Are there any difficulties with using external storage? And, for example, could we have multiple instances of the same application? What could happen in this case?

Using external storage with multiple application instances can lead to several challenges:

• Data consistency: Concurrent access by multiple instances to a single storage can result in race conditions and conflicting records.
• Caching: If each instance caches data locally, desynchronization between the cache and storage may occur.
• Performance: External storage can become a bottleneck if it does not scale with the load of all instances.
• Session management: If user sessions are stored in the memory of instances, load balancing can cause access issues to sessions.

To address these issues, the following are used:

• Locking mechanisms or transactions in storage.
• Centralized caching (e.g., Redis).
• Horizontal scaling and load balancing.
• Storing sessions in a shared storage.

Thus, it is important to consider the features of distributed architecture and ensure consistency and scalability.