science

Slony-I

Slony-I is a science topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Slony-I rather than just read about it. In short: Slony-I is an asynchronous master-slave replication system for the PostgreSQL DBMS, providing support for cascading and failover. Asynchronous means that when a database transaction has been committed to the master server, it is not yet guaranteed to be available in slaves.

Key takeaways

Slony-I belongs to science; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Slony-I to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Slony-I from memory before moving on to harder problems.

Reference excerpt

Slony-I is an asynchronous master-slave replication system for the PostgreSQL DBMS, providing support for cascading and failover. Asynchronous means that when a database transaction has been committed to the master server, it is not yet guaranteed to be available in slaves. Cascading means that replicas can be created (and updated) via other replicas, i.e. they needn't directly connect to the master.

Terminology The name "slony" comes from the Russian word "слоны" which means "elephants". This is a reference to the PostgreSQL elephant logo, as well as being a "tip of the hat" to Vadim Mikheev, who came up with some of the core ideas Slony-I uses to work. Note that there is a whole set of related terminology:

slony is the plural word for elephants, and indicates that a cluster consists of multiple databases slon is the singular word for elephant; each replication node is managed by a program named "slon", which aggregate together into the aforementioned "cluster of elephants" slonik is the word for a "little elephant," and is the name of the program used to configure the cluster. In effect, the "little elephant" tells the cluster, "here's what you need to do!"

Unique features Unlike many other replication solutions for PostgreSQL, Slony-I is not tied to any particular version of the database, which makes it possible to upgrade a database cluster one node at a time, without material downtime. (Version 2.0.0 requires PostgreSQL 8.3+, though)

See also PostgreSQL Replication (computer science)

References

External links Slony-I official website Slony tutorial: How to Setup Slony-I Replication with Postgres Plus

Worked examples

Example 1 — a first encounter with Slony-I

Start with the simplest possible case. Write down what Slony-I claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In science, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Slony-I before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Slony-I ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Slony-I

In research: Slony-I appears in science research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Slony-I in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Slony-I is common in secondary-school and first-year university syllabi. It links to neighbouring topics Data synchronization, Database stubs, PostgreSQL, so understanding it makes those chapters shorter.
In everyday life: Look for Slony-I outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study Slony-I in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Slony-I means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Slony-I out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Slony-I in simple terms?

Why does Slony-I matter?

Because it connects several science ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Slony-I?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Slony-I.