对Slony-I中wait on的理解

4.1.2. Event Confirmations

When an event is processed by the slon process for a remote node, a CONFIRM message is generated by

inserting a tuple into the sl_confirm table.

This tuple indicates that a particular event has been confirmed by a particular receiver node.

Confirmation messages are then transferred back to all other nodes in the cluster.

For instance, many people have run afoul of problems with SLONIK SUBSCRIBE SET, when subscribing a new
set.

Be aware (and beware!) that a SLONIK SUBSCRIBE SET request will return the event confirmation almost

immediately, even though there might be several hours of work to do before the subscription is ready.

The trouble with SLONIK SUBSCRIBE SET is that it is processed as two events, one on the origin node,

with a second event, to enable the subscription, on the subscriber. 

In order to more reliably monitor from within a slonik script that SLONIK SUBSCRIBE SET is complete,

you may submit a SLONIK SYNC event after the subscription, and have the WAIT request wait on the SYNC

event

...

SYNC

...

Generates a SYNC event on a specified node.

ID = ival

The node on which to generate the SYNC event.

Example

      SYNC (ID = 1);

      WAIT FOR EVENT (ORIGIN = 1, CONFIRMED = 2, WAIT ON=1); ...

 Slonik does not wait for event confirmations before performing this command.

SUBSCRIBE SET (ID = 999, PROVIDER = 1, RECEIVER = 2);

SYNC (ID = 1)

WAIT FOR EVENT (ORIGIN = 1, CONFIRMED = ALL, WAIT ON=1);

SUBSCRIBE SET (ID = 999, PROVIDER = 1, RECEIVER = 2);

SYNC (ID = 2)

WAIT FOR EVENT (ORIGIN = 1, CONFIRMED = ALL, WAIT ON=2);