CacheBackend: Use a configurable setting to specify cache key (#336)

The internal cache key used to write to and read from the cache backend could
conflict with the main application.
By introducing a Django setting it allows the developer to specify one
different from the default value.

Co-authored-by: Markus Richter <markus.richter@scale.eu>

Author: Flauschbaellchen

README.md

@@ -111,6 +111,13 @@ on django.conf.settings with the required format to connect to your redis server
     REDIS_URL = redis://localhost:6370
 ```
 
+The cache healthcheck tries to write and read a specific key within the cache backend.
+It can be customized by setting `HEALTHCHECK_CACHE_KEY` to another value:
+
+```python
+    HEALTHCHECK_CACHE_KEY = custom_healthcheck_key
+```
+
 ## Setting up monitoring
 
 You can use tools like Pingdom, StatusCake or other uptime robots to monitor service status.

docs/contrib.rst

@@ -56,3 +56,9 @@ all the time.
 
 You may also use both of them. To use these checks add them to `INSTALLED_APPS` in your
 Django settings module.
+
+``cache``
+---------
+
+The key `djangohealtcheck_test` will be written to the cache backend to validate that the cache is working.  
+The name of the key can be customized by setting `HEALTHCHECK_CACHE_KEY` to another value.

docs/settings.rst

@@ -71,7 +71,7 @@ exceeds 90% or available memory drops below 100 MB.
    memory falls below the specified value, a warning will be reported.
 
 Celery Health Check
-----------------------
+-------------------
 Using `django.settings` you may exert more fine-grained control over the behavior of the celery health check
 
 .. list-table:: Additional Settings
@@ -82,6 +82,10 @@ Using `django.settings` you may exert more fine-grained control over the behavio
      - Type
      - Default
      - Description
+   * - `HEALTHCHECK_CACHE_KEY`
+     - String
+     - djangohealtcheck_test
+     - Specifies the name of the key to write to and read from to validate that the cache is working.
    * - `HEALTHCHECK_CELERY_QUEUE_TIMEOUT`
      - Number
      - 3

health_check/cache/backends.py

@@ -1,13 +1,15 @@
 from django.core.cache import CacheKeyWarning, caches
+from django.conf import settings
 
 from health_check.backends import BaseHealthCheckBackend
 from health_check.exceptions import ServiceReturnedUnexpectedResult, ServiceUnavailable
 
-
 class CacheBackend(BaseHealthCheckBackend):
+
     def __init__(self, backend="default"):
         super().__init__()
         self.backend = backend
+        self.cache_key = getattr(settings, "HEALTHCHECK_CACHE_KEY", "djangohealtcheck_test")
 
     def identifier(self):
         return f"Cache backend: {self.backend}"
@@ -16,9 +18,9 @@ def check_status(self):
         cache = caches[self.backend]
 
         try:
-            cache.set("djangohealtcheck_test", "itworks")
-            if not cache.get("djangohealtcheck_test") == "itworks":
-                raise ServiceUnavailable("Cache key does not match")
+            cache.set(self.cache_key, "itworks")
+            if not cache.get(self.cache_key) == "itworks":
+                raise ServiceUnavailable(f"Cache key {self.cache_key} does not match")
         except CacheKeyWarning as e:
             self.add_error(ServiceReturnedUnexpectedResult("Cache key warning"), e)
         except ValueError as e: