Faith & Code | Reflections by Errong

ParallelStream vs Virtual Threads in Java 21: What Actually Works for Database Workloads Java offers multiple ways to parallelize work, but not all concurrency models are suitable for database-heavy workloads. In modern systems, especially with large batch processing (e.g., 10,000–50,000 IDs), choosing the right model can drastically change performance. This article compares ParallelStream and Virtual Threads (Java 21) using a real-world scenario: batch database queries in Oracle. 🧠 The Problem: Large Batch Database Queries A common backend scenario is fetching data using large ID lists: Example: - 40,000 transaction IDs - Chunked into 1,000 per query - ~40 database queries executed The key question becomes: how do we execute these 40 queries efficiently? ⚙️ Approach 1: ParallelStream IntStream.range(0, batchCount) .parallel() .mapToObj(i -> repository.query(batch)) How it works Uses ForkJoin common pool Thread count ≈ number of CP...