86 lines
2.8 KiB
Markdown
86 lines
2.8 KiB
Markdown
+++
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title = "Leveraging Env Vars in Rust Apps"
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[categories]
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tags = [ "rust", "config" ]
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categories = [ "programming", "apps"]
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+++
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Environment variable have gained a lot of importance since the rise of
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the container based deployments and (consequently) the popularity of the
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[12 factor app](https://12factor.net/).
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It also has become very practical with the widespread support of the
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*.env* file in the project folder which makes configuring apps during
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development very practical.
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# Using environment in Rust
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The [std::env](https://doc.rust-lang.org/std/env/index.html) package
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gives access to the environment variables, and also information about
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the working directory, the location of the program executing, temp
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folder, etc...
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The method we really are interested in is
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[var](https://doc.rust-lang.org/std/env/fn.var.html).
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``` example
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match env::var("MQTT_BROKER") {
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Ok(mqtt_broker) => mqtt_init(&mqtt_broker).await,
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Err(e) => error!("No broker specified in MQTT_BROKER environment variable.({})", e)
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}
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```
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It returns a *Result\<String, VarError\>* which we can easily pattern
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match on and give readable feedback to the user.
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I thing this is perfectly fine for simple, small apps I am likely to
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write in the foreseeable future.
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# Controlling Logging from the Environment
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Another thing needed for smallisch apps is a logging system with the
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following requirements:
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- Controllable via environment
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- Add a timestamp
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- Output to stdout or stderr (a 12 factor thing)
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- Namespace modules
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- Override config for specific modules
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Rust has a standard logging API defined in the [log
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crate](https://docs.rs/log/0.4.11/log/) crate for which a large
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selection of implementations is available.
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The first one on the [list with
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implementations](https://docs.rs/log/0.4.11/log/#available-logging-implementations)
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fit all my requirements, so that\'s fine.
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All we need to do is initialize it after reading the environment
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variables from the *.env* file :
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``` example
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async fn main() {
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dotenv::dotenv().ok();
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env_logger::init();
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...
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```
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and we are logging using the standard `debug!`{.verbatim},
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`info!`{.verbatim}, `warn!`{.verbatim}, ... macros.
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# Scaling to larger apps
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When apps grow (or just when they live long enough) they tend to
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accumulate config options and layers of modules making logging also a
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headache.
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When confronted with these issues I saw that the *config* and *envy*
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crates offer nice layered configuration support and straightforward
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pouring in type safe structs.
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Similarly there are more flexible, and consequently more complex,
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logging frameworks like *log4rs*. There are also structured logging
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libraries but I still need to see how these can work in containers
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without adding additional hoops to jump through.
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Let\'s hope my apps stay small and simple and do not need this
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additional complexity.
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