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ALTER EVENT SESSION (Transact-SQL)

Starts or stops an event session or changes an event session configuration

Topic link iconTransact-SQL Syntax Conventions


ALTER EVENT SESSION event_session_name
ON SERVER
{
    [ [ {  <add_drop_event> [ ,...n] }   
       | { <add_drop_event_target> [ ,...n ] } ] 
    [ WITH ( <event_session_options> [ ,...n ] ) ]
    ]
    | [ STATE = { START | STOP } ]
}

<add_drop_event>::=
{
    [ ADD EVENT <event_specifier> 
         [ ( { 
                 [ SET { event_customizable_attribute = <value> [ ,...n ] } ]
                 [ ACTION ( { [event_module_guid].event_package_name.action_name [ ,...n ] } ) ]
                 [ WHERE <predicate_expression> ]
        } ) ]
   ] 
   | DROP EVENT <event_specifier> }

<event_specifier> ::=
{
[event_module_guid].event_package_name.event_name
}

<predicate_expression> ::= 
{
    [ NOT ] <predicate_factor> | {( <predicate_expression> ) } 
    [ { AND | OR } [ NOT ] { <predicate_factor> | ( <predicate_expression> ) } ] 
    [ ,...n ]
}
 
<predicate_factor>::= 
{
    <predicate_leaf> | ( <predicate_expression> )
}

<predicate_leaf>::=
{
      <predicate_source_declaration> { = | < > | ! = | > | > = | < | < = } <value> 
    | [event_module_guid].event_package_name.predicate_compare_name( <predicate_source_declaration>, <value> ) 
}

<predicate_source_declaration>::= 
{
    event_field_name | ( [event_module_guid].event_package_name.predicate_source_name)
}

<value>::= 
{
    number | 'string'
}

<add_drop_event_target>::=
{
    ADD TARGET <event_target_specifier>
        [ ( SET { target_parameter_name= <value> [ ,...n] } ) ]
    | DROP TARGET <event_target_specifier>
}

<event_target_specifier>::=
{
    [event_module_guid].event_package_name.target_name
}

<event_session_options>::=
{
    [    MAX_MEMORY =size [ KB | MB ] ]
    [ [,] EVENT_RETENTION_MODE = { ALLOW_SINGLE_EVENT_LOSS | ALLOW_MULTIPLE_EVENT_LOSS | NO_EVENT_LOSS } ]
    [ [,] MAX_DISPATCH_LATENCY = { seconds SECONDS | INFINITE } ]
    [ [,] MAX_EVENT_SIZE =size [ KB | MB ] ]
    [ [,] MEMORY_PARTITION_MODE = { NONE | PER_NODE | PER_CPU } ]
    [ [,] TRACK_CAUSALITY = { ON | OFF } ]
    [ [,] STARTUP_STATE = { ON | OFF } ]
}

Term

Definition

event_session_name

Is the name of an existing event session.

STATE = START | STOP

Starts or stops the event session. This argument is only valid when ALTER EVENT SESSION is applied to an event session object.

ADD EVENT <event_specifier>

Associates the event identified by <event_specifier>with the event session.

[event_module_guid].event_package_name.event_name

Is the name of an event in an event package, where:

  • event_module_guid is the GUID for the module that contains the event.

  • event_package_name is the package that contains the action object.

  • event_name is the event object.

Events appear in the sys.dm_xe_objects view as object_type 'event'.

SET { event_customizable_attribute= <value> [ ,...n] }

Specifies customizable attributes for the event. Customizable attributes appear in the sys.dm_xe_object_columns view as column_type 'customizable ' and object_name = event_name.

ACTION ( { [event_module_guid].event_package_name.action_name [ ,...n] } )

Is the action to associate with the event session, where:

  • event_module_guid is the GUID for the module that contains the event.

  • event_package_name is the package that contains the action object.

  • action_name is the action object.

Actions appear in the sys.dm_xe_objects view as object_type 'action'.

WHERE <predicate_expression>

Specifies the predicate expression used to determine if an event should be processed. If <predicate_expression> is true, the event is processed further by the actions and targets for the session. If <predicate_expression> is false, the event is dropped by the session before being processed by the actions and targets for the session. Predicate expressions are limited to 3000 characters, which limits string arguments.

event_field_name

Is the name of the event field that identifies the predicate source.

[event_module_guid].event_package_name.predicate_source_name

Is the name of the global predicate source where:

  • event_module_guid is the GUID for the module that contains the event.

  • event_package_name is the package that contains the predicate object.

  • predicate_source_name is defined in the sys.dm_xe_objects view as object_type 'pred_source'

[event_module_guid].event_package_name.predicate_compare_name

Is the name of the predicate object to associate with the event, where:

  • event_module_guid is the GUID for the module that contains the event.

  • event_package_name is the package that contains the predicate object.

  • predicate_compare_name is a global source defined in the sys.dm_xe_objects view as object_type 'pred_compare'.

DROP EVENT <event_specifier>

Drops the event identified by <event_specifier>. <event_specifier> must be valid in the event session.

ADD TARGET <event_target_specifier>

Associates the target identified by <event_target_specifier>with the event session.

[event_module_guid].event_package_name.target_name

Is the name of a target in the event session, where:

  • event_module_guid is the GUID for the module that contains the event.

  • event_package_name is the package that contains the action object.

  • target_name is the action. Actions appear in sys.dm_xe_objects view as object_type 'target'.

SET { target_parameter_name= <value> [, ...n] }

Sets a target parameter. Target parameters appear in the sys.dm_xe_object_columns view as column_type 'customizable' and object_name = target_name.

Important noteImportant
If you are using the ring buffer target, we recommend that you set the max_memory target parameter to 2048 kilobytes (KB) to help avoid possible data truncation of the XML output. For more information about when to use the different target types, see SQL Server Extended Events Targets.

DROP TARGET <event_target_specifier>

Drops the target identified by <event_target_specifier>. <event_target_specifier> must be valid in the event session.

EVENT_RETENTION_MODE = { ALLOW_SINGLE_EVENT_LOSS | ALLOW_MULTIPLE_EVENT_LOSS | NO_EVENT_LOSS }

Specifies the event retention mode to use for handling event loss.

ALLOW_SINGLE_EVENT_LOSS

An event can be lost from the session. A single event is only dropped when all the event buffers are full. Losing a single event when event buffers are full allows for acceptable SQL Server performance characteristics, while minimizing the loss of data in the processed event stream.

ALLOW_MULTIPLE_EVENT_LOSS

Full event buffers containing multiple events can be lost from the session. The number of events lost is dependent upon the memory size allocated to the session, the partitioning of the memory, and the size of the events in the buffer. This option minimizes performance impact on the server when event buffers are quickly filled, but large numbers of events can be lost from the session.

NO_EVENT_LOSS

No event loss is allowed. This option ensures that all events raised will be retained. Using this option forces all tasks that fire events to wait until space is available in an event buffer. This may cause detectable performance issues while the event session is active. User connections may stall while waiting for events to be flushed from the buffer.

MAX_DISPATCH_LATENCY = { seconds SECONDS | INFINITE }

Specifies the amount of time that events are buffered in memory before being dispatched to event session targets. The minimum latency value is 1 second. However, 0 can be used to specify INFINITE latency. By default, this value is set to 30 seconds.

seconds SECONDS

The time, in seconds, to wait before starting to flush buffers to targets. seconds is a whole number.

INFINITE

Flush buffers to targets only when the buffers are full, or when the event session closes.

NoteNote
MAX_DISPATCH_LATENCY = 0 SECONDS is equivalent to MAX_DISPATCH_LATENCY = INFINITE.

MAX_EVENT_SIZE =size [ KB | MB ]

Specifies the maximum allowable size for events. MAX_EVENT_SIZE should only be set to allow single events larger than MAX_MEMORY; setting it to less than MAX_MEMORY will raise an error. size is a whole number and can be a kilobyte (KB) or a megabyte (MB) value. If size is specified in kilobytes, the minimum allowable size is 64 KB. When MAX_EVENT_SIZE is set, two buffers of size are created in addition to MAX_MEMORY. This means that the total memory used for event buffering is MAX_MEMORY + 2 * MAX_EVENT_SIZE.

MEMORY_PARTITION_MODE ={ NONE | PER_NODE | PER_CPU }

Specifies the location where event buffers are created.

NONE

A single set of buffers is created within the SQL Server instance.

PER_NODEA set of buffers is created for each NUMA node. For more information, see Understanding Non-uniform Memory Access.
PER_CPUA set of buffers is created for each CPU.

TRACK_CAUSALITY ={ ON | OFF }

Specifies whether causality is tracked. If enabled, causality allows related events on different server connections to be correlated together.

STARTUP_STATE ={ ON | OFF }

Specifies whether to start this event session automatically when SQL Server starts.

NoteNote
If STARTUP_STATE = ON, the event session will only start if SQL Server is stopped and then restarted.
TermDefinition
ONThe event session is started at startup.
OFFThe event session is not started at startup.

The ADD and DROP arguments cannot be used in the same statement.

Requires CONTROL SERVER permission.

The following example starts an event session, obtains some live session statistics, and then adds two events to the existing session.

-- Start the event session
ALTER EVENT SESSION test_session
ON SERVER
STATE = start
GO
-- Obtain live session statistics 
SELECT * FROM sys.dm_xe_sessions
SELECT * FROM sys.dm_xe_session_events
GO

-- Add new events to the session
ALTER EVENT SESSION test_session ON SERVER
ADD EVENT sqlserver.database_transaction_begin,
ADD EVENT sqlserver.database_transaction_end
GO

Updated content

Corrected the default value for the MAX_DISPATCH_LATENCY argument.

Added Important note to the description for SET { target_parameter_name= <value> [, ...n] }.

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