[poc] Elixir SDK #208

schmit · 2025-02-17T05:15:39Z

This implements the basics of an Elixir SDK that can evaluate feature flags.

UFC test cases are added and all pass.
Can be configured as a singleton using a GenServer
Documentation is auto-generated
Github workflow for automated testing

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elixir-sdk/native/sdk_core/src/lib.rs

rasendubi · 2025-02-17T13:52:57Z

elixir-sdk/test/sdk_core_test.exs

+  test "init with valid config succeeds" do
+    config = %SdkCore.Config{
+      api_key: "test-key",
+      base_url: "https://api.eppo.cloud",


How do you mock a configuration so it is possible to write more tests locally?

We have a mock-server implementation that exposes all configurations from sdk-test-data on different URLs.

You can start it with npm run start-mock-server and pass different base urls to the client:

http://127.0.0.1:8378/ufc/api

http://127.0.0.1:8378/obfuscated/api

http://127.0.0.1:8378/bandit/api

rasendubi · 2025-02-17T14:00:31Z

elixir-sdk/test/sdk_core_test.exs

+  test "init with valid config succeeds" do
+    config = %SdkCore.Config{
+      api_key: "test-key",
+      base_url: "https://api.eppo.cloud",


I got get_string_assignment to work locally in a script (although the targeting rules don’t seem to be working; I got stuck trying to figure out how to debug).

For debugging, I think getting logs working is most important as they'll tell you what's going on under the hood and maybe even what's wrong.

The second tip for debugging is trying evaluation details methods — they give quite a bit of insight on why targeting didn't work (e.g., whether it's missing configuration, rule condition failed, etc. In multiplatform, it even shows individual attribute values that it considered, so if you mess up elixir-rust conversion, it'll probably show it)

For this particular failure, I think base url might be to blame — it's missing trailing /api here and the default value is just an empty string, so not correct either. So what I think is happening is that it can't fetch configuration and is returning default values.

Good catch and good tip! I did fix the base_url and verified that the config is downloaded successfully (get_string_assignment returns a non-default variation). I think there is a bug with the conversion of attributes but I will implement get_details and try to use that to debug

elixir-sdk/native/sdk_core/src/lib.rs

rasendubi · 2025-02-17T14:58:54Z

elixir-sdk/README.md

My main question is, given this steel thread, how much more work is there to do? I think there’s quite a lot (I’m guessing somewhere between 5x and 20x) and wanted to understand whether it is feasible for me to keep working on it at random hours and make meaningful progress.

I actually think that you already implemented the hardest part — figuring out how to interact between the languages. After you get the targeting bug resolved (which I hope is easy), what's left is:

get assignment logger work (shall be quite trivial as well once you pass event to elixir side)

define the rest of API and write all the boilerplate code (not familiar with elixir but it'll probably take between a day and three if you want to get fancy). Bandits with their "context attributes" may get tricky but maybe not given that Elixir seems loosely typed. (e.g., for Dart, it felt awkward to have generic attributes and context attributes as different types, so I found a way to unify them into a single attribute type)

packaging and releasing (very much depends on the tooling if that's easy or not)

rasendubi · 2025-02-17T15:08:56Z

elixir-sdk/native/sdk_core/src/lib.rs

+    // Set global instance
+    let mut instance = CLIENT_INSTANCE
+        .write()
+        .map_err(|e| format!("Failed to acquire write lock: {}", e))?;
+
+    if let Some(existing) = instance.take() {
+        // Shutdown existing client
+        drop(existing);
+    }
+
+    *instance = Some(client.clone());


minor: if you can, I would like to push singleton up as much as possible. Ideally, having a non-singleton EppoClient "class" in Elixir with a thin wrapper to initialize/get instance.

This shall make it easier to support multiple clients in the future. Not sure about Elixir's threading model but that may also help us to get rid of locks (most of core is designed to allow concurrent access from multiple threads). Non-singletons are also kind of easier to reason about

Hmm Elixir is functional and hence the singleton pattern does not make as much sense; hence I figured it would be easiest to completely hide the client object in Elixir. There's something called a GenServer we could try using, but I feel like it'd needlessly complicate things (and you can only have 1 with a given name, so wouldn't support multiple clients easily either).

There's probably a good option available, I just don't know it

Fixed this using a GenServer, actually is pretty nice, hope you like it!

schmit · 2025-02-17T19:32:41Z

eppo_core/src/ufc/models.rs

@@ -96,7 +96,7 @@ pub(crate) struct FlagWire {
 }

 /// Type of the variation.
-#[derive(Debug, Serialize, Deserialize, Clone, Copy, PartialEq, Eq)]
+#[derive(Debug, Serialize, Deserialize, Clone, Copy, PartialEq, Eq, rustler::NifUnitEnum)]


@rasendubi it feels like a bit of a smell to add this but it seemed like the easiest way to make sure we can convert Elixir atoms (such as :string to a VariationType). Let me know if you want me to avoid adding rustler and this derive in eppo_core

I think that's ok'ish. We have some derives for python/ruby in core as well. Unfortunately, because of rust impl rules, we can't add these implementation outside of core so I think that's fine for now.

The only thing is that this should depend on a feature flag, so we don't pull any of elixir libraries into other languages SDKs.

Suggested change

#[derive(Debug, Serialize, Deserialize, Clone, Copy, PartialEq, Eq, rustler::NifUnitEnum)]

#[derive(Debug, Serialize, Deserialize, Clone, Copy, PartialEq, Eq)]

#[cfg_attr(feature = "rustler", derive(rustler::NifUnitEnum))]

I couldn't get this to work yet, might need some extra eyes later.

schmit · 2025-02-17T19:38:59Z

elixir-sdk/native/sdk_core/src/lib.rs

+    if let Some(event) = event {
+        let json_value = serde_json::to_value(&event)
+            .map_err(|e| rustler::Error::Term(Box::new(format!("Failed to serialize event: {:?}", e))))?;
+        if let serde_json::Value::Object(map) = json_value {
+            let converted: HashMap<String, String> = map.into_iter()
+                .map(|(k, v)| (k, v.to_string()))
+                .collect();
+            Ok(converted.encode(env))
+        } else {
+            Err(rustler::Error::Term(Box::new("Event did not serialize to an object".to_string())))
+        }


This is pretty gross -- I wonder what's best here, perhaps just send the json as a string and then convert to an object in Elixir (losing the performance of Rust)? Or maybe just keep it as a string (because we'll be logging a string anyway), that seems gross too

Have you looked into serde_rustler? (I didn't really but it looks like something that should help converting between rust/elixir given we already have Serialize/Deserialize implementations)

Good call, the repo is archived and hasn't been updated; also doesn't seem to be working with more recent Elixir versions. Sounds like there were some plans to merge this into Rustler but the issue is still open: rusterlium/rustler#200

oh, that sucks. it looks like there's some experimental serde support in rustler already: https://docs.rs/rustler/latest/rustler/serde/index.html

We may write our own encoder from events to elixir terms. Shouldn't be hard — just boring

But I think to json -> from json approach is also viable for the first version.

I think another option is keeping event in a resource arc, so the user can decide whether they want to serialize it to string or query data from it

For now, opted to send raw json to Elixir, and convert using Elixir's JSON library. Helps with nesting too. We can optimize later.

schmit · 2025-02-19T02:56:34Z

Current status: got get_X_assignment and get_X_assignment_details to work locally. Up next:

Write tests using sdk-test-data
Create some examples (script, Phoenix app)

schmit · 2025-02-20T17:18:19Z

elixir-sdk/lib/assignment_logger.ex

@@ -0,0 +1,12 @@
+defmodule Eppo.AssignmentLogger do


Unsure whether to use Eppo.X or EppoSDK.X

Suggested change

defmodule Eppo.AssignmentLogger do

defmodule EppoSDK.AssignmentLogger do

Curious what other think

In particular:

Avoid namespace conflicts with existing packages. Plug owns the Plug namespace, if you have an authentication package for Plug use the namespace PlugAuth instead of Plug.Auth.
This guidelines holds for all of your modules, so if your package is plug_auth then all of your modules (except for special ones like mix tasks) should start with PlugAuth. (e.g. PlugAuth.Utils, PlugAuth.User). This is important because there can only be one module with a given name running in a BEAM system, and if multiple packages define the same module, then you cannot use both packages because only one version of the module will be used.

So we can not create another Eppo package if we ever wanted to.

Defaulting to call it EppoSdk for now, seems most consistent but open to other suggestions

schmit · 2025-02-20T17:18:38Z

elixir-sdk/lib/client.ex

@@ -0,0 +1,461 @@
+defmodule Eppo.Client do


This module is the main interface for the client

schmit · 2025-02-20T17:19:51Z

elixir-sdk/lib/server.ex

+  end
+
+  @impl true
+  def init(%Eppo.Client.Config{} = config) do


Alternatively, we could require passing a client rather than a config?

felipecsl

Awesome work. I'm not really familiar with Elixir, but on a high level looks good to me.

felipecsl · 2025-02-20T18:54:18Z

elixir-sdk/native/sdk_core/src/lib.rs

+    });
+
+    Ok(client)
+}


it seems like we have this same init routine repeated 3 times for each SDK now, might be a good candidate for moving to eppo-core

felipecsl · 2025-02-20T19:00:47Z

I didn't approve yet since it seems like you're still iterating on it and it's probably a good idea getting another set of eyes, but no blockers from me

rasendubi reviewed Feb 17, 2025

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rasendubi reviewed Feb 17, 2025

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rasendubi reviewed Feb 17, 2025

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schmit commented Feb 17, 2025

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schmit added 20 commits February 19, 2025 21:47

Elixir: init and get_instance

9f24ad5

get_string_assignment and get_boolean_assignment

91b8c7a

clean-up tests

770e6b2

Use assignment event

04c207c

Use generic get_assignment interface

60eeaf1

add get_assignment_details

3a985bd

Add some documentation

823ac88

Proper object conversion with Jason

a43fb01

Add a note on how to load the sdk-test-data submodule

547772a

Add npm command to run elixir tests

b26b0a5

WIP: converting subject attributes and adding UFC tests

ff8e7ea

Pass UFC tests

50d32ef

small refactor

2d066f1

Move singleton pattern to Elixir using a GenServer

f2affe6

Better AssignmentLogger

086d75f

Add UFC tests for get_assignment_details

70d65df

comment out unused code to make compiler happy

ea8b920

Add github workflow for elixir tests

e353a3a

More modern elixir versions for workflow

ed42930

use only compatible versions

88a84ce

schmit added 5 commits February 19, 2025 21:47

Only test most recent versions for macos

9de0c73

Only run elixir tests when relevant

84b58db

Automatically generate docs

9d1134f

More documentation

0d0ad6f

Improve docs

df33bef

schmit force-pushed the sps/elixir-sdk branch from 0989da2 to df33bef Compare February 20, 2025 05:47

schmit marked this pull request as ready for review February 20, 2025 05:47

schmit requested review from felipecsl and rasendubi February 20, 2025 05:48

No support for testing macos in github workflow

783ccda

schmit commented Feb 20, 2025

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elixir-sdk/lib/client.ex Outdated

@@ -0,0 +1,461 @@

defmodule Eppo.Client do

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schmit Feb 20, 2025

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This module is the main interface for the client

schmit commented Feb 20, 2025

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schmit added 3 commits February 20, 2025 10:29

Rename Eppo -> EppoSdk for consistency

7fc4a70

clean up: no testing of details return

ca160dd

Add more paths to CI workflow

b6e7c59

felipecsl reviewed Feb 20, 2025

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[poc] Elixir SDK #208

[poc] Elixir SDK #208

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felipecsl commented Feb 20, 2025

	#[derive(Debug, Serialize, Deserialize, Clone, Copy, PartialEq, Eq, rustler::NifUnitEnum)]
	#[derive(Debug, Serialize, Deserialize, Clone, Copy, PartialEq, Eq)]
	#[cfg_attr(feature = "rustler", derive(rustler::NifUnitEnum))]

	defmodule Eppo.AssignmentLogger do
	defmodule EppoSDK.AssignmentLogger do

[poc] Elixir SDK #208

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[poc] Elixir SDK #208

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