feat(read-preference): unify means of read preference resolution (#1738)

* feat(read-preference): unify means of read preference resolution

NODE-1515

lib/collection.js

@@ -17,7 +17,7 @@ const ordered = require('./bulk/ordered');
 const ChangeStream = require('./change_stream');
 const executeOperation = require('./utils').executeOperation;
 const applyWriteConcern = require('./utils').applyWriteConcern;
-const getReadPreference = require('./utils').getReadPreference;
+const resolveReadPreference = require('./utils').resolveReadPreference;
 
 // Operations
 const bulkWrite = require('./operations/collection_ops').bulkWrite;
@@ -340,7 +340,10 @@ Collection.prototype.find = function(query, options, callback) {
   newOptions.slaveOk = options.slaveOk != null ? options.slaveOk : this.s.db.slaveOk;
 
   // Add read preference if needed
-  newOptions = getReadPreference(this, newOptions, this.s.db);
+  newOptions.readPreference = resolveReadPreference(newOptions, {
+    db: this.s.db,
+    collection: this
+  });
 
   // Set slave ok to true if read preference different from primary
   if (
@@ -1223,7 +1226,7 @@ Collection.prototype.listIndexes = function(options) {
   // Clone the options
   options = Object.assign({}, options);
   // Determine the read preference in the options.
-  options = getReadPreference(this, options, this.s.db, this);
+  options.readPreference = resolveReadPreference(options, { db: this.s.db, collection: this });
   // Set the CommandCursor constructor
   options.cursorFactory = CommandCursor;
   // Set the promiseLibrary
@@ -1738,7 +1741,7 @@ Collection.prototype.aggregate = function(pipeline, options, callback) {
 
   options = Object.assign({}, options);
   // Ensure we have the right read preference inheritance
-  options = getReadPreference(this, options, this.s.db, this);
+  options.readPreference = resolveReadPreference(options, { db: this.s.db, collection: this });
 
   // If explain has been specified add it
   if (options.explain) {
@@ -1836,7 +1839,7 @@ Collection.prototype.parallelCollectionScan = function(options, callback) {
 
   options = Object.assign({}, options);
   // Ensure we have the right read preference inheritance
-  options = getReadPreference(this, options, this.s.db, this);
+  options.readPreference = resolveReadPreference(options, { db: this.s.db, collection: this });
 
   // Add a promiseLibrary
   options.promiseLibrary = this.s.promiseLibrary;

lib/db.js

@@ -15,7 +15,7 @@ const Collection = require('./collection');
 const mergeOptionsAndWriteConcern = require('./utils').mergeOptionsAndWriteConcern;
 const executeOperation = require('./utils').executeOperation;
 const applyWriteConcern = require('./utils').applyWriteConcern;
-const convertReadPreference = require('./utils').convertReadPreference;
+const resolveReadPreference = require('./utils').resolveReadPreference;
 const ChangeStream = require('./change_stream');
 
 // Operations
@@ -477,11 +477,10 @@ Db.prototype.listCollections = function(filter, options) {
   options.promiseLibrary = this.s.promiseLibrary;
 
   // Ensure valid readPreference
-  if (options.readPreference) {
-    options.readPreference = convertReadPreference(options.readPreference);
-  } else {
-    options.readPreference = this.s.readPreference || ReadPreference.primary;
-  }
+  options.readPreference = resolveReadPreference(options, {
+    db: this,
+    default: ReadPreference.primary
+  });
 
   // Cursor options
   let cursor = options.batchSize ? { batchSize: options.batchSize } : {};
@@ -673,11 +672,7 @@ Db.prototype.collections = function(options, callback) {
 Db.prototype.executeDbAdminCommand = function(selector, options, callback) {
   if (typeof options === 'function') (callback = options), (options = {});
   options = options || {};
-
-  // Convert read preference
-  if (options.readPreference) {
-    options.readPreference = convertReadPreference(options.readPreference);
-  }
+  options.readPreference = resolveReadPreference(options);
 
   return executeOperation(this.s.topology, executeDbAdminCommand, [
     this,

lib/operations/collection_ops.js

@@ -12,7 +12,7 @@ const evaluate = require('./db_ops').evaluate;
 const executeCommand = require('./db_ops').executeCommand;
 const executeDbAdminCommand = require('./db_ops').executeDbAdminCommand;
 const formattedOrderClause = require('../utils').formattedOrderClause;
-const getReadPreference = require('../utils').getReadPreference;
+const resolveReadPreference = require('../utils').resolveReadPreference;
 const handleCallback = require('../utils').handleCallback;
 const indexInformationDb = require('./db_ops').indexInformation;
 const isObject = require('../utils').isObject;
@@ -195,7 +195,7 @@ function count(coll, query, options, callback) {
   if (hint) cmd.hint = hint;
 
   // Ensure we have the right read preference inheritance
-  options = getReadPreference(coll, options, coll.s.db);
+  options.readPreference = resolveReadPreference(options, { db: coll.s.db, collection: coll });
 
   // Do we have a readConcern specified
   decorateWithReadConcern(cmd, coll, options);
@@ -359,7 +359,7 @@ function distinct(coll, key, query, options, callback) {
 
   options = Object.assign({}, options);
   // Ensure we have the right read preference inheritance
-  options = getReadPreference(coll, options, coll.s.db, coll);
+  options.readPreference = resolveReadPreference(options, { db: coll.s.db, collection: coll });
 
   // Add maxTimeMS if defined
   if (typeof maxTimeMS === 'number') cmd.maxTimeMS = maxTimeMS;
@@ -634,7 +634,7 @@ function geoHaystackSearch(coll, x, y, options, callback) {
 
   options = Object.assign({}, options);
   // Ensure we have the right read preference inheritance
-  options = getReadPreference(coll, options, coll.s.db, coll);
+  options.readPreference = resolveReadPreference(options, { db: coll.s.db, collection: coll });
 
   // Do we have a readConcern specified
   decorateWithReadConcern(commandObject, coll, options);
@@ -694,7 +694,7 @@ function group(coll, keys, condition, initial, reduce, finalize, command, option
 
     options = Object.assign({}, options);
     // Ensure we have the right read preference inheritance
-    options = getReadPreference(coll, options, coll.s.db, coll);
+    options.readPreference = resolveReadPreference(options, { db: coll.s.db, collection: coll });
 
     // Do we have a readConcern specified
     decorateWithReadConcern(selector, coll, options);
@@ -891,7 +891,7 @@ function mapReduce(coll, map, reduce, options, callback) {
   options = Object.assign({}, options);
 
   // Ensure we have the right read preference inheritance
-  options = getReadPreference(coll, options, coll.s.db, coll);
+  options.readPreference = resolveReadPreference(options, { db: coll.s.db, collection: coll });
 
   // If we have a read preference and inline is not set as output fail hard
   if (
@@ -1280,7 +1280,7 @@ function stats(coll, options, callback) {
 
   options = Object.assign({}, options);
   // Ensure we have the right read preference inheritance
-  options = getReadPreference(coll, options, coll.s.db, coll);
+  options.readPreference = resolveReadPreference(options, { db: coll.s.db, collection: coll });
 
   // Execute the command
   executeCommand(coll.s.db, commandObject, options, callback);

lib/operations/db_ops.js

@@ -2,7 +2,7 @@
 
 const applyWriteConcern = require('../utils').applyWriteConcern;
 const Code = require('mongodb-core').BSON.Code;
-const convertReadPreference = require('../utils').convertReadPreference;
+const resolveReadPreference = require('../utils').resolveReadPreference;
 const crypto = require('crypto');
 const Db = require('../db');
 const debugOptions = require('../utils').debugOptions;
@@ -482,11 +482,7 @@ function executeCommand(db, command, options, callback) {
   }
 
   // Convert the readPreference if its not a write
-  if (options.readPreference) {
-    options.readPreference = convertReadPreference(options.readPreference);
-  } else {
-    options.readPreference = ReadPreference.primary;
-  }
+  options.readPreference = resolveReadPreference(options, { default: ReadPreference.primary });
 
   // Debug information
   if (db.s.logger.isDebug())

lib/utils.js

@@ -496,60 +496,57 @@ function applyWriteConcern(target, sources, options) {
 }
 
 /**
- * Ensures provided read preference is properly converted into an object
- * @param {(ReadPreference|string|object)} readPreference the user provided read preference
- * @return {ReadPreference}
+ * Resolves a read preference based on well-defined inheritance rules. This method will not only
+ * determine the read preference (if there is one), but will also ensure the returned value is a
+ * properly constructed instance of `ReadPreference`.
+ *
+ * @param {Object} options The options passed into the method, potentially containing a read preference
+ * @param {Object} sources Sources from which we can inherit a read preference
+ * @returns {(ReadPreference|null)} The resolved read preference
  */
-function convertReadPreference(readPreference) {
-  if (readPreference) {
-    if (typeof readPreference === 'string') {
-      return new ReadPreference(readPreference);
-    } else if (
-      readPreference &&
-      !(readPreference instanceof ReadPreference) &&
-      typeof readPreference === 'object'
-    ) {
-      const mode = readPreference.mode || readPreference.preference;
-      if (mode && typeof mode === 'string') {
-        return new ReadPreference(mode, readPreference.tags, {
-          maxStalenessSeconds: readPreference.maxStalenessSeconds
-        });
-      }
-    } else if (!(readPreference instanceof ReadPreference)) {
-      throw new TypeError('Invalid read preference: ' + readPreference);
-    }
-  }
+function resolveReadPreference(options, sources) {
+  options = options || {};
+  sources = sources || {};
 
-  return readPreference;
-}
+  const db = sources.db;
+  const coll = sources.collection;
+  const defaultReadPreference = sources.default;
 
-// Figure out the read preference
-function getReadPreference(coll, options, db) {
-  let r = null;
+  let readPreference;
   if (options.readPreference) {
-    r = options.readPreference;
-  } else if (coll.s.readPreference) {
-    r = coll.s.readPreference;
-  } else if (db.s.readPreference) {
-    r = db.s.readPreference;
-  } else {
-    return options;
-  }
-
-  if (typeof r === 'string') {
-    options.readPreference = new ReadPreference(r);
-  } else if (r && !(r instanceof ReadPreference) && typeof r === 'object') {
-    const mode = r.mode || r.preference;
+    readPreference = options.readPreference;
+  } else if (coll && coll.s.readPreference) {
+    readPreference = coll.s.readPreference;
+  } else if (db && db.s.readPreference) {
+    readPreference = db.s.readPreference;
+  } else if (defaultReadPreference) {
+    readPreference = defaultReadPreference;
+  }
+
+  // do we even have a read preference?
+  if (readPreference == null) {
+    return null;
+  }
+
+  // now attempt to convert the read preference if necessary
+  if (typeof readPreference === 'string') {
+    readPreference = new ReadPreference(readPreference);
+  } else if (
+    readPreference &&
+    !(readPreference instanceof ReadPreference) &&
+    typeof readPreference === 'object'
+  ) {
+    const mode = readPreference.mode || readPreference.preference;
     if (mode && typeof mode === 'string') {
-      options.readPreference = new ReadPreference(mode, r.tags, {
-        maxStalenessSeconds: r.maxStalenessSeconds
+      readPreference = new ReadPreference(mode, readPreference.tags, {
+        maxStalenessSeconds: readPreference.maxStalenessSeconds
       });
     }
-  } else if (!(r instanceof ReadPreference)) {
-    throw new TypeError('Invalid read preference: ' + r);
+  } else if (!(readPreference instanceof ReadPreference)) {
+    throw new TypeError('Invalid read preference: ' + readPreference);
   }
 
-  return options;
+  return readPreference;
 }
 
 /**
@@ -625,9 +622,8 @@ module.exports = {
   translateReadPreference,
   executeOperation,
   applyWriteConcern,
-  convertReadPreference,
+  resolveReadPreference,
   isPromiseLike,
-  getReadPreference,
   decorateWithCollation,
   decorateWithReadConcern
 };