Transactions and Exception Handling Eric Allsop SQLBits 6 th October 2007

Transactions Lock Manager Locks, lockable resources and modes Managing locks Concurrency vs. Isolation Blocks and Deadlocks Old School Exception Handling TRY/CATCH Managing Exceptions in Transactions Transactions and Exception Handling

Atomicity Consistency Isolation Durability What is a Transaction?

Implementation –Auto commit –Implicit (IMPLICIT_TRANSACTION) –Explicit (BEGAN TRAN etc.) Scope –Local –Distributed Types of Transaction

Transaction Processing

Internal SQL Server service Manages access to resources Services lock and latch requests Enforces isolation level In memory service Fixed memory allocation Lock Manager

Common Access –RID / KEY –Page –Table –Database Space Management –Extent –HOBT –Allocation Unit –File Other –Application –Metadata Lockable Resources Hierarchy of lockable resources Locks applied at most suitable level to maximise concurrency Automatic lock escalation driven by memory limits Lock hints

Existing Granted Mode Requested ModeISSUIXSIXX Intent shared (IS)   Shared (S)   Update (U)   Intent exclusive (IX)     Shared with intent exclusive (SIX)   Exclusive (X)  Lock Modes Shared locks (S) Exclusive locks (X) Lock compatibility Update locks (U) Intent locks (IS, IX, SIX, IU, SIU, UIX)

Schema locks (Sch-S, Sch-M) Bulk Update locks (BU) Key Range locks (RangeS-S, …) Lock hints Lock Modes contd.

Pre 2005 –sp_lock –sp_who / sp_who2 –DBCC INPUTBUFFER / fn_get_sql SQL 2005 –sys.dm_tran_locks –sys.dm_exec_connections –sys.dm_exec_requests –sys.dm_exec_sql_text Viewing Locking Information

Lost updates Uncommitted dependencies (Dirty reads) Inconsistent Analysis (Non repeatable reads) Phantom Reads Concurrency Effects

Pessimistic – use locks –Lower concurrency –Blocking –Deadlocking Optimistic – use snapshots –Higher concurrency –Can be resource intensive –May need to manage conflict Concurrency Model

LevelConcurrency Model Dirty Reads Lost Updates Non repeatable Reads Phantoms Read Uncommitted Pessimistic Yes Read Committed Pessimistic NoYes Read Committed Snapshot Optimistic NoYes Repeatable Read Pessimistic No Yes SerializablePessimistic No SnapshotOptimistic No Isolation Levels

Lock wait <> Deadlock Deadlock is an irresolvable chain of blocking Lock manager automatically resolves deadlock by selecting deadlock victim Most deadlock situations can be architected out Resolving deadlocks Deadlocks

LOCK_TIMEOUT Read-only filegroups Nested transactions Save points Transaction marks XACT_ABORT Transaction Bits and Pieces

Error vs. exception Severity vs. error number Some errors are too severe to handle User defined errors Exception Handling

and are volatile Limited handling capabilities Limited information available Repetitive code blocks Unstructured code with GOTO Potential need to manage open transaction in caller Exception handling with the

Put suspect code in TRY block Put exception handling in CATCH block CATCH block directly follows TRY block in same batch Manage many more exceptions Throw error to caller using RAISERROR Exception handling functions provide detail of exception TRY/CATCH Methodology

XACT_STATE 0 – no active transactions 1 – open committable transaction -1 – open doomed transaction Exceptions in Transactions

Transactions Lock Manager Locks, lockable resources and modes Managing locks Concurrency vs. Isolation Blocks and Deadlocks Old School Exception Handling TRY/CATCH Managing Exceptions in Transactions Transactions and Exception Handling

Resources –Inside MS SQL Server 2005 Series T-SQL Programming –Itzik Ben-Gan et al. The Storage Engine –Kalen Delaney –Books Online Contact Resources and Contact Details