Avoid blocking other requests while validating large output

lib/responseHelper.js

@@ -21,7 +21,13 @@ responseHelper._enforceSchemaOnArray = (items, schema, callback) => {
   if (!(items instanceof Array)) {
     items = [ items ]
   }
-  async.map(items, (item, done) => responseHelper._enforceSchemaOnObject(item, schema, done), (err, results) => {
+  // the validation process can take a lengthy amount of time when processing large responses.
+  // to prevent other requests from being blocked by this syncronous process, we break it up using
+  // setImmediate (see _enforceSchemaOnObject below), and force this map to run in series instead of in parallel.
+  // It really will not make much of a difference to individual requests, but
+  // splitting it up this way makes a HUGE difference if a small request arrives on the server while a large request's
+  // output is still being validated.
+  async.mapSeries(items, (item, done) => responseHelper._enforceSchemaOnObject(item, schema, done), (err, results) => {
     if (err) return callback(err)
 
     results = results.filter(result => !!result)
@@ -30,15 +36,21 @@ responseHelper._enforceSchemaOnArray = (items, schema, callback) => {
 }
 
 responseHelper._enforceSchemaOnObject = (item, schema, callback) => {
-  debug.validationOutput(JSON.stringify(item))
-  Joi.validate(item, schema, (err, sanitisedItem) => {
-    if (err) {
-      debug.validationError(err.message, JSON.stringify(item))
-      return callback(null, null)
-    }
+  setImmediate(() => {
+    debug.validationOutput(JSON.stringify(item))
+    Joi.validate(item, schema, (err, sanitisedItem) => {
+      if (err) {
+        debug.validationError(err.message, JSON.stringify(item))
+        // is silently dropping an output just because it doesn't validate really the right thing to do here?
+        // if a dev isn't watching their logs carefully, this could cause unexpected output.
+        // that said, Joi.validate does do some nice things, like normalizing data types and setting defaults, etc.
+        // not sure what the right answer is here.
+        return callback(null, null)
+      }
 
-    const dataItem = responseHelper._generateDataItem(sanitisedItem, schema)
-    return callback(null, dataItem)
+      const dataItem = responseHelper._generateDataItem(sanitisedItem, schema)
+      return callback(null, dataItem)
+    })
   })
 }