store: Allow disabling some notifications

The notifications we send when transacting entity changes are only needed
for updating GraphQL subscriptions. When that is not needed, disabling
these notifications will reduce the load on Postgres' message queue

Author: lutter

docs/environment-variables.md

@@ -105,6 +105,10 @@ those.
 - `GRAPH_SQL_STATEMENT_TIMEOUT`: the maximum number of seconds an
   individual SQL query is allowed to take during GraphQL
   execution. Default: unlimited
+- `GRAPH_DISABLE_SUBSCRIPTION_NOTIFICATIONS`: disables the internal
+  mechanism that is used to trigger updates on GraphQL subscriptions. When
+  this variable is set to any value, `graph-node` will still accept GraphQL
+  subscriptions, but they won't receive any updates.
 
 ## Miscellaneous
 

store/postgres/src/subgraph_store.rs

@@ -49,6 +49,12 @@ pub struct Shard(String);
 lazy_static! {
     /// The name of the primary shard that contains all instance-wide data
     pub static ref PRIMARY_SHARD: Shard = Shard("primary".to_string());
+    /// Whether to disable the notifications that feed GraphQL
+    /// subscriptions; when the environment variable is set, no updates
+    /// about entity changes will be sent to query nodes
+    pub static ref SEND_SUBSCRIPTION_NOTIFICATIONS: bool = {
+      std::env::var("GRAPH_DISABLE_SUBSCRIPTION_NOTIFICATIONS").ok().is_none()
+    };
 }
 
 /// How long to cache information about a deployment site
@@ -1072,7 +1078,11 @@ impl WritableStoreTrait for WritableStore {
         let event = self
             .writable
             .revert_block_operations(self.site.clone(), block_ptr_to)?;
-        self.store.send_store_event(&event)
+        if *SEND_SUBSCRIPTION_NOTIFICATIONS {
+            self.store.send_store_event(&event)
+        } else {
+            Ok(())
+        }
     }
 
     fn unfail(&self) -> Result<(), StoreError> {
@@ -1117,7 +1127,11 @@ impl WritableStoreTrait for WritableStore {
         )?;
 
         let _section = stopwatch.start_section("send_store_event");
-        self.store.send_store_event(&event)
+        if *SEND_SUBSCRIPTION_NOTIFICATIONS {
+            self.store.send_store_event(&event)
+        } else {
+            Ok(())
+        }
     }
 
     fn get_many(