Add a stdout sink

README.md

@@ -153,6 +153,16 @@ To use the loki driver, you'll need to set up:
 - `--loki-url` (or the env var `VERCEL_LOG_DRAIN_LOKI_URL`)
 - (optional, if you have basic auth) `--loki-basic-auth-user` and `--loki-basic-auth-pass` (or the corresponding env vars `VERCEL_LOG_DRAIN_LOKI_USER` and `VERCEL_LOG_DRAIN_LOKI_PASS`)
 
+### Standard Out
+
+You can enable the `stdout` driver by simply adding `--enable-stdout` which will print the each `Message` payload as json to stdout. Application logging will be printed to `stderr`.
+
+> [!IMPORTANT]
+> If you have a previous deployment and enable the `stdout` driver it may effect your logging setup.
+> The default (without stdout driver) application logs from the `vercel-log-drain` go to stdout.
+> With this driver enabled app will now go to `stderr` and the Vercel messages are printed to `stdout`.
+
+
 ## Configuration
 
 | CLI Flag                 | Environment Variable                 | Default Value | Description                              |

src/drivers/mod.rs

@@ -1,8 +1,12 @@
+mod stdout;
+
 #[cfg(feature = "cloudwatch")]
 mod cloudwatch;
 #[cfg(feature = "loki")]
 mod loki;
 
+pub use stdout::StdOutDriver;
+
 #[cfg(feature = "cloudwatch")]
 pub use cloudwatch::CloudWatchDriver;
 #[cfg(feature = "loki")]

src/drivers/stdout.rs

@@ -0,0 +1,25 @@
+use crate::types::{LogDriver, Message};
+use anyhow::Result;
+use async_trait::async_trait;
+use tracing::debug;
+
+pub struct StdOutDriver;
+
+impl StdOutDriver {
+    pub fn new() -> Self {
+        Self {}
+    }
+}
+#[async_trait]
+impl LogDriver for StdOutDriver {
+    async fn init(&mut self) -> Result<()> {
+        debug!("init stdout driver");
+        Ok(())
+    }
+
+    async fn send_log(&mut self, message: &Message) -> Result<()> {
+        let log_line = serde_json::to_string(message)?;
+        println!("{}", log_line);
+        Ok(())
+    }
+}

src/main.rs

@@ -9,6 +9,7 @@ use crate::types::LogDriver;
 use axum::routing::get;
 use axum_prometheus::PrometheusMetricLayerBuilder;
 use clap::Parser;
+use std::io;
 use tokio::signal::{unix, unix::SignalKind};
 use tokio::sync::mpsc;
 use tracing::{debug, info, Level};
@@ -38,6 +39,9 @@ struct Args {
     #[arg(long, env = "VERCEL_LOG_DRAIN_METRICS_PREFIX", default_value = "drain")]
     metrics_prefix: String,
 
+    #[arg(long, env = "VERCEL_LOG_DRAIN_ENABLE_STDOUT")]
+    enable_stdout: bool,
+
     #[cfg(feature = "cloudwatch")]
     #[arg(long, env = "VERCEL_LOG_DRAIN_ENABLE_CLOUDWATCH")]
     enable_cloudwatch: bool,
@@ -59,15 +63,27 @@ struct Args {
 #[tokio::main]
 async fn main() -> anyhow::Result<()> {
     let args = Args::parse();
-    tracing_subscriber::fmt()
-        .json()
-        .with_max_level(args.log)
-        .init();
+    if args.enable_stdout {
+        tracing_subscriber::fmt()
+            .json()
+            .with_max_level(args.log)
+            .with_writer(io::stderr)
+            .init();
+    } else {
+        tracing_subscriber::fmt()
+            .json()
+            .with_max_level(args.log)
+            .init();
+    }
 
     let (tx, rx) = mpsc::unbounded_channel::<types::Message>();
 
     let mut drivers: Vec<Box<dyn LogDriver>> = Vec::new();
 
+    if args.enable_stdout {
+        drivers.push(Box::new(StdOutDriver::new()));
+    }
+
     #[cfg(feature = "cloudwatch")]
     if args.enable_cloudwatch {
         let config = aws_config::load_defaults(aws_config::BehaviorVersion::v2024_03_28()).await;