Implement am proxy command

.gitignore

@@ -4,3 +4,5 @@ target
 # This directory is created by the prometheus process
 # Could be removed once we persist the data somewhere else
 data
+
+.vscode

CHANGELOG.md

@@ -12,6 +12,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 - Update all depdencies (#124)
 - Fix multiarch docker image for arm64 users (#125)
 - Update markdown reference generator command to disable TOC (#127)
+- Add `am proxy` command (#128)
 
 ## [0.3.0]
 

src/bin/am/commands.rs

@@ -7,6 +7,7 @@ use tracing::info;
 
 mod explore;
 mod init;
+mod proxy;
 pub mod start;
 pub mod system;
 pub mod update;
@@ -46,6 +47,9 @@ pub enum SubCommands {
     #[clap(alias = "explorer")]
     Explore(explore::Arguments),
 
+    /// Use am as a proxy to another prometheus instance
+    Proxy(proxy::CliArguments),
+
     /// Create a new `am.toml` file interactively with sensible defaults
     Init(init::Arguments),
 
@@ -65,6 +69,7 @@ pub async fn handle_command(app: Application, config: AmConfig, mp: MultiProgres
         SubCommands::Start(args) => start::handle_command(args, config, mp).await,
         SubCommands::System(args) => system::handle_command(args, mp).await,
         SubCommands::Explore(args) => explore::handle_command(args).await,
+        SubCommands::Proxy(args) => proxy::handle_command(args).await,
         SubCommands::Init(args) => init::handle_command(args).await,
         SubCommands::Discord => {
             const URL: &str = "https://discord.gg/kHtwcH8As9";

src/bin/am/commands/proxy.rs

@@ -0,0 +1,80 @@
+use crate::server::start_web_server;
+use anyhow::{bail, Context, Result};
+use clap::Parser;
+use directories::ProjectDirs;
+use std::net::SocketAddr;
+use tokio::select;
+use tokio::sync::watch;
+use tracing::info;
+use url::Url;
+
+#[derive(Parser, Clone)]
+pub struct CliArguments {
+    /// The listen address for the web server of am.
+    ///
+    /// This includes am's HTTP API, the explorer and the proxy to the Prometheus, Gateway, etc.
+    #[clap(
+        short,
+        long,
+        env,
+        default_value = "127.0.0.1:6789",
+        alias = "explorer-address"
+    )]
+    listen_address: SocketAddr,
+
+    /// The upstream Prometheus URL
+    #[clap(long, env, alias = "prometheus-address")]
+    prometheus_url: Option<Url>,
+}
+
+#[derive(Debug, Clone)]
+struct Arguments {
+    listen_address: SocketAddr,
+    prometheus_url: Option<Url>,
+}
+
+impl Arguments {
+    fn new(args: CliArguments) -> Self {
+        Arguments {
+            listen_address: args.listen_address,
+            prometheus_url: args.prometheus_url,
+        }
+    }
+}
+
+pub async fn handle_command(args: CliArguments) -> Result<()> {
+    let args = Arguments::new(args);
+
+    // First let's retrieve the directory for our application to store data in.
+    let project_dirs =
+        ProjectDirs::from("", "autometrics", "am").context("Unable to determine home directory")?;
+    let local_data = project_dirs.data_local_dir().to_owned();
+
+    // Make sure that the local data directory exists for our application.
+    std::fs::create_dir_all(&local_data)
+        .with_context(|| format!("Unable to create data directory: {:?}", local_data))?;
+
+    let (tx, _) = watch::channel(None);
+
+    // Start web server for hosting the explorer, am api and proxies to the enabled services.
+    let web_server_task = async move {
+        start_web_server(&args.listen_address, false, false, args.prometheus_url, tx).await
+    };
+
+    select! {
+        biased;
+
+        _ = tokio::signal::ctrl_c() => {
+            info!("SIGINT signal received, exiting...");
+            Ok(())
+        }
+
+        Err(err) = web_server_task => {
+            bail!("Web server exited with an error: {err:?}");
+        }
+
+        else => {
+            Ok(())
+        }
+    }
+}

src/bin/am/commands/start.rs

@@ -261,8 +261,16 @@ pub async fn handle_command(args: CliArguments, config: AmConfig, mp: MultiProgr
     let (tx, rx) = watch::channel(None);
 
     // Start web server for hosting the explorer, am api and proxies to the enabled services.
-    let web_server_task =
-        async move { start_web_server(&args.listen_address, args.pushgateway_enabled, tx).await };
+    let web_server_task = async move {
+        start_web_server(
+            &args.listen_address,
+            true,
+            args.pushgateway_enabled,
+            None,
+            tx,
+        )
+        .await
+    };
 
     // Start Prometheus server
     let prometheus_args = args.clone();

src/bin/am/server.rs

@@ -4,8 +4,10 @@ use axum::response::Redirect;
 use axum::routing::{any, get};
 use axum::{Router, Server};
 use std::net::SocketAddr;
+use std::sync::Arc;
 use tokio::sync::watch::Sender;
 use tracing::{debug, info};
+use url::Url;
 
 mod explorer;
 mod prometheus;
@@ -14,9 +16,13 @@ mod util;
 
 pub(crate) async fn start_web_server(
     listen_address: &SocketAddr,
+    enable_prometheus: bool,
     enable_pushgateway: bool,
+    prometheus_proxy_url: Option<Url>,
     tx: Sender<Option<SocketAddr>>,
 ) -> Result<()> {
+    let is_proxying_prometheus = prometheus_proxy_url.is_some();
+    let should_enable_prometheus = enable_prometheus && !is_proxying_prometheus;
     let mut app = Router::new()
         // Any calls to the root should be redirected to the explorer which is most likely what the user wants to use.
         .route("/", get(|| async { Redirect::temporary("/explorer/") }))
@@ -32,9 +38,51 @@ pub(crate) async fn start_web_server(
             }),
         )
         .route("/explorer/", get(explorer::handler))
-        .route("/explorer/*path", get(explorer::handler))
-        .route("/prometheus/*path", any(prometheus::handler))
-        .route("/prometheus", any(prometheus::handler));
+        .route("/explorer/*path", get(explorer::handler));
+
+    // Proxy `/prometheus` to the upstream (local) prometheus instance
+    if should_enable_prometheus {
+        app = app
+            .route("/prometheus/*path", any(prometheus::handler))
+            .route("/prometheus", any(prometheus::handler));
+    }
+
+    // NOTE - this will override local prometheus routes if specified
+    if is_proxying_prometheus {
+        let prometheus_upstream_base = Arc::new(prometheus_proxy_url.clone().unwrap());
+
+        // Define a handler that will proxy to an external Prometheus instance
+        let handler = move |mut req: http::Request<Body>| {
+            let upstream_base = prometheus_upstream_base.clone();
+            // 1. Get the path and query from the request, since we need to strip out `/prometheus`
+            let path_and_query = req
+                .uri()
+                .path_and_query()
+                .map(|pq| pq.as_str())
+                .unwrap_or("");
+            if let Some(stripped_path) = path_and_query.strip_prefix("/prometheus") {
+                let stripped_path_str = stripped_path.to_string();
+                // 2. Remove the `/prometheus` prefix.
+                let new_path_and_query =
+                    http::uri::PathAndQuery::from_maybe_shared(stripped_path_str)
+                        .expect("Invalid path");
+
+                // 3. Create a new URI with the modified path.
+                let mut new_uri_parts = req.uri().clone().into_parts();
+                new_uri_parts.path_and_query = Some(new_path_and_query);
+
+                let new_uri = http::Uri::from_parts(new_uri_parts).expect("Invalid URI");
+
+                // 4. Replace the request's URI with the modified URI.
+                *req.uri_mut() = new_uri;
+            }
+            async move { prometheus::handler_with_url(req, &upstream_base).await }
+        };
+
+        app = app
+            .route("/prometheus/*path", any(handler.clone()))
+            .route("/prometheus", any(handler));
+    }
 
     if enable_pushgateway {
         app = app
@@ -52,7 +100,14 @@ pub(crate) async fn start_web_server(
     debug!("Web server listening on {}", server.local_addr());
 
     info!("Explorer endpoint: http://{}", server.local_addr());
-    info!("Prometheus endpoint: http://127.0.0.1:9090/prometheus");
+
+    if should_enable_prometheus {
+        info!("Prometheus endpoint: http://127.0.0.1:9090/prometheus");
+    }
+
+    if is_proxying_prometheus {
+        info!("Proxying to prometheus: {}", prometheus_proxy_url.unwrap());
+    }
 
     if enable_pushgateway {
         info!("Pushgateway endpoint: http://127.0.0.1:9091/pushgateway");

src/bin/am/server/prometheus.rs

@@ -4,6 +4,13 @@ use axum::response::IntoResponse;
 use url::Url;
 
 pub(crate) async fn handler(req: http::Request<Body>) -> impl IntoResponse {
-    let upstream_base = Url::parse("http://localhost:9090").unwrap();
+    let upstream_base = url::Url::parse("http://localhost:9090").unwrap();
     proxy_handler(req, upstream_base).await
 }
+
+pub(crate) async fn handler_with_url(
+    req: http::Request<Body>,
+    upstream_base: &Url,
+) -> impl IntoResponse {
+    proxy_handler(req, upstream_base.clone()).await
+}