| SLONIK SUBSCRIBE SET(7) | Slony-I 2.2.10 Documentation | SLONIK SUBSCRIBE SET(7) |
SUBSCRIBE SET - Start replication of Slony-I set
SUBSCRIBE SET (options);
This performs one of two actions:
Causes a node (subscriber) to start replicating a set of tables either from the origin or from another provider node, which must itself already be be an active, forwarding subscriber.
The application tables contained in the set must already exist and should ideally be empty. The current version of Slony-I will not attempt to copy the schema of the set. The replication daemon will start copying the current content of the set from the given provider and then try to catch up with any update activity that happened during that copy process. After successful subscription, the tables are guarded on the subscriber, using triggers, against accidental updates by the application.
If the tables on the subscriber are not empty, then the COPY SET event (which is part of the subscription process) may wind up doing more work than should be strictly necessary:
This operation can take a (potentially distinctly) non-zero period of time. If you have a great deal of data in a particular set of tables, it may take hours or even (if ‘a great deal’ indicates ‘tens or hundreds gigabytes of data’) possibly multiple days for this event to complete.
The SUBSCRIBE SET request will, nonetheless, return fairly much immediately, even though the work, being handled by the COPY SET event, is still in progress. If you need to set up subscriptions for a set of cascading nodes, you will need to wait for each subscriber to complete subscribing before submitting requests for subscriptions that use that node as a provider.
Slony-I: provider 2 is not an active forwarding node for replication set 1
In effect, such subscription requests will be ignored until the provider is ready.
If you need to revise subscription information for a set,provider,receiver combination you must submit the new information using this command, and the new configuration will be propagated throughout the replication network. The normal reason to revise this information is if you want to change the FORWARD status of a set on a particular receiver node. If you want to change the provider used by a particular node you should use the SLONIK RESUBSCRIBE NODE(7) command.
This is notably useful for the following sorts of cases:
SUBSCRIBE SET (
ID = 1,
PROVIDER = 1,
RECEIVER = 3,
FORWARD = YES );
WAIT FOR EVENT(
ORIGIN=1,
CONFIRMED=ALL,
WAIT ON=1 );
The FORWARD=boolean flag indicates whether the subscriber will store log information in tables “sl_log_1” [not available as a man page] and “sl_log_2” [not available as a man page]. Several implications fall from this...
By storing the data in these tables on the subscriber, there is some additional processing burden. If you are certain that you would never want to SLONIK MOVE SET(7) or SLONIK FAILOVER(7) to a particular subscriber, it is worth considering turning off forwarding on that node.
There is, however, a case where having forwarding turned off opens up a perhaps-unexpected failure condition; a rule of thumb should be that all nodes that connect directly to the origin should have forwarding turned on. Supposing one such ‘direct subscriber’ has forwarding turned off, it is possible for that node to be forcibly lost in a case of failover. The problem comes if that node gets ahead of other nodes.
Let's suppose that the origin, node 1 is at SYNC number 88901, a non-forwarding node, node 2 has processed up to SYNC 88897, and other forwarding nodes, 3, 4, and 5, have only processed data up to SYNC 88895. At that moment, the disk system on the origin node catches fire. Node 2 has the data up to SYNC 88897, but there is no remaining node that contains, in “sl_log_1” [not available as a man page] or “sl_log_2” [not available as a man page], the data for SYNCs 88896 and 88897, so there is no way to bring nodes 3-5 up to that point.
At that point, there are only two choices: To drop node 2, because there is no way to continue managing it, or to drop all nodes but 2, because there is no way to bring them up to SYNC 88897.
That dilemma may be avoided by making sure that all nodes directly subscribing to the origin have forwarding turned on.
Processing of the subscription involves two events; the SUBSCRIBE_SET, initiated on the set origin node, and an ENABLE_SUBSCRIPTION. This means that SLONIK WAIT FOR EVENT(7) must be used following a SUBSCRIBE SET to wait until the last event on the set origin completes.
This operation does not require acquiring any locks on the provider node.
On the subscriber node, it will have the effect of locking every table in the replication set. In version 1.2 and later, exclusive locks are acquired at the beginning of the process.
Slonik waits until the provider has confirmed all outstanding configuration events from any other node before contacting the provider to determine the set origin. Slonik then waits for the command submitted to the previous event node to be confirmed on the origin before submitting this command to the origin.
This command was introduced in Slony-I 1.0
The OMIT COPY option was introduced in Slony-I 2.0.3.
In Slony-I 2.0.5 the SUBSCRIBE SET command gets submitted directly against the set origin. Prior to this change the SUBSCRIBE SET was submitted against the provider
Prior to Slony-I 2.2.0 the SUBSCRIBE SET command could be used to change the provider of an already subscribed node. As of 2.2.0 the SLONIK RESUBSCRIBE NODE(7) command must be used.
| 18 January 2021 |