diff --git a/README.md b/README.md
index fbdb421..cead0c7 100644
--- a/README.md
+++ b/README.md
@@ -10,12 +10,13 @@ assumptions about a microservice:
    singular queue (RabbitMQ).
 2. Incoming requests are in the form of a 64-bit unsigned integer (`u64`).
 2. Microservices process requests via a `process` function, which takes three
-    arguments: the incoming request (`u64`), a `read_file` function, and a
-    `write_file` function.
+   arguments: the incoming request (`u64`), a `read_file` function, and a
+   `write_file` function.
 3. The `process` function returns a set of IDs (also `u64`) that are the result
    of processing the incoming request.  Each of these IDs is also associated
    with a "case variable" that is used for routing the result to the
-   appropriate outbound queues.
+   appropriate outbound queues. Case variables for routing must be one of:
+   boolean, integer, or string.
 4. Rather than hard-coding the inbound and outbound queues, the
    microservice communicates with a self-contained configuration service shared
    across all microservices.
@@ -101,7 +102,7 @@ and a mapping of case variables to outbound queue names. For example:
 }
 ```
 
-The case variables can be any primitive type (e.g. string, integer, boolean).
+The case variables used for routing can be one of: string, integer, or boolean.
 E.g. a binary classification microservice might decide on which outbound queue
 to send results to based on a case variable that is either `false` or `true`:
 
diff --git a/src/lib.rs b/src/lib.rs
index af3f789..8d7129b 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -31,7 +31,7 @@ pub type AnyError = Box<dyn Error + Send + Sync + 'static>;
 pub type ReadFile = Box<dyn Read + Send + 'static>;
 pub type ReadFileFn = dyn Fn(&str, u64) -> Result<ReadFile, AnyError> + Send + Sync + 'static;
 pub type WriteFileFn = dyn Fn(&str, u64) -> Result<File, AnyError> + Send + Sync + 'static;
-type ProcessFn = dyn Fn(u64, &ReadFileFn, &WriteFileFn) -> Result<Vec<(u64, Value)>, AnyError>
+type ProcessFn = dyn Fn(u64, &ReadFileFn, &WriteFileFn) -> Result<Vec<(u64, CaseKey)>, AnyError>
 	+ Send
 	+ Sync
 	+ 'static;
@@ -57,6 +57,13 @@ struct OutboundCase {
 	queues: Vec<String>,
 }
 
+#[derive(Clone, Debug, Eq, Hash, PartialEq)]
+enum CaseKey {
+	Bool(bool),
+	Int(i128),
+	String(String),
+}
+
 /// A simple queue-driven microservice runtime.
 ///
 /// The microservice:
@@ -88,16 +95,16 @@ impl Microservice {
 			request: u64,
 			read_file: &ReadFileFn,
 			write_file: &WriteFileFn,
-		| -> Result<Vec<(u64, Value)>, AnyError> {
+		| -> Result<Vec<(u64, CaseKey)>, AnyError> {
 			let outputs = process(request, read_file, write_file)?;
-			Ok(outputs
-				.into_iter()
-				.map(|(id, case)| {
-					let value = serde_json::to_value(case)
-						.expect("case variable must be serializable to JSON");
-					(id, value)
-				})
-				.collect())
+			let mut mapped = Vec::with_capacity(outputs.len());
+			for (id, case) in outputs {
+				let value = serde_json::to_value(case)
+					.map_err(|e| format!("case variable must be serializable to JSON: {}", e))?;
+				mapped.push((id, case_key_from_value(&value)?));
+			}
+
+			Ok(mapped)
 		};
 
 		Self {
@@ -110,7 +117,7 @@ impl Microservice {
 	/// Start the microservice. This call blocks while the consumer loop runs.
 	pub fn start(&self) -> Result<(), AnyError> {
 		let config = self.fetch_config()?;
-		let route_map = build_route_map(&config.out);
+		let route_map = build_route_map(&config.out)?;
 		let amqp_url = fetch_rabbitmq_url_from_sys_map()?;
 
 		let runtime = tokio::runtime::Builder::new_multi_thread()
@@ -132,7 +139,7 @@ impl Microservice {
 	async fn run_consumer(
 		&self,
 		inbound_queue: String,
-		route_map: HashMap<String, Vec<String>>,
+		route_map: HashMap<CaseKey, Vec<String>>,
 		amqp_url: String,
 		s3_client: Arc<Client>,
 	) -> Result<(), AnyError> {
@@ -515,22 +522,35 @@ fn bucket_mapping_url(host: &str, microservice_name: &str, key: &str) -> String
 	format!("{}/{}", config_url(host, microservice_name), key.trim_matches('/'))
 }
 
-fn build_route_map(outbound: &[OutboundCase]) -> HashMap<String, Vec<String>> {
+fn build_route_map(outbound: &[OutboundCase]) -> Result<HashMap<CaseKey, Vec<String>>, AnyError> {
 	let mut map = HashMap::new();
 	for entry in outbound {
-		map.insert(case_key(&entry.case_value), entry.queues.clone());
+		map.insert(case_key_from_value(&entry.case_value)?, entry.queues.clone());
 	}
-	map
+	Ok(map)
 }
 
-fn case_key(case_value: &Value) -> String {
-	case_value.to_string()
+fn case_key_from_value(case_value: &Value) -> Result<CaseKey, AnyError> {
+	match case_value {
+		Value::Bool(value) => Ok(CaseKey::Bool(*value)),
+		Value::String(value) => Ok(CaseKey::String(value.clone())),
+		Value::Number(value) => {
+			if let Some(v) = value.as_i64() {
+				Ok(CaseKey::Int(v as i128))
+			} else if let Some(v) = value.as_u64() {
+				Ok(CaseKey::Int(v as i128))
+			} else {
+				Err(format!("case variable '{}' must be an integer number", value).into())
+			}
+		}
+		_ => Err("case variable must be one of: bool, int, string".into()),
+	}
 }
 
 async fn declare_queues(
 	channel: &Channel,
 	inbound_queue: &str,
-	route_map: &HashMap<String, Vec<String>>,
+	route_map: &HashMap<CaseKey, Vec<String>>,
 ) -> Result<(), AnyError> {
 	channel
 		.queue_declare(
@@ -557,12 +577,12 @@ async fn declare_queues(
 
 async fn publish_outputs(
 	channel: &Channel,
-	outputs: Vec<(u64, Value)>,
-	route_map: &HashMap<String, Vec<String>>,
+	outputs: Vec<(u64, CaseKey)>,
+	route_map: &HashMap<CaseKey, Vec<String>>,
 ) -> Result<(), AnyError> {
 	for (result_id, case_var) in outputs {
-		info!("Shuttle output result_id={}, case_var={}", result_id, case_var);
-		if let Some(outbound_queues) = route_map.get(&case_key(&case_var)) {
+		info!("Shuttle output result_id={}, case_var={:?}", result_id, case_var);
+		if let Some(outbound_queues) = route_map.get(&case_var) {
 			for queue in outbound_queues {
 				let payload = result_id.to_string();
 				let confirm = channel