Enable cache time-based expiry by default

changelog.d/11849.misc

@@ -0,0 +1 @@
+Enable cache time-based expiry by default.

docs/sample_config.yaml

@@ -751,11 +751,16 @@ caches:
   per_cache_factors:
     #get_users_who_share_room_with_user: 2.0
 
-  # Controls how long an entry can be in a cache without having been
-  # accessed before being evicted. Defaults to None, which means
-  # entries are never evicted based on time.
+  # Controls whether cache entries are evicted after a specified time
+  # period. Defaults to true. Uncomment to disable this feature.
   #
-  #expiry_time: 30m
+  #expire_caches: false
+
+  # If expire_caches is enabled, this flag controls how long an entry can
+  # be in a cache without having been accessed before being evicted.
+  # Defaults to 30m. Uncomment to set a different time to live for cache entries.
+  #
+  #cache_entry_ttl: 30m
 
   # Controls how long the results of a /sync request are cached for after
   # a successful response is returned. A higher duration can help clients with

docs/upgrade.md

@@ -86,6 +86,13 @@ process, for example:
     ```
 # Upgrading to v1.(next)
 
+## Time-based cache expiry is now enabled by default
+
+Formerly, entries in the cache were not evicted regardless of whether they were accessed after storing.
+This behavior has now changed. By default entries in the cache are now evicted after 30m of not being accessed. 
+To change the default behavior, go to the `caches` section of the config and change the `expire_caches` and 
+`cache_entry_ttl` flags as necessary. Please note that these flags replace the `expiry_time` flag in the config.  
+
 ## Stablisation of MSC3231
 
 The unstable validity-check endpoint for the 

synapse/config/cache.py

@@ -148,11 +148,16 @@ def generate_config_section(self, **kwargs) -> str:
           per_cache_factors:
             #get_users_who_share_room_with_user: 2.0
 
-          # Controls how long an entry can be in a cache without having been
-          # accessed before being evicted. Defaults to None, which means
-          # entries are never evicted based on time.
+          # Controls whether cache entries are evicted after a specified time
+          # period. Defaults to true. Uncomment to disable this feature.
           #
-          #expiry_time: 30m
+          #expire_caches: false
+
+          # If expire_caches is enabled, this flag controls how long an entry can
+          # be in a cache without having been accessed before being evicted.
+          # Defaults to 30m. Uncomment to set a different time to live for cache entries.
+          #
+          #cache_entry_ttl: 30m
 
           # Controls how long the results of a /sync request are cached for after
           # a successful response is returned. A higher duration can help clients with
@@ -217,12 +222,19 @@ def read_config(self, config, **kwargs) -> None:
                     e.message  # noqa: B306, DependencyException.message is a property
                 )
 
-        expiry_time = cache_config.get("expiry_time")
-        if expiry_time:
-            self.expiry_time_msec: Optional[int] = self.parse_duration(expiry_time)
+        expire_caches = cache_config.get("expire_caches", True)
+        cache_entry_ttl = cache_config.get("cache_entry_ttl", "30m")
+
+        if expire_caches:
+            self.expiry_time_msec: Optional[int] = self.parse_duration(cache_entry_ttl)
         else:
             self.expiry_time_msec = None
 
+        # Backwards compatibility support for the now-removed "expiry_time" config flag.
+        expiry_time = cache_config.get("expiry_time")
+        if expiry_time:
+            self.expiry_time_msec = self.parse_duration(expiry_time)
+
         self.sync_response_cache_duration = self.parse_duration(
             cache_config.get("sync_response_cache_duration", 0)
         )