ivankelly commented on a change in pull request #1466: Topic compaction
documentation
URL: https://github.com/apache/incubator-pulsar/pull/1466#discussion_r190841526
##########
File path: site/docs/latest/getting-started/ConceptsAndArchitecture.md
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@@ -522,18 +541,55 @@ while (true) {
To create a reader that will read from the latest available message:
```java
-MessageId id = MessageId.latest;
-Reader reader = pulsarClient.createReader(topic, id, new
ReaderConfiguration());
+Reader<byte[]> reader = pulsarClient.newReader()
+ .topic(topic)
+ .startMessageId(MessageId.latest)
+ .create();
```
To create a reader that will read from some message between earliest and
latest:
```java
byte[] msgIdBytes = // Some byte array
MessageId id = MessageId.fromByteArray(msgIdBytes);
-Reader reader = pulsarClient.createReader(topic, id, new
ReaderConfiguration());
+Reader<byte[]> reader = pulsarClient.newReader()
+ .topic(topic)
+ .startMessageId(id)
+ .create();
```
+## Topic compaction {#compaction}
+
+Pulsar was built with highly scalable [persistent
storage](#persistent-storage) of message data as a primary objective. Pulsar {%
popover topics %} enable you to persistently store as many unacknowledged
messages as you need while preserving message ordering. By default, Pulsar
stores *all* unacknowledged/unprocessed messages produced on a topic.
Accumulating many unacknowledged messages on a topic is necessary for many
Pulsar use cases but it can also be very time intensive for Pulsar {% popover
consumers %} to "rewind" through the entire log of messages.
+
+{% include admonition.html type="success" content="For a more practical guide
to topic compaction, see the [Topic compaction
cookbook](../../cookbooks/compaction)." %}
+
+For some use cases, however, consumers don't need a complete "image" of the
topic log. They may only need a few values to construct a more "shallow" image
of the log, perhaps even just the most recent value. For these kinds of use
cases Pulsar offers **topic compaction**. When you run compaction on a topic,
Pulsar goes through a topic's backlog and removes messages that are *obscured*
by later messages, i.e. it goes through the topic on a per-key basis and leaves
only the most recent message associated with that key.
+
+Pulsar's topic compaction feature:
+
+* Can help preserve disk space and allow for much more efficient "rewind" of
topic logs
Review comment:
It doesn't help with disk space, as we don't delete the old data. It only
helps with rewind.
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