add: Publish GHC runtime metrics to Prometheus metrics endpoint

CHANGELOG.md

@@ -14,6 +14,7 @@ All notable changes to this project will be documented in this file. From versio
 - Add `Vary` header to responses by @develop7 in #4609
 - Add config `db-timezone-enabled` for optional querying of timezones by @taimoorzaeem in #4751
 - Log schema cache queries timings on `log-level=debug` by @steve-chavez in #4805
+- Add GHC runtime metrics to the metrics endpoint by @mkleczek in #4862
 
 ### Fixed
 

docs/postgrest.dict

@@ -45,7 +45,9 @@ filename
 FreeBSD
 fts
 fullstack
+GC
 GeoJSON
+GHC
 Github
 Google
 grantor
@@ -147,6 +149,7 @@ RESTful
 RLS
 RPC
 RSA
+RTS
 safeupdate
 savepoint
 schemas

docs/references/observability.rst

@@ -238,6 +238,45 @@ pgrst_jwt_cache_evictions_total
 
 The total number of JWT cache evictions.
 
+GHC Runtime Metrics
+-------------------
+
+PostgREST can also expose GHC runtime system metrics. These use the ``ghc_*``
+prefix and include
+`GHC RTS statistics <https://ghc.gitlab.haskell.org/ghc/doc/libraries/base-4.22.0.0-inplace/GHC-Stats.html#g:1>`_
+for runtime allocation, garbage collection, memory, and CPU/elapsed time.
+
+These are useful for monitoring PostgREST process health and diagnosing memory
+pressure or GC behavior.
+
+To expose these metrics, enable GHC RTS statistics when starting PostgREST:
+
+.. code-block:: bash
+
+  postgrest +RTS -T -RTS
+
+When enabled, the admin ``/metrics`` endpoint includes samples such as:
+
+.. code-block:: text
+
+  # HELP ghc_gcs_total Total number of GCs
+  # TYPE ghc_gcs_total counter
+  ghc_gcs_total 1
+  # HELP ghc_allocated_bytes_total Total bytes allocated
+  # TYPE ghc_allocated_bytes_total counter
+  ghc_allocated_bytes_total 12345678
+
+Other available GHC runtime metrics include:
+
+- ``ghc_gcs_total``
+- ``ghc_major_gcs_total``
+- ``ghc_allocated_bytes_total``
+- ``ghc_max_live_bytes``
+- ``ghc_max_mem_in_use_bytes``
+- ``ghc_mutator_cpu_seconds_total``
+- ``ghc_gc_cpu_seconds_total``
+- ``ghc_elapsed_seconds_total``
+
 Traces
 ======
 

postgrest.cabal

@@ -136,6 +136,7 @@ library
                     , parsec                    >= 3.1.11 && < 3.2
                     , postgresql-libpq          >= 0.10
                     , prometheus-client         >= 1.1.1 && < 1.2.0
+                    , prometheus-metrics-ghc    >= 1.0.1.2 && < 1.2
                     , protolude                 >= 0.3.1 && < 0.4
                     , regex-tdfa                >= 1.2.2 && < 1.4
                     , retry                     >= 0.7.4 && < 0.10

src/PostgREST/Metrics.hs

@@ -13,10 +13,13 @@ module PostgREST.Metrics
 import qualified Data.ByteString.Lazy   as LBS
 import qualified Hasql.Pool.Observation as SQL
 
+import           GHC.Stats             (getRTSStatsEnabled)
 import           Prometheus
+import qualified Prometheus.Metric.GHC as PMG
 
 import PostgREST.Observation
 
+
 import Protolude
 
 data MetricsState =
@@ -34,6 +37,7 @@ data MetricsState =
 
 init :: Int -> IO MetricsState
 init configDbPoolSize = do
+  whenM getRTSStatsEnabled $ void $ register PMG.ghcMetrics
   metricState <- MetricsState <$>
     register (counter (Info "pgrst_db_pool_timeouts_total" "The total number of pool connection timeouts")) <*>
     register (gauge (Info "pgrst_db_pool_available" "Available connections in the pool")) <*>

test/io/postgrest.py

@@ -84,6 +84,7 @@ def run(
     configpath=None,
     stdin=None,
     env=None,
+    args=None,
     port=None,
     admin_port=None,
     host=None,
@@ -122,6 +123,9 @@ def run(
         command = [POSTGREST_BIN]
         env["HPCTIXFILE"] = hpctixfile()
 
+        if args:
+            command.extend(args)
+
         if configpath:
             command.append(configpath)
 

test/io/test_io.py

@@ -1693,6 +1693,30 @@ def test_admin_metrics(defaultenv):
         assert "pgrst_db_pool_timeouts_total" in response.text
 
 
+def test_admin_metrics_include_ghc_runtime_metrics(defaultenv):
+    "Should get GHC runtime metrics from the admin endpoint when RTS stats are enabled"
+
+    with run(env=defaultenv, args=["+RTS", "-T", "-RTS"], port=freeport()) as postgrest:
+        response = postgrest.admin.get("/metrics")
+        assert response.status_code == 200
+        assert "# HELP ghc_gcs_total Total number of GCs" in response.text
+        assert "# TYPE ghc_gcs_total counter" in response.text
+        assert re.search(r"^ghc_gcs_total \d+(?:\.\d+)?$", response.text, re.MULTILINE)
+        assert re.search(
+            r"^ghc_allocated_bytes_total \d+(?:\.\d+)?$", response.text, re.MULTILINE
+        )
+
+
+def test_admin_metrics_exclude_ghc_runtime_metrics_by_default(defaultenv):
+    "Should not get GHC runtime metrics unless RTS stats are enabled"
+
+    with run(env=defaultenv, port=freeport()) as postgrest:
+        response = postgrest.admin.get("/metrics")
+        assert response.status_code == 200
+        assert "ghc_gcs_total" not in response.text
+        assert "ghc_allocated_bytes_total" not in response.text
+
+
 def test_schema_cache_startup_load_with_in_db_config(defaultenv, metapostgrest):
     "verify that the Schema Cache loads correctly at startup, using the in-db `pgrst.db_schemas` config"