Database have two copies of the data.
Only one copy is writable, the over one are readable with multiple threads.

Once the transaction is written, the pointer to the readable copy is swapped to the new version of the database.
Then the transaction is replicated to the second copy.
After that, the system is ready to apply the next transaction from the transaction log.

Before the swap, we should wait to finish all readers in order to preserve consistency of data.
It is possible to write all transactions from transaction log that were not yet written while we wait for all readers to finish.