feat: composable threshold_config list for per-host threshold layering

threshold_config in the hosts section now accepts a list of named
configs applied left-to-right on top of the defaults, so focused
override profiles can be mixed without duplication. Single-string
and legacy host_threshold_mapping forms are unchanged.

- Add threshold_raw_configs to store per-config overrides separately
- Normalise threshold_config to list on parse (string or list)
- get_thresholds_for_host folds the list over the default base
- Update README and docs/THRESHOLD_ALERTING.md with examples

Co-Authored-By: Claude Sonnet 4.6 (1M context) <noreply@anthropic.com>

README.md

@@ -267,6 +267,41 @@ All plugin metrics can be thresholded:
 - **Network**: errors_total, dropped packets, connection counts
 - **Nagios**: exit_code mapping (0=OK, 1=WARNING, 2=CRITICAL)
 
+### Per-Host Threshold Profiles
+
+Named threshold configurations let different hosts use different limits. A host's `threshold_config` can be a single name or a **list** — lists are applied left-to-right so profiles compose without duplication:
+
+```yaml
+threshold_configs:
+  default:
+    thresholds:
+      cpu_monitor:
+        cpu_percent: {warning: 80, critical: 90}
+      memory_monitor:
+        memory_percent: {warning: 85, critical: 95}
+
+  tight_cpu:           # override CPU limits only
+    thresholds:
+      cpu_monitor:
+        cpu_percent: {warning: 60, critical: 75}
+
+  db_disk:             # add a database partition check
+    thresholds:
+      disk_monitor:
+        partitions:
+          /var/lib/postgresql:
+            percent: {warning: 75, critical: 88}
+
+hosts:
+  web-01:
+    threshold_config: default          # single profile
+
+  db-01:
+    threshold_config: [tight_cpu, db_disk]   # layered: CPU override + extra disk check
+```
+
+Each named config's overrides are applied in order on top of the defaults. Metrics not mentioned in a profile are inherited unchanged.
+
 See [docs/THRESHOLD_ALERTING.md](docs/THRESHOLD_ALERTING.md) for comprehensive documentation including best practices, troubleshooting, and advanced configuration.
 
 ---

docs/THRESHOLD_ALERTING.md

@@ -814,34 +814,32 @@ Planned features:
 
 ## Multi-Threshold Configuration
 
-**New in version 2.0**: Support for multiple named threshold configurations with per-host mapping.
+Support for multiple named threshold configurations with per-host mapping and composable layering.
 
 ### Overview
 
 The multi-threshold feature allows you to:
-- Define multiple sets of threshold configurations
+- Define multiple named threshold configurations
-- Map different hosts to different threshold sets
+- Assign one or more configurations to each host
+- Compose configurations by layering — each named config's overrides are applied in order on top of the defaults
 - Use different sensitivity levels for different environments
-- Maintain a default configuration for unmapped hosts
 
 ### Configuration Structure
 
+Named configurations are defined under `threshold_configs`. Each host selects which ones to use via `threshold_config` in the `hosts` section (a string for a single config, or a list to layer multiple):
+
 ```yaml
-# Optional: Set the default configuration name (defaults to "default")
+# Optional: set the default configuration name (defaults to "default")
 default_threshold_config: "default"
 
-# Define multiple named threshold configurations
 threshold_configs:
-  # Configuration name 1
   default:
     thresholds:
-      # Standard threshold definitions
       cpu_monitor:
         cpu_percent:
           warning: 80.0
           critical: 90.0
 
-  # Configuration name 2
   high_sensitivity:
     thresholds:
       cpu_monitor:
@@ -849,7 +847,6 @@ threshold_configs:
           warning: 60.0
           critical: 75.0
 
-  # Configuration name 3
   low_sensitivity:
     thresholds:
       cpu_monitor:
@@ -857,14 +854,77 @@ threshold_configs:
           warning: 90.0
           critical: 95.0
 
-# Map specific hosts to specific configurations
+hosts:
-host_threshold_mapping:
+  prod-web-01:
-  prod-web-01: high_sensitivity
+    threshold_config: high_sensitivity   # single config
-  prod-web-02: high_sensitivity
+
-  dev-server-01: low_sensitivity
+  dev-server-01:
-  # Unmapped hosts use default_threshold_config
+    threshold_config: low_sensitivity
+
+  # Hosts with no threshold_config use default_threshold_config
 ```
 
+### Composable Configurations (list form)
+
+`threshold_config` can be a list. Configs are applied **left to right**: the defaults are the base, then each named config's overrides are layered on top. Later entries in the list win on any metric they define.
+
+```yaml
+threshold_configs:
+  default:
+    thresholds:
+      cpu_monitor:
+        cpu_percent: {warning: 80, critical: 90}
+      memory_monitor:
+        memory_percent: {warning: 85, critical: 95}
+      disk_monitor:
+        partitions:
+          /:
+            percent: {warning: 80, critical: 90}
+
+  # Tighter CPU limits for busy servers
+  high_cpu_load:
+    thresholds:
+      cpu_monitor:
+        cpu_percent: {warning: 60, critical: 75}
+
+  # Tighter disk limits for data-heavy servers
+  busy_disk:
+    thresholds:
+      disk_monitor:
+        partitions:
+          /:
+            percent: {warning: 70, critical: 85}
+
+hosts:
+  # Gets default thresholds only
+  web-01:
+    threshold_config: default
+
+  # Gets tighter CPU limits, default memory and disk
+  build-server:
+    threshold_config: high_cpu_load
+
+  # Layers both: tighter CPU AND tighter disk, default memory
+  db-01:
+    threshold_config: [high_cpu_load, busy_disk]
+
+  # Three layers: busy_disk overrides high_cpu_load if they conflict
+  storage-01:
+    threshold_config: [default, high_cpu_load, busy_disk]
+```
+
+**How layering works:**
+
+Starting from the `default` thresholds:
+
+| Layer | Applied config | Effect |
+|-------|---------------|--------|
+| Base  | `default` | all default thresholds |
+| +1    | `high_cpu_load` | cpu_percent overridden to 60/75 |
+| +2    | `busy_disk` | disk percent overridden to 70/85; cpu_percent stays at 60/75 |
+
+Each named config only overrides the metrics it explicitly defines. Metrics not mentioned in a config inherit from the layers beneath.
+
 ### Use Cases
 
 #### 1. Environment-Based Thresholds
@@ -887,11 +947,15 @@ threshold_configs:
           warning: 90.0   # More relaxed for dev
           critical: 98.0
 
-host_threshold_mapping:
+hosts:
-  prod-web-01: production
+  prod-web-01:
-  prod-web-02: production
+    threshold_config: production
-  dev-web-01: development
+  prod-web-02:
-  dev-web-02: development
+    threshold_config: production
+  dev-web-01:
+    threshold_config: development
+  dev-web-02:
+    threshold_config: development
 ```
 
 #### 2. Server Role-Based Thresholds
@@ -914,7 +978,7 @@ threshold_configs:
           warning: 70.0
           critical: 85.0
       memory_monitor:
-        percent:
+        memory_percent:
           warning: 90.0   # Databases can use high memory
           critical: 97.0
       disk_monitor:
@@ -927,17 +991,23 @@ threshold_configs:
   cache:
     thresholds:
       memory_monitor:
-        percent:
+        memory_percent:
           warning: 95.0   # Redis/Memcached can use very high memory
           critical: 99.0
 
-host_threshold_mapping:
+hosts:
-  web-01: webserver
+  web-01:
-  web-02: webserver
+    threshold_config: webserver
-  db-01: database
+  web-02:
-  db-02: database
+    threshold_config: webserver
-  redis-01: cache
+  db-01:
-  memcached-01: cache
+    threshold_config: database
+  db-02:
+    threshold_config: database
+  redis-01:
+    threshold_config: cache
+  memcached-01:
+    threshold_config: cache
 ```
 
 #### 3. Sensitivity Levels
@@ -952,7 +1022,7 @@ threshold_configs:
         partitions:
           /:
             percent:
-              warning: 70.0    # Very sensitive
+              warning: 70.0
               critical: 80.0
               hysteresis: 0.15
 
@@ -976,12 +1046,69 @@ threshold_configs:
               critical: 98.0
               hysteresis: 0.05
 
-host_threshold_mapping:
+hosts:
-  payment-gateway: critical
+  payment-gateway:
-  auth-server: critical
+    threshold_config: critical
-  web-01: standard
+  auth-server:
-  web-02: standard
+    threshold_config: critical
-  test-server: relaxed
+  web-01:
+    threshold_config: standard
+  web-02:
+    threshold_config: standard
+  test-server:
+    threshold_config: relaxed
+```
+
+#### 4. Composable Profiles
+
+Build host-specific thresholds by combining small, focused configs:
+
+```yaml
+threshold_configs:
+  # Baseline — everything at default levels
+  default:
+    thresholds:
+      cpu_monitor:
+        cpu_percent: {warning: 80, critical: 90}
+      memory_monitor:
+        memory_percent: {warning: 85, critical: 95}
+
+  # Overlay: tighter CPU only
+  tight_cpu:
+    thresholds:
+      cpu_monitor:
+        cpu_percent: {warning: 60, critical: 75}
+
+  # Overlay: tighter memory only
+  tight_memory:
+    thresholds:
+      memory_monitor:
+        memory_percent: {warning: 70, critical: 85}
+
+  # Overlay: extra disk partition for database servers
+  db_disk:
+    thresholds:
+      disk_monitor:
+        partitions:
+          /var/lib/postgresql:
+            percent: {warning: 75, critical: 88}
+
+hosts:
+  # Plain web server
+  web-01:
+    threshold_config: default
+
+  # Build server: tight CPU, default memory and disk
+  build-01:
+    threshold_config: tight_cpu
+
+  # Database: tight CPU + tight memory + extra disk partition
+  db-01:
+    threshold_config: [tight_cpu, tight_memory, db_disk]
+
+  # Replica database: tight memory + extra disk, normal CPU
+  db-02:
+    threshold_config: [tight_memory, db_disk]
 ```
 
 ### Backward Compatibility
@@ -1012,16 +1139,25 @@ threshold_configs:
 
 ### Configuration Priority
 
-1. **Host-specific mapping**: If host is in `host_threshold_mapping`, use that config
+1. **Host `threshold_config` (list)**: Layer each named config's overrides left-to-right on top of the defaults
-2. **Default config**: Use `default_threshold_config` 
+2. **Host `threshold_config` (string)**: Use that single named config directly
-3. **First alphabetically**: If default not found, use first config alphabetically
+3. **`host_threshold_mapping`** (legacy): Same as above, string only
-4. **Legacy fallback**: If `threshold_configs` not present, use `thresholds`
+4. **`default_threshold_config`**: Used for hosts with no mapping
+5. **First alphabetically**: If the default config is not found, use the first config alphabetically
+6. **Legacy `thresholds` section**: Used when `threshold_configs` is absent entirely
 
-### Example: Complete Multi-Threshold Setup
+### Backward Compatibility
 
-See `hbd/config_multi_threshold_example.yaml` for a complete example with:
+The legacy `host_threshold_mapping` top-level key and the flat `thresholds` section are still fully supported:
-- 4 named configurations (default, high_sensitivity, low_sensitivity, database)
+
-- Host-to-config mappings for production, development, and test systems
+```yaml
-- Specialized database server thresholds
+# Still works — equivalent to hosts: {prod-web-01: {threshold_config: high_sensitivity}}
-- Custom display messages with plugin data
+host_threshold_mapping:
+  prod-web-01: high_sensitivity
+
+# Still works — equivalent to threshold_configs: {default: {thresholds: ...}}
+thresholds:
+  cpu_monitor:
+    cpu_percent: {warning: 80, critical: 90}
+```
 

hbd/server/threshold.py

@@ -328,14 +328,17 @@ class ThresholdChecker:
             renotify_interval: Seconds between repeat notifications (default: 1 hour)
             journal: Optional MessageJournal instance for logging threshold events
         """
-        # Named threshold configurations: {config_name: {metric_path: ThresholdConfig}}
+        # Named threshold configurations (pre-merged: defaults + overrides): {config_name: {metric_path: ThresholdConfig}}
         self.threshold_configs = {}
 
+        # Raw overrides only for each named config (no defaults baked in): {config_name: {metric_path: ThresholdConfig}}
+        self.threshold_raw_configs: Dict[str, Dict[str, ThresholdConfig]] = {}
+
         # Single threshold set for backward compatibility: {metric_path: ThresholdConfig}
         self.thresholds = {}
 
-        # Host to config name mapping: {host_name: config_name}
+        # Host to ordered list of config names: {host_name: [config_name, ...]}
-        self.host_config_mapping = {}
+        self.host_config_mapping: Dict[str, List[str]] = {}
 
         # Default config name to use when no mapping exists
         self.default_config = "default"
@@ -372,6 +375,7 @@ class ThresholdChecker:
         
         # Clear old configuration
         self.threshold_configs.clear()
+        self.threshold_raw_configs.clear()
         self.thresholds.clear()
         self.host_config_mapping.clear()
         self.grace_seconds = float(config.get("grace", 2))
@@ -424,9 +428,10 @@ class ThresholdChecker:
                         self._parse_plugin_thresholds(plugin_name, plugin_thresholds, target_dict=effective_defaults)
 
         self.threshold_configs["default"] = dict(effective_defaults)
+        self.threshold_raw_configs["default"] = {}
         logger.info("Registered 'default' threshold config with %d metrics", len(effective_defaults))
 
-        # Parse each named configuration, seeding it with effective_defaults first
+        # Parse each named configuration
         for config_name, config_data in threshold_configs.items():
             if config_name == "default":
                 continue  # already handled above
@@ -440,33 +445,41 @@ class ThresholdChecker:
                 continue
 
             logger.info("Parsing threshold configuration: %s", config_name)
-            self.threshold_configs[config_name] = dict(effective_defaults)
 
+            # Raw overrides only (used for multi-config layering)
+            raw_overrides: Dict[str, ThresholdConfig] = {}
             thresholds_config = config_data["thresholds"]
             for plugin_name, plugin_thresholds in thresholds_config.items():
-                if not isinstance(plugin_thresholds, dict):
+                if isinstance(plugin_thresholds, dict):
-                    continue
+                    self._parse_plugin_thresholds(plugin_name, plugin_thresholds, target_dict=raw_overrides)
+            self.threshold_raw_configs[config_name] = raw_overrides
 
-                self._parse_plugin_thresholds(
+            # Pre-merged version (defaults + overrides) for single-config fast path
-                    plugin_name,
+            self.threshold_configs[config_name] = dict(effective_defaults)
-                    plugin_thresholds,
+            self.threshold_configs[config_name].update(raw_overrides)
-                    target_dict=self.threshold_configs[config_name]
-                )
 
-        # Parse host to config mapping from two possible sources
+        # Parse host → config list mapping from two possible sources
-        # 1. New format: hosts section with threshold_config attribute
+
+        def _normalise(value) -> List[str]:
+            """Accept a string or list; always return a list."""
+            if isinstance(value, list):
+                return [str(v) for v in value]
+            return [str(value)]
+
+        # 1. hosts section with threshold_config attribute (string or list)
         if "hosts" in config:
             hosts_config = config["hosts"]
             if isinstance(hosts_config, dict):
                 for host_name, host_attrs in hosts_config.items():
                     if isinstance(host_attrs, dict) and "threshold_config" in host_attrs:
-                        self.host_config_mapping[host_name] = host_attrs["threshold_config"]
+                        self.host_config_mapping[host_name] = _normalise(host_attrs["threshold_config"])
 
-        # 2. Legacy format: host_threshold_mapping section (for backward compatibility)
+        # 2. Legacy host_threshold_mapping section (string values only)
         if "host_threshold_mapping" in config:
             legacy_mapping = config.get("host_threshold_mapping", {})
             if isinstance(legacy_mapping, dict):
-                self.host_config_mapping.update(legacy_mapping)
+                for host_name, value in legacy_mapping.items():
+                    self.host_config_mapping[host_name] = _normalise(value)
         
         # Set default config (first one alphabetically or explicitly set)
         self.default_config = config.get("default_threshold_config", "default")
@@ -664,7 +677,10 @@ class ThresholdChecker:
         )
     
     def get_thresholds_for_host(self, host_name: str) -> Dict[str, ThresholdConfig]:
-        """Get the appropriate threshold configuration for a host.
+        """Get the effective threshold configuration for a host.
+
+        When threshold_config is a list, configs are applied left-to-right on top
+        of the default thresholds so earlier entries can be overridden by later ones.
 
         Args:
             host_name: Name of the host
@@ -676,23 +692,40 @@ class ThresholdChecker:
         if self.thresholds and not self.threshold_configs:
             return self.thresholds
 
-        # Multi-config mode: look up host-specific configuration
+        if not self.threshold_configs:
-        if self.threshold_configs:
+            return {}
-            config_name = self.host_config_mapping.get(host_name, self.default_config)
 
-            if config_name in self.threshold_configs:
+        config_names = self.host_config_mapping.get(host_name)
-                return self.threshold_configs[config_name]
+
-            else:
+        # No host-specific mapping → return pre-merged default
+        if not config_names:
+            return self.threshold_configs.get(self.default_config, {})
+
+        # Single config → fast path using pre-merged copy
+        if len(config_names) == 1:
+            name = config_names[0]
+            if name in self.threshold_configs:
+                return self.threshold_configs[name]
             logger.warning(
                 "Threshold config '%s' not found for host '%s', using default '%s'",
-                    config_name,
+                name, host_name, self.default_config,
-                    host_name,
-                    self.default_config
             )
             return self.threshold_configs.get(self.default_config, {})
 
-        # No thresholds configured
+        # Multiple configs → start from defaults, layer raw overrides in order
-        return {}
+        result = dict(self.threshold_configs.get(self.default_config, {}))
+        for name in config_names:
+            if name == self.default_config:
+                continue  # defaults already the base
+            raw = self.threshold_raw_configs.get(name)
+            if raw is None:
+                logger.warning(
+                    "Threshold config '%s' not found for host '%s', skipping",
+                    name, host_name,
+                )
+            else:
+                result.update(raw)
+        return result
     
     def check_value(
         self,