diff --git a/examples/roundrobinrouter.cpp b/examples/roundrobinrouter.cpp
index bd79aa02a..73663b61d 100644
--- a/examples/roundrobinrouter.cpp
+++ b/examples/roundrobinrouter.cpp
@@ -572,7 +572,7 @@ static MXS_ROUTER_SESSION* newSession(MXS_ROUTER* instance, MXS_SESSION* session
 static void closeSession(MXS_ROUTER* instance, MXS_ROUTER_SESSION* session);
 static void freeSession(MXS_ROUTER* instance, MXS_ROUTER_SESSION* session);
 static int routeQuery(MXS_ROUTER* instance, MXS_ROUTER_SESSION* session, GWBUF* querybuf);
-static json_t* diagnostics(MXS_ROUTER* instance);
+static json_t* diagnostics(const MXS_ROUTER* instance);
 static void clientReply(MXS_ROUTER* instance, MXS_ROUTER_SESSION* router_session,
                         GWBUF* resultbuf, DCB* backend_dcb);
 static void handleError(MXS_ROUTER* instance, MXS_ROUTER_SESSION* router_session,
@@ -759,9 +759,9 @@ static int routeQuery(MXS_ROUTER* instance, MXS_ROUTER_SESSION* session, GWBUF*
  * @param   instance    The router instance
  * @param   dcb         The DCB for diagnostic output
  */
-static json_t* diagnostics(MXS_ROUTER* instance)
+static json_t* diagnostics(const MXS_ROUTER* instance)
 {
-    RRRouter* router = static_cast<RRRouter*>(instance);
+    const RRRouter* router = static_cast<const RRRouter*>(instance);
     json_t* rval = json_object();
 
     json_object_set_new(rval, "queries_ok", json_integer(router->m_routing_s));
diff --git a/examples/testfilter.c b/examples/testfilter.c
index ac825df45..7c97f6d35 100644
--- a/examples/testfilter.c
+++ b/examples/testfilter.c
@@ -35,7 +35,7 @@ static void closeSession(MXS_FILTER *instance, MXS_FILTER_SESSION *session);
 static void freeSession(MXS_FILTER *instance, MXS_FILTER_SESSION *session);
 static void setDownstream(MXS_FILTER *instance, MXS_FILTER_SESSION *fsession, MXS_DOWNSTREAM *downstream);
 static int routeQuery(MXS_FILTER *instance, MXS_FILTER_SESSION *fsession, GWBUF *queue);
-static json_t* diagnostic(MXS_FILTER *instance, MXS_FILTER_SESSION *fsession);
+static json_t* diagnostic(const MXS_FILTER *instance, const MXS_FILTER_SESSION *fsession);
 static uint64_t getCapabilities(MXS_FILTER* instance);
 static void destroyInstance(MXS_FILTER *instance);
 
@@ -221,7 +221,7 @@ routeQuery(MXS_FILTER *instance, MXS_FILTER_SESSION *session, GWBUF *queue)
  * @param   fsession    Filter session, may be NULL
  * @param   dcb     The DCB for diagnostic output
  */
-static json_t* diagnostic(MXS_FILTER *instance, MXS_FILTER_SESSION *fsession)
+static json_t* diagnostic(const MXS_FILTER *instance, const MXS_FILTER_SESSION *fsession)
 {
     return NULL;
 }
diff --git a/examples/testroute.c b/examples/testroute.c
index b3d70aa92..74572a933 100644
--- a/examples/testroute.c
+++ b/examples/testroute.c
@@ -21,7 +21,7 @@ static void closeSession(MXS_ROUTER *instance, MXS_ROUTER_SESSION *session);
 static void freeSession(MXS_ROUTER *instance, MXS_ROUTER_SESSION *session);
 static int routeQuery(MXS_ROUTER *instance, MXS_ROUTER_SESSION *session, GWBUF *queue);
 static void clientReply(MXS_ROUTER *instance, MXS_ROUTER_SESSION *session, GWBUF *queue, DCB*);
-static json_t* diagnostic(MXS_ROUTER *instance);
+static json_t* diagnostic(const MXS_ROUTER *instance);
 static uint64_t getCapabilities(MXS_ROUTER* instance);
 static void handleError(MXS_ROUTER *instance,
                         MXS_ROUTER_SESSION *router_session,
@@ -145,7 +145,7 @@ void clientReply(MXS_ROUTER* instance, MXS_ROUTER_SESSION* session, GWBUF* queue
  * @param   instance    The router instance
  * @param   dcb     The DCB for diagnostic output
  */
-static json_t* diagnostic(MXS_ROUTER *instance)
+static json_t* diagnostic(const MXS_ROUTER *instance)
 {
     return NULL;
 }
diff --git a/include/maxscale/authenticator.h b/include/maxscale/authenticator.h
index a37e4b143..e77abb52c 100644
--- a/include/maxscale/authenticator.h
+++ b/include/maxscale/authenticator.h
@@ -95,7 +95,7 @@ typedef struct mxs_authenticator
      *
      * @see jansson.h
      */
-    json_t*  (*diagnostic)(struct servlistener *listener);
+    json_t*  (*diagnostic)(const struct servlistener *listener);
 
     /** This entry point was added to avoid calling authenticator functions
      * directly when a COM_CHANGE_USER command is executed. */
diff --git a/include/maxscale/filter.h b/include/maxscale/filter.h
index 82ba99d89..73b9e6b06 100644
--- a/include/maxscale/filter.h
+++ b/include/maxscale/filter.h
@@ -180,7 +180,7 @@ typedef struct mxs_filter_object
      *
      * @see jansson.h
      */
-    json_t* (*diagnostics)(MXS_FILTER *instance, MXS_FILTER_SESSION *fsession);
+    json_t* (*diagnostics)(const MXS_FILTER *instance, const MXS_FILTER_SESSION *fsession);
 
     /**
      * @brief Called to obtain the capabilities of the filter
diff --git a/include/maxscale/filter.hh b/include/maxscale/filter.hh
index ec8ddd2f5..69b2cd414 100644
--- a/include/maxscale/filter.hh
+++ b/include/maxscale/filter.hh
@@ -145,7 +145,7 @@ public:
     /**
      * Called for obtaining diagnostics about the filter session.
      */
-    json_t* diagnostics();
+    json_t* diagnostics() const;
 
 protected:
     FilterSession(MXS_SESSION* pSession);
@@ -272,19 +272,19 @@ public:
         return rv;
     }
 
-    static json_t* diagnostics(MXS_FILTER* pInstance, MXS_FILTER_SESSION* pData)
+    static json_t* diagnostics(const MXS_FILTER* pInstance, const MXS_FILTER_SESSION* pData)
     {
         json_t* rval = NULL;
 
         if (pData)
         {
-            FilterSessionType* pFilterSession = static_cast<FilterSessionType*>(pData);
+            const FilterSessionType* pFilterSession = static_cast<const FilterSessionType*>(pData);
 
             MXS_EXCEPTION_GUARD(rval = pFilterSession->diagnostics());
         }
         else
         {
-            FilterType* pFilter = static_cast<FilterType*>(pInstance);
+            const FilterType* pFilter = static_cast<const FilterType*>(pInstance);
 
             MXS_EXCEPTION_GUARD(rval = pFilter->diagnostics());
         }
diff --git a/include/maxscale/router.h b/include/maxscale/router.h
index 96a2edcbe..e3af7c6df 100644
--- a/include/maxscale/router.h
+++ b/include/maxscale/router.h
@@ -154,7 +154,7 @@ typedef struct mxs_router_object
      *
      * @see jansson.h
      */
-    json_t*     (*diagnostics)(MXS_ROUTER *instance);
+    json_t*  (*diagnostics)(const MXS_ROUTER *instance);
 
     /**
      * @brief Called for each reply packet
diff --git a/include/maxscale/router.hh b/include/maxscale/router.hh
index a05d1522f..ec51e7275 100644
--- a/include/maxscale/router.hh
+++ b/include/maxscale/router.hh
@@ -173,9 +173,9 @@ public:
         return rv;
     }
 
-    static json_t* diagnostics(MXS_ROUTER* pInstance)
+    static json_t* diagnostics(const MXS_ROUTER* pInstance)
     {
-        RouterType* pRouter = static_cast<RouterType*>(pInstance);
+        const RouterType* pRouter = static_cast<const RouterType*>(pInstance);
 
         json_t* rval = NULL;
 
diff --git a/include/maxscale/users.h b/include/maxscale/users.h
index 8d5fa69f8..ad8384596 100644
--- a/include/maxscale/users.h
+++ b/include/maxscale/users.h
@@ -117,7 +117,7 @@ int users_default_loadusers(SERV_LISTENER *port);
  *
  * @param port Port whose data is to be printed
  */
-json_t* users_default_diagnostic(SERV_LISTENER *port);
+json_t* users_default_diagnostic(const SERV_LISTENER *port);
 
 /**
  * Print details of the users storage mechanism
diff --git a/server/core/filter.cc b/server/core/filter.cc
index cae496c76..d225f45ba 100644
--- a/server/core/filter.cc
+++ b/server/core/filter.cc
@@ -585,7 +585,7 @@ int FilterSession::clientReply(GWBUF* pPacket)
     return m_up.clientReply(pPacket);
 }
 
-json_t* FilterSession::diagnostics()
+json_t* FilterSession::diagnostics() const
 {
     return NULL;
 }
diff --git a/server/core/users.cc b/server/core/users.cc
index a5e5db868..b66a22102 100644
--- a/server/core/users.cc
+++ b/server/core/users.cc
@@ -109,7 +109,7 @@ void usersPrint(const USERS *users)
     hashtable_stats(users->data);
 }
 
-json_t* users_default_diagnostic(SERV_LISTENER *port)
+json_t* users_default_diagnostic(const SERV_LISTENER *port)
 {
     json_t* rval = json_array();
 
diff --git a/server/modules/authenticator/MySQLAuth/mysql_auth.c b/server/modules/authenticator/MySQLAuth/mysql_auth.c
index 2a79d3b50..61ba21f69 100644
--- a/server/modules/authenticator/MySQLAuth/mysql_auth.c
+++ b/server/modules/authenticator/MySQLAuth/mysql_auth.c
@@ -58,7 +58,7 @@ static int mysql_auth_set_client_data(
     MySQLProtocol *protocol,
     GWBUF         *buffer);
 
-json_t* mysql_auth_diagnostic(SERV_LISTENER *port);
+json_t* mysql_auth_diagnostic(const SERV_LISTENER *port);
 
 int mysql_auth_reauthenticate(DCB *dcb, const char *user,
                               uint8_t *token, size_t token_len,
@@ -691,7 +691,7 @@ int diag_cb(void *data, int columns, char **row, char **field_names)
     return 0;
 }
 
-json_t* mysql_auth_diagnostic(SERV_LISTENER *port)
+json_t* mysql_auth_diagnostic(const SERV_LISTENER *port)
 {
     json_t* rval = json_array();
 
diff --git a/server/modules/filter/cache/cachefilter.cc b/server/modules/filter/cache/cachefilter.cc
index b3aa5f95a..68d2f5615 100644
--- a/server/modules/filter/cache/cachefilter.cc
+++ b/server/modules/filter/cache/cachefilter.cc
@@ -306,7 +306,7 @@ CacheFilterSession* CacheFilter::newSession(MXS_SESSION* pSession)
 }
 
 // static
-json_t* CacheFilter::diagnostics()
+json_t* CacheFilter::diagnostics() const
 {
     return m_sCache->show();
 }
diff --git a/server/modules/filter/cache/cachefilter.hh b/server/modules/filter/cache/cachefilter.hh
index 0b6aedece..df06d92e2 100644
--- a/server/modules/filter/cache/cachefilter.hh
+++ b/server/modules/filter/cache/cachefilter.hh
@@ -37,7 +37,7 @@ public:
 
     CacheFilterSession* newSession(MXS_SESSION* pSession);
 
-    json_t* diagnostics();
+    json_t* diagnostics() const;
 
     uint64_t getCapabilities();
 
diff --git a/server/modules/filter/cache/cachefiltersession.cc b/server/modules/filter/cache/cachefiltersession.cc
index a36a69241..a74629c0a 100644
--- a/server/modules/filter/cache/cachefiltersession.cc
+++ b/server/modules/filter/cache/cachefiltersession.cc
@@ -447,7 +447,7 @@ int CacheFilterSession::clientReply(GWBUF* pData)
     return rv;
 }
 
-json_t* CacheFilterSession::diagnostics()
+json_t* CacheFilterSession::diagnostics() const
 {
     // Not printing anything. Session of the same instance share the same cache, in
     // which case the same information would be printed once per session, or all
diff --git a/server/modules/filter/cache/cachefiltersession.hh b/server/modules/filter/cache/cachefiltersession.hh
index f8a68a287..cd153ad2f 100644
--- a/server/modules/filter/cache/cachefiltersession.hh
+++ b/server/modules/filter/cache/cachefiltersession.hh
@@ -83,7 +83,7 @@ public:
     /**
      * Print diagnostics of the session cache.
      */
-    json_t* diagnostics();
+    json_t* diagnostics() const;
 
 private:
     int handle_expecting_fields();
diff --git a/server/modules/filter/ccrfilter/ccrfilter.c b/server/modules/filter/ccrfilter/ccrfilter.c
index 51f3b2b58..24640c9ef 100644
--- a/server/modules/filter/ccrfilter/ccrfilter.c
+++ b/server/modules/filter/ccrfilter/ccrfilter.c
@@ -55,7 +55,7 @@ static  void   closeSession(MXS_FILTER *instance, MXS_FILTER_SESSION *session);
 static  void   freeSession(MXS_FILTER *instance, MXS_FILTER_SESSION *session);
 static  void   setDownstream(MXS_FILTER *instance, MXS_FILTER_SESSION *fsession, MXS_DOWNSTREAM *downstream);
 static  int    routeQuery(MXS_FILTER *instance, MXS_FILTER_SESSION *fsession, GWBUF *queue);
-static  json_t*   diagnostic(MXS_FILTER *instance, MXS_FILTER_SESSION *fsession);
+static  json_t*   diagnostic(const MXS_FILTER *instance, const MXS_FILTER_SESSION *fsession);
 static uint64_t getCapabilities(MXS_FILTER* instance);
 
 #define CCR_DEFAULT_TIME "60"
@@ -358,7 +358,7 @@ routeQuery(MXS_FILTER *instance, MXS_FILTER_SESSION *session, GWBUF *queue)
  * @param fsession  Filter session, may be NULL
  * @param dcb       The DCB for diagnostic output
  */
-static json_t* diagnostic(MXS_FILTER *instance, MXS_FILTER_SESSION *fsession)
+static json_t* diagnostic(const MXS_FILTER *instance, const MXS_FILTER_SESSION *fsession)
 {
     CCR_INSTANCE *my_instance = (CCR_INSTANCE *)instance;
     json_t* rval = json_object();
diff --git a/server/modules/filter/dbfwfilter/dbfwfilter.c b/server/modules/filter/dbfwfilter/dbfwfilter.c
index 22bb392e0..7691bef5a 100644
--- a/server/modules/filter/dbfwfilter/dbfwfilter.c
+++ b/server/modules/filter/dbfwfilter/dbfwfilter.c
@@ -101,7 +101,7 @@ static void closeSession(MXS_FILTER *instance, MXS_FILTER_SESSION *session);
 static void freeSession(MXS_FILTER *instance, MXS_FILTER_SESSION *session);
 static void setDownstream(MXS_FILTER *instance, MXS_FILTER_SESSION *fsession, MXS_DOWNSTREAM *downstream);
 static int routeQuery(MXS_FILTER *instance, MXS_FILTER_SESSION *fsession, GWBUF *queue);
-static json_t* diagnostic(MXS_FILTER *instance, MXS_FILTER_SESSION *fsession);
+static json_t* diagnostic(const MXS_FILTER *instance, const MXS_FILTER_SESSION *fsession);
 static uint64_t getCapabilities(MXS_FILTER* instance);
 
 /**
@@ -2504,7 +2504,7 @@ routeQuery(MXS_FILTER *instance, MXS_FILTER_SESSION *session, GWBUF *queue)
  * @param   dcb     The DCB for diagnostic output
  */
 static json_t*
-diagnostic(MXS_FILTER *instance, MXS_FILTER_SESSION *fsession)
+diagnostic(const MXS_FILTER *instance, const MXS_FILTER_SESSION *fsession)
 {
     return rules_to_json(thr_rules);
 }
diff --git a/server/modules/filter/hintfilter/hintfilter.c b/server/modules/filter/hintfilter/hintfilter.c
index ad7ed6040..52acf2bf5 100644
--- a/server/modules/filter/hintfilter/hintfilter.c
+++ b/server/modules/filter/hintfilter/hintfilter.c
@@ -32,7 +32,7 @@ static void closeSession(MXS_FILTER *instance, MXS_FILTER_SESSION *session);
 static void freeSession(MXS_FILTER *instance, MXS_FILTER_SESSION *session);
 static void setDownstream(MXS_FILTER *instance, MXS_FILTER_SESSION *fsession, MXS_DOWNSTREAM *downstream);
 static int routeQuery(MXS_FILTER *instance, MXS_FILTER_SESSION *fsession, GWBUF *queue);
-static json_t* diagnostic(MXS_FILTER *instance, MXS_FILTER_SESSION *fsession);
+static json_t* diagnostic(const MXS_FILTER *instance, const MXS_FILTER_SESSION *fsession);
 static uint64_t getCapabilities(MXS_FILTER* instance);
 
 /**
@@ -222,7 +222,7 @@ routeQuery(MXS_FILTER *instance, MXS_FILTER_SESSION *session, GWBUF *queue)
  * @param   fsession    Filter session, may be NULL
  */
 static json_t*
-diagnostic(MXS_FILTER *instance, MXS_FILTER_SESSION *fsession)
+diagnostic(const MXS_FILTER *instance, const MXS_FILTER_SESSION *fsession)
 {
     return NULL;
 }
diff --git a/server/modules/filter/insertstream/insertstream.c b/server/modules/filter/insertstream/insertstream.c
index 9a3fbd74e..8f09a3d18 100644
--- a/server/modules/filter/insertstream/insertstream.c
+++ b/server/modules/filter/insertstream/insertstream.c
@@ -37,7 +37,7 @@ static void freeSession(MXS_FILTER *instance, MXS_FILTER_SESSION *session);
 static void setDownstream(MXS_FILTER *instance, MXS_FILTER_SESSION *fsession, MXS_DOWNSTREAM *downstream);
 static void setUpstream(MXS_FILTER *instance, MXS_FILTER_SESSION *session, MXS_UPSTREAM *upstream);
 static int32_t routeQuery(MXS_FILTER *instance, MXS_FILTER_SESSION *fsession, GWBUF *queue);
-static json_t* diagnostic(MXS_FILTER *instance, MXS_FILTER_SESSION *fsession);
+static json_t* diagnostic(const MXS_FILTER *instance, const MXS_FILTER_SESSION *fsession);
 static uint64_t getCapabilities(MXS_FILTER *instance);
 static int32_t clientReply(MXS_FILTER* instance, MXS_FILTER_SESSION *session, GWBUF *reply);
 static bool extract_insert_target(GWBUF *buffer, char* target, int len);
@@ -498,7 +498,7 @@ static int32_t clientReply(MXS_FILTER* instance, MXS_FILTER_SESSION *session, GW
  * @param   instance The filter instance
  * @param   fsession Filter session, may be NULL
  */
-static json_t* diagnostic(MXS_FILTER *instance, MXS_FILTER_SESSION *fsession)
+static json_t* diagnostic(const MXS_FILTER *instance, const MXS_FILTER_SESSION *fsession)
 {
     DS_INSTANCE *my_instance = (DS_INSTANCE*)instance;
 
diff --git a/server/modules/filter/luafilter/luafilter.c b/server/modules/filter/luafilter/luafilter.c
index 22fe06f96..672359247 100644
--- a/server/modules/filter/luafilter/luafilter.c
+++ b/server/modules/filter/luafilter/luafilter.c
@@ -63,7 +63,7 @@ static void setDownstream(MXS_FILTER *instance, MXS_FILTER_SESSION *fsession,  M
 static void setUpstream(MXS_FILTER *instance, MXS_FILTER_SESSION *fsession,  MXS_UPSTREAM *upstream);
 static int32_t routeQuery(MXS_FILTER *instance, MXS_FILTER_SESSION *fsession, GWBUF *queue);
 static int32_t clientReply(MXS_FILTER *instance, MXS_FILTER_SESSION *fsession, GWBUF *queue);
-static json_t* diagnostic(MXS_FILTER *instance, MXS_FILTER_SESSION *fsession);
+static json_t* diagnostic(const MXS_FILTER *instance, const MXS_FILTER_SESSION *fsession);
 static uint64_t getCapabilities(MXS_FILTER *instance);
 
 /**
@@ -624,7 +624,7 @@ static int32_t routeQuery(MXS_FILTER *instance, MXS_FILTER_SESSION *session, GWB
  * @param instance The filter instance
  * @param fsession Filter session, may be NULL
  */
-static json_t* diagnostic(MXS_FILTER *instance, MXS_FILTER_SESSION *fsession)
+static json_t* diagnostic(const MXS_FILTER *instance, const MXS_FILTER_SESSION *fsession)
 {
     LUA_INSTANCE *my_instance = (LUA_INSTANCE *)instance;
     json_t* rval = json_object();
diff --git a/server/modules/filter/masking/maskingfilter.cc b/server/modules/filter/masking/maskingfilter.cc
index a02595249..58b0a545e 100644
--- a/server/modules/filter/masking/maskingfilter.cc
+++ b/server/modules/filter/masking/maskingfilter.cc
@@ -138,7 +138,7 @@ MaskingFilterSession* MaskingFilter::newSession(MXS_SESSION* pSession)
 }
 
 // static
-json_t* MaskingFilter::diagnostics()
+json_t* MaskingFilter::diagnostics() const
 {
     return NULL;
 }
diff --git a/server/modules/filter/masking/maskingfilter.hh b/server/modules/filter/masking/maskingfilter.hh
index a977373a0..db464c678 100644
--- a/server/modules/filter/masking/maskingfilter.hh
+++ b/server/modules/filter/masking/maskingfilter.hh
@@ -33,7 +33,7 @@ public:
 
     MaskingFilterSession* newSession(MXS_SESSION* pSession);
 
-    json_t* diagnostics();
+    json_t* diagnostics() const;
 
     uint64_t getCapabilities();
 
diff --git a/server/modules/filter/maxrows/maxrows.c b/server/modules/filter/maxrows/maxrows.c
index 1dbd2b65b..67dd9bdbe 100644
--- a/server/modules/filter/maxrows/maxrows.c
+++ b/server/modules/filter/maxrows/maxrows.c
@@ -67,8 +67,8 @@ static int     routeQuery(MXS_FILTER *instance,
 static int     clientReply(MXS_FILTER *instance,
                            MXS_FILTER_SESSION *sdata,
                            GWBUF *queue);
-static json_t* diagnostics(MXS_FILTER *instance,
-                           MXS_FILTER_SESSION *sdata);
+static json_t* diagnostics(const MXS_FILTER *instance,
+                           const MXS_FILTER_SESSION *sdata);
 static uint64_t getCapabilities(MXS_FILTER *instance);
 
 enum maxrows_return_mode
@@ -487,7 +487,7 @@ static int clientReply(MXS_FILTER *instance,
  * @param fsession  Filter session, may be NULL
  * @param dcb       The DCB for diagnostic output
  */
-static json_t* diagnostics(MXS_FILTER *instance, MXS_FILTER_SESSION *sdata)
+static json_t* diagnostics(const MXS_FILTER *instance, const MXS_FILTER_SESSION *sdata)
 {
     return NULL;
 }
diff --git a/server/modules/filter/mqfilter/mqfilter.c b/server/modules/filter/mqfilter/mqfilter.c
index 13adb8384..a7faa99f3 100644
--- a/server/modules/filter/mqfilter/mqfilter.c
+++ b/server/modules/filter/mqfilter/mqfilter.c
@@ -94,7 +94,7 @@ static void setDownstream(MXS_FILTER *instance, MXS_FILTER_SESSION *fsession, MX
 static void setUpstream(MXS_FILTER *instance, MXS_FILTER_SESSION *fsession, MXS_UPSTREAM *upstream);
 static int routeQuery(MXS_FILTER *instance, MXS_FILTER_SESSION *fsession, GWBUF *queue);
 static int clientReply(MXS_FILTER *instance, MXS_FILTER_SESSION *fsession, GWBUF *queue);
-static json_t* diagnostic(MXS_FILTER *instance, MXS_FILTER_SESSION *fsession);
+static json_t* diagnostic(const MXS_FILTER *instance, const MXS_FILTER_SESSION *fsession);
 static uint64_t getCapabilities(MXS_FILTER *instance);
 
 /**
@@ -1484,7 +1484,7 @@ static int clientReply(MXS_FILTER* instance, MXS_FILTER_SESSION *session, GWBUF
  * @param       fsession        Filter session, may be NULL
  */
 static json_t*
-diagnostic(MXS_FILTER *instance, MXS_FILTER_SESSION *fsession)
+diagnostic(const MXS_FILTER *instance, const MXS_FILTER_SESSION *fsession)
 {
     MQ_INSTANCE *my_instance = (MQ_INSTANCE*)instance;
     json_t* rval = json_object();
diff --git a/server/modules/filter/namedserverfilter/namedserverfilter.cc b/server/modules/filter/namedserverfilter/namedserverfilter.cc
index 4b16ec99a..7309829fd 100644
--- a/server/modules/filter/namedserverfilter/namedserverfilter.cc
+++ b/server/modules/filter/namedserverfilter/namedserverfilter.cc
@@ -311,7 +311,7 @@ RegexHintFilter::create(const char* name, char** options, MXS_CONFIG_PARAMETER*
  *
  * @param   dcb     The DCB for diagnostic output
  */
-json_t* RegexHintFSession::diagnostics()
+json_t* RegexHintFSession::diagnostics() const
 {
 
     json_t* rval = m_fil_inst.diagnostics(); /* Print overall diagnostics */
@@ -329,7 +329,7 @@ json_t* RegexHintFSession::diagnostics()
  *
  * @param   dcb     The DCB for diagnostic output
  */
-json_t* RegexHintFilter::diagnostics()
+json_t* RegexHintFilter::diagnostics() const
 {
     json_t* rval = json_object();
 
@@ -340,12 +340,12 @@ json_t* RegexHintFilter::diagnostics()
     {
         json_t* arr = json_array();
 
-        for (MappingArray::iterator it = m_mapping.begin(); it != m_mapping.end(); it++)
+        for (MappingArray::const_iterator it = m_mapping.begin(); it != m_mapping.end(); it++)
         {
             json_t* obj = json_object();
             json_t* targets = json_array();
 
-            for (StringArray::iterator it2 = it->m_targets.begin(); it2 != it->m_targets.end(); it2++)
+            for (StringArray::const_iterator it2 = it->m_targets.begin(); it2 != it->m_targets.end(); it2++)
             {
                 json_array_append_new(targets, json_string(it2->c_str()));
             }
diff --git a/server/modules/filter/namedserverfilter/namedserverfilter.hh b/server/modules/filter/namedserverfilter/namedserverfilter.hh
index 3b3f012e6..b5a868344 100644
--- a/server/modules/filter/namedserverfilter/namedserverfilter.hh
+++ b/server/modules/filter/namedserverfilter/namedserverfilter.hh
@@ -57,7 +57,7 @@ public:
     static RegexHintFilter* create(const char* zName, char** pzOptions,
                                    MXS_CONFIG_PARAMETER* ppParams);
     RegexHintFSession* newSession(MXS_SESSION *session);
-    json_t* diagnostics();
+    json_t* diagnostics() const;
     uint64_t getCapabilities();
     const RegexToServers* find_servers(char* sql, int sql_len, pcre2_match_data* mdata);
 
@@ -86,7 +86,7 @@ public:
                       pcre2_match_data* md);
     ~RegexHintFSession();
 
-    json_t* diagnostics();
+    json_t* diagnostics() const;
     int routeQuery(GWBUF* buffer);
 };
 
diff --git a/server/modules/filter/nullfilter/nullfilter.cc b/server/modules/filter/nullfilter/nullfilter.cc
index 346bf5b4b..6032d1518 100644
--- a/server/modules/filter/nullfilter/nullfilter.cc
+++ b/server/modules/filter/nullfilter/nullfilter.cc
@@ -142,7 +142,7 @@ NullFilterSession* NullFilter::newSession(MXS_SESSION* pSession)
 }
 
 // static
-json_t* NullFilter::diagnostics()
+json_t* NullFilter::diagnostics() const
 {
     return NULL;
 }
diff --git a/server/modules/filter/nullfilter/nullfilter.hh b/server/modules/filter/nullfilter/nullfilter.hh
index b2fabae7b..e0c96173e 100644
--- a/server/modules/filter/nullfilter/nullfilter.hh
+++ b/server/modules/filter/nullfilter/nullfilter.hh
@@ -24,7 +24,7 @@ public:
 
     NullFilterSession* newSession(MXS_SESSION* pSession);
 
-    json_t* diagnostics();
+    json_t* diagnostics() const;
 
     uint64_t getCapabilities();
 
diff --git a/server/modules/filter/qlafilter/qlafilter.c b/server/modules/filter/qlafilter/qlafilter.c
index 77f0d5578..b6aa6ec94 100644
--- a/server/modules/filter/qlafilter/qlafilter.c
+++ b/server/modules/filter/qlafilter/qlafilter.c
@@ -81,7 +81,7 @@ static void closeSession(MXS_FILTER *instance, MXS_FILTER_SESSION *session);
 static void freeSession(MXS_FILTER *instance, MXS_FILTER_SESSION *session);
 static void setDownstream(MXS_FILTER *instance, MXS_FILTER_SESSION *fsession, MXS_DOWNSTREAM *downstream);
 static int routeQuery(MXS_FILTER *instance, MXS_FILTER_SESSION *fsession, GWBUF *queue);
-static json_t* diagnostic(MXS_FILTER *instance, MXS_FILTER_SESSION *fsession);
+static json_t* diagnostic(const MXS_FILTER *instance, const MXS_FILTER_SESSION *fsession);
 static uint64_t getCapabilities(MXS_FILTER* instance);
 
 /**
@@ -579,7 +579,7 @@ routeQuery(MXS_FILTER *instance, MXS_FILTER_SESSION *session, GWBUF *queue)
  * @param   instance    The filter instance
  * @param   fsession    Filter session, may be NULL
  */
-static json_t* diagnostic(MXS_FILTER *instance, MXS_FILTER_SESSION *fsession)
+static json_t* diagnostic(const MXS_FILTER *instance, const MXS_FILTER_SESSION *fsession)
 {
     QLA_INSTANCE *my_instance = (QLA_INSTANCE*)instance;
     QLA_SESSION *my_session = (QLA_SESSION*)fsession;
diff --git a/server/modules/filter/regexfilter/regexfilter.c b/server/modules/filter/regexfilter/regexfilter.c
index 3252a262b..9fdf13713 100644
--- a/server/modules/filter/regexfilter/regexfilter.c
+++ b/server/modules/filter/regexfilter/regexfilter.c
@@ -48,7 +48,7 @@ static void closeSession(MXS_FILTER *instance, MXS_FILTER_SESSION *session);
 static void freeSession(MXS_FILTER *instance, MXS_FILTER_SESSION *session);
 static void setDownstream(MXS_FILTER *instance, MXS_FILTER_SESSION *fsession, MXS_DOWNSTREAM *downstream);
 static int routeQuery(MXS_FILTER *instance, MXS_FILTER_SESSION *fsession, GWBUF *queue);
-static json_t* diagnostic(MXS_FILTER *instance, MXS_FILTER_SESSION *fsession);
+static json_t* diagnostic(const MXS_FILTER *instance, const MXS_FILTER_SESSION *fsession);
 static uint64_t getCapabilities(MXS_FILTER* instance);
 
 static char *regex_replace(const char *sql, pcre2_code *re, pcre2_match_data *study,
@@ -377,7 +377,7 @@ routeQuery(MXS_FILTER *instance, MXS_FILTER_SESSION *session, GWBUF *queue)
  * @param   instance    The filter instance
  * @param   fsession    Filter session, may be NULL
  */
-static json_t* diagnostic(MXS_FILTER *instance, MXS_FILTER_SESSION *fsession)
+static json_t* diagnostic(const MXS_FILTER *instance, const MXS_FILTER_SESSION *fsession)
 {
     REGEX_INSTANCE *my_instance = (REGEX_INSTANCE*)instance;
     REGEX_SESSION *my_session = (REGEX_SESSION*)fsession;
diff --git a/server/modules/filter/tee/tee.c b/server/modules/filter/tee/tee.c
index 24d2ad8c4..d09dde1ae 100644
--- a/server/modules/filter/tee/tee.c
+++ b/server/modules/filter/tee/tee.c
@@ -106,7 +106,7 @@ static void setDownstream(MXS_FILTER *instance, MXS_FILTER_SESSION *fsession, MX
 static void setUpstream(MXS_FILTER *instance, MXS_FILTER_SESSION *fsession, MXS_UPSTREAM *upstream);
 static int routeQuery(MXS_FILTER *instance, MXS_FILTER_SESSION *fsession, GWBUF *queue);
 static int clientReply(MXS_FILTER *instance, MXS_FILTER_SESSION *fsession, GWBUF *queue);
-static json_t* diagnostic(MXS_FILTER *instance, MXS_FILTER_SESSION *fsession);
+static json_t* diagnostic(const MXS_FILTER *instance, const MXS_FILTER_SESSION *fsession);
 static uint64_t getCapabilities(MXS_FILTER* instance);
 
 /**
@@ -698,7 +698,7 @@ clientReply(MXS_FILTER* instance, MXS_FILTER_SESSION *session, GWBUF *reply)
  * @param   instance    The filter instance
  * @param   fsession    Filter session, may be NULL
  */
-static json_t* diagnostic(MXS_FILTER *instance, MXS_FILTER_SESSION *fsession)
+static json_t* diagnostic(const MXS_FILTER *instance, const MXS_FILTER_SESSION *fsession)
 {
     TEE_INSTANCE *my_instance = (TEE_INSTANCE*)instance;
     TEE_SESSION *my_session = (TEE_SESSION*)fsession;
diff --git a/server/modules/filter/topfilter/topfilter.c b/server/modules/filter/topfilter/topfilter.c
index 56b0ea5a4..24cf86465 100644
--- a/server/modules/filter/topfilter/topfilter.c
+++ b/server/modules/filter/topfilter/topfilter.c
@@ -58,7 +58,7 @@ static void setDownstream(MXS_FILTER *instance, MXS_FILTER_SESSION *fsession, MX
 static void setUpstream(MXS_FILTER *instance, MXS_FILTER_SESSION *fsession, MXS_UPSTREAM *upstream);
 static int routeQuery(MXS_FILTER *instance, MXS_FILTER_SESSION *fsession, GWBUF *queue);
 static int clientReply(MXS_FILTER *instance, MXS_FILTER_SESSION *fsession, GWBUF *queue);
-static json_t* diagnostic(MXS_FILTER *instance, MXS_FILTER_SESSION *fsession);
+static json_t* diagnostic(const MXS_FILTER *instance, const MXS_FILTER_SESSION *fsession);
 static uint64_t getCapabilities(MXS_FILTER* instance);
 
 /**
@@ -574,7 +574,7 @@ clientReply(MXS_FILTER *instance, MXS_FILTER_SESSION *session, GWBUF *reply)
  * @param   instance    The filter instance
  * @param   fsession    Filter session, may be NULL
  */
-static json_t* diagnostic(MXS_FILTER *instance, MXS_FILTER_SESSION *fsession)
+static json_t* diagnostic(const MXS_FILTER *instance, const MXS_FILTER_SESSION *fsession)
 {
     TOPN_INSTANCE *my_instance = (TOPN_INSTANCE*)instance;
     TOPN_SESSION *my_session = (TOPN_SESSION*)fsession;
diff --git a/server/modules/filter/tpmfilter/tpmfilter.c b/server/modules/filter/tpmfilter/tpmfilter.c
index 88494539e..d0697cd79 100644
--- a/server/modules/filter/tpmfilter/tpmfilter.c
+++ b/server/modules/filter/tpmfilter/tpmfilter.c
@@ -85,7 +85,7 @@ static  void    setDownstream(MXS_FILTER *instance, MXS_FILTER_SESSION *fsession
 static  void    setUpstream(MXS_FILTER *instance, MXS_FILTER_SESSION *fsession, MXS_UPSTREAM *upstream);
 static  int routeQuery(MXS_FILTER *instance, MXS_FILTER_SESSION *fsession, GWBUF *queue);
 static  int clientReply(MXS_FILTER *instance, MXS_FILTER_SESSION *fsession, GWBUF *queue);
-static  json_t*    diagnostic(MXS_FILTER *instance, MXS_FILTER_SESSION *fsession);
+static  json_t*    diagnostic(const MXS_FILTER *instance, const MXS_FILTER_SESSION *fsession);
 static uint64_t getCapabilities(MXS_FILTER* instance);
 static  void checkNamedPipe(void *args);
 
@@ -574,7 +574,7 @@ clientReply(MXS_FILTER *instance, MXS_FILTER_SESSION *session, GWBUF *reply)
  * @param   fsession    Filter session, may be NULL
  */
 static json_t*
-diagnostic(MXS_FILTER *instance, MXS_FILTER_SESSION *fsession)
+diagnostic(const MXS_FILTER *instance, const MXS_FILTER_SESSION *fsession)
 {
     TPM_INSTANCE *my_instance = (TPM_INSTANCE*)instance;
 
diff --git a/server/modules/routing/avrorouter/avro.c b/server/modules/routing/avrorouter/avro.c
index 2807eb90d..4bb1ab0cb 100644
--- a/server/modules/routing/avrorouter/avro.c
+++ b/server/modules/routing/avrorouter/avro.c
@@ -76,7 +76,7 @@ static MXS_ROUTER_SESSION *newSession(MXS_ROUTER *instance, MXS_SESSION *session
 static void closeSession(MXS_ROUTER *instance, MXS_ROUTER_SESSION *router_session);
 static void freeSession(MXS_ROUTER *instance, MXS_ROUTER_SESSION *router_session);
 static int routeQuery(MXS_ROUTER *instance, MXS_ROUTER_SESSION *router_session, GWBUF *queue);
-static json_t* diagnostics(MXS_ROUTER *instance);
+static json_t* diagnostics(const MXS_ROUTER *instance);
 static void clientReply(MXS_ROUTER *instance, MXS_ROUTER_SESSION *router_session, GWBUF *queue,
                         DCB *backend_dcb);
 static void errorReply(MXS_ROUTER *instance, MXS_ROUTER_SESSION *router_session, GWBUF *message,
@@ -807,7 +807,7 @@ routeQuery(MXS_ROUTER *instance, MXS_ROUTER_SESSION *router_session, GWBUF *queu
  *
  * @param instance  Instance of the router
  */
-static json_t* diagnostics(MXS_ROUTER *router)
+static json_t* diagnostics(const MXS_ROUTER *router)
 {
     AVRO_INSTANCE *router_inst = (AVRO_INSTANCE *)router;
 
diff --git a/server/modules/routing/binlogrouter/blr.c b/server/modules/routing/binlogrouter/blr.c
index c83268c7a..4ae20776e 100644
--- a/server/modules/routing/binlogrouter/blr.c
+++ b/server/modules/routing/binlogrouter/blr.c
@@ -95,7 +95,7 @@ static  MXS_ROUTER_SESSION *newSession(MXS_ROUTER *instance, MXS_SESSION *sessio
 static  void closeSession(MXS_ROUTER *instance, MXS_ROUTER_SESSION *router_session);
 static  void freeSession(MXS_ROUTER *instance, MXS_ROUTER_SESSION *router_session);
 static  int routeQuery(MXS_ROUTER *instance, MXS_ROUTER_SESSION *router_session, GWBUF *queue);
-static  json_t* diagnostics(MXS_ROUTER *instance);
+static  json_t* diagnostics(const MXS_ROUTER *instance);
 static  void clientReply(MXS_ROUTER *instance,
                          MXS_ROUTER_SESSION *router_session,
                          GWBUF *queue,
@@ -1287,7 +1287,7 @@ spin_reporter(void *dcb, char *desc, int value)
  * @param instance  Instance of the router
  * @param dcb       DCB to send diagnostics to
  */
-static json_t* diagnostics(MXS_ROUTER *router)
+static json_t* diagnostics(const MXS_ROUTER *router)
 {
     ROUTER_INSTANCE *router_inst = (ROUTER_INSTANCE *)router;
     int minno = 0;
diff --git a/server/modules/routing/cli/cli.c b/server/modules/routing/cli/cli.c
index 9d13bce48..fe7e4e797 100644
--- a/server/modules/routing/cli/cli.c
+++ b/server/modules/routing/cli/cli.c
@@ -48,7 +48,7 @@ static  MXS_ROUTER_SESSION *newSession(MXS_ROUTER *instance, MXS_SESSION *sessio
 static  void closeSession(MXS_ROUTER *instance, MXS_ROUTER_SESSION *router_session);
 static  void freeSession(MXS_ROUTER *instance, MXS_ROUTER_SESSION *router_session);
 static  int execute(MXS_ROUTER *instance, MXS_ROUTER_SESSION *router_session, GWBUF *queue);
-static  json_t* diagnostics(MXS_ROUTER *instance);
+static  json_t* diagnostics(const MXS_ROUTER *instance);
 static  uint64_t getCapabilities(MXS_ROUTER* instance);
 
 extern int execute_cmd(CLI_SESSION *cli);
@@ -284,7 +284,7 @@ execute(MXS_ROUTER *instance, MXS_ROUTER_SESSION *router_session, GWBUF *queue)
  * @param instance  Instance of the router
  * @param dcb       DCB to send diagnostics to
  */
-static  json_t* diagnostics(MXS_ROUTER *instance)
+static  json_t* diagnostics(const MXS_ROUTER *instance)
 {
     return NULL;
 }
diff --git a/server/modules/routing/debugcli/debugcli.c b/server/modules/routing/debugcli/debugcli.c
index c238e929e..83e6afa82 100644
--- a/server/modules/routing/debugcli/debugcli.c
+++ b/server/modules/routing/debugcli/debugcli.c
@@ -47,7 +47,7 @@ static  MXS_ROUTER_SESSION *newSession(MXS_ROUTER *instance, MXS_SESSION *sessio
 static  void closeSession(MXS_ROUTER *instance, MXS_ROUTER_SESSION *router_session);
 static  void freeSession(MXS_ROUTER *instance, MXS_ROUTER_SESSION *router_session);
 static  int execute(MXS_ROUTER *instance, MXS_ROUTER_SESSION *router_session, GWBUF *queue);
-static  json_t* diagnostics(MXS_ROUTER *instance);
+static  json_t* diagnostics(const MXS_ROUTER *instance);
 static  uint64_t getCapabilities(MXS_ROUTER* instance);
 
 extern int execute_cmd(CLI_SESSION *cli);
@@ -287,7 +287,7 @@ execute(MXS_ROUTER *instance, MXS_ROUTER_SESSION *router_session, GWBUF *queue)
  * @param instance  Instance of the router
  * @param dcb       DCB to send diagnostics to
  */
-static  json_t* diagnostics(MXS_ROUTER *instance)
+static  json_t* diagnostics(const MXS_ROUTER *instance)
 {
     return NULL;
 }
diff --git a/server/modules/routing/hintrouter/hintrouter.cc b/server/modules/routing/hintrouter/hintrouter.cc
index 5e932de76..061ff8693 100644
--- a/server/modules/routing/hintrouter/hintrouter.cc
+++ b/server/modules/routing/hintrouter/hintrouter.cc
@@ -144,7 +144,7 @@ HintRouterSession* HintRouter::newSession(MXS_SESSION *pSession)
     return rval;
 }
 
-json_t* HintRouter::diagnostics()
+json_t* HintRouter::diagnostics() const
 {
     HR_ENTRY();
 
diff --git a/server/modules/routing/hintrouter/hintrouter.hh b/server/modules/routing/hintrouter/hintrouter.hh
index aebd2ea8a..f4dcad722 100644
--- a/server/modules/routing/hintrouter/hintrouter.hh
+++ b/server/modules/routing/hintrouter/hintrouter.hh
@@ -22,7 +22,7 @@ class HintRouter : public maxscale::Router<HintRouter, HintRouterSession>
 public:
     static HintRouter* create(SERVICE* pService, char** pzOptions);
     HintRouterSession* newSession(MXS_SESSION *pSession);
-    json_t* diagnostics();
+    json_t* diagnostics() const;
     uint64_t getCapabilities() const
     {
         return RCAP_TYPE_NONE;
diff --git a/server/modules/routing/maxinfo/maxinfo.c b/server/modules/routing/maxinfo/maxinfo.c
index d9b157947..80164a065 100644
--- a/server/modules/routing/maxinfo/maxinfo.c
+++ b/server/modules/routing/maxinfo/maxinfo.c
@@ -69,7 +69,7 @@ static  MXS_ROUTER_SESSION *newSession(MXS_ROUTER *instance, MXS_SESSION *sessio
 static  void closeSession(MXS_ROUTER *instance, MXS_ROUTER_SESSION *router_session);
 static  void freeSession(MXS_ROUTER *instance, MXS_ROUTER_SESSION *router_session);
 static  int execute(MXS_ROUTER *instance, MXS_ROUTER_SESSION *router_session, GWBUF *queue);
-static  json_t* diagnostics(MXS_ROUTER *instance);
+static  json_t* diagnostics(const MXS_ROUTER *instance);
 static  uint64_t getCapabilities(MXS_ROUTER* instance);
 static  void handleError(MXS_ROUTER     *instance,
                          MXS_ROUTER_SESSION  *router_session,
@@ -369,7 +369,7 @@ execute(MXS_ROUTER *rinstance, MXS_ROUTER_SESSION *router_session, GWBUF *queue)
  * @param dcb       DCB to send diagnostics to
  */
 static  json_t*
-diagnostics(MXS_ROUTER *instance)
+diagnostics(const MXS_ROUTER *instance)
 {
     return NULL;
 }
diff --git a/server/modules/routing/readconnroute/readconnroute.c b/server/modules/routing/readconnroute/readconnroute.c
index 264c2d6a8..7573da13b 100644
--- a/server/modules/routing/readconnroute/readconnroute.c
+++ b/server/modules/routing/readconnroute/readconnroute.c
@@ -93,7 +93,7 @@ static MXS_ROUTER_SESSION *newSession(MXS_ROUTER *instance, MXS_SESSION *session
 static void closeSession(MXS_ROUTER *instance, MXS_ROUTER_SESSION *router_session);
 static void freeSession(MXS_ROUTER *instance, MXS_ROUTER_SESSION *router_session);
 static int routeQuery(MXS_ROUTER *instance, MXS_ROUTER_SESSION *router_session, GWBUF *queue);
-static json_t* diagnostics(MXS_ROUTER *instance);
+static json_t* diagnostics(const MXS_ROUTER *instance);
 static void clientReply(MXS_ROUTER *instance, MXS_ROUTER_SESSION *router_session, GWBUF *queue,
                         DCB *backend_dcb);
 static void handleError(MXS_ROUTER *instance, MXS_ROUTER_SESSION *router_session, GWBUF *errbuf,
@@ -615,7 +615,7 @@ return_rc:
  * @param instance  Instance of the router
  * @param dcb       DCB to send diagnostics to
  */
-static json_t* diagnostics(MXS_ROUTER *router)
+static json_t* diagnostics(const MXS_ROUTER *router)
 {
     ROUTER_INSTANCE *router_inst = (ROUTER_INSTANCE *)router;
     json_t* rval = json_object();
diff --git a/server/modules/routing/readwritesplit/readwritesplit.c b/server/modules/routing/readwritesplit/readwritesplit.c
index cb9dd7ba3..f6ebdd1a3 100644
--- a/server/modules/routing/readwritesplit/readwritesplit.c
+++ b/server/modules/routing/readwritesplit/readwritesplit.c
@@ -74,7 +74,7 @@ static MXS_ROUTER_SESSION *newSession(MXS_ROUTER *instance, MXS_SESSION *session
 static void closeSession(MXS_ROUTER *instance, MXS_ROUTER_SESSION *session);
 static void freeSession(MXS_ROUTER *instance, MXS_ROUTER_SESSION *session);
 static int routeQuery(MXS_ROUTER *instance, MXS_ROUTER_SESSION *session, GWBUF *queue);
-static json_t* diagnostics(MXS_ROUTER *instance);
+static json_t* diagnostics(const MXS_ROUTER *instance);
 static void clientReply(MXS_ROUTER *instance, MXS_ROUTER_SESSION *router_session, GWBUF *queue,
                         DCB *backend_dcb);
 static void handleError(MXS_ROUTER *instance, MXS_ROUTER_SESSION *router_session,
@@ -646,7 +646,7 @@ static int routeQuery(MXS_ROUTER *instance, MXS_ROUTER_SESSION *router_session,
  * @param   instance    The router instance
  * @param   dcb     The DCB for diagnostic output
  */
-static json_t* diagnostics(MXS_ROUTER *instance)
+static json_t* diagnostics(const MXS_ROUTER *instance)
 {
     ROUTER_INSTANCE *router = (ROUTER_INSTANCE *)instance;
 
diff --git a/server/modules/routing/schemarouter/schemarouterinstance.cc b/server/modules/routing/schemarouter/schemarouterinstance.cc
index 95a0e4177..ac28ee1df 100644
--- a/server/modules/routing/schemarouter/schemarouterinstance.cc
+++ b/server/modules/routing/schemarouter/schemarouterinstance.cc
@@ -302,7 +302,7 @@ SchemaRouterSession* SchemaRouter::newSession(MXS_SESSION* pSession)
     return rval;
 }
 
-json_t* SchemaRouter::diagnostics()
+json_t* SchemaRouter::diagnostics() const
 {
     double sescmd_pct = m_stats.n_sescmd != 0 ?
         100.0 * ((double)m_stats.n_sescmd / (double)m_stats.n_queries) :
diff --git a/server/modules/routing/schemarouter/schemarouterinstance.hh b/server/modules/routing/schemarouter/schemarouterinstance.hh
index a0b3afae3..62e1a152f 100644
--- a/server/modules/routing/schemarouter/schemarouterinstance.hh
+++ b/server/modules/routing/schemarouter/schemarouterinstance.hh
@@ -37,7 +37,7 @@ public:
     ~SchemaRouter();
     static SchemaRouter* create(SERVICE* pService, char** pzOptions);
     SchemaRouterSession* newSession(MXS_SESSION* pSession);
-    json_t* diagnostics();
+    json_t* diagnostics() const;
     uint64_t getCapabilities();
 
 private: