Manage Mirror Topics for Cluster Linking on Confluent Platform¶
This page provides a concept guide, walkthroughs, and examples for creating, configuring, and managing mirror topics on cluster links.
What are mirror topics?¶
Mirror topics are the building blocks for moving data with Cluster Linking. They are read-only topics that are created and owned by a cluster link.
The sections below provide a conceptual overview of mirror topics, how they are created, configured, and how they work in operation, that is applicable to both Confluent Platform and Confluent Cloud.
Overview¶
A cluster link connects a mirror topic to its source topic. Any messages produced to the source topic are mirrored over the cluster link to the mirror topic.
A mirror topic syncs many of its configurations from its source topic. It can also sync ACLs and consumer group offsets from its source topic, if you enable those features on the cluster link.
You can convert a mirror topic to a regular topic and permanently stop the
mirroring relationship, using the Cluster Linking promote
and failover
commands.
The diagram below shows how mirror topics work, including the relationship between the mirror topic and its source topic, and the syncing of ACLs and consumer offsets.
![../../_images/cluster-link-mirror-topics-example.png](../../_images/cluster-link-mirror-topics-example.png)
Mirror Topic Fundamentals¶
Properties¶
Mirror topics have these unique properties:
- Mirror topics are created by and owned by a cluster link.
- Mirror topics get their messages from their source topic. They are byte-for-byte, offset-preserving asynchronous copies of their source topics.
- Mirror topics are read-only; you can consume them the same as any other topic, but you cannot produce into them. If a producer tries to produce a message into a mirror topic, the action will fail. The only way to get a message into a mirror topic is to produce the message to the mirror topic’s source topic.
- Many of the mirror topic’s configurations are copied and synced from the source topic. A full list is at the end of this page.
Mirror topic creation¶
You can create a mirror topic using the Confluent Cloud Console, the Confluent Cloud REST API, the Confluent CLI, the Confluent Platform AdminClient API, or Confluent for Kubernetes.
Alternatively, you can configure your cluster link to automatically create mirror topics that match certain prefixes.
Requirements¶
Mirror topics can only be created with the mirror topic command or by enabling auto-create mirror topics on the cluster link. A mirror topic cannot be pre-created by a non mirror topic command.
- Creating a mirror topic requires an existing cluster link. The mirror topic will be created on the destination cluster of the cluster link. The user must have access to the destination cluster.
- A mirror topic is always created with the same name as its source topic. There must be a topic of that name on the source cluster. The only exception is when a cluster link has
link.prefix
configured, which will add a prefix to the name of the mirror topic. - The destination cluster must be able to reach the source cluster and verify there is a suitable topic. A mirror topic cannot be created if the source cluster is unreachable.
- Creating a mirror topic on Confluent Cloud requires the user to have the CloudClusterAdmin, EnvironmentAdmin, or OrgAdmin role over the destination cluster–that is, the cluster the mirror topic will be created on. Alternatively, the user can have the appropriate ACLs.
- The cluster link’s principal must have both DeveloperRead and DeveloperManage on the relevant source topic on the source cluster. Alternatively, it could have ResourceOwner or the appropriate ACLs on that topic. The user does not need any permissions on the source cluster.
Tip
- You can also have the cluster link automatically create mirror topics.
- For more about configuring access on Confluent Cloud, see Security for Cluster Linking on Confluent Cloud.
- For more about configuring security on Confluent Platform, see Cluster Linking Security.
Create a mirror topic on Confluent Cloud Console¶
If the destination cluster is a Confluent Cloud cluster, you can view and create mirror topics on the Confluent Cloud Console:
On the top-level page showing your Environments, click Cluster links tab.
Click on an existing cluster link, or create a new one. (If you choose to create a new cluster link, follow the prompts as given.)
To add a mirror topic to an existing cluster link, click Add mirror topic.
- If the source cluster is a Confluent Cloud cluster that the current user has access to, the dialog will have a dropdown with a list of all of the source topics on it.
- If the source cluster is external to Confluent Cloud or the user does not have access to it, then you will see a text box instead in which to add the name of the source topic.
Enter the source topic name and click the Add to create the mirror topic.
Tip
For cluster links with link.prefix
configured, enter the name of the source topic in this dialog. The mirror topic’s name will automatically have the prefix added after you click the Add.
![../../_images/cluster-link-mirror-topics-add-1.png](../../_images/cluster-link-mirror-topics-add-1.png)
![../../_images/cluster-link-mirror-topics-add-2.png](../../_images/cluster-link-mirror-topics-add-2.png)
![../../_images/cluster-link-mirror-topics-add-3.png](../../_images/cluster-link-mirror-topics-add-3.png)
Create a mirror topic with the Confluent CLI¶
To create a mirror topic with the Confluent CLI, the general syntax is:
confluent kafka mirror create <mirror-topic-name> --link <link-name>
The command must be run against the destination cluster. If needed you can specify the destination cluster with --cluster <destination-cluster-id>
.
To learn more, see confluent kafka mirror create in the command reference.
If the cluster link is configured with link.prefix
, then --source-topic source-topic-name
must be passed, too. For example:
confluent kafka mirror create west.clicks --link from-west --source-topic clicks
On Confluent Platform clusters, you can use either the Confluent CLI or the bin/kafka-mirrors
script. The general syntax to create a mirror topic is:
kafka-mirrors --create --mirror-topic <topic-name> \
--link <link-name> \
--bootstrap-server <host:port>
To learn more, see Cluster Linking on Confluent Platform.
Create a mirror topic with the REST API¶
On Confluent Cloud:
To create a mirror topic, send a
POST
request to the destination cluster’s REST API endpoint at:/kafka/v3/clusters/{cluster_id}/links/{link_name}/mirrors
.Include the following in the payload:
{ "source_topic_name": "<source-topic-name>", "mirror_topic_name": "<mirror-topic-name>", // (only required if link.prefix is configured) }
The above shows the only required parameters. More options are available to override topic configurations.
To learn more, see the Create a mirror topic in the Confluent Cloud API reference .
Examples¶
For examples of how to create mirror topics on Confluent Platform, see Create the cluster link and mirror topic (step 2, “Initialize the mirror topic”) in the basic tutorial and Creating a mirror topic in the Commands documentation.
For examples of how to create mirror topics on Confluent Cloud, see the following sections:
- Create source and mirror topics in the Quick Start for Cluster Linking on Confluent Cloud
- Mirror a topic on the Confluent Cloud Cluster Linking tutorial
- examples on the Confluent CLI command reference
Create a mirror topic with the AdminClient API¶
On Confluent Platform, you can use the AdminClient API to create mirror topics. To learn more, see ConfluentAdmin API reference.
Additional requirements when prefixing is enabled¶
When a cluster link has a prefix set, the specified prefix will be added to the beginning of
mirror topic names. For example; if you set the prefix to west
, the source topic orders
will be mirrored as west.orders
.
If the cluster link is configured for prefixing mirror topic names, then to create a mirror topic you must pass both the mirror topic name and the source topic name (instead of just the source topic name).
To learn more about prefixing, see Prefix Mirror Topics and Consumer Group Names.
Bidirectional cluster linking¶
To establish bidirectional linking between two clusters, you must use two cluster links. You cannot establish bi-directional linking with a single cluster link. For an example of bidirectional linking, see the Hybrid tutorial (on either Confluent Cloud or Confluent Platform), which sets up bidirectional linking between on-premises and cloud clusters.
Bidirectional linking is supported for different topics. For a specific topic, only unidirectional linking is supported.
Cherry pick which topics to mirror¶
To cherry pick topics to be mirrored, you can use any of the following methods:
- set filters
- manually mirror individual topics with
confluent mirror
command as shown in the tutorials (quick start for Confluent Cloud, tutorial for Confluent Platform) - use the Confluent Cloud Cluster Linking (v3) REST API (see “Create a mirror topic”).
Support for compacted topics¶
Cluster Linking supports compacted topics. A compacted topic is mirrored as such from source to destination. To learn more, see the FAQs for Confluent Cloud and Confluent Platform.
Auto-Create mirror topics¶
A cluster link is able to automatically create mirror topics on the destination cluster for any topics that exist on the source cluster. This is called “auto-creating” mirror topics. This saves time and effort, because you do not have to create mirror topics by hand. This functionality can be scoped down to a specific set of topics by matching on the topics’ names.
Enable auto-create mirror topics¶
To enable this functionality, you must set two properties on the cluster link. You can set these properties when a cluster link is created, or update an existing cluster link with these properties. These properties are:
auto.create.mirror.topics.enable
Whether or not to auto-create mirror topics based on topics on the source cluster. When set to “true”, mirror topics will be auto-created. Setting this option to “false” disables mirror topic creation and clears any existing filters.
- Type: boolean
- Default: false
auto.create.mirror.topics.filters
- A JSON object with one property,
topicFilters
, that contains an array of filters to apply to indicate which topics should be mirrored. Filters are described below. - This list must have at least one filter.
- Ordering of the filters in this array does not matter.
- Type: array
- Default: empty
Syntax
{ “topicFilters”: [ <each filter to apply> ] }
- A JSON object with one property,
Schedule and frequency of mirror topic auto-create task¶
The auto-topic creation schedule and frequency is driven by metadata.max.age.ms
,
which controls the delay between subsequent auto mirror tasks.
metadata.max.age.ms
Maximum amount of time in milliseconds that the client can use a cached metadata value before it is refreshed from the brokers.
- Type: integer
- Default: 5 minutes (300000 milliseconds)
The default value for metadata.max.age.ms
is 5 minutes (300000 milliseconds).
Using the default, clients will cache metadata about the cluster for up to 5 minutes before they refresh it.
Reducing the value specified for metadata.max.age.ms
will increase the frequency
with which mirror topics are auto-created.
If the metadata.max.age.ms
value is too low, it can cause a refresh of the metadata
too frequently, which can impact the performance of the auto-create mirror topic feature.
In this case, you may see errors or delays in the creation of mirror topics.
On the other hand, if the metadata.max.age.ms
value is too high, it can result in
stale metadata, which can lead to inconsistencies between the source and mirror topics.
Filters for auto-create mirror topics¶
In both Confluent Cloud and Confluent Platform, auto-creating mirror topics automatically excludes Confluent internal topics and topics prefixed with _confluent
even if these are user-created.
For full detail on this, see Topics not mirrored.
All other filtering options described below also are available in both Confluent Cloud and current releases of Confluent Platform.
The topic __consumer_timestamps
internal topic is used by Confluent Replicator for consumer offset translation;
this topic should not be mirrored. Therefore, you must filter this topic out using the auto-create mirror topics EXCLUDE filters, as described below.
Other topics can be excluded using filters. For example, if a different topic name is used for Schema Registry storage, instead of _schemas
,
it can be excluded by using filters. Further detail on how to filter topics for auto-create mirror topics is provided below.
You can select exactly which source topics to automatically mirror through a list of filters. There is no limit to the number of filters you can add on a cluster link.
Each filter is a JSON object with the following fields:
name
- Text that will be matched against the name of the topic. Set
name
to the wildcard,*
, to apply to all topics. patternType
Either
LITERAL
orPREFIXED
- If
name
is set tofoo
, then settingpatternType
toLITERAL
will only match a topic namedfoo
. - Setting
patternType
toPREFIXED
will match any topic names that begin with “foo”, for example, “foo”, “football”, and “foo.fighters”.
- If
filterType
Either
INCLUDE
orEXCLUDE
- If
filterType
is set toINCLUDE
, any topic names on the source cluster that match this filter will be created as mirror topics. - If
filterType
is set toEXCLUDE
, any matching topic names will not be created as mirror topics. In other words, prevents auto mirror topic creation for the specified topic names.EXCLUDE
filters override any overlappingINCLUDE
filters. For example, if you have anINCLUDE
filter for the prefix “foo” but have anEXCLUDE
filter for the prefix “foo.bar,” then a topic on the source cluster named “foo.fighters” would be mirrored automatically, but a topic named “foo.bar.fighters” would not be mirrored automatically.
- If
There is no limit to the number of filters you can add on a cluster link.
Example filters¶
Mirror all topics¶
This filter will create mirror topics for all current and future source cluster topics:
{ "topicFilters": [ {"name": "*", "patternType": "LITERAL", "filterType": "INCLUDE"} ] }
Mirror all topics that begin with a given string¶
This filter will mirror all topics that begin with “foo”:
{ "topicFilters": [ {"name": "foo", "patternType": "PREFIXED", "filterType": "INCLUDE"} ] }
Mirror all topics except those that begin with “secret”¶
This filter will mirror all topics except those that begin with “secret”:
{ "topicFilters": [ {"name": "*", "patternType": "LITERAL", "filterType": "INCLUDE"}, \
{"name": "secret", "patternType": "PREFIXED", "filterType": "EXCLUDE"} ] }
Mirror named topics if they exist on the source cluster¶
This filter will mirror three topics, “liz”, “jack”, and “kenneth”, if they exist on the source cluster:
{ "topicFilters": [ {"name": "liz", "patternType": "LITERAL", "filterType": "INCLUDE"}, \
{"name": "jack", "patternType": "LITERAL", "filterType": "INCLUDE"}, \
{"name": "kenneth", "patternType": "LITERAL", "filterType": "INCLUDE"} ] }
How a mirror topic is auto-created¶
For a given topic on a cluster link’s source cluster (the “source topic”), a new mirror topic will be auto-created by the cluster link if all of these conditions are true:
auto.create.mirror.topics.enable
is set totrue
auto.create.mirror.topics.filters
has filters which INCLUDE the source topic name- The cluster link’s security credential is authorized (via source cluster ACLs) to read the source topic.
- There is no topic by that name already on the destination cluster.
- If prefixing is enabled on the cluster link, then the source topic cannot be a mirror topic. You cannot “chain” mirror
topics when both
auto.create.mirror.topics.enable
and prefixing are enabled.
If any of the above conditions are false, then a mirror topic will not be auto-created for the given source topic.
Override topic configurations when using auto-create mirror topics¶
To override a topic configuration when using auto-create mirror topics, you have two options:
- Change the topic configuration after the mirror topic is automatically created.
- Use the CLI or API to manually create the mirror topic, and override the configuration. Even if a topic matches the auto-create mirror topic filters, it can still be manually created as a mirror topic before the cluster link creates it automatically. Auto-create mirror topics runs once every 5 minutes, so the mirror topic can be manually created soon after the cluster link is created or soon after the source topic is created.
Delete topics that were auto-created¶
You cannot delete a mirror topic that matches the auto-create mirror topics
filters. If you deleted such a topic, and there was a topic of the same name on
the source cluster, the mirror topic would be automatically re-created and sync
all its history (if mirror.start.offset.spec
is set to the default). This would render the delete operation futile.
To delete a mirror topic while auto-create mirror topics is enabled, you have three options: delete the source topic first, exclude the topic’s name from the auto-create mirror topics filters, or disable auto-create mirror topics.
- Option 1. Delete the source topic first - Given a source topic named
cool-topic
, if you delete the source topic and then want to subsequently delete the associated mirror topic (cool-topic
on the destination), wait until the mirror topic becomes aFAILED
mirror topic (which may take up to five minutes), after which point you can delete it. You can also callfailover
orpromote
on the mirror topic to transition it to theSTOPPED
state. BothFAILED
andSTOPPED
mirror topics can be deleted. - Option 2. Exclude the topic name from the auto-create mirror topics filters - This strategy will prevent
the mirror topic from overlapping with the auto-create filters. An easy way to remove a given topic from the
filters is to add an
EXCLUDE
filter for that topic name. You can addcool-topic
to theEXCLUDE
filters, even if no such source topic exists. After editing the auto-create mirror topic filters, you can delete the mirror topic. - Option 3: Disable auto-create mirror topics on the cluster link - After the setting has been disabled, the mirror topic can be deleted. If needed, auto-create mirror topics can be immediately re-enabled on the cluster link. To learn more, see Disable auto-create mirror topics and Mirror topic deletion.
Disable auto-create mirror topics¶
To disable auto-create mirror topics entirely, set this property on the cluster link:
auto.create.mirror.topics.enable=false
Here’s an example of how to set that property with the CLI:
echo "auto.create.mirror.topics.enable=false" > tmp.txt
confluent kafka link update <link-name> --config-file tmp.txt
rm tmp.txt
Prefix Mirror Topics and Consumer Group Names¶
Note
Prefixing is not available on Confluent Platform version 7.1 or earlier. It is available on Confluent Cloud and in Confluent Platform starting with release 7.2.0.
Cluster links can be configured with a prefix (cluster.link.prefix
) that is
applied to the names of the mirror topics and, optionally, the names of the
consumer groups that are managed by the cluster link at the destination cluster.
This enables topics and consumer groups from different source clusters that have
the same name to be synced to the destination without name clashes. It also
enables all mirror topics from a cluster link to be categorized and managed
under one prefix on the destination.
For example, consider two links, link-1 and link-2. link-1 is linking data from
cluster s1 to destination and link-2 is linking data from s2 to destination, and
furthermore s1 and s2 both contain a topic “clicks”. Without prefixing, it would
be impossible for both links to sync data for their own “clicks” topic as they would
have the same name on the destination cluster. With prefixing, each link can
have its own unique prefix that is applied to the topic name as its mirrored.
link-1 could have prefix usa_
and link-2 could have prefix eu_
. Finally,
at the destination cluster there would be two topics, usa_clicks
and
eu_clicks
.
If the link is configured with a prefix, when a mirror topic is
created (for example, with confluent kafka mirror create
) then the mirror
topic name must begin with the prefix (otherwise, the operation will fail). If
auto-create mirror topics is used,
the topics created on the destination will automatically be named with the prefix.
The prefix can optionally be applied to the consumer groups that are created on
the destination cluster because of consumer group offset syncing. When offsets
are synced, consumer groups are created on the destination; with this feature
it’s possible to prefix the consumer group name on the destination. This enables
consumer group offsets to be synced even when two (or more) consumer groups from
two (or more) different source clusters have the same name. For example, if
link-1 had consumer group g1 and link-2 had consumer group g1, then prefixing
would result in two consumer groups at the destination: usa_g1
and eu_g1
.
By default, consumer group names are not prefixed with the prefix; consumer.group.prefix.enable
must be set to true
in the cluster link config to enable this.
Here’s an example configuration file for Confluent Enterprise, containing just elements relevant to prefixing:
bootstrap.servers=localhost:9092
cluster.link.prefix=usa_
consumer.offset.sync.enable=true
auto.create.mirror.topics.enable=true
auto.create.mirror.topics.filters={"topicFilters":[{"name": "*","patternType": "LITERAL","filterType": "INCLUDE"}]}
consumer.group.prefix.enable=false
acl.sync.enable=false
Here, a prefix of usa_
has been configured and consumer.group.prefix.enable
has been set to false
(which is the default, but shown here for context). All mirror
topic names on the destination will start with the prefix; consumer group names
will remain the same as they are on the source. Also note, acl.sync.enable is
set to false which is required because auto.create.mirror.topics.enable
is set
to true
and prefixing is enabled; see limitations below.
On Confluent Cloud, these configurations are specified on the command line or the Confluent Cloud console.
Limitations on prefixing¶
- The prefix cannot be changed after the cluster link is created.
- ACL syncing and prefixing cannot be enabled at the same time. Note, ACLs can always be synced on a separate link; just create a new link and configure it to sync ACLs.
- Consumer group prefixing cannot be enabled for bidirectional links. Setting
consumer.group.prefix.enable
totrue
on a bidirectional cluster link will result in an “invalid configuration” error stating that the cluster link cannot be validated due to this limitation. - Prefixing cannot be combined with chaining and auto-create mirror topics at the
same time. When auto-mirroring and prefixing is configured, a link cannot mirror
a topic that is itself a mirror topic at the source cluster. For example,
consider the same links above, link-1 and link-2. If a new link-3 was created,
auto-mirroring would not be able to mirror data from
usa_clicks
oreu_clicks
or any mirror topic on the destination (even if it didn’t have a prefix) because they are mirror topics. This is done as a safeguard to prevent auto-mirroring from creating an infinite number of topics due to cyclical cluster link connections.
Tip
Prefixed chained mirror topics can still be created by hand, for
example via confluent kafka mirror create
.
Aggregate multiple source cluster topics into a single topic¶
Cluster Linking can be used for aggregating data from multiple identical source clusters into one destination cluster. For example, each source cluster may be running in a different region, collecting local data, and Cluster Linking can stream data from each local cluster to a central, aggregate cluster.
Note that every topic, on every source cluster, that you wish to aggregate will need its own uniquely named mirror topic on the aggregate cluster. An easy way to accomplish this is by setting a unique prefix on each cluster link.
If a consumer group needs to read the data from all source clusters (for example, from all regions), it can easily consume multiple mirror topics at the same time by consuming from a regular expression (“regex”) topic pattern that matches the all mirror topic names you want to source from (rather than consuming from a single topic name). Most open source Kafka clients support consuming from a regex topic pattern.
![../../_images/cluster-link-aggregate-topics.png](../../_images/cluster-link-aggregate-topics.png)
Tip
If you can’t consume from a regex pattern, ksqlDB INSERT
queries can be used to merge the mirror topics
into a single aggregated topic for each data type.
Topics not mirrored¶
By design, the following topics are not mirrored (synced). Mirrors will not be created for these topics, even if they match the topic name filter.
- Internal or “system” topics (for example, any topic prefixed with
_confluent
) - Any topic prefixed with
_confluent
, even if these are user-created and not system topics. confluent-audit-log-events
- The topic that holds schemas (default name
_schemas
)
For more background detail, some of the internal topics not mirrored include the following:
__consumer_offsets
: This topic stores consumer group offsets. Cluster Linking uses a built-in offset sync mechanism, rather than “mirror” this topic in the standard sense.__transaction_state
: This topic stores transaction states and helps maintain transactional integrity. This topic is not replicated because Cluster Linking does not support transaction.__cluster_metadata
: In KRaft mode, this topic is used for internal cluster coordination. There is no need to mirror this topic.
Mirroring lag¶
The mirror process is asynchronous in operation. Therefore, there will often be some mirroring lag between the source topic and the mirror topic. The most recent messages on the source topic may not yet have been mirrored to the mirror topic, so the mirror topic may often be slightly behind the source topic.
The same is true for syncing the topic configuration, the consumer group offsets, and the ACLs. All of these processes are asynchronous, so the changes will happen first on the source topic, and then on the mirror topic shortly after.
Sync consumer group offsets¶
You can configure your cluster link to sync consumer group offsets from its source topic to the destination topic.
Enabling consumer group offset sync and specifying filters¶
To set this up, you configure the following the following properties (as described in Migrating consumer groups from source to destination cluster):
consumer.offset.sync.enable
- Set this totrue
to sync consumer group offsets. (The default isfalse
.)consumer.offset.group.filters
- Pass in a JSON file with a pattern that is matched against consumer group names to identify which groups to mirror.
If these two properties are set, the cluster link syncs the consumer group offsets of any matching consumer groups for all mirror topics that the link mirrors.
Note
Consumer group filters should not include groups that are being used on the destination. This will help ensure that the system does not override offsets committed by other consumers on the destination, or overwrite the consumer offsets while consumer groups are consuming from the mirror topic. If you are unsure about which consumer groups are being used on the destination, consumer offset sync on the cluster link until you verify this.
Why consumer offsets may be clamped in the event of failover, promote, or consumer group migration¶
When either failover
or promote
is called on a mirror topic or when a consumer group moves
to the destination cluster and consumes its first message from a mirror topic, if consumer offset sync
is enabled on the cluster link, then the consumer offsets for that topic may be “clamped”. That is,
the consumer offsets for the topic that the cluster link synced will not
be allowed to be larger than the last offsets on the mirror topic (the “log end
offset”). If any of these consumer offsets are larger / further than the last
offsets (Log End Offset), then those consumer offsets will be reset to the Log End Offset.
(The use of failover
and promote
are covered in the next section on Convert a mirror topic to a normal topic.)
To illustrate with an example: suppose a source topic with one partition had messages up to offset 100 and is being mirrored over a cluster link. However, there was mirroring lag on the cluster link, and only the messages up to offset 90 were mirrored when a disaster hit the source topic. At this point, you call failover on the mirror topic. Consumer group A was at offset 80 on the source cluster, so it will remain at offset 80 on the destination cluster, since that offset was mirrored to the mirror topic. But consumer group B was at offset 95, which was not mirrored to the mirror topic. If consumer group B started consuming at offset 95 on the mirror topic, then it would miss any messages at offsets 90-94 that were produced to the topic. To avoid that problem, the cluster link “clamps” consumer group B’s offsets down to 90, which is the highest offset on the mirror topic.
Where to learn more¶
To learn more, see Migrating consumer groups from source to destination cluster and the command descriptions under Configuration Options in Configure Cluster Linking on Confluent Platform.
Reverse a source and mirror topic¶
The source topic -> mirror topic relationship can be reversed using the reverse-and-start
or reverse-and-pause
commands.
These cause the source topic to become the mirror topic, and the mirror topic to become the source topic.
The following diagram shows how a reverse-and-pause
command works. The reverse-and-start
command behaves in a similar fashion,
except that instead of the extra manual step to resume the mirror topic to become an active mirror topic, the command automatically converts
the topic to an active mirror topic.
![../../_images/cluster-link-reverse-mirror.png](../../_images/cluster-link-reverse-mirror.png)
How it works¶
The reverse-and-start command leaves the new mirror topic in an active mirroring state, whereas reverse-and-pause leaves the new mirror topic in a paused state until the resume command is called.
These commands are available in the Confluent CLI at confluent kafka mirror,
the REST API at reverse,
and kafka-mirrors
in Confluent Platform 7.7+.
Cluster Linking ensures that both topics have the same data and metadata at the point of change, so no data are left behind. Once the command is called, the source topic will not accept new writes, which allows the mirror topic to catch up and perform the reversal. After the reversal is complete, data written to the (new) source topic will then flow to the (new) mirror topic. This provides a fast and efficient failback for a planned failover mechanism, allowing you to quickly failover to the mirror site, produce new data, and then quickly failback to the original site.
Requirements for using “reverse” commands¶
- The cluster link must be in Bidirectional mode.
- Both clusters must be Confluent Cloud Dedicated clusters.
- Both clusters must be healthy and able to communicate over the network.
- You must have the CLUSTER:ALTER ACL or the Admin role on the cluster where this command is run.
Process flow for “reverse” commands¶
Calling the reverse
commands follows this chain of events:
Call
reverse-and-start
orreverse-and-pause
on the mirror topic.- Make sure this command is used against the cluster that hosts the mirror topic; for example, the disaster recovery (DR) cluster
- Multiple topics can be reversed at once using the REST API
- You must have the CLUSTER:ALTER ACL or the Admin role on the cluster where this command is run.
The mirror topic enters
PENDING_SYNCHRONIZE
state, and the source topic entersPENDING_MIRROR
state.During this time:
- The source topic will not accept any new writes (produce requests).
- The mirror topic will fetch all data from the source topic until it is up to date.
- Tip: The larger the mirroring lag on the mirror topic, the longer this step will take. To minimize the amount of time when both topics are in a read-only state, call the command at a time when mirroring lag is at zero, or very low.
Once the data has been synchronized, the (old) mirror topic enters
PENDING_STOPPED
state.- During this time, the old mirror topic fetches any last metadata (such as, consumer offsets).
- Monitor for errors that could cause this step to hang using the state transition error API or Metrics.
The (new) source topic enters the
STOPPED
mirror state, and accepts writes as a regular topic.- The (new) mirror topic enters the
ACTIVE
orPAUSED
state, depending on which command was called. The mirroring relationship is reversed.
- The (new) mirror topic enters the
Limitations¶
- The “reverse” commands currently do not work with hybrid links. In other words, this will not work on a bidirectional link where one cluster is on-premises and the other cluster is in Confluent Cloud.
- The “reverse” commands cannot be used in Confluent Platform when unclean leader election is enabled.
- Make sure you have monitoring in place to check all of the different states the topics will be going through (as described in Process flow for “reverse” commands).
If at any point in time there are any issues in the process, you can always use the
failover
command to get a mirror topic to a writable state. - You should run this command with only one topic at a time for transactional producers, and you must monitor each topic to the end state before running the command for the next topic. If you run the command in batch, you must make sure that all topics are transitioned to a writable state before restarting the application for production. Otherwise, if the applications are restarted before the topics are transitioned into the end state, this can result in the new records not being persisted in Kafka because it starts writing to an immutable topic.
Convert a mirror topic to a normal topic¶
If you want to convert a mirror topic into a normal topic that you can produce
into, you can call the failover
or the promote
command on the mirror topic.
In Confluent Platform, you call kafka-mirrors --promote
or kafka-mirrors --failover
.
In Confluent Cloud, you call confluent kafka mirror promote <topic-name>
or confluent kafka mirror failover <topic-name>
.
Both commands occur on the destination cluster (the mirror topic’s cluster) and require you to pass in the cluster link’s name.
The promote
option is often used for migrations. It checks that there is no mirroring lag, config sync lag, or
consumer offset lag between the source topic and the mirror topic. Then, it converts the mirror topic into a full topic,
with the assurance that this topic was exactly the same as its source topic.
Note
The destination cluster’s brokers must be able to reach the source cluster’s brokers in order to make this check, so your source cluster must be online.
The failover
option is often used when a disaster has hit the source cluster (for example, a cloud region outage)
and you want to shift operations from the source topic to the mirror topic. This command will succeed regardless of the
mirroring lag or the source cluster’s reachability.
Both the promote
and failover
commands are irreversible. There is no way to change this topic back into a mirror topic.
If you want a mirror topic of the same name as the one you promoted or failed over on this cluster, you must delete the converted topic, and create a new mirror topic of the same name.
There is no way to change a mirror topic to use a different cluster link or make changes to the link itself, other than to recreate the mirror topic on a different link.
Important
- You can run
mirror describe
(confluent kafka mirror describe <mirror-topic-name> --link <link>
) on a promoted or failed over mirror topic, if you do not delete the cluster link. If you delete the cluster link, you will lose the history and, therefore,mirror describe
will not find data on promoted or failed over topics. - You cannot delete a cluster link that still has mirror topics on it (the delete operation will fail).
- If you are using Confluent for Kubernetes (CFK), and you delete your cluster link resource, any mirror topics still attached to that cluster link
will be forcibly converted to regular topics by use of the
failover
API. To learn more, see Modify a mirror topic in Cluster Linking using Confluent for Kubernetes.
![../../_images/cluster-link-migrate.png](../../_images/cluster-link-migrate.png)
Example of Topic Migration¶
![../../_images/cluster-link-failover.png](../../_images/cluster-link-failover.png)
Example of Failing Over a Topic¶
Mirror topic states and statuses¶
When you describe a mirror topic, it will return one of these states:
ACTIVE
- The mirror is running normally, and messages are being mirrored from the source topic to the destination topic.
PAUSED
A user has paused mirroring for this mirror topic.
To reach this state, a user must either pause this specific topic, or pause its cluster link.
Caution
Confluent Cloud cluster links cannot be paused. On Confluent Cloud, users can only pause the individual mirror topics, as described in confluent kafka mirror pause.
PENDING_SYNCHRONIZE
- This topic is in the process of becoming a regular topic that will be mirrored to the remote cluster. (Previous to this, the topic was a mirror topic.)
- This topic is currently READ ONLY. (It will not accept produce, but it can be consumed from).
- The
PENDING_SYNCHRONIZE
state occurs when areverse
command is called on a topic. - Allowed operations to a topic in this state are as follows:
pause
: to pause the reversal processfailover
: to permanently abort the reversal process and convert this to a writable, non-mirror topic.
- The topic will automatically transition to the
STOPPED
state when ready.
PENDING_MIRROR
- This topic is in the process of becoming a mirror topic (it was formerly a writable topic).
- It will not accept produce, but it can be consumed from (it is READ ONLY).
- The
PENDING_MIRROR
state occurs when a reverse command is called on the remote mirror topic for this source topic. - Allowed operations to a topic in this state are as follows:
failover
: to permanently abort the reversal process and convert this to a writable, non-mirror topic.
- When its conversion to a mirror topic has completed, the topic will automatically transition to the
ACTIVE
orPAUSED
state, depending on which reverse command was used.
PENDING_STOPPED
- A user has stopped this mirror topic with the
promote
command, and this topic will soon be in theSTOPPED
state. - To force the mirror topic to immediately go from the
PENDING_STOPPED
state to theSTOPPED
state, call thefailover
command on it. Doing this cancels any synchronization that was happening between the source cluster and the destination cluster, and eliminates any guarantees that thepromote
command gives.
- A user has stopped this mirror topic with the
STOPPED
- Mirroring has permanently stopped for this topic. It will no longer receive messages from its source topic. The topic is now writable and can receive messages produced directly to it.
- To get into this state, a user must call either
promote
orfailover
on this mirror topic. - Even though a
STOPPED
topic is no longer a mirror topic, it will still be listed in output for the commandsconfluent kafka mirror list
andconfluent kafka mirror describe <destination-topic-name> --link <link>
for as long as the cluster link exists. This is useful because the topic will return the last offset it fetched from its source topic (Last Source Fetch Offset) for each partition, and the time at which it was stopped (Status Time).
SOURCE_UNAVAILABLE
- The mirror topic is unable to reach the source topic, and is not mirroring messages from the source topic. This could happen if the source cluster is experiencing an outage or if the network between the destination cluster and the source cluster is unstable.
- Mirroring will resume once the issue is resolved and the destination cluster can reach the source cluster.
Note
Using a Confluent Platform 7.0.x source cluster with a source-initiated link to a KRaft destination cluster will
generate a SOURCE_UNAVAILABLE
error. Cluster Linking between a source cluster running
Confluent Platform 7.0.x or earlier (non-KRaft) and a destination cluster running in KRaft mode is not supported.
To solve for this, upgrade the source cluster to Confluent Platform 7.1.0 or later.
LINK_FAILED
- An error has broken the mirror topic’s cluster link, and no data is being mirrored. A user needs to manually re-configure the link.
FAILED
- The mirror topic has permanently failed. It will no longer mirror data. This can happen if the cluster link ACLs are removed from the source cluster, or if the source topic is deleted. In both cases, the failed status takes effect only after cluster.link.retry.timeout.ms is reached (by default, the system retries the link for 5 minutes).
- You can stop this mirror with the
failover
command, and it will become a regular topic. - If you want to restore mirroring for this topic, you must delete the legacy mirror topic and create a new mirror topic with the same name.
View mirror topic state transition errors¶
You can use the following commands to view mirror topic state transition errors.
For example, when a mirror topic is promoted, it transitions from the
PENDING_STOPPED
state to STOPPED
state. During that process, various
actions are performed to implement the transition and errors can occur during
that implementation. The following APIs allow you to view these errors and
unblock the mirror topic state transitions. For example, if you see an
authentication issue, you can reconfigure the link’s credentials to allow the
mirror topic to be fully promoted.
For a full list of possible task states and error codes, see Troubleshooting Cluster Linking on Confluent Cloud.
Important
These CLI commands are not yet available on Confluent Platform.
To view errors associated with a state transition:
confluent kafka mirror state-transition-error list <topic-name> --link <link-name>
./bin/kafka-mirrors.sh ... --list-state-transition-errors --topics <topic-name>
See Describe the mirror topic in the Confluent Cloud REST API documentation.
To view a mirror topic status, send a GET request to <REST-Endpoint>/clusters/<cluster-ID>/links/<link-name>/mirrors/<mirror_topic_name>?include_state_transition_errors=true
.
Examples¶
confluent kafka mirror state-transition-error list topic-1 --link link-1
Mirror State Transition Error | Mirror State Transition Error
Code | Message
---------------------------------+---------------------------------
AUTHENTICATION_ERROR | Failed to describe topic
| configs due to authentication
| issues.
---------------------------------+---------------------------------
./bin/kafka-mirrors.sh --bootstrap-server pkc-j581r8.us-west2.gcp.confluent.cloud:9092 --command-config
command-config.properties --list-state-transition-errors --topics topic-1
Topic: topic-1 State: PENDING_STOPPED
Error Code: AUTHENTICATION_ERROR Error Message: "Failed to describe topic configs due to authentication issues."
curl -H "Authorization: Basic XXX" --request GET \
--url 'https://pkc-j581r8.us-west2.gcp.confluent.cloud:443/kafka/v3/clusters/lkc-ok51xj/links/link-1/mirrors/topic-2?include_state_transition_errors=true' | jq
% Total % Received % Xferd Average Speed Time Time Time Current
Dload Upload Total Spent Left Speed
100 844 0 844 0 0 2217 0 --:--:-- --:--:-- --:--:-- 2215
{
"kind": "KafkaMirrorData",
"metadata": {
"self": "https://pkc-j581r8.us-west2.gcp.confluent.cloud/kafka/v3/clusters/lkc-ok51xj/links/link-1/mirrors/topic-2"
},
"link_name": "link-1",
"mirror_topic_name": "topic-2",
"source_topic_name": "topic-2",
"num_partitions": 6,
"mirror_lags": [
{
"partition": 5,
"lag": 0,
"last_source_fetch_offset": -1
},
{
"partition": 4,
"lag": 0,
"last_source_fetch_offset": -1
},
{
"partition": 3,
"lag": 0,
"last_source_fetch_offset": -1
},
{
"partition": 2,
"lag": 0,
"last_source_fetch_offset": -1
},
{
"partition": 1,
"lag": 0,
"last_source_fetch_offset": -1
},
{
"partition": 0,
"lag": 0,
"last_source_fetch_offset": -1
}
],
"mirror_status": "PENDING_STOPPED",
"mirror_topic_error": "NO_ERROR",
"state_time_ms": 1706820222493,
"mirror_state_transition_errors": [
{
"error_code": "AUTHENTICATION_ERROR",
"error_message": "Failed to describe topic configs due to authentication issues."
}
]
}
Mirror topic deletion¶
You may safely delete a mirror topic. Deleting a mirror topic permanently stops data mirroring to that topic. If you create a new normal topic of the same name on the same cluster, data will not be mirrored to it.
To delete a mirror topic, use the same command you would use to delete a normal topic:
confluent kafka topic delete <topic-name>
To learn more, see Delete topics that were auto-created.
Important
- When deleting a cluster link, first check that all mirror topics are in the
STOPPED
state. If any are in thePENDING_STOPPED
state, deleting a cluster link can cause irrecoverable errors on those mirror topics due to a temporary limitation. - You cannot delete a cluster link that is attached to any mirror topics. You must first delete, failover, or promote all of the mirror topics, and then you can delete the cluster link.
Source topic deletion¶
Caution
Do not delete a source topic that is being mirrored by a mirror topic. Doing so can lead to unpredictable truncation and data loss on the mirror topic. You should always stop mirroring to all associated mirror topics before deleting a source topic.
While it may be possible to delete a source topic that is being mirrored by a mirror topic and a cluster link, doing so is not recommended. In particular, unpredictable behavior can occur if a source topic is deleted, and a topic by the same name is then created within a few minutes time. This scenario can cause permanent data loss on any mirror topics that are still mirroring from that source topic, and can also cause performance issues on the source cluster or destination cluster.
Before deleting a source topic, you should stop any mirroring to associated mirror topics. You can stop mirroring on a mirror topic in one of these ways:
- Delete the mirror topic.
- Call
promote
orfailover
so the mirror topic enters theSTOPPED
state. - Revoke the security permissions for the cluster link to read the source topic.
You can do this in one of three ways: (1) delete the cluster link’s
ALLOW
ACL for the source topic, (2) create aDENY
ACL for the source topic, or (3) delete the cluster link’s API key.
For further discussion about Kafka limitations with topic deletion and how topic IDs will help, see KIP-516.
How schemas work with mirror topics¶
Cluster Linking preserves the schema ID stored in each message. Therefore, to consume from a mirror topic that is using schemas, the consumer clients must use a Schema Registry context with the same schema IDs as on the Schema Registry context used by the producers to the source topic. Consequently, consuming from a mirror topic that uses schemas should be done either by:
- (Option 1) using the same Schema Registry as the producers used
- (Option 2) using a Schema Registry context that was synced through Schema Linking from the Schema Registry that the producers used
Caution
When using Schema Linking: To use a mirror topic that has a schema with Confluent Cloud Connect, ksqlDB, broker-side schema ID validation, or the topic viewer, make sure that Schema Linking puts the schema in the default context of the Confluent Cloud Schema Registry. These fully-managed Confluent Cloud features require schemas to be in the default context of the Confluent Cloud Schema Registry in their Environment.
![../../_images/mirror-topics-and-schemas.png](../../_images/mirror-topics-and-schemas.png)
Mirror Topics and Schemas¶
Advanced mirror topic architectures¶
Chaining¶
A mirror topic can be a source topic itself. A mirror topic can be mirrored by different cluster links, allowing you to “chain” cluster links and mirror topics together.
For example, Topic A (source topic) on Cluster 1 —cluster link—> Topic A (mirror topic and source topic) on Cluster 2 —cluster link—> Topic A (mirror topic) on Cluster 3
Tip
You can safely create these chained topics and cluster links without creating a circular dependency between mirror topics and cluster links. You can create mirror topics without the fear of creating an infinite loop.
![../../_images/cluster-link-chain.png](../../_images/cluster-link-chain.png)
Chaining Example¶
Fanning out¶
A source topic can be mirrored to multiple mirror topics. These mirror topics must exist on multiple different clusters.
For example, Topic A on Cluster 1 —cluster link—> Topic A on Cluster 4, and Topic A on Cluster 1 —cluster link—> Topic A on Cluster 5
Tip
If you plan to use failover
or promote
on a cluster link (for example, for Disaster Recovery or Migration), then chained or
fanned-out mirror topics will not automatically retain their shape. For example, if you fan out A –> B and A –> C, if A has an outage
and you call failover on B, there is no way to automatically mirror B –> C. You will need to reconstruct the appropriate mirroring
relationship for your use case using brand new topics.
![../../_images/cluster-link-fan-out.png](../../_images/cluster-link-fan-out.png)
Fan-Out Example¶
Configurations¶
The following sections provide a quick reference of which Cluster Linking configurations are synced from the source to the mirror topic, overrides, and concepts related to syncing. For a full configuration reference, see Configure Cluster Linking on Confluent Platform.
Synced mirror topic configurations for Confluent Platform¶
These configurations are always synced from the source topic to the mirror topic. Mirror topics will always have the same value as their source topic, in order to ensure the properties of mirror topics are met.
- number of partitions
max.message.bytes
cleanup.policy
message.timestamp.type
message.timestamp.difference.max.ms
By default, the following configurations also are synced from the source topic to the mirror topic
unless they are explicitly removed from topic.config.sync.include
, as described in the following section.
retention.bytes
retention.ms
delete.retention.ms
min.compaction.lag.ms
max.compaction.lag.ms
Setting retention configurations to always sync keeps the source and destination data identical. With this default configuration, the starting offset is also synced from source to mirror topics. By maintaining consistent log start offsets, Cluster Linking guarantees that records deleted from the source cluster are also deleted from the destination cluster. This may be a regulatory requirement for some customers.
Override default syncing to specify independent mirror topic behavior¶
Some use cases require independent retention for source and destination topics. For example, when mirroring data from small edge clusters to large centralized clusters, low-footprint edge clusters may use very small retention, but rely on the data being available for a long time on the destination cluster.
To satisfy these cases, you can override the defaults by explicitly setting the following property to specify only those topic configs you want synced from source to destination:
topic.config.sync.include
- The list of topic configs to sync from the source topic.
For example, the topic configurations could be set to the following (which does not include the retention properties):
topic.config.sync.include=max.message.bytes,cleanup.policy,message.timestamp.type,message.timestamp.difference.max.ms,min.compaction.lag.ms,max.compaction.lag.ms
Use kafka-configs
to dynamically override the configuration of an existing topic,
or create new mirror topics and specify an override using --config
as described in
Creating a Mirror Topic
and confluent kafka mirror create.
With these overrides in place, mirror topics will have independent retention periods and starting offsets instead of syncing with their source topics.
Important
Keep in mind, configuration overrides are specified at the cluster link level, so will apply to all mirror topics on the cluster.
If independent retention is specified (by omission in topic.config.sync.include
), you must either specify the retention value or use the Kafka defaults.
Overridable mirror topic configurations¶
These configurations are configurable in Confluent Platform, meaning you can override the source configurations for these on the mirror topic.
segment.jitter.ms
segment.index.bytes
flush.messages
flush.ms
index.interval.bytes
min.cleanable.dirty.ratio
file.delete.delay.ms
preallocatemessage.format.version
confluent.segment.speculative.prefetch.enable
compression.type
Mirror topic configurations not synced for Confluent Platform¶
Any configuration that is not in the list above will not be synced to a mirror topic in Confluent Platform. Therefore, the mirror topic’s configuration could be different from the source topic’s configuration. If you don’t override the mirror topic’s configuration, then it will inherit its cluster’s default.
A few important examples of configurations that are not synced to mirror topics in Confluent Platform:
min.insync.replicas
confluent.placement.constraints
confluent.tier.enable
confluent.key.schema.validation
confluent.value.schema.validation
replication.factor
Tip
No replication factors are synced to mirror topics. Mirror topics use the
default.replication.factor
configured on the brokers. If not explicitly set, this defaults to a replication factor of1
, and consequently the mirror topics will pick up a replication factor of1
. For a description of this option, seedefault.replication.factor
in Kafka Broker and Controller Configuration Reference for Confluent Platform.
Hybrid cloud configuration syncs¶
Confluent Platform and Confluent Cloud have different policies for which mirror topic configurations are synced. If you cluster link between Confluent Platform and Confluent Cloud, then the destination cluster’s policy is enforced.
For example, if you cluster link from a Confluent Platform source cluster to a Confluent Cloud destination cluster,
then the value of compression.type
will not be synced. But if you cluster link from a Confluent Cloud source cluster
to a Confluent Platform destination cluster, then compression.type
will be synced.