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https://issues.apache.org/jira/browse/RATIS-2350?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=18048481#comment-18048481
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Tsz-wo Sze commented on RATIS-2350:
-----------------------------------
Sorry that I may have misunderstood the following sentence from the [Amazon
blog|https://aws.amazon.com/blogs/aws/amazon-s3-update-strong-read-after-write-consistency/]
- "... What you write is what you will read ..."
{quote}... "After a successful write of a new object, ... any subsequent read
request immediately receives the latest version of the object."
{quote}
As you mentioned, this is just strong consistency, i.e. it won't read stale
data. Ratis is strong consistency.
{quote}"read-after-write" can mean strong consistency or "read-your-own-write"
consistency.
{quote}
The read-after-write consistency feature in Ratis is for async requests – a
client could send an async write and then an async read. The read could be
faster than the write and get the result before the write. Read-after-write
consistency is to make the read waits for write to complete.
Suppose there are two related clients.
# Initially, x=0
# Client 1 async writes x=1
# Client 1 tells Client 2 to read x
# Client 2 reads x
Then, Client 2 could get x=0 or x=1. The Ratis read-after-write consistency
feature won't work in this case.
> Fix readAfterWrite bugs
> -----------------------
>
> Key: RATIS-2350
> URL: https://issues.apache.org/jira/browse/RATIS-2350
> Project: Ratis
> Issue Type: Bug
> Components: server
> Reporter: Tsz-wo Sze
> Assignee: Tsz-wo Sze
> Priority: Major
> Fix For: 3.3.0
>
> Attachments: screenshot-1.png, screenshot-2.png
>
> Time Spent: 1.5h
> Remaining Estimate: 0h
>
> There are bugs in handling readAfterWrite requests:
> # In LeaderStateImpl.getReadIndex(..), it should use the max of
> readAfterWriteConsistentIndex and commitIndex.
> # In WriteIndexCache.add(..), it should combine the current future with
> previous future when the previous future exists.
> Improvement:
> - Add lastAppliedIndex to ReadIndexQueue
> - Replace Consumer<Long> with LongConsumer
> Bug in tests:
> - In LinearizableReadTests.runTestReadAfterWrite(..), it tries to assert the
> following:
> {quote}Assertion: _read-after-write is more consistent than linearizable read_
> {quote}
> Recall the definitions:
> {quote}Read-after-write: _Within the same client, the read called after write
> must able to see the change of the write._
> {quote}
> {quote}Linearizable read: _The read is linearizable (i.e. it won't read stale
> data)._
> {quote}
> Suppose readIndex is 9 and writeIndex is 10. By definition, read-after-write
> must return any state at log index A >= 10 while linearizable read must
> return any state at log index L >= 9. The assertion incorrectly check if A >=
> L, which is not a requirement. It is perfectly fine, for example, if A=11 <
> L=12.
> ----
> Original Summary: TestReadOnlyRequestWithGrpc may fail intermittently
> Original Description:
> {code:java}
> org.apache.ratis.grpc.TestReadOnlyRequestWithGrpc.testReadAfterWrite -- Time
> elapsed: 1.572 s <<< FAILURE!
> org.opentest4j.AssertionFailedError: expected: <true> but was: <false>
> at
> org.apache.ratis.ReadOnlyRequestTests.testReadAfterWriteImpl(ReadOnlyRequestTests.java:314)
> at
> org.apache.ratis.server.impl.MiniRaftCluster$Factory$Get.runWithNewCluster(MiniRaftCluster.java:143)
> at
> org.apache.ratis.server.impl.MiniRaftCluster$Factory$Get.runWithNewCluster(MiniRaftCluster.java:121)
> at
> org.apache.ratis.ReadOnlyRequestTests.testReadAfterWrite(ReadOnlyRequestTests.java:289)
> {code}
> It failed 8 in [this 10x10
> run|https://github.com/apache/ratis/actions/runs/19023405871/job/54322726144].
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