ivankelly commented on a change in pull request #1466: Topic compaction
documentation
URL: https://github.com/apache/incubator-pulsar/pull/1466#discussion_r191704435
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File path: site/docs/latest/cookbooks/compaction.md
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+---
+title: Topic compaction cookbook
+tags: [admin, clients, compaction]
+---
+
+Pulsar's [topic
compaction](../../getting-started/ConceptsAndArchitecture#compaction) feature
enables you to create **compacted** topics in which older, "obscured" entries
are pruned from the topic, allowing for faster reads through the topic's
history (which messages are deemed obscured/outdated/irrelevant will depend on
your use case).
+
+To use compaction:
+
+* You need to give messages keys, as topic compaction in Pulsar takes place on
a *per-key basis* (i.e. messages are compacted based on their key). For a stock
ticker use case, the stock symbol---e.g. `AAPL` or `GOOG`---could serve as the
key (more on this [below](#when)). Messages without keys will be left alone by
the compaction process.
+* You must manually [trigger](#trigger) compaction using the Pulsar
administrative API. This will both run a compaction operation *and* mark the
topic as a compacted topic.
+* Your {% popover consumers %} must be [configured](#config) to read from
compacted topics ([Java consumers](#java), for example, have a `readCompacted`
setting that must be set to `true`). If this configuration is not set,
consumers will still be able to read from the non-compacted topic.
+
+## When should I use compacted topics? {#when}
+
+The classic example of a topic that could benefit from compaction would be a
stock ticker topic through which {% popover consumers %} can access up-to-date
values for specific stocks. On a stock ticker topic you only care about the
most recent value of each stock; "historical values" don't matter, so there's
no need to read through outdated data when processing a topic's messages.
+
+{% include admonition.html type="info" content="For topics where older values
are important, for example when you need to process a long series of messages
in order, many of which have the same key, compaction is unnecessary and could
possibly even be harmful." %}
Review comment:
Compaction isn't harmful in this case. They will still have access to all
messages.
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