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https://issues.apache.org/jira/browse/FLINK-9637?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16561606#comment-16561606
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ASF GitHub Bot commented on FLINK-9637:
---------------------------------------
StefanRRichter commented on a change in pull request #6379: [FLINK-9637] Add
public user documentation for state TTL feature
URL: https://github.com/apache/flink/pull/6379#discussion_r206045195
##########
File path: docs/dev/stream/state/state.md
##########
@@ -266,6 +266,92 @@ a `ValueState`. Once the count reaches 2 it will emit the
average and clear the
we start over from `0`. Note that this would keep a different state value for
each different input
key if we had tuples with different values in the first field.
+### State time-to-live (TTL)
+
+A time-to-live (TTL) can be assigned to the keyed state value.
+In this case it will expire after the configured TTL
+and its stored value will be cleaned up based on the best effort.
+Depending on configuration, the expired state can become unavailable for read
access
+even if it is not cleaned up yet. In this case it behaves as if it does not
exist any more.
+
+The collection types of state support TTL on entry level:
+separate list elements and map entries expire independently.
+
+The behaviour of state with TTL firstly should be configured by building
`StateTtlConfiguration`:
+
+<div class="codetabs" markdown="1">
+<div data-lang="java" markdown="1">
+{% highlight java %}
+StateTtlConfiguration ttlConfig = StateTtlConfiguration
+ .newBuilder(Time.seconds(1))
+ .setTtlUpdateType(StateTtlConfiguration.TtlUpdateType.OnCreateAndWrite)
+
.setStateVisibility(StateTtlConfiguration.TtlStateVisibility.NeverReturnExpired)
+ .build();
+{% endhighlight %}
+</div>
+
+<div data-lang="scala" markdown="1">
+{% highlight scala %}
+val ttlConfig = StateTtlConfiguration
+ .newBuilder(Time.seconds(1))
+ .setTtlUpdateType(StateTtlConfiguration.TtlUpdateType.OnCreateAndWrite)
+
.setStateVisibility(StateTtlConfiguration.TtlStateVisibility.NeverReturnExpired)
+ .build()
+{% endhighlight %}
+</div>
+</div>
+
+It has several options to consider.
+The first parameter of `newBuilder` method is mandatory, it is a value of
time-to-live itself.
+
+The update type configures when the time-to-live of state value is prolonged
(default `OnCreateAndWrite`):
+
+ - `StateTtlConfiguration.TtlUpdateType.OnCreateAndWrite` - only on creation
and write access,
+ - `StateTtlConfiguration.TtlUpdateType.OnReadAndWrite` - also on read access.
+
+The state visibility configures whether the expired value is returned on read
access
+if it is not cleaned up yet (default `NeverReturnExpired`):
+
+ - `StateTtlConfiguration.TtlStateVisibility.NeverReturnExpired` - expired
value is never returned,
+ - `StateTtlConfiguration.TtlStateVisibility.ReturnExpiredIfNotCleanedUp` -
returned if still available.
+
+The TTL can be enabled in descriptor for any type of state:
+
+<div class="codetabs" markdown="1">
+<div data-lang="java" markdown="1">
+{% highlight java %}
+StateTtlConfiguration ttlConfig =
StateTtlConfiguration.newBuilder(Time.seconds(1)).build();
+ValueStateDescriptor<String> stateDescriptor = new
ValueStateDescriptor<>("text state", String.class);
+stateDescriptor.enableTimeToLive(ttlConfig);
+{% endhighlight %}
+</div>
+
+<div data-lang="scala" markdown="1">
+{% highlight scala %}
+val ttlConfig = StateTtlConfiguration.newBuilder(Time.seconds(1)).build()
+val stateDescriptor = new ValueStateDescriptor[String]("text state",
classOf[String])
+stateDescriptor.enableTimeToLive(ttlConfig)
+{% endhighlight %}
+</div>
+</div>
+
+**Notes:**
+
+- The state backends store the timestamp of last modification along with the
user value,
+which means that enabling this feature increases consumption of state storage.
+
+- As of current implementation the state storage is cleaned up of expired
value
+only on its explicit read access per key, e.g. calling `ValueState.value()`.
+This might change in future releases, e.g. additional strategies might be
added in background to speed up cleanup.
+
+- Only *processing time* scale is currently supported for TTL.
+
+- Trying to restore state, which was previously configured without TTL, using
TTL enabled descriptor or vice versa
+will lead to compatibility failure.
Review comment:
maybe name the concrete exception
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> Add public user documentation for TTL feature
> ---------------------------------------------
>
> Key: FLINK-9637
> URL: https://issues.apache.org/jira/browse/FLINK-9637
> Project: Flink
> Issue Type: Sub-task
> Components: State Backends, Checkpointing
> Affects Versions: 1.6.0
> Reporter: Andrey Zagrebin
> Assignee: Andrey Zagrebin
> Priority: Major
> Labels: pull-request-available
> Fix For: 1.6.0
>
>
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