If two services want to write the same record to the database simultaneously, what will happen?

When two services attempt to write the same record to a database simultaneously, the behavior depends on the locking mechanisms and transaction isolation levels in the DBMS.

In most cases:

• One of the services will acquire a lock for writing (e.g., at the row or table level).
• The second service will wait for the lock to be released or will receive an error (e.g., conflict or deadlock).

If a transaction with an isolation level such as "Read Committed" or "Serializable" is used, the database guarantees data consistency.

Example:

• Service A starts a transaction and updates a record.
• Service B attempts to update the same record and waits until Service A completes the transaction.
• After the transaction completes, Service B either applies its changes or receives an error if the data has changed.

To avoid issues, the following are often used:

• Optimistic locking mechanisms (version check of the record).
• Explicit locks (SELECT ... FOR UPDATE).
• Queues or other synchronization tools between services.

Thus, concurrent writing requires proper management of concurrent data access.