Status
Current state: Discussion
Discussion thread: here
JIRA: - KAFKA-4682Getting issue details... STATUS
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Motivation
The offset of a topic partition within a consumer group expires when the expiration timestamp associated with that partition is reached. This expiration timestamp is usually affected by the broker config offsets.retention.minutes
, unless user overrides that default and uses a custom retention. This is an overview of how offset expiration works today with respect to different versions of the OffsetCommit
protocol:
- Version 0: Offsets are stored in ZooKeeper. ZooKeeper based offset storage is not in scope of this KIP.
- Version 1: An optional commit timestamp is associated with each topic partition in the request. The broker’s
offsets.retention.minutes
is added to the commit timestamp to determine the expiration timestamp of the partition. In this case, clients cannot override the default retention enforced by the broker. - Version 2, 3: Similar to Version 1 except there is no explicit commit timestamp for each partition.The
retention_time
field in the request replaces the value of broker’s offset retention config in calculating the expiration timestamp.
The following table summarizes this:
Version of OffsetCommit | Commit Timestamp | Offset Retention | Expiration Timestamp |
---|---|---|---|
0 | [ZooKeeper based offset management - out of scope] | ||
1 - no explicit commit timestamp | Current timestamp | Broker's offsets.retention.minutes | Commit Timestamp + Offset Retention |
1 - with explicit commit timestamp | Partition-specific timestamp in the request | Broker’s offsets.retention.minutes | |
2, 3 | Current timestamp | Request’s retention_time |
For versions 1-3, once the expiration timestamp is reached, the offset is removed from the offset cache (during the next cleanup) regardless of the group state. KAFKA-4682 reports an issue related to this offset expiration, where committed offsets are removed even when there are still active, but rarely committing, consumers in the (Stable
) group.
In other words, if the corresponding retention period or longer has passed since an active consumer has committed offset for a topic partition, that committed offset will be removed from the consumer group metadata. If then there is a rebalance or the consumer restarts the last committed offset for that topic partition will not be found, and the consumer is forced to start from the start or end of the log (depending on the value of auto.offset.reset
configuration) leading to potential duplicate consumption or missing records. This situation can be avoided if the offsets are preserved beyond their expiration timestamp if the group is still in a Stable
state.
There are workarounds to this issue and some of them are described in KAFKA-4682, but they come with their own limitations and drawbacks, as discussed in that JIRA.
Public Interfaces
This is the current OffsetCommit
protocol:
OffsetCommit Request (Version: 3) => group_id group_generation_id member_id retention_time [topics] group_id => STRING group_generation_id => INT32 member_id => STRING retention_time => INT64 topics => topic [partitions] topic => STRING partitions => partition offset metadata partition => INT32 offset => INT64 metadata => NULLABLE_STRING OffsetCommit Response (Version: 3) => throttle_time_ms [responses] throttle_time_ms => INT32 responses => topic [partition_responses] topic => STRING partition_responses => partition error_code partition => INT32 error_code => INT16
The only change made to this protocol is dropping the field retention_time
from the request. Retention time will be enforced through the broker config offsets.retention.minutes
in the new version of the protocol and normally takes effect once the group transitions into Empty
state.
OffsetCommit Request (Version: 4) => group_id group_generation_id member_id [topics] group_id => STRING group_generation_id => INT32 member_id => STRING topics => topic [partitions] topic => STRING partitions => partition offset metadata partition => INT32 offset => INT64 metadata => NULLABLE_STRING OffsetCommit Response (Version: 4) => throttle_time_ms [responses] throttle_time_ms => INT32 responses => topic [partition_responses] topic => STRING partition_responses => partition error_code partition => INT32 error_code => INT16
Proposed Changes
A more viable solution for KAFKA-4682 can be achieved by changing how group offset expiration works: preserve committed offsets as long as the group is active (has consumers). The expiration timer should start ticking the moment all group members are gone and the group transitions into Empty
state. This expiration semantics implies that there is no longer a need to enforce individual offset retention times and keep individual expiration timestamps for each topic partition in the group. This is because all committed offsets in the group will expire at the same time.
This proposed change has an impact on the existing offset commit value schema. There is an expire_timestamp
field in this schema that, as a result of expiring all group offsets at the same time, would become redundant (as it would repeat the same value for each offset in the group).
Offset Commit Value Schema (Version: 1) => offset => Long metadata => String commit_timestamp => Long expire_timestamp => Long
The proposal is to create a new version of this schema and drop the expire_timestamp
field:
Offset Commit Value Schema (Version: 2) => offset => Long metadata => String commit_timestamp => Long
To make up for the per-offset expiration timestamp we lose in the new version of offset commit value schema, a new field in the group metadata value schema that indicates when the group last transitioned to Empty
state, that defaults to Long.MaxValue
.
Group Metadata Value Schema (Version: 1) => protocol_type => String generation => Int protocol => String leader => String members => [member] ...
Group Metadata Value Schema (Version: 2) => protocol_type => String generation => Int protocol => String leader => String empty_state_timestamp => Long members => [member] ...
The rest of this section explains how these suggested changes help in implementing the new group expiration semantics.
Transitioning to Empty
State
The expiration time of offsets in a group will be when the group becomes Empty
plus retention time of offsets.retention.minutes
(assuming during that time the group does not become active again). Whenever the group transitions to Empty
state, empty_state_timestamp
resets to the value of current timestamp. Then, during any scheduled offset cleanup task, if "current timestamp" minus empty_state_timestamp
is greater than or equal to broker's offsets.retention.minutes
for any group, all offsets in that group will be removed and the group will transition to Dead
state.
Note that consumers may rejoin the group while the group is in Empty
state. As soon as that happens and the group state changes, empty_state_timestamp
resets to the default Long.MaxValue
which practically disables the expiration timer. This is a breakdown of group states and how the offsets expiration works in those states:
Stable
: Group offsets will not expire in this state. The fieldempty_state_timestamp
isLong.MaxValue
.PreparingRebalance
: Group offsets will not expire in this state. The fieldempty_state_timestamp
isLong.MaxValue
.CompletingRebalance
: Group offsets will not expire in this state. The fieldempty_state_timestamp
isLong.MaxValue
.Empty
: The fieldempty_state_timestamp
is set to when group last transitioned to this state. If the group stays in this foroffsets.retention.minutes
, the following offset cleanup scheduled task will remove all offsets in the group (as explained above).Dead
: Group offsets have already expired in this state.
The default retention time for group offsets can be customized through the existing offsets.retention.minutes
broker configuration. If, in the future, a need arises for enforcing a per group retention configuration, it can be implemented via a separate KIP.
There is also a particular corner case that needs to be addressed with this new semantics:
If a group consumer unsubscribes from a topic but continues to consume from other subscribed topics, the offset information of that unsubscribed topic’s partitions should be deleted at the appropriate time.
Unsubscribing from a Topic
If the group state is not Empty
, when there is a change in subscribed topics of a group consumer, and, as a result, the group stops consuming from a topic, the associated offsets for that topic should go through the expiry process – to avoid unnecessary expansion of the offset cache.
Unfortunately, there is no notification mechanism in place for member subscription change within a group. Therefore, a poll mechanism can be implemented to run at specific intervals and check whether group subscription has deviated from what is stored in the cache. One place to do this is the repeating offset cleanup scheduled jobs, which by default run every 10 minutes, making them a good choice as the group subscription check will not be executed very frequently. At every execution of this job we collect a list of all topic partitions the group is consuming from (this can be calculated based on the the data in each group member’s metadata), and cross reference it with the stored offsets for the group. If there are partitions the group has offset for but no longer consumes from, and offsets.retention.minutes
has passed since their last commit timestamp, the corresponding offsets will be removed from the offset cache.
The following table summarize how the new offset expiration semantics would be implemented.
Group State | Additional Check in Offset Cleanup Job | Action if Check Holds |
---|---|---|
= Empty | current timestamp - |
|
≠ Empty | (Non-subscribed partitions = partitions group has offset for - partitions the group is consuming from) ∀partition ∈ non-subscribed partitions:
| Remove offset of partition |
Compatibility, Deprecation, and Migration Plan
The new protocol does not allow clients to customize the retention time of specific offsets in the group. The old consumers, however, could still commit offsets with a customized retention time. Such old consumers will continue to be supported:
- If a consumer uses the old API without customizing the retention time, the new approach will be applied; i.e., the broker’s config will be used as the retention time of its offsets once it becomes Empty. The same retention will be used for offsets of partitions the group no longer consumes from (or is subscribed to).
- If a consumer uses the old API with a customized retention time, the provided retention time will become the retention time of the offsets in question from the offset commit timestamp (this fully matches the current behavior). In this scenario, version 1 of offset commit value schema (with the
expire_timestamp
field) will be used.
Rejected Alternatives
- Removing the partition-level expire_timestamp from the offset schema and making all group offsets expire at the same time. Even though this is a good solution for when the group becomes Empty, it fails to address the scenario where the group stops consuming from a particular partition and causes those offsets to remain while the group exists, which leads to unnecessary expansion of the group metadata cache.