Stream Triggers¶

BDR introduces new types of triggers which can be used for additional data processing on the downstream/target node.

Conflict Triggers
Transform Triggers

Together, these types of triggers are known as Stream Triggers.

Stream Triggers are designed to be trigger-like in syntax, they leverage the PostgreSQL BEFORE trigger architecture, and are likely to have similar performance characteristics as PostgreSQL BEFORE Triggers.

One trigger function can be used by multiple trigger definitions, just as with normal PostgreSQL triggers. A trigger function is simply a program defined in this form: CREATE FUNCTION ... RETURNS TRIGGER . Creating the actual trigger does not require use of the CREATE TRIGGER command. Instead, stream triggers are created using the special BDR functions bdr.create_conflict_trigger() and bdr.create_transform_trigger() .

Once created, the trigger will be visible in the catalog table pg_trigger . The stream triggers will be marked as tgisinternal = true and tgenabled = 'D' and will have name suffix ‘_bdrc’ or ‘_bdrt’. The view bdr.triggers provides information on the triggers in relation to the table, the name of the procedure that is being executed, the event that triggers it, and the trigger type.

Note that stream triggers are NOT therefore enabled for normal SQL processing. Because of this the ALTER TABLE ... ENABLE TRIGGER is blocked for stream triggers in both its specific name variant and the ALL variant, to prevent the trigger from executing as a normal SQL trigger.

Note that these triggers execute on the downstream or target node. There is no option for them to execute on the origin node, though one may wish to consider the use of row_filter expressions on the origin.

Also, any DML which is applied during the execution of a stream trigger will not be replicated to other BDR nodes, and will not trigger the execution of standard local triggers. This is intentional, and can be used for instance to log changes or conflicts captured by a stream trigger into a table that is crash-safe and specific of that node; a working example is provided at the end of this chapter.

Trigger execution during Apply¶

Transform triggers execute first, once for each incoming change in the triggering table. These triggers fire before we have even attempted to locate a matching target row, allowing a very wide range of transforms to be applied efficiently and consistently.

Next, for UPDATE and DELETE changes we locate the target row. If there is no target row, then there is no further processing for those change types.

We then execute any normal triggers that previously have been explicitly enabled as replica triggers at table-level:

ALTER TABLE tablename
ENABLE REPLICA TRIGGER trigger_name;

We then decide whether a potential conflict exists and if so, we then call any conflict trigger that exists for that table.

Missing Column Conflict Resolution¶

Before transform triggers are executed, PostgreSQL tries to match the incoming tuple against the rowtype of the target table.

Any column that exists on the input row but not on the target table will trigger a conflict of type target_column_missing ; conversely, a column existing on the target table but not in the incoming row triggers a source_column_missing conflict. The default resolutions for those two conflict types are respectively ignore_if_null and use_default_value .

This is relevant in the context of rolling schema upgrades; for instance, if the new version of the schema introduces a new column. When replicating from an old version of the schema to a new one, the source column is missing, and the use_default_value strategy is appropriate, as it populates the newly introduced column with the default value.

However, when replicating from a node having the new schema version to a node having the old one, the column is missing from the target table, and the ignore_if_null resolver is not appropriate for a rolling upgrade, because it will break replication as soon as the user inserts, in any of the upgraded nodes, a tuple with a non-NULL value in the new column.

In view of this example, the appropriate setting for rolling schema upgrades is to configure each node to apply the ignore resolver in case of a target_column_missing conflict.

This is done with the following query, that must be executedseparatelyoneachnode , after replacing node1 with the actual node name:

SELECT bdr.alter_node_set_conflict_resolver(node1,
    target_column_missing, ignore);

Data Loss and Divergence Risk¶

In this section, we show how setting the conflict resolver to ignore can lead to data loss and cluster divergence.

Consider the following example: table t exists on nodes 1 and 2, but its column col only exists on node 1.

If the conflict resolver is set to ignore , then there can be rows on node 1 where c is not null, e.g. (pk=1, col=100) . That row will be replicated to node 2, and the value in column c will be discarded, e.g. (pk=1) .

If column c is then added to the table on node 2, it will initially be set to NULL on all existing rows, and the row considered above becomes (pk=1, col=NULL) : the row having pk=1 is no longer identical on all nodes, and the cluster is therefore divergent.

Note that the default ignore_if_null resolver is not affected by this risk, because any row that is replicated to node 2 will have col=NULL .

Based on this example, we recommend running LiveCompare against the whole cluster at the end of a rolling schema upgrade where the ignore resolver was used, to make sure that any divergence is detected and fixed.

Terminology of row-types¶

This document uses these row-types:

SOURCE_OLD is the row before update, i.e. the key.
SOURCE_NEW is the new row coming from another node.
TARGET is row that exists on the node already, i.e. conflicting row.

Conflict Triggers¶

Conflict triggers are executed when a conflict is detected by BDR, and are used to decide what happens when the conflict has occurred.

If the trigger function returns a row, the action will be applied to the target.
If the trigger function returns NULL row, the action will be skipped.

To clarify, if the trigger is called for a DELETE , the trigger should return NULL if it wants to skip the DELETE . If you wish the DELETE to proceed, then return a row value - either SOURCE_OLD or TARGET will work. When the conflicting operation is either INSERT or UPDATE , and the chosen resolution is the deletion of the conflicting row, the trigger must explicitly perform the deletion and return NULL. The trigger function may perform other SQL actions as it chooses, but those actions will only be applied locally, not replicated.

When a real data conflict occurs between two or more nodes, there will be two or more concurrent changes occurring. When we apply those changes, the conflict resolution occurs independently on each node. This means the conflict resolution will occur once on each node, and can occur with a significant time difference between then. As a result, there is no possibility of communication between the multiple executions of the conflict trigger. It is the responsibility of the author of the conflict trigger to ensure that the trigger gives exactly the same result for all related events, otherwise data divergence will occur. Technical Support recommends that all conflict triggers are formally tested using the isolationtester tool supplied with BDR.

Warning

Multiple conflict triggers can be specified on a single table, but they should match distinct event, i.e. each conflict should only match a single conflict trigger. - Multiple triggers matching the same event on the same table are not recommended; they might result in inconsistent behaviour, and will be forbidden in a future release.

If the same conflict trigger matches more than one event, the TG_OP variable can be used within the trigger to identify the operation that produced the conflict.

By default, BDR detects conflicts by observing a change of replication origin for a row, hence it is possible for a conflict trigger to be called even when there is only one change occurring. Since in this case there is no real conflict, we say that this conflict detection mechanism can generate false positive conflicts. The conflict trigger must handle all of those identically, as mentioned above.

Note that in some cases, timestamp conflict detection will not detect a conflict at all. For example, in a concurrent UPDATE/DELETE where the DELETE occurs just after the UPDATE, any nodes that see first the UPDATE and then the DELETE will not see any conflict. If no conflict is seen, the conflict trigger will never be called. The same situation, but using row version conflict detection, will see a conflict, which can then be handled by a conflict trigger.

The trigger function has access to additional state information as well as the data row involved in the conflict, depending upon the operation type:

On INSERT , conflict triggers would be able to access SOURCE_NEW row from source and TARGET row
On UPDATE , conflict triggers would be able to access SOURCE_OLD and SOURCE_NEW row from source and TARGET row
On DELETE , conflict triggers would be able to access SOURCE_OLD row from source and TARGET row

The function bdr.trigger_get_row() can be used to retrieve SOURCE_OLD , SOURCE_NEW or TARGET rows, if a value exists for that operation.

Changes to conflict triggers happen transactionally and are protected by Global DML Locks during replication of the configuration change, similarly to how some variants of ALTER TABLE are handled.

If primary keys are updated inside a conflict trigger, it can sometimes leads to unique constraint violations error due to a difference in timing of execution. Hence, users should avoid updating primary keys within conflict triggers.

Transform Triggers¶

These triggers are similar to the Conflict Triggers, except they are executed for every row on the data stream against the specific table. The behaviour of return values and the exposed variables are similar, but transform triggers execute before a target row is identified, so there is no TARGET row.

Specify multiple Transform Triggers on each table in BDR, if desired. Transform triggers execute in alphabetical order.

A transform trigger can filter away rows, and it can do additional operations as needed. It can alter the values of any column, or set them to NULL . The return value decides what further action is taken:

If the trigger function returns a row, it will be applied to the target.
If the trigger function returns a NULL row, there is no further action to be performed and as-yet unexecuted triggers will never execute.
The trigger function may perform other actions as it chooses.

The trigger function has access to additional state information as well as rows involved in the conflict:

On INSERT , transform triggers would be able to access SOURCE_NEW row from source.
On UPDATE , transform triggers would be able to access SOURCE_OLD and SOURCE_NEW row from source.
On DELETE , transform triggers would be able to access SOURCE_OLD row from source.

The function bdr.trigger_get_row() can be used to retrieve SOURCE_OLD or SOURCE_NEW rows; TARGET row is not available, since this type of trigger executes before such a target row is identified, if any.

Transform Triggers look very similar to normal BEFORE row triggers, but have these important differences:

Transform trigger gets called for every incoming change. BEFORE triggers will not be called at all for UPDATE and DELETE changes if we don’t find a matching row in a table.
Transform triggers are called before partition table routing occurs.
Transform triggers have access to the lookup key via SOURCE_OLD, which is not available to normal SQL triggers.

Stream Triggers Variables¶

Both Conflict Trigger and Transform Triggers have access to information about rows and metadata via the predefined variables provided by trigger API and additional information functions provided by BDR.

In PL/pgSQL, the following predefined variables exist:

TG_NAME¶

Data type name; variable that contains the name of the trigger actually fired. Note that the actual trigger name has a ‘_bdrt’ or ‘_bdrc’ suffix (depending on trigger type) compared to the name provided during trigger creation.

TG_WHEN¶

Data type text; this will say BEFORE for both Conflict and Transform triggers. The stream trigger type can be obtained by calling the bdr.trigger_get_type() information function (see below).

TG_LEVEL¶

Data type text; a string of ROW .

TG_OP¶

Data type text; a string of INSERT , UPDATE or DELETE telling for which operation the trigger was fired.

TG_RELID¶

Data type oid; the object ID of the table that caused the trigger invocation.

TG_TABLE_NAME¶

Data type name; the name of the table that caused the trigger invocation.

TG_TABLE_SCHEMA¶

Data type name; the name of the schema of the table that caused the trigger invocation. For partitioned tables, this is the name of the root table.

TG_NARGS¶

Data type integer; the number of arguments given to the trigger function in the bdr.create_conflict_trigger() or bdr.create_transform_trigger() statement.

TG_ARGV[]¶

Data type array of text; the arguments from the bdr.create_conflict_trigger() or bdr.create_transform_trigger() statement. The index counts from 0. Invalid indexes (less than 0 or greater than or equal to TG_NARGS ) result in a NULL value.

Information functions¶

bdr.trigger_get_row¶

This function returns the contents of a trigger row specified by an identifier as a RECORD . This function returns NULL if called inappropriately, i.e. called with SOURCE_NEW when the operation type (TG_OP) is DELETE.

Synopsis¶

bdr.trigger_get_row(row_id text)

Parameters¶

row_id - identifier of the row; can be any of SOURCE_NEW, SOURCE_OLD and TARGET, depending on the trigger type and operation (see documentation of individual trigger types).

bdr.trigger_get_committs¶

This function returns the commit timestamp of a trigger row specified by an identifier. If not available because a row is frozen or row is not available, this will return NULL. Always returns NULL for row identifier SOURCE_OLD.

Synopsis¶

bdr.trigger_get_committs(row_id text)

Parameters¶

row_id - identifier of the row; can be any of SOURCE_NEW, SOURCE_OLD and TARGET, depending on trigger type and operation (see documentation of individual trigger types).

bdr.trigger_get_xid¶

This function returns the local transaction id of a TARGET row specified by an identifier. If not available because a row is frozen or row is not available, this will return NULL. Always returns NULL for SOURCE_OLD and SOURCE_NEW row identifiers.

This is only available for conflict triggers.

Synopsis¶

bdr.trigger_get_xid(row_id text)

Parameters¶

row_id - identifier of the row; can be any of SOURCE_NEW, SOURCE_OLD and TARGET, depending on trigger type and operation (see documentation of individual trigger types).

bdr.trigger_get_type¶

This function returns the current trigger type, which can be either CONFLICT or TRANSFORM . Returns null if called outside a Stream Trigger.

Synopsis¶

bdr.trigger_get_type()

bdr.trigger_get_conflict_type¶

This function returns the current conflict type if called inside a conflict trigger, or NULL otherwise.

See List of conflict types for possible return values of this function.

Synopsis¶

bdr.trigger_get_conflict_type()

bdr.trigger_get_origin_node_id¶

This function returns the node id corresponding to the origin for the trigger row_id passed in as argument. If the origin is not valid (which means the row has originated locally), return the node id of the source or target node, depending on the trigger row argument. Always returns NULL for row identifier SOURCE_OLD. This can be used to define conflict triggers to always favour a trusted source node. See the example given below.

Synopsis¶

bdr.trigger_get_origin_node_id(row_id text)

Parameters¶

row_id - identifier of the row; can be any of SOURCE_NEW, SOURCE_OLD and TARGET, depending on trigger type and operation (see documentation of individual trigger types).

bdr.ri_fkey_on_del_trigger¶

When called as a BEFORE trigger, this function will use FOREIGN KEY information to avoid FK anomalies.

Synopsis¶

bdr.ri_fkey_on_del_trigger()

Row Contents¶

The SOURCE_NEW, SOURCE_OLD and TARGET contents depend on the operation, REPLICA IDENTITY setting of a table, and the contents of the target table.

The TARGET row is only available in conflict triggers. The TARGET row only contains data if a row was found when applying UPDATE or DELETE in the target table; if the row is not found, the TARGET will be NULL.

Triggers Notes¶

Execution order for triggers:

Transform triggers - execute once for each incoming row on the target
Normal triggers - execute once per row
Conflict triggers - execute once per row where a conflict exists

Stream Triggers Manipulation Interfaces¶

Stream Triggers can only be created on tables with REPLICA IDENTITY FULL or tables without any TOAST able columns.

bdr.create_conflict_trigger¶

This function creates a new conflict trigger.

Synopsis¶

bdr.create_conflict_trigger(trigger_name text,
                            events text[],
                            relation regclass,
                            function regprocedure,
                            args text[] DEFAULT {})

Parameters¶

trigger_name - name of the new trigger
events - array of events on which to fire this trigger; valid values are ‘INSERT’, ‘UPDATE’ and ‘DELETE’
relation - for which relation to fire this trigger
function - which function to execute
args - optional; specifies the array of parameters the trigger function will receive upon execution (contents of TG_ARGV variable)

Notes¶

This function uses the same replication mechanism as DDL statements. This means that the replication is affected by the ddl filters configuration.

The function will take a global DML lock on the relation on which the trigger is being created.

This function is transactional - the effects can be rolled back with the ROLLBACK of the transaction, and the changes are visible to the current transaction.

Similarly to normal PostgreSQL triggers, the bdr.create_conflict_trigger function requires TRIGGER privilege on the relation and EXECUTE privilege on the function. This applies with a bdr.backwards_compatibility of 30619 or above. Additional security rules apply in BDR to all triggers including conflict triggers; see the security chapter on triggers .

bdr.create_transform_trigger¶

This function creates a new transform trigger.

Synopsis¶

bdr.create_transform_trigger(trigger_name text,
                             events text[],
                             relation regclass,
                             function regprocedure,
                             args text[] DEFAULT {})

Parameters¶

trigger_name - name of the new trigger
events - array of events on which to fire this trigger, valid values are ‘INSERT’, ‘UPDATE’ and ‘DELETE’
relation - for which relation to fire this trigger
function - which function to execute
args - optional, specify array of parameters the trigger function will receive upon execution (contents of TG_ARGV variable)

Notes¶

This function uses the same replication mechanism as DDL statements. This means that the replication is affected by the ddl filters configuration.

The function will take a global DML lock on the relation on which the trigger is being created.

This function is transactional - the effects can be rolled back with the ROLLBACK of the transaction, and the changes are visible to the current transaction.

Similarly to normal PostgreSQL triggers, the bdr.create_transform_trigger function requires the TRIGGER privilege on the relation and EXECUTE privilege on the function. Additional security rules apply in BDR to all triggers including transform triggers; see the security chapter on triggers .

bdr.drop_trigger¶

This function removes an existing stream trigger (both conflict and transform).

Synopsis¶

bdr.drop_trigger(trigger_name text,
                 relation regclass,
                 ifexists boolean DEFAULT false)

Parameters¶

trigger_name - name of an existing trigger
relation - which relation is the trigger defined for
ifexists - when set to true true , this command will ignore missing triggers

Notes¶

This function uses the same replication mechanism as DDL statements. This means that the replication is affected by the ddl filters configuration.

The function will take a global DML lock on the relation on which the trigger is being created.

This function is transactional - the effects can be rolled back with the ROLLBACK of the transaction, and the changes are visible to the current transaction.

The bdr.drop_trigger function can be only executed by the owner of the relation .

Stream Triggers Examples¶

A conflict trigger which provides similar behaviour as the update_if_newer conflict resolver:

CREATE OR REPLACE FUNCTION update_if_newer_trig_func
RETURNS TRIGGER
LANGUAGE plpgsql
AS $$
BEGIN
    IF (bdr.trigger_get_committs(TARGET) >
        bdr.trigger_get_committs(SOURCE_NEW)) THEN
    RETURN TARGET;
    ELSIF
        RETURN SOURCE;
    END IF;
END;
$$;

A conflict trigger which applies a delta change on a counter column and uses SOURCE_NEW for all other columns:

CREATE OR REPLACE FUNCTION delta_count_trg_func
RETURNS TRIGGER
LANGUAGE plpgsql
AS $$
DECLARE
    DELTA bigint;
    SOURCE_OLD record;
    SOURCE_NEW record;
    TARGET record;
BEGIN
    SOURCE_OLD := bdr.trigger_get_row(SOURCE_OLD);
    SOURCE_NEW := bdr.trigger_get_row(SOURCE_NEW);
    TARGET := bdr.trigger_get_row(TARGET);

    DELTA := SOURCE_NEW.counter - SOURCE_OLD.counter;
    SOURCE_NEW.counter = TARGET.counter + DELTA;

    RETURN SOURCE_NEW;
END;
$$;

A transform trigger which logs all changes to a log table instead of applying them:

CREATE OR REPLACE FUNCTION log_change
RETURNS TRIGGER
LANGUAGE plpgsql
AS $$
DECLARE
    SOURCE_NEW record;
    SOURCE_OLD record;
    COMMITTS timestamptz;
BEGIN
    SOURCE_NEW := bdr.trigger_get_row(SOURCE_NEW);
    SOURCE_OLD := bdr.trigger_get_row(SOURCE_OLD);
    COMMITTS := bdr.trigger_get_committs(SOURCE_NEW);

    IF (TG_OP = INSERT) THEN
        INSERT INTO log SELECT I, COMMITTS, row_to_json(SOURCE_NEW);
    ELSIF (TG_OP = UPDATE) THEN
        INSERT INTO log SELECT U, COMMITTS, row_to_json(SOURCE_NEW);
    ELSIF (TG_OP = DELETE) THEN
        INSERT INTO log SELECT D, COMMITTS, row_to_json(SOURCE_OLD);
    END IF;

    RETURN NULL; -- do not apply the change
END;
$$;

The example below shows a conflict trigger that implements Trusted Source conflict detection, also known as trusted site, preferred node or Always Wins resolution. This uses the bdr.trigger_get_origin_node_id() function to provide a solution that works with 3 or more nodes.

CREATE OR REPLACE FUNCTION test_conflict_trigger()
RETURNS TRIGGER
LANGUAGE plpgsql
AS $$
DECLARE
    SOURCE  record;
    TARGET  record;

    TRUSTED_NODE    bigint;
    SOURCE_NODE     bigint;
    TARGET_NODE     bigint;
BEGIN
    TARGET := bdr.trigger_get_row(TARGET);
    IF (TG_OP = DELETE)
        SOURCE := bdr.trigger_get_row(SOURCE_OLD);
    ELSE
        SOURCE := bdr.trigger_get_row(SOURCE_NEW);
    END IF;

    TRUSTED_NODE := current_setting(customer.trusted_node_id);

    SOURCE_NODE := bdr.trigger_get_origin_node_id(SOURCE_NEW);
    TARGET_NODE := bdr.trigger_get_origin_node_id(TARGET);

    IF (TRUSTED_NODE = SOURCE_NODE) THEN
        RETURN SOURCE;
    ELSIF (TRUSTED_NODE = TARGET_NODE) THEN
        RETURN TARGET;
    ELSE
        RETURN NULL; -- do not apply the change
    END IF;
END;
$$;