Provided by: slony1-2-doc_2.1.4-1ubuntu1_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 nodes.
If you need to revise subscription information for a node, you also 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 that you want a
node to subscribe to a different provider node, or for a node to become a ‘forwarding’
subscriber so it may later become the provider for a later subscriber.
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
6 February 2014 SLONIK SUBSCRIBE SET(7)