The marketplace system slowed down under load. Suspecting a database issue. What will you look for?

When suspecting database issues in a slow marketplace system, consider checking the following aspects:

1. Database Load:

   • Number of concurrent connections.
   • CPU and memory usage on the database server.

2. Indexes:

   • Absence or inefficiency of indexes leading to full table scans.

3. Slow Queries:

   • Analyze execution plans (EXPLAIN) to identify bottlenecks.
   • Queries with long execution times.

4. Locks and Contention:

   • Presence of locks, deadlocks.
   • Contention for table or row resources.

5. Database Configuration:

   • Caching parameters, connection pool settings.
   • Parallelism settings.

6. Logs and Metrics:

   • Error and warning logs.
   • Performance metrics.

7. Hardware Resources:

   • Disk I/O, network.

After identifying problematic areas, measures can be taken: optimize queries, add indexes, increase resources, configure connection pools, etc.