diff --git a/src/main.py b/src/main.py
index b156727..717334d 100644
--- a/src/main.py
+++ b/src/main.py
@@ -4,6 +4,7 @@
 import datetime
 import json
 import logging
+import math
 import os
 from pathlib import Path
 import sys
@@ -121,15 +122,21 @@ class InstrumentConfig:
 
     instrument: str
     """The instrument whose metrics are held by this object (`str`)."""
-    url: str
-    """The address of the Knative Serving instance for this instrument (`str`)."""
+    urls: collections.abc.Sequence[str]
+    """The addresses of the Knative Serving instance for this instrument (sequence [`str`])."""
     detectors: collections.abc.Sequence[int]
     """The active detectors for this instrument (sequence [`int`])."""
 
     def __init__(self, conf, instrument):
         super().__setattr__("instrument", instrument)
-        super().__setattr__("url", conf["knative-urls"][instrument])
+        super().__setattr__("urls", conf["knative-urls"][instrument])
+        if not self.urls:
+            raise ValueError(f"Must provide at least one URL for the {instrument} service.")
         super().__setattr__("detectors", self.detector_load(conf, instrument))
+        if not self.detectors:
+            raise ValueError(f"Must provide at least one detector for the {instrument} service. "
+                             "If the service is not yet ready, remove it from the supported "
+                             "instruments list.")
 
     @staticmethod
     def detector_load(conf: dict, instrument: str) -> list[int]:
@@ -300,10 +307,10 @@ def fan_out(next_visit, inst_config):
 
     Returns
     -------
-    fanned_out : `Submission`
+    fanned_out : collection [`Submission`]
         The submission information for the fanned-out messages.
     """
-    return Submission(inst_config.url, next_visit.add_detectors(inst_config.detectors))
+    return fan_out_hsc(next_visit, inst_config, inst_config.detectors)
 
 
 def fan_out_hsc(next_visit, inst_config, detectors):
@@ -320,16 +327,48 @@ def fan_out_hsc(next_visit, inst_config, detectors):
 
     Returns
     -------
-    fanned_out : `Submission`
+    fanned_out : collection [`Submission`]
         The submission information for the fanned-out messages.
     """
-    return Submission(inst_config.url, next_visit.add_detectors(detectors))
+    n_services = len(inst_config.urls)
+    n_detectors = len(detectors)
+    # Block-based assignment is more intuitive for humans monitoring the services.
+    # The last blocks may be underfilled or (if n_detectors < n_services) empty.
+    block_size = int(math.ceil(n_detectors / n_services))
+
+    blocks = _batched(detectors, block_size)
+    # len(blocks) = min(n_services, n_detectors)
+    return [Submission(url, next_visit.add_detectors(block))
+            for url, block in zip(inst_config.urls, blocks)]
+
+
+def _batched(seq, size):
+    """Batch a sequence data into smaller sequences.
+
+    Parameters
+    ----------
+    seq : sequence
+        The sequence to batch.
+    size : `int`
+        The desired batch size.
+
+    Returns
+    -------
+    batched : sequence [sequence]
+        A sequence of consecutive subsequences, in the same order as the
+        original. Each sequence is of length ``size`` until there are
+        not enough elements.
+    """
+    batches = []
+    for i in range(0, len(seq), size):
+        batches.append(seq[i:i + size])
+    return batches
 
 
 def dispatch_fanned_out_messages(client: httpx.AsyncClient,
                                  topic: str,
                                  tasks: collections.abc.MutableSet[asyncio.Task],
-                                 send_info: Submission,
+                                 send_infos: collections.abc.Collection[Submission],
                                  gauges: collections.abc.Mapping[str, Metrics],
                                  ):
     """Package and send the fanned-out messages to Prompt Processing.
@@ -342,8 +381,8 @@ def dispatch_fanned_out_messages(client: httpx.AsyncClient,
         The topic to which to upload the messages.
     tasks : set [`asyncio.Task`]
         Collection for holding the requests.
-    send_info : `Submission`
-        The data and address to submit.
+    send_infos : collection [`Submission`]
+        The data and addresses to submit.
     gauges : mapping [`str`, `Metrics`]
         A mapping from instrument name to metrics for that instrument.
     """
@@ -353,29 +392,30 @@ def dispatch_fanned_out_messages(client: httpx.AsyncClient,
             "source": topic,
         }
 
-        for fan_out_message in send_info.fan_out_messages:
-            data = fan_out_message
-            data_json = json.dumps(data)
-
-            logging.info(f"data after json dump {data_json}")
-            event = CloudEvent(attributes, data_json)
-            headers, body = to_structured(event)
-            info = {
-                key: data[key] for key in ["instrument", "groupId", "detector"]
-            }
-
-            task = asyncio.create_task(
-                knative_request(
-                    gauges[fan_out_message["instrument"]].in_process,
-                    client,
-                    send_info.url,
-                    headers,
-                    body,
-                    str(info),
+        for send_info in send_infos:
+            for fan_out_message in send_info.fan_out_messages:
+                data = fan_out_message
+                data_json = json.dumps(data)
+
+                logging.info(f"data after json dump {data_json}")
+                event = CloudEvent(attributes, data_json)
+                headers, body = to_structured(event)
+                info = {
+                    key: data[key] for key in ["instrument", "groupId", "detector"]
+                }
+
+                task = asyncio.create_task(
+                    knative_request(
+                        gauges[fan_out_message["instrument"]].in_process,
+                        client,
+                        send_info.url,
+                        headers,
+                        body,
+                        str(info),
+                    )
                 )
-            )
-            tasks.add(task)
-            task.add_done_callback(tasks.discard)
+                tasks.add(task)
+                task.add_done_callback(tasks.discard)
 
     except ValueError:
         logging.exception("Error while sending fanned-out messages.")