diff --git a/src/server.rs b/src/server.rs
index 4abeb2c..45c2fd7 100644
--- a/src/server.rs
+++ b/src/server.rs
@@ -22,6 +22,60 @@ pub struct AppState {
     pub config: Arc<Config>,
 }
 
+// --- Pure decision functions ---
+
+/// Pick the first volume from the record that appears in the healthy set.
+fn pick_healthy_volume<'a>(
+    record_volumes: &'a [String],
+    healthy: &HashSet<String>,
+) -> Option<&'a str> {
+    record_volumes
+        .iter()
+        .find(|v| healthy.contains(v.as_str()))
+        .map(|v| v.as_str())
+}
+
+/// Compute the placement plan for a PUT: which path and which volumes.
+struct PutPlan {
+    path: String,
+    target_volumes: Vec<String>,
+}
+
+fn plan_put(ring: &Ring, key: &str, replication: usize) -> Result<PutPlan, AppError> {
+    let path = Ring::key_path(key);
+    let target_volumes = ring.get_volumes(key, replication);
+    if target_volumes.len() < replication {
+        return Err(AppError::VolumeError(format!(
+            "need {replication} volumes but only {} available",
+            target_volumes.len()
+        )));
+    }
+    Ok(PutPlan {
+        path,
+        target_volumes,
+    })
+}
+
+/// Evaluate fan-out PUT results. Returns the list of volumes that succeeded,
+/// or an error if any failed.
+fn evaluate_fanout(results: Vec<Result<(), String>>, volumes: &[String]) -> Result<Vec<String>, Vec<String>> {
+    let mut succeeded = Vec::new();
+    let mut errors = Vec::new();
+    for (i, result) in results.into_iter().enumerate() {
+        match result {
+            Ok(()) => succeeded.push(volumes[i].clone()),
+            Err(e) => errors.push(e),
+        }
+    }
+    if errors.is_empty() {
+        Ok(succeeded)
+    } else {
+        Err(errors)
+    }
+}
+
+// --- Handlers ---
+
 /// GET /:key — look up key, redirect to a healthy volume
 pub async fn get_key(
     State(state): State<AppState>,
@@ -36,20 +90,14 @@ pub async fn get_key(
         .await?;
 
     let healthy = state.healthy_volumes.read().await;
+    let vol = pick_healthy_volume(&record.volumes, &healthy).ok_or(AppError::NoHealthyVolume)?;
+    let location = format!("{vol}{}", record.path);
 
-    // Pick the first healthy volume
-    for vol in &record.volumes {
-        if healthy.contains(vol) {
-            let location = format!("{}{}", vol, record.path);
-            return Ok((
-                StatusCode::FOUND,
-                [(axum::http::header::LOCATION, location)],
-            )
-                .into_response());
-        }
-    }
-
-    Err(AppError::NoHealthyVolume)
+    Ok((
+        StatusCode::FOUND,
+        [(axum::http::header::LOCATION, location)],
+    )
+        .into_response())
 }
 
 /// PUT /:key — store blob on volumes, record in index
@@ -58,27 +106,17 @@ pub async fn put_key(
     Path(key): Path<String>,
     body: Bytes,
 ) -> Result<Response, AppError> {
-    let replication = state.config.server.replication_factor;
-    let path = Ring::key_path(&key);
-
-    let target_volumes = {
+    let plan = {
         let ring = state.ring.read().await;
-        ring.get_volumes(&key, replication)
+        plan_put(&ring, &key, state.config.server.replication_factor)?
     };
 
-    if target_volumes.len() < replication {
-        return Err(AppError::VolumeError(format!(
-            "need {replication} volumes but only {} available",
-            target_volumes.len()
-        )));
-    }
-
     // Fan out PUTs to all target volumes concurrently
-    let mut handles = Vec::with_capacity(target_volumes.len());
-    for vol in &target_volumes {
+    let mut handles = Vec::with_capacity(plan.target_volumes.len());
+    for vol in &plan.target_volumes {
         let client = state.volume_client.clone();
         let vol = vol.clone();
-        let path = path.clone();
+        let path = plan.path.clone();
         let key = key.clone();
         let data = body.clone();
         handles.push(tokio::spawn(async move {
@@ -86,37 +124,33 @@ pub async fn put_key(
         }));
     }
 
-    let mut succeeded = Vec::new();
-    let mut failed = false;
-    for (i, handle) in handles.into_iter().enumerate() {
-        match handle.await.unwrap() {
-            Ok(()) => succeeded.push(target_volumes[i].clone()),
-            Err(e) => {
+    let mut results = Vec::with_capacity(handles.len());
+    for handle in handles {
+        results.push(handle.await.unwrap());
+    }
+
+    match evaluate_fanout(results, &plan.target_volumes) {
+        Ok(succeeded_volumes) => {
+            let size = Some(body.len() as i64);
+            state
+                .writer
+                .put(key, succeeded_volumes, plan.path, size)
+                .await?;
+            Ok(StatusCode::CREATED.into_response())
+        }
+        Err(errors) => {
+            for e in &errors {
                 tracing::error!("PUT to volume failed: {e}");
-                failed = true;
             }
+            // Rollback: best-effort delete from any volumes that may have succeeded
+            for vol in &plan.target_volumes {
+                let _ = state.volume_client.delete(vol, &plan.path).await;
+            }
+            Err(AppError::VolumeError(
+                "not all volume writes succeeded".into(),
+            ))
         }
     }
-
-    if failed {
-        // Rollback: delete from volumes that succeeded
-        for vol in &succeeded {
-            if let Err(e) = state.volume_client.delete(vol, &path).await {
-                tracing::error!("Rollback DELETE failed: {e}");
-            }
-        }
-        return Err(AppError::VolumeError(
-            "not all volume writes succeeded".into(),
-        ));
-    }
-
-    let size = Some(body.len() as i64);
-    state
-        .writer
-        .put(key, target_volumes, path, size)
-        .await?;
-
-    Ok(StatusCode::CREATED.into_response())
 }
 
 /// DELETE /:key — remove from volumes and index
@@ -195,3 +229,83 @@ pub async fn list_keys(
     let body = keys.join("\n");
     Ok((StatusCode::OK, body).into_response())
 }
+
+// --- Tests for pure functions ---
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn test_pick_healthy_volume_first_match() {
+        let volumes = vec!["http://vol1".into(), "http://vol2".into(), "http://vol3".into()];
+        let healthy: HashSet<String> = ["http://vol2".into(), "http://vol3".into()].into();
+
+        assert_eq!(pick_healthy_volume(&volumes, &healthy), Some("http://vol2"));
+    }
+
+    #[test]
+    fn test_pick_healthy_volume_none_healthy() {
+        let volumes = vec!["http://vol1".into(), "http://vol2".into()];
+        let healthy: HashSet<String> = HashSet::new();
+
+        assert_eq!(pick_healthy_volume(&volumes, &healthy), None);
+    }
+
+    #[test]
+    fn test_pick_healthy_volume_all_healthy() {
+        let volumes = vec!["http://vol1".into(), "http://vol2".into()];
+        let healthy: HashSet<String> = ["http://vol1".into(), "http://vol2".into()].into();
+
+        // Should return the first one
+        assert_eq!(pick_healthy_volume(&volumes, &healthy), Some("http://vol1"));
+    }
+
+    #[test]
+    fn test_plan_put_sufficient_volumes() {
+        let volumes: Vec<String> = (1..=3).map(|i| format!("http://vol{i}")).collect();
+        let ring = Ring::new(&volumes, 50);
+
+        let plan = plan_put(&ring, "test-key", 2).unwrap();
+        assert_eq!(plan.target_volumes.len(), 2);
+        assert!(!plan.path.is_empty());
+    }
+
+    #[test]
+    fn test_plan_put_insufficient_volumes() {
+        let volumes: Vec<String> = vec!["http://vol1".into()];
+        let ring = Ring::new(&volumes, 50);
+
+        let result = plan_put(&ring, "test-key", 2);
+        assert!(result.is_err());
+    }
+
+    #[test]
+    fn test_evaluate_fanout_all_ok() {
+        let volumes = vec!["http://vol1".into(), "http://vol2".into()];
+        let results = vec![Ok(()), Ok(())];
+
+        let outcome = evaluate_fanout(results, &volumes);
+        assert_eq!(outcome.unwrap(), volumes);
+    }
+
+    #[test]
+    fn test_evaluate_fanout_partial_failure() {
+        let volumes = vec!["http://vol1".into(), "http://vol2".into()];
+        let results = vec![Ok(()), Err("connection refused".into())];
+
+        let outcome = evaluate_fanout(results, &volumes);
+        let errors = outcome.unwrap_err();
+        assert_eq!(errors.len(), 1);
+        assert_eq!(errors[0], "connection refused");
+    }
+
+    #[test]
+    fn test_evaluate_fanout_all_fail() {
+        let volumes = vec!["http://vol1".into(), "http://vol2".into()];
+        let results = vec![Err("err1".into()), Err("err2".into())];
+
+        let outcome = evaluate_fanout(results, &volumes);
+        assert_eq!(outcome.unwrap_err().len(), 2);
+    }
+}