Summary of Lock Behavior

This table summarizes the locking behavior for each type of lock that Microsoft Jet uses.


Lock name		 What sets the lock What is locked What the lock prevents Persistence of the lock Shared or exclusive Byte range examples
User User opens a database N/A Nothing Until user closes the database N/A 10000001 –10000001
Write Insert, update, or delete All available pages Updates or deletes to data and sometimes inserts to a table Controlled by locking mode set by user or the duration of a transaction Exclusive 2000A601 –2000A701
Read
(version 2.x)		 Reads on long
value or index
pages		 Long value, index, or table header pages Updates or deletes to long value or index pages Controlled by LockedPage-
Timeout setting or the Idle method of the DBEngine object with the dbFreeLocks constant		 Shared 30001E01 –30001E01
Read
(versions 3.0 and 3.5)		 Reads on certain long value or index pages when referential integrity is being enforced Long value, index, or directory pages Updates or deletes to long value or index pages Until a read or a transaction is complete or the user moves to a new record Shared 20063801 –20063801
Commit (version 2.x only) Writes to long
value or index
pages		 Long value or index pages Prevents reads when data is being written to disk Held until data is written to disk Exclusive 30001A01 –30001B01


Lock name		 What sets the lock What is locked What the lock prevents Persistence of the lock Shared or exclusive Byte range examples
Table-read Shared lock is obtained when table is opened. Exclusive lock is obtained when Default Record Locking in Microsoft Access is set to All Records or a recordset is opened through DAO using the dbDenyRead constant for the options argument of the OpenRecordset method. Table header page Prevents exclusive read lock from being set As long as the table is kept open by the user Both 4000C801 –4000C801
Table-write Shared lock is obtained when table is opened. Exclusive lock is obtained when Default Record Locking in Microsoft Access is set to All Records or a recordset is opened through DAO using the dbDenyRead constant for the options argument of the OpenRecordset method. Table header page Prevents exclusive write lock from being set; doesn’t prevent table deny-write lock As long as the table is kept open by the user Both 5000C801 –5000C801
Table deny-write Can be set only through DAO Table header page Prevents all writes to the table As long as the table is kept open by the user Both 6000C801 –6000C801