Transaction Control

Transaction Types

All database access occurs within transactions which are defined by the enumerated values described in the following sections.

The following eXtremeDB Transaction Types are defined by enum MCO_TRANS_TYPE in mco.h:

     
    typedef enum MCO_TRANS_TYPE_E_
    {
        MCO_READ_ONLY = 0, MCO_UPDATE = 1, MCO_READ_WRITE = 2, MCO_EXCLUSIVE = 3
         
    } MCO_TRANS_TYPE;
     

 

MCO_READ_ONLY	Transaction for READ_ONLY access to data.
MCO_UPDATE	Transaction for "read with intent to update" instead of READ_ONLY. If the intent is to find a database object for modification (Update of Delete) the MCO_UPDATE transaction type should be specified. To update a database object a new READ_WRITE transaction must be started, or the current transaction must be upgraded by calling function mco_trans_upgrade().
MCO_READ_WRITE	Transaction for READ_WRITE access to data.
MCO_EXCLUSIVE	(AKA "Global Lock") No other transaction can run in parallel even if the Transaction Manager would normally allow it: MVCC will not allow READ_ONLY or READ_WRITE transactions while EXCLUSIVE transaction is working, MURSIW will postpone execution on other READ_ONLY transactions while EXCLUSIVE transaction is working (EXCLUSIVE Transaction Manager ignores any type of transaction because it runs just one transaction at a time regardless of type)

 

Transaction Priorities

Transaction scheduling is partially determined by the transaction priority. Enum MCO_TRANS_PRIORITY is defined in mco.h as follows:

     
    typedef enum MCO_TRANS_PRIORITY_E_
    {
        MCO_TRANS_IDLE       = -2,   /* Least important transactions */
        MCO_TRANS_BACKGROUND = -1,
        MCO_TRANS_FOREGROUND = 0,    /* Normal priority transactions */
        MCO_TRANS_HIGH       = 1,
        MCO_TRANS_ISR        = 2     /* Very Important Transactions */
     
    } MCO_TRANS_PRIORITY;
     

Transaction priority can be set to one of the following values:

MCO_TRANS_IDLE	Least important transactions
MCO_TRANS_BACKGROUND	Less important then foreground but more important then idle
MCO_TRANS_FOREGROUND	Normal priority transactions
MCO_TRANS_HIGH	More important then foreground but less important then ISR
MCO_TRANS_ISR	Very Important Transactions

 

Transaction Scheduling Policy

The transaction scheduling policy for transactions with the same priority can be set to one of the following values as defined by enum MCO_TRANS_SCHED_POLICY in mco.h:

     
    typedef enum MCO_TRANS_SCHED_POLICY_E_
    {
        MCO_SCHED_FIFO = 0,                    /* First In First Out */
        MCO_SCHED_READER_FAVOR = 1,            /* Run read-only transactions first */
        MCO_SCHED_WRITER_FAVOR = 2             /* Run read-write transactions first */
     
    } MCO_TRANS_SCHED_POLICY;
     

 

MCO_SCHED_FIFO	First In First Out
MCO_SCHED_READER_FAVOR	Run read-only transactions first
MCO_SCHED_WRITER_FAVOR	Run read-write transactions first

 

Transaction Object States

Within a transaction, objects can have one of the following states as defined by enum MCO_TRANS_OBJ_STATE in mco.h:

     
    typedef enum MCO_TRANS_OBJ_STATE_
    {
        MCO_TRANS_OBJ_CREATED     = 0x01,
        MCO_TRANS_OBJ_DELETED     = 0x02,
        MCO_TRANS_OBJ_ALL_DELETED = 0x04
     
    } MCO_TRANS_OBJ_STATE;
     

 

MCO_TRANS_OBJ_CREATED	This object is being created.
MCO_TRANS_OBJ_DELETED	This object is being deleted.
MCO_TRANS_OBJ_ALL_DELETED	All objects of this class are being deleted.

 

Transaction Isolation Levels

The transaction isolation level can be or one of the following types as defined by enum MCO_TRANS_ISOLATION_LEVEL in mco.h::

     
    typedef enum MCO_TRANS_ISOLATION_LEVEL_E_
    {
        MCO_DEFAULT_ISOLATION_LEVEL = 0x0, /* MCO_REPEATABLE_READ for MVCC,  MCO_SERIALIZABLE for GlobalLock / MURSIW */
        MCO_READ_COMMITTED          = 0x1, /* Guarantees that any data read was committed at the moment is read. 
                            It restricts the reader from seeing any uncommitted data. */
        MCO_REPEATABLE_READ         = 0x2, /* In addition to MCO_READ_COMMITTED guarantees that any data read cannot change, 
                            if the transaction reads the same data again,
                            it will find the previously read data in place, unchanged, 
                            and available to read. */
        MCO_SERIALIZABLE            = 0x4  /* Emulates serial transaction execution */
     
    } MCO_TRANS_ISOLATION_LEVEL;
     

 

MCO_DEFAULT_ISOLATION_LEVEL	MCO_REPEATABLE_READ for MVCC, MCO_SERIALIZABLE for GlobalLock / MURSIW
MCO_READ_COMMITTED	Guarantees that any data read was committed at the moment was read. It restricts the reader from seeing any uncommitted data
MCO_REPEATABLE_READ	In addition to MCO_READ_COMMITTED guarantees that any data read cannot change; if the transaction reads the same data again, it will find the previously read data in place, unchanged, and available to read
MCO_SERIALIZABLE	Emulates serial transaction execution

 

Transaction Commit Policy

The transaction commit policy can be one of the following:

 
    typedef enum MCO_COMMIT_POLICY_E
    {
        MCO_COMMIT_SYNC_FLUSH, /* Flush to disk on each commit (default)  */
        MCO_COMMIT_BUFFERED,   /* Runtime buffered transactions */
        MCO_COMMIT_DELAYED,    /* Runtime buffered transactions, with commit criteria */
        MCO_COMMIT_NO_SYNC     /* Changes are not synchronously written to the disk */
     
    } MCO_COMMIT_POLICY;
     

 

MCO_COMMIT_SYNC_FLUSH	Flush to disk on each commit (default)
MCO_COMMIT_BUFFERED	Runtime buffered transactions
MCO_COMMIT_DELAYED	Runtime buffered transactions with commit criteria
MCO_COMMIT_NO_SYNC	Changes are not synchronously written to the disk