SLONIK_SUBSCRIBE_SET(7)

💖 SLONIK SUBSCRIBE SET(7) 💖

okay bestie, pull up a chair, we're doing SUBSCRIBE SET from the Slony-I 2.2.11 Documentation era and it's honestly kind of a big deal 🫶

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✨ NAME ✨

SUBSCRIBE SET — start replication of a Slony-I set, girl, it's time 💅

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🌸 SYNOPSIS 🌸

SUBSCRIBE SET (options);

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🦋 DESCRIPTION 🦋

so this command does one of two things, depending on the vibe:

• Initiates replication for a replication set

It makes a node (the subscriber) start replicating a set of tables, either straight from the origin or from another provider node — and that provider has to already be an active, forwarding subscriber itself, no shortcuts. 👑

The application tables in the set must already exist and should <em>ideally</em> be empty. Lowkey important: 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 a successful subscription, the tables on the subscriber get guarded using triggers, so the application can't accidentally mess with them. 🛡️

And if the tables on the subscriber are <em>not</em> empty? Then the COPY SET event (which is part of the subscription process) may wind up doing more work than should be strictly necessary, and here's why:

• It attempts to TRUNCATE the table, which will be efficient. slay.
• 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. like. DAYS. bestie. 😭

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. So 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. no skipping the line, babe 🎀

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. the audacity, honestly.

• Revising subscription information for an 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. But heads up: if you want to change the provider used by a particular node you should use the SLONIK RESUBSCRIBE NODE(7) command instead. 🫡

• 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 a possible candidate for the provider role for future nodes. Any node that is intended to be a candidate for FAILOVER must have FORWARD = yes. periodt.
• OMIT COPY = boolean — Flag whether or not the subscription process should omit doing the COPY of the existing data in the set. In effect, using 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) 🧬
• (that's it, that's the list — the original leaves it hanging like that, iconic honestly)

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🌈 EXAMPLE 🌈

SUBSCRIBE SET (
   ID = 1,
   PROVIDER = 1,
   RECEIVER = 3,
   FORWARD = YES
);
  WAIT FOR EVENT(
    ORIGIN=1,
    CONFIRMED=ALL,
    WAIT ON=1
  );

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🧚 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. ✨ that's the whole tea.

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💀 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. rip to that data fr fr 😭
• 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. 🔫🦶 be careful bestie.

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🔒 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. 🔐

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⭐ 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. it's giving patient queen energy 💅

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👑 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. glow-up behavior honestly ✨

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and that's the whole page, bestie 💖 go forth and subscribe responsibly 🦋

this lil doc came from the Slony-I 2.2.11 Documentation set, dated 19 September 2024 — mwah, stay iconic 👑💅