diff --git a/Cargo.lock b/Cargo.lock
index fd93b9c36..1426c11a9 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -2918,6 +2918,7 @@ dependencies = [
  "once_cell",
  "reqwest",
  "serde_json",
+ "serial_test",
  "tokio",
  "tokio-util",
  "tracing",
diff --git a/crates/db_schema/src/newtypes.rs b/crates/db_schema/src/newtypes.rs
index e0c516037..8f2e99cc5 100644
--- a/crates/db_schema/src/newtypes.rs
+++ b/crates/db_schema/src/newtypes.rs
@@ -131,7 +131,7 @@ pub struct CommentReplyId(i32);
 #[cfg_attr(feature = "full", derive(DieselNewType, TS))]
 #[cfg_attr(feature = "full", ts(export))]
 /// The instance id.
-pub struct InstanceId(i32);
+pub struct InstanceId(pub i32);
 
 #[derive(
   Debug, Copy, Clone, Hash, Eq, PartialEq, Serialize, Deserialize, Default, PartialOrd, Ord,
diff --git a/crates/federate/Cargo.toml b/crates/federate/Cargo.toml
index 0c3baf348..2ca17c037 100644
--- a/crates/federate/Cargo.toml
+++ b/crates/federate/Cargo.toml
@@ -34,3 +34,6 @@ tokio = { workspace = true, features = ["full"] }
 tracing.workspace = true
 moka.workspace = true
 tokio-util = "0.7.10"
+
+[dev-dependencies]
+serial_test = { workspace = true }
diff --git a/crates/federate/src/lib.rs b/crates/federate/src/lib.rs
index febdb34e1..87c642998 100644
--- a/crates/federate/src/lib.rs
+++ b/crates/federate/src/lib.rs
@@ -44,6 +44,9 @@ pub struct SendManager {
 
 impl SendManager {
   pub fn new(opts: Opts, context: FederationConfig<LemmyContext>) -> Self {
+    assert!(opts.process_count > 0);
+    assert!(opts.process_index > 0);
+
     let (stats_sender, stats_receiver) = unbounded_channel();
     Self {
       opts,
@@ -153,59 +156,102 @@ mod test {
 
   use super::*;
   use activitypub_federation::config::Data;
-use tokio::{spawn, time::sleep};
+  use lemmy_utils::error::LemmyError;
+  use serial_test::serial;
+  use std::sync::{Arc, Mutex};
+  use tokio::{spawn, time::sleep};
 
   struct TestData {
     send_manager: SendManager,
     context: Data<LemmyContext>,
-    instances: Vec<Instance>
+    instances: Vec<Instance>,
   }
-
-  async fn init() -> LemmyResult<TestData> {
-    let context = LemmyContext::init_test_context().await;
-    let opts = Opts {
-      process_count: 1,
-      process_index: 1,
-    };
-    let federation_config = FederationConfig::builder()
-      .domain("local.com")
-      .app_data(context.clone())
-      .build()
-      .await?;
+  impl TestData {
+    async fn init(process_count: i32, process_index: i32) -> LemmyResult<Self> {
+      let context = LemmyContext::init_test_context().await;
+      let opts = Opts {
+        process_count,
+        process_index,
+      };
+      let federation_config = FederationConfig::builder()
+        .domain("local.com")
+        .app_data(context.clone())
+        .build()
+        .await?;
 
       let pool = &mut context.pool();
-    let instances = vec![
-      Instance::read_or_create(pool, "alpha.com".to_string()).await?,
-      Instance::read_or_create(pool, "beta.com".to_string()).await?,
-      Instance::read_or_create(pool, "gamma.com".to_string()).await?,
-    ];
+      let instances = vec![
+        Instance::read_or_create(pool, "alpha.com".to_string()).await?,
+        Instance::read_or_create(pool, "beta.com".to_string()).await?,
+        Instance::read_or_create(pool, "gamma.com".to_string()).await?,
+      ];
 
-    let send_manager = SendManager::new(opts, federation_config);
-    Ok(TestData {send_manager,
-      context,instances
-    })
+      let send_manager = SendManager::new(opts, federation_config);
+      Ok(Self {
+        send_manager,
+        context,
+        instances,
+      })
+    }
+
+    async fn run(&mut self) -> LemmyResult<()> {
+      // start it and cancel after workers are running
+      let cancel = CancellationToken::new();
+      let cancel_ = cancel.clone();
+      spawn(async move {
+        sleep(Duration::from_millis(100)).await;
+        cancel_.cancel();
+      });
+      self.send_manager.do_loop(cancel.clone()).await?;
+      Ok(())
+    }
+
+    async fn cleanup(self) -> LemmyResult<()> {
+      self.send_manager.cancel().await?;
+      Instance::delete_all(&mut self.context.pool()).await?;
+      Ok(())
+    }
+  }
+
+  // check that correct number of instance workers was started
+  // TODO: need to wrap in Arc or something similar
+  // TODO: test with different `opts`, dead/blocked instances etc
+
+  #[tokio::test]
+  #[serial]
+  async fn test_send_manager() -> LemmyResult<()> {
+    let mut data = TestData::init(1, 1).await?;
+
+    data.run().await?;
+    assert_eq!(3, data.send_manager.workers.len());
+
+    data.cleanup().await?;
+    Ok(())
   }
 
   #[tokio::test]
-  async fn test_start_stop_federation_workers() -> LemmyResult<()> {
-    let mut data = init().await?;
+  #[serial]
+  async fn test_send_manager_processes() -> LemmyResult<()> {
+    let active = Arc::new(Mutex::new(vec![]));
+    let execute = |count, index, len, active: Arc<Mutex<Vec<InstanceId>>>| async move {
+      let mut data = TestData::init(count, index).await?;
+      data.run().await?;
+      assert_eq!(len, data.send_manager.workers.len());
+      for k in data.send_manager.workers.keys() {
+        active.lock().unwrap().push(*k);
+      }
+      data.cleanup().await?;
+      Ok::<(), LemmyError>(())
+    };
+    execute(3, 1, 1, active.clone()).await?;
+    execute(3, 2, 1, active.clone()).await?;
+    execute(3, 3, 1, active.clone()).await?;
+    execute(3, 4, 0, active.clone()).await?;
+    execute(3, 6, 0, active.clone()).await?;
 
-    // start it and wait a moment
-    let cancel = CancellationToken::new();
-    let cancel_ = cancel.clone();
-    spawn(async move {
-      sleep(Duration::from_millis(100)).await;
-      cancel_.cancel();
-    });
-    data.send_manager.do_loop(cancel.clone()).await.unwrap();
-    assert_eq!(3, data.send_manager.workers.len());
+    // Should run exactly three workers
+    assert_eq!(3, active.lock().unwrap().len());
 
-    // check that correct number of instance workers was started
-    // TODO: need to wrap in Arc or something similar
-    // TODO: test with different `opts`, dead/blocked instances etc
-
-    // cleanup
-    Instance::delete_all(&mut data.context.pool()).await?;
     Ok(())
   }
 }