How to Parse Structured Environment Variables in Rust

30 May 2024

I'm in the process of adding more components to my OpenTelemetry demo (again!). The new design deploys several warehouse services behind the inventory service so the latter can query the former for data via their respective HTTP interface. I implemented each warehouse on top of a different technology stack. This way, I can show OpenTelemetry traces across several stacks.

Anyone should be able to add a warehouse in their favorite tech stack if it returns the correct JSON payload to the inventory. For this, I want to make the inventory’s configuration "easy"; add a new warehouse with a simple environment variable pair, i.e., the endpoint and its optional country.

The main issue is that environment variables are not structured. I searched for a while and found a relevant post. Its idea is simple but efficient; here's a sample from the post:

FOO__1__BAR=setting-1         #1
FOO__1__BAZ=setting-2         #1
FOO__2__BAR=setting-3         #1
FOO__2__QUE=setting-4         #1

FIZZ__1=setting-5             #2
FIZZ__2=setting-6             #2

BILL=setting-7                #3
  1. Map-like structure
  2. Table-like structure
  3. Just a value

With this approach, I could configure the inventory like this:

    image: otel-inventory:1.0
      WAREHOUSE__0__ENDPOINT: http://apisix:9080/warehouse/us #1
      WAREHOUSE__0__COUNTRY: USA                              #2
      WAREHOUSE__1__ENDPOINT: http://apisix:9080/warehouse/eu #1
      WAREHOUSE__2__ENDPOINT: http://warehouse-jp:8080        #1
      WAREHOUSE__2__COUNTRY: Japan                            #2
      OTEL_EXPORTER_OTLP_ENDPOINT: http://jaeger:4317
  1. Warehouse endpoint
  2. Set country

You can see the three warehouses configured in the above. Each has an endpoint/optional country pair.

My first attempt looked like the following:

lazy_static::lazy_static! {                                                     //1
    static ref REGEXP_WAREHOUSE: Regex = Regex::new(r"^WAREHOUSE__(\d)__.*").unwrap();

    .filter(|(key, _)| REGEXP_WAREHOUSE.find(key.as_str()).is_some())           //2
    .group_by(|(key, _)| key.split("__").nth(1).unwrap().to_string())           //3
    .into_iter()                                                                //4
    .map(|(_, mut group)| {                                                     //5
        let some_endpoint = group.find(|item| item.0.ends_with("ENDPOINT"));    //6
        let endpoint = some_endpoint.unwrap().1;
        let some_country = group                                                //7
            .find(|item| item.0.ends_with("COUNTRY"))
            .map(|(_, country)| country);
        println! {"Country pair is: {:?}", some_country};
            (endpoint, some_country).into()                                     //8
  1. For making constants out of code evaluated at runtime
  2. Filter out warehouse-related environment variable
  3. Group by index
  4. Back to an Iter with the help of itertools
  5. Consist of just the endpoint or the endpoint and the country
  6. Get the endpoint
  7. Get the country
  8. Into a structure - irrelevant

I encountered issues several times when I started the demo. The code somehow didn't find the endpoint at all. I chose this approach because I've been taught that it's more performant to iterate throughout the key-value pairs of a map than iterate through its key only and then get the value in the map. I tried to change to the latter.

lazy_static! {
    static ref REGEXP_WAREHOUSE_ENDPOINT: Regex =
        Regex::new(r"^WAREHOUSE__(?<index>\d)__ENDPOINT.*").unwrap();           //1
    .filter(|(key, _)| REGEXP_WAREHOUSE_ENDPOINT.find(key.as_str()).is_some())  //2
    .map(|(key, endpoint)| {
        let some_warehouse_index = REGEXP_WAREHOUSE_ENDPOINT.captures(key.as_str()).unwrap(); //3//4
        println!("some_warehouse_index: {:?}", some_warehouse_index);
        let index ="index").unwrap().as_str();
        let country_key = format!("WAREHOUSE__{}__COUNTRY", index);             //5
        let some_country = var(country_key);                                    //6
        println!("endpoint: {}", endpoint);
        (endpoint, some_country).into()
  1. Change the regex to capture only the endpoint-related variables
  2. Filter out warehouse-related environment variable
  3. I'm aware that the filter_map() function exists, but I think it's clearer to separate them here
  4. Capture the index
  5. Create the country environment variable from a known string, and the index
  6. Get the country

With this code, I didn't encounter any issues.

Now that it works, I'm left with two questions:

  • Why doesn't the group()/find() version work in the deployed Docker Compose despite working in the tests?
  • Is anyone interested in making a crate out of it?

To go further: