Provided by: slony1-2-doc_2.2.11-4_all 
NAME
SUBSCRIBE SET - Start replication of Slony-I set
SYNOPSIS
SUBSCRIBE SET (options);
DESCRIPTION
This performs one of two actions:
• Initiates replication for a replication set
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:
• It attempts to TRUNCATE the table, which will be efficient.
• If that fails (a foreign key relationship might prevent TRUNCATE from working), it uses DELETE to
delete all ‘old’ entries in the table
• Those old entries clutter up the table until it is next VACUUMed after the subscription process is
complete
• The indices for the table will contain entries for the old, deleted entries, which will slow the
process of inserting new entries into the index.
Warning
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.
• Revising subscription information for already-subscribed provider/receiver pair of nodes.
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.
ID = ival
ID of the set to subscribe
PROVIDER = ival
Node ID of the data provider from which this node draws data.
RECEIVER = ival
Node ID of the new subscriber
FORWARD = boolean
Flag whether or not the new subscriber should store the log information during replication to make it
possible candidate for the provider role for future nodes. Any node that is intended to be a candidate
for FAILOVER must have FORWARD = yes.
OMIT COPY = boolean
Flag whether or not the subscription process should omit doing the COPY of the existing data in the
set. In effect, use this option indicates ‘Trust me, the data is already in sync!’
This is notably useful for the following sorts of cases:
• Major inter-version upgrades (e.g. - as from Slony-I 1.2 to 2.0) may be done quickly.
• Cloning a ‘master node’. SLONIK CLONE PREPARE(7)/SLONIK CLONE FINISH(7)
•
EXAMPLE
SUBSCRIBE SET (
ID = 1,
PROVIDER = 1,
RECEIVER = 3,
FORWARD = YES
);
WAIT FOR EVENT(
ORIGIN=1,
CONFIRMED=ALL,
WAIT ON=1
);
FORWARDING BEHAVIOUR
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.
DANGEROUS/UNINTUITIVE BEHAVIOUR
• The fact that the request returns immediately even though the subscription may take considerable time
to complete may be a bit surprising.
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 command has two purposes; setting up subscriptions (which should be unsurprising) and revising
subscriptions, which isn't so obvious to intuition.
• New subscriptions are set up by using DELETE or TRUNCATE to empty the table on a subscriber. If you
created a new node by copying data from an existing node, it might ‘seem intuitive’ that that data
should be kept; that is not the case - the former contents are discarded and the node is populated from
scratch.
• The OMIT COPY option has the potential to be a large ‘foot gun’ in that it allows the administrator to
push replication sets out of sync.
LOCKING BEHAVIOUR
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 EVENT CONFIRMATION BEHAVIOUR
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.
VERSION INFORMATION
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.
28 July 2024 SLONIK SUBSCRIBE SET(7)