| Table of Contents |
|---|
Status
Current state: AcceptedUnder Discussion
Discussion thread: here
JIRA: KAFKA-3163
...
- The log retention will be changed to base on the time index of a log segment instead of basing on the last modification time of the log segment file.
- The time based log rolling will have the following change: The log segment will roll out when log.roll.ms has elapsed since the largest timestamp of the messages in the log segment.
This KIP will indroduce introduce another index file for each log segment. So the number of open file handlers will increased increase by about 1/3.
Proposed Changes
...
Create another index file for each log segment with name SegmentBaseOffset.timeindex. The density of the index is defined upper bounded by index.interval.bytes configuration.
...
| Code Block | ||
|---|---|---|
| ||
Time Index Entry => Timestamp Offset Timestamp => int64 Offset => int32 |
- Timestamp - the biggest timestamp seen so far in this segment.
- Offset - the next offset when the time index entry is inserted.
- A time index entry (timestampT, offset) means that in this segment any message whose timestamp is greater than timestamp T come after offset.
Build the time index
...
- When a new log segment is created, the broker will create a time index file for the log segment.
- The default initial / max size of the time index files is the same as the offset index files. (time index entry is 1.5x of the size of offset index entry, user should set the configuration accordingly).
- Each log segment maintains the largest timestamp so far in that segment. The initial value of the largest timestamp is -1 for a newly created segment.
- When broker receives a message, if the message is not rejected due to timestamp exceeds threshold, the message will be appended to the log. (The timestamp will either be LogAppendTime or CreateTime depending on the configuration)
- When broker appends the message to the log segment, if an offset index entry is inserted, it will also insert a time index entry if the max timestamp so far is greater than the timestamp in the last time index entry.
- For message format v0, the timestamp is always -1, so no time index entry will be inserted when message is appended.
- When the current active segment is rolled out or closed. A time index entry will be inserted into the time index to ensure the last time index entry has the largest timestamp of the log segment.
- If largest timestamp in the segment is non-negative (at least one message has a timestamp), the entry will be (largest_timestamp_in_the_segment -> base_offset_of_the_next_segment)
- If largest timestamp in the segment is -1 (No message in the segment has a timestamp), the entry will be (last_modification_time_of_the_segment -> base_offset_of_the_next_segment) time index will be empty and the largest timestamp would be default to the segment last modification time.
The time index is not globally monotonically increasing for the segments of a partition. Instead, it is only monotonically increasing within each individual time index file. i.e. It is possible that the time index file for a later log segment contains smaller timestamp than some timestamp in the time index file of an earlier segment.
...
When the log is truncated, because the offset in the time index is also monotonically increasing, we will also truncate the time index entries whose corresponding messages have been truncated. The active segment will reload the largest timestamp after truncation just like it did during startupby scanning the messages from the offset of the last time index entry till the log end.
Enforce time based log retention
To enforce time based log retention, the broker will check from the oldest segment forward to the latest segment. For each segment, the broker checks the last time index entry of a log segment. The timestamp will be the latest timestamp of the messages in the log segment. So if that timestamp expires, the broker will delete the log segment. The broker will stop at the first segment which is not expired. i.e. the broker will not expire a segment even if it is expired, unless all the older segment has been expired.
Enforce time based log rollingEnforce time based log rolling
Currently time based log rolling is based on the creating time of the log segment. With this KIP, the time based rolling would be changed to only based on the largest timestamp ever seen in a log segment. message timestamp. More specifically, if the first message in the log segment has a timestamp, A new log segment will be rolled out if current time timestamp in the message about to be appended is greater than largest timestamp ever seen the timestamp of the first message in the log segment + log.roll.ms. When message.timestamp.type=CreateTime, user should set max.message.time.difference.ms appropriately together with log.roll.ms to avoid frequent log segment roll out.
During the migration phase, if the first message in a segment does not have a timestamp, the log rolling will still be based on the (current time - create time of the segment).
Search message by timestamp
...
Use case discussion
| Use case | Goal | Solution with Index based on LogAppendTime indexSolution with | Index based on CreateTime index | Comparison | |
|---|---|---|---|---|---|
| 1 | Search by timestamp | Not lose messages | If user want to search for a message with CreateTime CT. They can use CT to search in the LogAppendTime index. Because LogAppendTime > CT for the same message (assuming no skew clock). If the clock is skewed, people can search with CT - X where X is the max skew. If user want to search for a message with LogAppendTime LAT, they can just search with LAT and get a millisecond accuracy. | User can just search with CT and get a minute level granularity precise offset. | If the latency in the pipeline is greater than one minute, user might consume less message by using CreateTime index. Otherwise, LogAppendTime index is probably preferredUser may see duplicates when searching by CreateTime. Consider the following case:
If user want to search with CT after they consumed m2, they will have to reconsume from m1. Depending on how big LAT2 - LAT1 is, the amount of messages to be reconsumed can be very big. |
| 2 | Search by timestamp (bootstrap) |
| In bootstrap case, all the LAT would be close. For example If user want to process the data in last 3 days and did the following:
In this case, LogAppendTime index does not help too much. That means user needs to filter out the data older than 3 days before dumping them into Kafka. | In bootstrap case, the CreateTime will not change, if user follow the same procedure started in LogAppendTime index section. Searching by timestamp will work. | LogAppendTime index needs further attention from user. |
| 3 | Failover from cluster 1 to cluster 2 |
| Similar search by timestamp. User can choose to use CT or LAT of cluster 1 to search on cluster 2. In this case, searching with CT - MaxLatencyOfCluster will provide strong guarantee on not losing messages, but might have some duplicates depending on the difference in latency between cluster 1 and cluster 2. | User can use CT to search and get minute level granularityprecise offset. Duplicates are still not avoidable. There can be some tricky cases here. Consider the following case [1]:
In this case, m1 is created before m2. Due to latency difference, m1 arrives cluster 1 then m2 does, m2 arrives cluster 2 before m1 does. If a consumer consumed m2 in cluster 2 and fail over to cluster 1, simply search by CT2 will miss m1 because m1 has larger offset than m2 in cluster 2 but smaller offset than m2 in cluster 1. So the same trick or CT - MaxLatencyOfCluster is still needed. | In cross cluster fail over case, both solution can provide strong guarantee of not losing messages. But both needs to depend on the knowledge of MaxLatencyOfCluster. |
| 4 | Get lag for consumers by time | Know how long a consumer is lagging by time. | With LogAppendTime in the message, consumer can easily find out the lag by time and estimate how long it might need to reach the log end. | Not supported. | |
| 5 | Broker side latency metric | Let the broker to report latency of each topic. i.e. LAT - CT | The latency can be reported as LAT - CT. | The latency can be reported as System.currentTimeMillis - CT | The two solutions are the same. This latency information can be used for MaxLatencyOfCluster in use case 3. |
...
The change is backward compatible after KIP-31 and KIP-32 are checked in.
User may want to bump up the log.index.size.max.bytes to 1.5x because the time index may take up to 1.5x of the offset index. We will change the index size to appropriate value (3 MB for 1GB segment, 4KB index interval).
Broker will keep an Broker will keep keep an in-memory maxTimestampSoFar variable, which is initialized to -1 and only gets updated when a message with a larger timestamp is appended to the log segment.
If maxTimestampSoFar is -1, log retention will still be based on last_modification_time. And log rolling will still be based on log create time.
Broker will do the followings following during migration for log retention and searching by timestamp::
- The broker will create a time index for each segment if the segment does not have one.
- For the inactive log segments, the broker will append an entry (last_modification_time_of_the_segment -> offset_of_the_first_message_in_the_segment) to each empty time index.
- For the active log segments, the time index file will be left empty.
- The broker will not rebuild the time index unless the broker had a hard failure during the previous shutdown.
After the entire cluster is migrated to use time based log index for log retention,
- The . The broker will enforce log retention using time index. Given what we do in step 1, the behavior is:
- For segments only having messages whose versions are before 0.10.0, the entry with last modification time in the time index will be used for retention.
- For segments having at least one message with version 0.10.0, the max timestamp of the messages will be used for log retention.
...
- The broker will
...
- enforce the
...
- log rolling
...
- time when there is at least one message with format version 0.10.0 inserted, otherwise the log rolling is still based on segment create time.
- Searching by timestamp will behave the same as before because if there is no message with timestamp, the time index will only contain one entry with last_modification_time. If there are messages with timestamp inserted, those messages will be indexed for search
- On startup, broker will initially use the segment last modification time as the largest message timestamp.
- If a new message whose version is after 0.10.0 and its timestamp is greater than current max message timestamp. The broker updates the current largest message timestamp.
- Broker always use the difference between current time and largest timestamp of the segment to decide whether roll out a new log segment or not.
Rejected Alternatives
Add a timestamp field to log index entry
...