Hello, Ms Sql Server, as many dbms, has different types of lock: at row level, at page level, at table level, etc.
The isolation level defines the extension of lock.
For example, with "Serializable" level the db engine locks entirely all tables involved in the logic transaction, so that a second user cannot never change data managed by the first user, until the end of first transaction.
See detailed (and long) description:
https://learn.microsoft.com/en-us/sql/relational-databases/sql-server-transaction-locking-and-row-versioning-guide?view=sql-server-ver16And you have to understand the concept of logic transaction. Simplifying, a transaction contains all db (Zeos) instructions comprised between the Begin transaction and Commiy (or Rollback if you want to cancel all updates).
In short, the TZConnection component has a property TransactionIsolationLevel that you can set to the 4 possible values, as descripted in previous link.
From another discussion:
https://zeoslib.sourceforge.io/viewtopic.php?t=2644 a possible sequence of Zeos instructions:
//At connection:
TransactionIsolationLevel := tiNone;
//At StartTransaction:
TransactionIsolationLevel := tiReadCommited;
// do all select, update, delete, .... on various tables
//At Commit, that is when confirming the updates
Commit;
TransactionIsolationLevel := tiNone;
//Or, at Rollback, that is when you want to cancel the updates,
// for example in case of exception
RollBack;
TransactionIsolationLevel := tiNone;
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