ivankelly commented on a change in pull request #1466: Topic compaction
documentation
URL: https://github.com/apache/incubator-pulsar/pull/1466#discussion_r190840343
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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history (which messages are deemed obscured/outdated/irrelevant will depend on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 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 (or else the messages won't be properly
read/processed/acknowledged).
+
+In Pulsar, topic compaction takes place on a *per-key basis*, meaning that
messages are compacted based on their key. For the stock ticker use case, the
stock symbol---e.g. `AAPL` or `GOOG`---could serve as the key.
+
+## When should I use compacted topics?
+
+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no need to read through outdated data when processing a topic's messages. For
topics where older values are important, for example when you need to process
long series of messages in order, compaction is unnecessary and possibly even
harmful.
+
+{% include admonition.html type="warning" content="Compaction only works on
topics where each message has a key (as in the stock ticker example, where the
stock symbol serves as the key). Keys can be thought of as the axis along which
compaction is applied." %}
+
+## When should I trigger compaction?
+
+How often you trigger compaction will vary widely based on the use case. If
you want a compacted topic to be extremely speedy on read, then you should run
compaction fairly frequently.
+
+{% include admonition.html type="warning" title="No automatic compaction"
content="Currently, all topic compaction in Pulsar must be initialized
manually." %}
Review comment:
must be initiated manually, via the cli or rest api.
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