SLONIK SUBSCRIBE SET(7)Configuration and Action commandSLONIK SUBSCRIBE SET(7)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 dur‐
ing 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 imme‐
diately, 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 com‐
plete 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 informa‐
tion 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 and sl_log_2. 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 sub‐
scriber, 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 for‐
warding 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 or sl_log_2, 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 sub‐
scribing to the origin have forwarding turned on.
DANGEROUS/UNINTUITIVE BEHAVIOUR
· The fact that the request returns immediately even though the sub‐
scription may take considerable time to complete may be a bit sur‐
prising.
Processing of the subscription involves two events; the SUB‐
SCRIBE_SET, initiated on the set origin node, and an ENABLE_SUBSCRIP‐
TION. This means that SLONIK WAIT FOR EVENT(7) must be used follow‐
ing 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 dis‐
carded 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 configu‐
ration 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 submit‐
ting 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 sub‐
mitted 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 2015 SLONIK SUBSCRIBE SET(7)
