Introduction to DDT Architecture


The need for full migration and real-time synchronization of MongoDB databases led to the development of the new data migration project at our company, resulting in the DDT project.

Project Overview

  • Name: DDT (Document Data Transfer)
  • Language: Developed purely in Java
  • Purpose: Full migration and real-time synchronization of MongoDB databases

Functionality Overview

DDT is a MongoDB data synchronization component.

Migration features include:

Synchronization Modes

  • Full Synchronization: Syncs all data in tables. Only data from tables existing before the program startup is synchronized.

  • Real-time Synchronization: Real-time sync of oplogs (operation logs) generated by the source.

  • Full + Incremental Synchronization: After full synchronization, only operations on source tables during the sync period are synchronized. The start time of real-time sync is the start time of full synchronization, and the end time of real-time sync is the end time of full synchronization.

  • Full + Real-time Synchronization: After full synchronization, real-time sync begins. The start time of real-time sync is the start time of full synchronization.

Additional Features

  • Delayed Synchronization: During real-time sync, the reading of oplogs can be delayed.

  • Syncing DDL Operations: During real-time sync, users can customize the synchronization of certain DDL operations.

  • Source Table Indexes: During full synchronization, users can specify whether to sync the source table indexes list after 60% of the total data is synced.

  • Multi-Table Parallelism: During full synchronization, choose between synchronizing multiple source tables simultaneously or synchronizing them one by one.

  • Sync Source Table List: Use regular expressions to specify the desired list of tables to sync.

  • Time-Interval Real-time Sync: During real-time sync, you can set to sync oplogs within a certain time interval.

MongoDB Versions

  • Supported Versions: DDT supports MongoDB versions 3.2, 3.4, 3.6, 4.0, 4.4, 5.0, and 6.0.


DDT Architecture


  • A JVM container corresponds to multiple instances, with each instance corresponding to a migration program.
  • Each instance comprises three parts: a. Source (extracts data from the source database, supports full/real-time implementation) b. Cache (caches data from the source according to target requirements) c. Target (updates data to the target database, supports full/real-time/comparison implementation)

DDT Process Diagram

Real-Time Migration

Real-Time Migration


  • Use CAS mechanism to ensure that only one thread accesses each table's buckets at a time.
  • Use CAS mechanism to ensure that only one thread writes data to a bucket of a table at a time.
  • When splitting buckets for a table, if a DDL operation is encountered, the data before that DDL is written, followed by executing the DDL.

Full Migration

Full Migration


  • Use CAS mechanism to ensure that only one thread writes/reads bucket queues in the same partition at the same time.
  • Data from a source table can be placed in any partition. A target table can retrieve data from any partition. However, only one thread can operate on a partition at a time.

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