Server Side Databases

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Server-based Databases

Problems

Simultaneous access to a database can cause problems, but only under specific circumstances

two clients attempt to edit the same record at the same time, one of the edits will be lost as it is overwritten this is the lost update problem

Managing Concurrent Access

Transactions

Each query is passed to the database as a transaction

3 stages

submitted (sent to the database, with an attached timestamp)

executed (the processing takes place)

committed (the data is read/written)

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Record Locks

Once a transaction is submitted, the record is locked

No other transactions on that record can take place until the first transaction commits and the lock is released

Any updates sent during the lock period will fail, and will need to be re-attempted

Serialisation

the transactions are queued and executed in a specific order, rather than being blocked

Timestamp Ordering

database prioritises the timestamp when deciding what order to execute the transactions

Each transaction will be executed in the order they were submitted

Commitment Ordering

The database prioritises commitment when deciding what order to execute the transactions

Transactions are executed in the order they commit, rather then the order they were submitted

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Pros and Cons

Timestamp ordering is generally faster, but can result in anomalies if later transactions commit before earlier ones

Commitment ordering is more predictable, but can be slower, and may result in deadlocks, where one transaction is waiting for the commitment of another, which is waiting for the commitment of the first