value sets in consul, and then fsconsul will map that to files for your apps.
If that's off the table, then the question is how much inconsistencies are you willing to tolerate.
If you can stand a few seconds of inconsistencies, your best bet might be to put a TTL (time-to-live) on your in-memory config. You'll still have the watch on consul but you combine it with evicting your in-memory cache every few seconds, as a fallback in case the watch fails (or stalls) for some reason. This should give you a worst-case few seconds inconsistencies (depending on the value you set for your TTL), but normal case (I think) should be fast.
If that's not acceptable (does downloading the zip take a lot of time, maybe?), you can go down the route you mentioned. To update a value atomically you can use their cas (check-and-set) operation. It will give you an error if an update had happened between the time you sent the request and the time consul tried to apply it. Then you need to pull the list of machines, and apply your change again and retry (until it succeeds).
I don't see why you would need 2 directories, but maybe I'm misunderstanding the question: when your app starts, before you do anything else, you check if there's a new config and if there is you download it and load it to memory. So you shouldn't have a "default config" if you want to be consistent. After you downloaded the config on startup, you're up and alive. When your watch signals a key change you can download the config to directly override your old config. This is assuming you're running the watch triggered code on a single thread, so you're not going to be downloading the file multiple times in parallel. If the download failed, it's not like you're going to load the corrupt file to your memory. And if you crashed mid-download, then you'll download again on startup, so should be fine.

Using Consul for dynamic configuration management

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