wolf4earth

What to use to rehydrate process state

We have a stateful Elixir service at work running in a Kubernetes cluster which keeps some user related ephemeral data in processes (think of it as a session process). At the moment we lose this state on a redeployment, which hasn’t been an issue yet. We do plan to keep additional information in there which is why we now want to investigate on how to keep this state around beyond a deployment.

The current idea is to run multiple instances of the app and also sync the process state into an in-memory data store. If a node then goes down - for example due to a deployment - new processes can be spun up on other nodes which rehydrate their state through this synced state.

I’ve spent some time looking into our options:

mnesia
riak_core
redis

mnesia and riak_core both have the charm that they run alongside our application without needing to spin up something separate like in the case of redis. Now I have some thoughts on each of these options:

mnesia

From what I’ve read so far mnesia seems like a solid choice as it’s comes with OTP but has one caveat: it has no built-in support for handling split-brain scenarios.

Since we’re running this app in a Kubernetes cluster and not on a telephone switch with a shared backplane, split-brain scenarios are not something we can ignore. This is not necessarily a deal breaker for us but it makes the next contender much more interesting.

riak_core

My current understanding is that riak_core is what powers riak KV, riak TS etc… It’s also better equipped to handle a split-brain scenario compared to mnesia is.

What has been confusing me though is: how the hell do I use it?

there is the “official” riak_core repository which seems to under active development but can’t be found on hex
there’s also a fork (riak_core_ng) which can be found on hex but development seems to be stalled

I’ve tried to install riak_core_ng but immediately ran into issues as it depends on a fork of poolboy (version spec ~> 0.8.4) and we’re using the “real” poolboy at version 1.5.2. We’re also running on OTP 23 which I expect to give us trouble as the latest release of riak_core_ng was mid 2018.

redis

Last but not least. Redis is a pretty straight-forward choice but kinda makes me sad. We can’t run it inside the same BEAM instance and it would require encoding and decoding our state (which should be manageable with erlang:term_to_binary/1 but still).

Right now it seems like the simple choice though, and I like simple.

I’d be interested to hear your perspective on this, big bonus when you have actual prod experience to back it up.

15 comments

#riak

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2021-02-02 22:16:51 UTC

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chulkilee

Some notes from a few prod services running elixir on k8s

If you need some data after restart, those data won’t be ephemeral anymore, and you have to deal with it as like other persistent data, but probably with different performance/reliability requirement in this case. You mentioned you need to handle split-brain scenarios - then how are you handling the split-brain scenario with other persistent data?

The easiest way is to use the same persistent data (e.g. database for example). This is good unless the “less” important data causes bottleneck. In that case, you may just run a separate instance of the same kind of storage - instead of introducing new kind of storage.

You mentioned Redis - that’s a good choice (super easy to run). However introducing new type of external system needs good justification - for both app developer and infra management.

One of my projects actually stores all data in the database. We had to make sure new cluster should not start the job before existing cluster stopped and pushed all data into the database. With some work, if you carefully divide features, you can actually achieve zero-down time with some degraded performance on some features requiring pass-over the data.

In another project - I simply use oban background job processing instead of running a long-running process and worrying about redeployment. This project has relatively low volume so the choice was obvious, and it works great.

Oh, and yes, we also hit the network split in the k8s which led to split erlang cluster I highly recommend to just stick with simple centralized storage, instead of baking some tools inside your app erlang cluster. You should avoid adding more responsibility and requirement to your application when it’s not the core job.

If you move to erlang cluster wth hot-code reloading, some problems will become very easy to solve - but you also need to solve new problems, which are already covered by k8s and your existing k8s setup (such as CI/CD). If your app’s scale and challenges can justify the investment, that would be great. In my case this was not an option.

Post #11

LostKobrakai

This one might be interesting to you as well

Post #4

dimitarvp

No, it doesn’t. It’s more like a per-node pure Elixir mini DB.

Post #5

dimitarvp

I am afraid I can’t give you a very informed take. I used DeltaCRDT once in a hobby project and I liked it a lot; that being said, I’ve heard people being displeased with CRDTs in general and that you still have to resolve some conflicts manually (imagine if two clients without internet made slightly conflicting changes and then both tried to synchronize them after they got back online).

However, I was left with the impression (after I tinkered on my hobby project for a while) that the CRDTs are very applicable in many areas. It just seems that people want to try them in scenarios that are either impossible (parallel conflicting changes) or not meant for them (offline-first apps).

But again, take this with a big spoon of salt. I am not an expert, just a guy who decided to see what’s this thing so many people talked about. They don’t seem very applicable to decentralized / federated scenarios is all. I liked using CRDTs and I can see applications but can’t talk informed about subtler nuances.

Post #7