This post continues my previous post on the subject. In that post I describe the data structures, schema, data transfer object, indexing algorithm, filtering algorithms; also in that post, I review the literature and analyze complexity. Here, I show the actual CRUD operations. The approach is scalable, JPQL-only, and uses very few pessimistic locks. Spring Initializr code can be found here. Let’s recall the requirements for this task.

The Requirements

We build a CRM system for a mid-size online education center. The system is implemented with Spring Boot 2, H2(development), and (for now) Ms-SQL(production) technology. Among many functionalities, the system must be able to create, read, update, and delete nested comments from/to a relational database. The functional requirements for the comment functionality are:

1. The most common operations are to find all descendants of a particular comment and to find all root comments,
2. All new and edited comments are moderated before being published and saved into the database,
3. If a comment is deleted, all its child comments are deleted as well,
4. The system should support full-text search over comments’ content.

We assume the following about the comments:

1. The comments are text strings no more than a certain length (no pictures or video fragments for now),
2. Every comment has 0 or 1 parent,
3. Most of the comments are questions/answers/hints on assignments and classes, so there going to be a lot more reads than writes,
4. Ones saved, the comments are rarely updated and even more rarely, if ever, deleted,
5. The older the comment gets, the lesser it is commented; the total number of child comments of a particular comment rarely exceeds a certain (large) number; the number depends on the depth of the comment.
6. For every root comment, there are a lot more sub root comments. This can be achieved, for example, if only users of a certain role or reputation can create root comments.
7. The majority of the comments are just few levels deep. Much deeper comments are rare.

The following are the implementation details for the comment functionality:

1. the functionalities need to be implemented with JPQL only and no native queries,
2. there should be as few pessimistic locks as possible.

These requirements are not easy to satisfy simultaneously.

The Challenges

To implement these requirements, we face the following challenges:

1. The data structures must be compatible with the relational database (rows and columns), yet implement a deeply nested tree (this challenge is addressed in Part 1).
2. There should be efficient indexes to quickly find all children of a particular comment, yet no dialect-specific database features should be used (this challenge is addressed in Part 1).
3. If all comments are stored in a single table, all root comments can not be found quickly without advanced dialect-specific database features. Yet, if all root comments are stored in a separate table, we have to use transactions (and pessimistic locks).

Notice, that if we use a JPQL-only approach, then transactions are inevitable since we have to process at least some data on the server side. To make as few transactions as possible, we need to separate methods, where transactions are absolutely necessary, from methods that can work without transactions. Let’s see how to use our requirements, data structures, and algorithms to achieve this.

#spring data jpa #spring boot 2.2 #hierarchical data #jpql