ivankelly commented on a change in pull request #1466: Topic compaction
documentation
URL: https://github.com/apache/incubator-pulsar/pull/1466#discussion_r190841826
##########
File path: site/docs/latest/getting-started/ConceptsAndArchitecture.md
##########
@@ -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
+* Applies only to [persistent topics](#persistent-storage)
+* Is triggered manually via the command line. See the [Topic compaction
cookbook](../../cookbooks/compaction)
+* Is conceptually and operationally distinct from [retention and
expiry](#message-retention-and-expiry)
Review comment:
However, it does respect retention. So if retention has removed a message
from the message backlog, this message will also be removed from the compacted
topic ledger (or rather, it won't be readable).
----------------------------------------------------------------
This is an automated message from the Apache Git Service.
To respond to the message, please log on GitHub and use the
URL above to go to the specific comment.
For queries about this service, please contact Infrastructure at:
us...@infra.apache.org
With regards,
Apache Git Services
