Concurrency control contd … Database recovery

Concurrency control contd…Database recovery 17. 02 . 2011

RECAP • Concurrency control concepts • Motivation • If we insist only one transaction can execute at a time, in serial order, then performance will be quite poor. • Concurrency Control is a method for controlling or scheduling the operations of transactions in such a way that concurrent transactions can be executed safely (i.e., without causing the database to reach an inconsistent state) • If we do concurrency control properly, then we can maximize transaction throughput while avoiding any chance of corrupting the database RECAP

There are several categories of problems encountered when concurrent access is required: • Lost Update Problem • Dirty Read problem • Incorrect Summary Problem • Non-Repeatable Read problem RECAP

Implementing Concurrency Control • Locking • Locks seek to guarantee that concurrent transactions can be serialized. • Locks may be applied to data items in two ways: • Implicit Locks are applied by the DBMS • Explicit Locks are applied by application programs • What is locking? RECAP

Lock granularity • Locks may be applied to: • a single data item (value) • an entire row of a table • a page (memory segment) (many rows worth) • an entire table • an entire database • Escalation of lock granularity What do you remember from our discussions on that RECAP

Locks types: • An Exclusive Lock • A Shared Lock depending on the requirements of the transaction • Downside of locks? RECAP

The basis of concurrency control is protocols to maintain serialization in DBMSs • E.g. protocols • Two-Phase Locking (2PL) • Timestamps Ordering • Optimistic Concurrency Control (OCC) • Mulitversion Concurrency • Multiversion based on Timestamp • Multiversion 2PL with certify locks

Two Phased Locking (2PL) • 2PL’s two phases: Growing and shrinking. • Variations of 2PL: Conservative 2PL, Strict 2PL RECAP

Variations of 2PL • In 2PL, transactions may get locks as they need them and release them as soon as possible by either: • the transaction pre-declares which items it will work with and it acquires all locks before any work is done (Conservative 2PL) • once all operations are completed, all of the locks are released after commit (Strict 2PL,) RECAP

Example (Exercise) • Consider T: Ra WaRbWbRcWc Rd Wd • Further, given the symbols: R = Read. W = write. L = Lock. U = Unlock. • Generate example schedules (one implementing conservative 2PL and another strict 2PL) RECAP

Conservative: • TLaTLbTLcTldTRaTwaTUaRbWbTUbRcWcTUc Rd Wd TUd • Strict: • TLaTRaTwaTLbTLcRbWbTLdRcWc Rd Wd TUaTUbTUcTUd

Timestamp Ordering • The timestamp of a transaction T is the time at which that transaction was initiated in the DBMS: TS(T) • We can use clock time or an incremental identifier (counter) for TS(T) • Two timestamps are also associated with each data item x. • 1. read_TS(x) is the TS(T) of the last transaction T to read from x. • 2. write_TS(x) is the TS(T) of the last transaction T to write to x. RECAP

Two simple rules to follow: • 1. Before T issues a write(x), check to see if • TS(T) < read_TS(x) or if • TS(T) < write_TS(x) • If so, then abort transaction T. • If not, then perform write(x) and set write_TS(x) = TS(T). Why Abort? RECAP

Two simple rules to follow: • 2. Before T issues a read(x), check to see if • TS(T) < write_TS(x) Then abort transaction T. • if TS(T) >= write_TS(x) then execute read(x) and set read_TS(x) = TS(T) only if TS(T) is greater than the current read_TS(x) - ? - ? ABORT or EXECUTE? RECAP

Two simple rules to follow: • 2. Before T issues a read(x), check to see if • TS(T) < write_TS(x) Then abort transaction T. • if TS(T) >= write_TS(x) then execute read(x) and set read_TS(x) = TS(T) only if TS(T) is greater than the current read_TS(x)

When a transaction is aborted, it is the restarted and issued a new TS(T). • Note that with timestamp ordering, deadlock can not occur. • However, starvation is possible i.e., a transaction keeps getting aborted over and over.

Timestamp ordering can also produce cascading rollbacks: • Assume transaction T begins executing and performs some read and write operations on data items a, b and c • However, T then reaches a data item it can not read or write and T must then be aborted. • Any effects of transaction T must then be rolled back. • Before T aborts, however, other transactions (T1, T2 and T3) have read and written data items a, b and c so these other transactions must also be rolled back. • There may be other transactions (T4 and T5) that worked with data items read or written by T1, T2 and T3, etc.

Optimistic Concurrency Control • Two Phase Locking (2PL) and Timestamp Ordering (TO) are pessimistic concurrency control protocols - they assume transactions will conflict and take steps to avoid it. i.e., they address the concurrency issues before while the transaction is executing and before the transaction commits. • 2PL and TO are also syntactic concurrency control protocols as they deal only with the syntax (set of read and write operations) of the transactions.

Optimistic Concurrency Control (2) • In an optimistic concurrency control protocol, we assume that most of the time, transactions will not conflict thus all of the locking and timestamp checking are not necessary. • There are a number of different optimistic CC protocols. Elmasri/Navathe describe an optimistic/syntactic concurrency control protocol.

In an optimistic concurrency control protocol, we assume that most of the time, transactions will not conflict thus all of the locking and timestamp checking are not necessary. • No checking for serialization is done while the transaction is executing • During transaction execution, all updates are applied to local copies of the data items that are kept for the transaction • During a validation phase the transactions updates are check to see if they violate serializability

The idea behind OCC is to do all the checks at once • If there is little interference between transactions, most will be validated successfully. • Extra requirements for OCC: • Local Copy • Transaction Timestamps • Must keep track of write_set & read_set

Optimistic Concurrency Control in three stages: • Read Stage: Transactions can read any data item. Writes are done to a local copy of the data item e.g., recorded in a log. • Validation stage: Transactions containing Write operations that are about to commit are validated to see if the schedule meets the serializabilityrequirements. • Write stage: If the transaction will not conflict with other transactions, then it will be committed (writes to local copy applied to the database). Otherwise, the transaction will be rolled back.

There are also a number of optimistic/semantic concurrency control protocols that take into account the semantics of the database application and schedule transactions accordingly. • Examples include databases that are "read mostly" or "read only". RECAP END

Concurrency control contd…Database recoveryDatabase back up

Database recovery • Overview: • Why DB Recovery • Recovery techniques: • Manual Reprocessing • Automated Recovery • Checkpoints • Deferred Update Recovery • Immediate Update Recovery

Database recovery • Motivations/ rationale • There are many situations in which a transaction may not reach a commit or abort point. • An operating system crash can terminate the DBMS processes • The DBMS can crash • The process issuing the transaction can abnormally terminate • The system might lose power • A disk may fail or other hardware may fail. • Human error can result in deletion of critical data. ?

In any of these situations, data in the database may become inconsistent or lost. • Two general classes of failures: • Process or Processor failures in which no data is lost but the database must be restarted and recovered. • Storage failure wherein data on disk is lost or corrupted. Data must be restored from backup and then the database must be restarted and recovered.

Types of Failure • The database may become unavailable for use due to • Transaction failure: Transactions may fail because of incorrect input, deadlock, incorrect synchronization. • System failure: System may fail because of addressing error, application error, operating system fault, RAM failure, etc. • Media failure: Disk head crash, power disruption, etc.

Definition: • Database Recovery is the process of restoring the database and the data to a consistent state. This may include restoring lost data up to the point of the event (e.g. system crash). • There are many different approaches to recover a database.

Database recovery • Recovery approaches: • Manual Reprocessing • Automated Recovery • Checkpoints • Deferred Update Recovery • Immediate Update Recovery

Manual Reprocessing • Overview: • The database is periodically backed up (a database save) and all transactions applied since the last save are recorded • If the system crashes, the latest database save is restored and all of the transactions are re-applied (by users) to bring the database back up to the point just before the crash. • Weaknesses: • Time required to re-apply transactions • Transactions might have other (physical) consequences • Re-applying concurrent transactions is not straight forward.

Database recovery • Recovery approaches: • Manual Reprocessing • Automated Recovery • Checkpoints • Deferred Update Recovery • Immediate Update Recovery

Automated Recovery • The DBMS is able to (re-)apply required changes in the event of a system failure • Recall, a transaction can be in one of the following states: • Active - when the transaction just begins • Partially Committed - after the last operation has completed (but before the commit point is reached) • Failed - Normal operation is prevented (e.g., in the event of a system crash) • Aborted - Transaction is rolled back. That is, all of its effects are undone • Committed - Transaction completes all operations and moved the database to the next consistent state

Automated Recovery • Automated recovery applies a similar technique to the manual recovery, i.e. Make periodic saves of the database (time consuming operation). However, maintain a more intelligent log (called the system log or transaction log) of the transactions that have been applied. Based on the “intelligent log ” then we can recover in the event of failure • NB: Recovery should be idempotent - that is, it should not matter if we recover many times; the effect to the database should be the same.

System log entries: • Each transaction writes the following information to the log: • Start(T) - the fact that transaction T has started • Write(T, X, old_value, new_value) - the fact that transaction T has written to item X with the new_value. old_value is also maintained. • Read(T, X) - the fact that transaction T has read data item X • Either: Commit(T) - transaction T committed, or Abort(T) - transaction T was aborted

Checkpoints • Checkpoint: A recovery point in the logs where all current transactions have terminated and all updates have been written to disk. Consists of 4 steps: • 1. Cease accepting new transactions • 2. Allow all unfinished transactions to complete (commit or abort) • 3. Write all pending changes to disk and to logs • 4. Resume accepting new transactions

Checkpoints (2) • In many environments, it is possible to take checkpoints each 15 minutes or half hour, etc. • Recovery must then only be done from the time of the last checkpoint (assuming no damage to storage media). What is the advantage of using time based checkpoints? Problem?

Database recovery • Overview: • Manual Reprocessing • Automated Recovery • Checkpoints • Deferred Update Recovery • Immediate Update Recovery

Deferred Update Recovery • Overview • During a transaction, only record the changes to data items in the log. • When the transaction commits, actually update the data items on disk. • Two main rules: • A transaction cannot change any items in the database until it commits. • A transaction may not commit until all of the write operations are successfully recorded in the log. This means that we must check to see that the log is actually written to disk.

Consider the following 4 transactions • T1: Ra Rd Wd C • T2: RbWb Rd Wd C • T3: Ra WaRcWc C • T4: RbWb Ra Wa C • We make a schedule: T1, T4, T2, T3

We make a schedule: T1, T4, T2, T3

T1: Ra Rd Wd C • T2: RbWb Rd Wd C • T3: Ra WaRcWc C • T4: RbWb Ra Wa C How do we recover based on this log?

Recovery from the log file • Since T1 and T4 committed, their changes were written to disk.However, T2 and T3 did not commit, hence their changes were not written to disk. To recover, we simply ignore those transactions that did not commit.

Advantages: • 1. Recovery is made easier: • Any transaction that reached the commit point (from the log) has its writes applied to the database (REDO). All other transactions are ignored. • 2. Cascading rollback does not occur because no other transaction sees the work of another until it is committed (no stale reads). • Disadvantages: • Concurrency is limited: Must employ Strict 2PL which limits concurrency.

Immediate Update Recovery • Overview • Immediate Update applies the write operations to the database as the transaction is executing (as opposed to at the end as with deferred update). • Writes are still saved in the log before being applied to the database - a write-ahead log (WAL) • Maintains two logs: • REDO log: A record of each new data item in the database. • UNDO log: A record of each updated data item (old values).

Two rules: • Transaction T may not update the database until all UNDO entries have been written to the UNDO log. • Transaction T is not allowed to commit until all REDO and UNDO log entries are written (forced-written to disk).

Concurrency control contd … Database recovery