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Fix testCDCIndexFileWriteOnSync and document cdc index file read edge…
… case

Patch by Yifan Cai; reviewed by Josh McKenzie for CASSANDRA-17416
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yifan-c committed May 6, 2022
1 parent acb5337 commit c48906394e01460382f4070ecc34f6f9754fc567
Showing 2 changed files with 30 additions and 6 deletions.
@@ -16,12 +16,17 @@ persisted to disk. Upon final segment flush, a second line with the
human-readable word "COMPLETED" will be added to the _cdc.idx file
indicating that Cassandra has completed all processing on the file.

We we use an index file rather than just encouraging clients to parse
We use an index file rather than just encouraging clients to parse
the log realtime off a memory mapped handle as data can be reflected in
a kernel buffer that is not yet persisted to disk. Parsing only up to
the listed offset in the _cdc.idx file will ensure that you only parse
CDC data for data that is durable.

Please note that in rare chances, e.g. slow disk, it is possible for the
consumer to read an empty value from the _cdc.idx file because update is
achieved with first truncating the file then write to the file. In such
case, the consumer should retry read the index file.

A threshold of total disk space allowed is specified in the yaml at
which time newly allocated CommitLogSegments will not allow CDC data
until a consumer parses and removes files from the specified cdc_raw
@@ -24,7 +24,9 @@
import java.nio.file.Files;
import java.nio.file.Path;
import java.util.*;
import java.util.concurrent.TimeUnit;

import org.junit.Assert;
@@ -157,11 +159,28 @@ public void testCDCIndexFileWriteOnSync() throws IOException
Assert.assertTrue("Index file not written: " + cdcIndexFile, cdcIndexFile.exists());

// Read index value and confirm it's == end from last sync
BufferedReader in = new BufferedReader(new FileReader(cdcIndexFile));
String input = in.readLine();
Integer offset = Integer.parseInt(input);
Assert.assertEquals(syncOffset, (long)offset);
String input = null;
// There could be a race between index file update (truncate & write) and read. See CASSANDRA-17416
// It is possible to read an empty line. In this case, re-try at most 5 times.
for (int i = 0; input == null && i < 5; i++)
if (i != 0) // add a little pause between each attempt
Uninterruptibles.sleepUninterruptibly(10, TimeUnit.MILLISECONDS);

try (BufferedReader in = new BufferedReader(new FileReader(cdcIndexFile)))
input = in.readLine();

if (input == null)
{"Unable to read the CDC index file after several attempts");

int indexOffset = Integer.parseInt(input);
Assert.assertTrue("The offset read from CDC index file should be equal or larger than the offset after sync. See CASSANDRA-17416",
syncOffset <= indexOffset);


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