Admin Backend

Read Concerns

• Represent different levels of "read isolation"
  • if data can be read by clients before it has been replicated to a majority of nodes it is considered a low level of "read isolation"
• Can be used to specify a consistent view of the database

ReadConcern local

• The default read concern level
• Does not check that data has been replicated

ReadConcern majority

• Allows more durable reads
• Only returns data that has been replicated to a majority of nodes (data might be more stale but more durable)
• Used for mission critical data

Other concern levels

There are more concern levels available. See MongoDB Documentation.

Ticket: User Report (Using group method)

Task: Build a pipeline that returns the 20 most frequent commenters on the MFlix site. You can do this by counting the number of occurrences of a user's email in the comments collection. In addition, set the ReadConcern on the _commentsCollection to ensure the most accurate reads occur.

Solution:

var result = await _commentsCollection
  .WithReadConcern(ReadConcern.Majority)
  .Aggregate()
  .Group(x => x.Email, value => new ReportProjection { Id = value.Key, Count = value.Count() })
  .Sort(Builders<ReportProjection>.Sort.Descending(c => c.Count))
  .Limit(20)
  .ToListAsync();

return new TopCommentsProjection(result);

Bulk Writes

• Bulk writes allow database clients to send multiple writes
• Can either be ordered or unordered
• More performant than executing a write operation for each document

Ordered Bulk Write

• The default setting for bulk writes in MongoDB
• Executes writes sequentially
  • Will end execution after first write failure
• Replica will execute writes in the order they were sent

Unordered Bulk Write

• Has to be specified with the flag { ordered: false }
• Executes writes in parallel

Using BulkWriteAsync to update many documents in a specific way

The BulkWriteAsync method takes an IEnumerable of the WriteModel abstract class as input. There are several derived classes of WriteModel, which allow you to perform different operations:

C# Driver	MongoDB Shell equivalent	Operation
InsertOneModel	insertOne	Inserts the document
UpdateOneModel	updateOne	Updates one document that matches the filter definition
UpdateManyModel	updateMany	Updates all the documents that match the filter definition
ReplaceOneModel	replaceOne	Replaces one document that matches the filter definition with the document provided
DeleteOneModel	deleteOne	Deletes one document that matches the filter definition
DeleteManyModel	deleteMany	Deletes all the documents that match the filter definition

These operations can be passed to the BulkWriteAsync method and will be executed. They are equivalent to the MongoDB db.collection.bulkWrite() shell method.

Example

var requests = new List<ReplaceOneModel<Movie>>();
datePipelineResults.ForEach(transformedMovie => requests.Add(new ReplaceOneModel<Movie>(Builders<Movie>.Filter.Where(movie => movie.Id == transformedMovie.Id), transformedMovie)));
var bulkWriteDatesResult = await _moviesCollection.BulkWriteAsync(requests);