Enable cache time-based expiry by default

changelog.d/11849.misc

@@ -1 +1 @@
-Enable cache time-based expiry by default.
+Enable cache time-based expiry by default. The `expiry_time` config flag will be superseded by `expire_caches` and `cache_entry_ttl`.

docs/upgrade.md

@@ -86,12 +86,6 @@ 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
 
@@ -109,6 +103,14 @@ Please update any relevant reverse proxy or firewall configurations appropriatel
 
 # Upgrading to v1.53.0
 
+## 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.  
+
+
 ## Dropping support for `webclient` listeners and non-HTTP(S) `web_client_location`
 
 Per the deprecation notice in Synapse v1.51.0, listeners of type  `webclient`

synapse/config/cache.py

@@ -12,6 +12,7 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
+import logging
 import os
 import re
 import threading
@@ -23,6 +24,8 @@
 
 from ._base import Config, ConfigError
 
+logger = logging.getLogger(__name__)
+
 # The prefix for all cache factor-related environment variables
 _CACHE_PREFIX = "SYNAPSE_CACHE_FACTOR"
 
@@ -232,7 +235,18 @@ def read_config(self, config, **kwargs) -> None:
 
         # Backwards compatibility support for the now-removed "expiry_time" config flag.
         expiry_time = cache_config.get("expiry_time")
+
+        if expiry_time and expire_caches:
+            logger.warning(
+                "You have set two incompatible flags, expiry_time and expire_caches. Please only use the "
+                "expire_caches and cache_entry_ttl flags and delete the expiry_time flag as it is "
+                "deprecated."
+            )
         if expiry_time:
+            logger.warning(
+                "Expiry_time is a deprecated flag, please use the expire_caches and cache_entry_ttl flags "
+                "instead."
+            )
             self.expiry_time_msec = self.parse_duration(expiry_time)
 
         self.sync_response_cache_duration = self.parse_duration(