ivankelly commented on a change in pull request #1466: Topic compaction
documentation
URL: https://github.com/apache/incubator-pulsar/pull/1466#discussion_r190840650
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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history (which messages are deemed obscured/outdated/irrelevant will depend on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no need to read through outdated data when processing a topic's messages.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." %}
+
+## Which messages get compacted?
+
+When you [trigger](#trigger) compaction on a topic, all messages with the
following
+
+{% include admonition.html type="warning" title="Message keys are required"
+content="Messages that don't have keys are simply left alone and *never*
compacted. In order to use compaction, you'll need to come up with some kind of
key-based scheme for messages on the topic." %}
+
+## Triggering compaction {#trigger}
+
+In order to run compaction on a topic, you need to use the [`topics
compact`](../../CliTools#pulsar-admin-topics-compact) command for the
[`pulsar-admin`](../../CliTools#pulsar-admin) CLI tool. Here's an example:
+
+```bash
+$ bin/pulsar-admin topics compact \
+ persistent://my-tenant/my-namespace/my-topic
+```
+
+The `pulsar-admin` tool runs compaction via the Pulsar [REST
API](../../reference/RestApi). To run compaction locally, i.e. *not* through
the REST API, you can use the [`pulsar
compact-topic`](../../CliTools#pulsar-compact-topic) command. Here's an example:
Review comment:
s/locally/in its own dedicated process/
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