Data Science Innovation Software Architecture

What are database 𝐒𝐬𝐨π₯𝐚𝐭𝐒𝐨𝐧 π₯𝐞𝐯𝐞π₯𝐬? What are they used for?

Database isolation allows a transaction to execute as if there are no other concurrently running transactions.

The diagram below illustrates four isolation levels.

πŸ”ΉSerializalble: This is the highest isolation level. Concurrent transactions are guaranteed to be executed in sequence.

πŸ”ΉRepeatable Read: Data read during the transaction stays the same as the transaction starts.

πŸ”ΉRead Committed: Data modification can only be read after the transaction is committed.

πŸ”ΉRead Uncommitted: The data modification can be read by other transactions before a transaction is committed.

The isolation is guaranteed by MVCC (Multi-Version Consistency Control) and locks.

The diagram below takes Repeatable Read as an example to demonstrate how MVCC works:

There are two hidden columns for each row: transaction_id and roll_pointer. When transaction A starts, a new Read View with transaction_id=201 is created. Shortly afterward, transaction B starts, and a new Read View with transaction_id=202 is created.

Now transaction A modifies the balance to 200, a new row of the log is created, and the roll_pointer points to the old row. Before transaction A commits, transaction B reads the balance data. Transaction B finds that transaction_id 201 is not committed, it reads the next committed record(transaction_id=200).

Even when transaction A commits, transaction B still reads data based on the Read View created when transaction B starts. So transaction B always reads the data with balance=100.

Over to you: have you seen isolation levels used in the wrong way? Did it cause serious outages?

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