PutIfExists fails if a record has been recently added - scalardb

Question

In ScalarDB, the library that add ACID functionality to Cassandra, I am getting the following error
2020-09-24 18:51:33,607 [WARN] from com.scalar.db.transaction.consensuscommit.CommitHandler in ScalaTest-run-running-AllRepositorySpecs - preparing records failed
com.scalar.db.exception.storage.NoMutationException: no mutation was applied.
I am running a test case in which I get a record to check that it doesn't exist, then I add the record, then I fetch it to see it was successfully added, then I update it and then I get it again to see that the value was updated.
"update an answer if the answer exists" in {
beforeEach()
embeddedCassandraManager.executeStatements(cqlStartupStatements)
val cassandraConnectionService = CassandraConnectionManagementService()
val (cassandraSession, cluster) = cassandraConnectionService.connectWithCassandra("cassandra://localhost:9042/codingjedi", "codingJediCluster")
//TODOM - pick the database and keyspace names from config file.
cassandraConnectionService.initKeySpace(cassandraSession.get, "codingjedi")
val transactionService = cassandraConnectionService.connectWithCassandraWithTransactionSupport("localhost", "9042", "codingJediCluster" /*,dbUsername,dbPassword*/)
val repository = new AnswersTransactionRepository("codingjedi", "answer_by_user_id_and_question_id")
val answerKey = AnswerKeys(repoTestEnv.answerTestEnv.answerOfAPracticeQuestion.answer_id.get,
repoTestEnv.answerTestEnv.answerOfAPracticeQuestion.question_id,
Some(repoTestEnv.answerTestEnv.answerOfAPracticeQuestion.answer_id.get))
logger.trace(s"checking if answer already exists")
val distributedTransactionBefore = transactionService.get.start()
val resultBefore = repository.get(distributedTransactionBefore, answerKey) //answer should not exist
distributedTransactionBefore.commit()
resultBefore.isLeft mustBe true
resultBefore.left.get.isInstanceOf[AnswerNotFoundException] mustBe true
logger.trace(s"no answer found. adding answer")
val distributedTransactionDuring = transactionService.get.start()
repository.add(distributedTransactionDuring, repoTestEnv.answerTestEnv.answerOfAPracticeQuestion)//add answer
distributedTransactionDuring.commit()
logger.trace(s"answer added")
val distributedTransactionAfter = transactionService.get.start()
val result = repository.get(distributedTransactionAfter, answerKey) //now answer should exist
distributedTransactionAfter.commit()
result mustBe (Right(repoTestEnv.answerTestEnv.answerOfAPracticeQuestion))
logger.trace(s"got answer from repo ${result}")
val updatedNotes = if(repoTestEnv.answerTestEnv.answerOfAPracticeQuestion.notes.isDefined)
Some(repoTestEnv.answerTestEnv.answerOfAPracticeQuestion.notes.get+"updated") else Some("updated notes")
val updatedAnswer = repoTestEnv.answerTestEnv.answerOfAPracticeQuestion.copy(notes=updatedNotes) //updated answer
logger.trace(s"old notes ${repoTestEnv.answerTestEnv.answerOfAPracticeQuestion.notes} vs new notes ${updatedNotes}")
logger.trace(s"updated answer ${updatedAnswer}")
val distributedTransactionForUpdate = transactionService.get.start()
val resultOfupdate = repository.update(distributedTransactionForUpdate,updatedAnswer) //update answer
distributedTransactionForUpdate.commit() //fails here
logger.trace(s"update done. getting answer again")
val distributedTransactionAfterUpdate = transactionService.get.start()
val resultAfterUpdate = repository.get(distributedTransactionAfterUpdate, answerKey)
distributedTransactionForUpdate.commit()
resultAfterUpdate mustBe (Right(updatedAnswer))
logger.trace(s"got result after update ${resultAfterUpdate}")
afterEach()
}
The update method calls add with putIfExists condition
def update(transaction:DistributedTransaction, answer:AnswerOfAPracticeQuestion) = {
logger.trace(s"updating answer value ${answer}")
//checktest-update an answer if the answer exists
add(transaction,answer, new PutIfExists)
}
def add(transaction:DistributedTransaction,answer:AnswerOfAPracticeQuestion,mutationCondition:MutationCondition = new PutIfNotExists()) = {
logger.trace(s"adding answer ${answer} with mutation state ${mutationCondition}")
val pAnswerKey = new Key(new TextValue("answered_by_user", answer.answeredBy.get.answerer_id.toString),
new TextValue("question_id",answer.question_id.toString))
//to check duplication, both partition and clustering keys need to be present
//val cAnswerKey = new Key(new TextValue("answer_id",answer.answer_id.toString))
//logger.trace(s"created keys. ${pAnswerKey}, ${cAnswerKey}")
val imageData = answer.image.map(imageList=>imageList).getOrElse(List())
logger.trace(s"will check in ${keyspaceName},${tablename}")
val putAnswer: Put = new Put(pAnswerKey/*,cAnswerKey*/)
.forNamespace(keyspaceName)
.forTable(tablename)
.withCondition(mutationCondition)
.withValue(new TextValue("answer_id", answer.answer_id.get.toString))
.withValue(new TextValue("image", convertImageToString(imageData)))
.withValue(new TextValue("answer", convertAnswersFromModelToString(answer.answer)))
.withValue(new BigIntValue("creation_year", answer.creationYear.getOrElse(0)))
.withValue(new BigIntValue("creation_month", answer.creationMonth.getOrElse(0)))
.withValue(new TextValue("notes", answer.notes.getOrElse("")))
logger.trace(s"putting answer ${putAnswer}")
//checktest-add answer to respository
//checktest-not add answer to respository if duplicate
transaction.put(putAnswer)
}
Why am I getting the error even though the notes field is changed between the existing answer and the updated answer
The error trace is (note that it says IF NOT EXISTS!). Shouldn't it be IF EXISTS? There is also a trace there was a hit in the statement cache for [INSERT INTO codingjedi.answer_by_user_id_and_question_id (answered_by_user,question_id,tx_id,tx_state,tx_prepared_at,answer_id,image,answer,creation_year,creation_month,notes,tx_version) VALUES (?,?,?,?,?,?,?,?,?,?,?,?) IF NOT EXISTS;]. Does it mean that the previous put is still in cache and is that causing the conflict?
2020-09-24 18:51:33,593 [DEBUG] from com.scalar.db.storage.cassandra.StatementHandler in ScalaTest-run-running-AllRepositorySpecs - query to prepare : [INSERT INTO codingjedi.answer_by_user_id_and_question_id (answered_by_user,question_id,tx_id,tx_state,tx_prepared_at,answer_id,image,answer,creation_year,creation_month,notes,tx_version) VALUES (?,?,?,?,?,?,?,?,?,?,?,?) IF NOT EXISTS;].
2020-09-24 18:51:33,593 [DEBUG] from com.scalar.db.storage.cassandra.StatementHandler in ScalaTest-run-running-AllRepositorySpecs - there was a hit in the statement cache for [INSERT INTO codingjedi.answer_by_user_id_and_question_id (answered_by_user,question_id,tx_id,tx_state,tx_prepared_at,answer_id,image,answer,creation_year,creation_month,notes,tx_version) VALUES (?,?,?,?,?,?,?,?,?,?,?,?) IF NOT EXISTS;].
2020-09-24 18:51:33,593 [DEBUG] from com.scalar.db.storage.cassandra.ValueBinder in ScalaTest-run-running-AllRepositorySpecs - Optional[11111111-1111-1111-1111-111111111111] is bound to 0
2020-09-24 18:51:33,593 [DEBUG] from com.scalar.db.storage.cassandra.ValueBinder in ScalaTest-run-running-AllRepositorySpecs - Optional[11111111-1111-1111-1111-111111111111] is bound to 1
2020-09-24 18:51:33,593 [DEBUG] from com.scalar.db.storage.cassandra.ValueBinder in ScalaTest-run-running-AllRepositorySpecs - Optional[468492df-0960-4160-8391-27fe7fa626c5] is bound to 2
2020-09-24 18:51:33,593 [DEBUG] from com.scalar.db.storage.cassandra.ValueBinder in ScalaTest-run-running-AllRepositorySpecs - 1 is bound to 3
2020-09-24 18:51:33,593 [DEBUG] from com.scalar.db.storage.cassandra.ValueBinder in ScalaTest-run-running-AllRepositorySpecs - 1600969893592 is bound to 4
2020-09-24 18:51:33,593 [DEBUG] from com.scalar.db.storage.cassandra.ValueBinder in ScalaTest-run-running-AllRepositorySpecs - Optional[11111111-1111-1111-1111-111111111111] is bound to 5
2020-09-24 18:51:33,593 [DEBUG] from com.scalar.db.storage.cassandra.ValueBinder in ScalaTest-run-running-AllRepositorySpecs - Optional[{"image":["image1binarydata","image2binarydata"]}] is bound to 6
2020-09-24 18:51:33,593 [DEBUG] from com.scalar.db.storage.cassandra.ValueBinder in ScalaTest-run-running-AllRepositorySpecs - Optional[{"answer":[{"filename":"c.js","answer":"some answer"}]}] is bound to 7
2020-09-24 18:51:33,593 [DEBUG] from com.scalar.db.storage.cassandra.ValueBinder in ScalaTest-run-running-AllRepositorySpecs - 2019 is bound to 8
2020-09-24 18:51:33,593 [DEBUG] from com.scalar.db.storage.cassandra.ValueBinder in ScalaTest-run-running-AllRepositorySpecs - 12 is bound to 9
2020-09-24 18:51:33,593 [DEBUG] from com.scalar.db.storage.cassandra.ValueBinder in ScalaTest-run-running-AllRepositorySpecs - Optional[some notesupdated] is bound to 10
2020-09-24 18:51:33,593 [DEBUG] from com.scalar.db.storage.cassandra.ValueBinder in ScalaTest-run-running-AllRepositorySpecs - 1 is bound to 11
2020-09-24 18:51:33,607 [WARN] from com.scalar.db.transaction.consensuscommit.CommitHandler in ScalaTest-run-running-AllRepositorySpecs - preparing records failed
com.scalar.db.exception.storage.NoMutationException: no mutation was applied.
UPDATE
Traces
For the 1st put, putAnswer is
putting answer Put{namespace=Optional[codingjedi], table=Optional[answer_by_user_id_and_question_id], partitionKey=Key{TextValue{name=answered_by_user, value=Optional[11111111-1111-1111-1111-111111111111]}, TextValue{name=question_id, value=Optional[11111111-1111-1111-1111-111111111111]}}, clusteringKey=Optional.empty, values={answer_id=TextValue{na
me=answer_id, value=Optional[11111111-1111-1111-1111-111111111111]}, image=TextValue{name=image, value=Optional[{"image":["image1binarydata","image2binarydata"]}]}, answer=TextValue{name=answer, value=Optional[{"answer":[{"filename":"c.j
s","answer":"some answer"}]}]}, creation_year=BigIntValue{name=creation_year, value=2019}, creation_month=BigIntValue{name=creation_month, value=12}, notes=TextValue{name=notes, value=Optional[some notes]}}, consistency=SEQUENTIAL, condi
tion=Optional[com.scalar.db.api.PutIfNotExists#21bf308]}
For the 2nd put, putAnswer is
putting answer Put{namespace=Optional[codingjedi], table=Optional[answer_by_user_id_and_question_id], partitionKey=Key{TextValue{name=answered_by_user, value=Optional[11111111-1111-1111-1111-111111111111]}, TextValue{name=question_id, value=Optional[11111111-1111-1111-1111-111111111111]}}, clusteringKey=Optional.empty, values={answer_id=TextValue{name=answer_id, value=Optional[11111111-1111-1111-1111-111111111111]}, image=TextValue{name=image, value=Optional[{"image":["image1binarydata","image2binarydata"]}]}, answer=TextValue{name=answer, value=Optional[{"answer":[{"filename":"c.js","answer":"some answer"}]}]}, creation_year=BigIntValue{name=creation_year, value=2019}, creation_month=BigIntValue{name=creation_month, value=12}, notes=TextValue{name=notes, value=Optional[some notesupdated]}}, consistency=SEQUENTIAL, condition=Optional[com.scalar.db.api.PutIfExists#2e057637]}
notes field has changed from notes=TextValue{name=notes, value=Optional[some notes]}}, to notes=TextValue{name=notes, value=Optional[some notesupdated]}}
When the 2nd put is executed, I can see that the mutation condition used is IfNotExists
2020-09-25 12:35:34,188 [DEBUG] from com.scalar.db.storage.cassandra.Cassandra in ScalaTest-run-running-AllRepositorySpecs - executing put operation with Put{namespace=Optional[codingjedi], table=Optional[answer_by_user_id_and_question_id], partitionKey=Key{TextValue{name=answered_by_user, value=Optional[11111111-1111-1111-1111-111111111111]}, TextValue{name=question_id, value=Optional[11111111-1111-1111-1111-111111111111]}}, clusteringKey=Optional.empty, values={tx_id=TextValue{name=tx_id, value=Optional[c6bc39e9-656a-440c-8f68-af6005f37f7c]}, tx_state=IntValue{name=tx_state, value=1}, tx_prepared_at=BigIntValue{name=tx_prepared_at, value=1601033734188}, answer_id=TextValue{name=answer_id, value=Optional[11111111-1111-1111-1111-111111111111]}, image=TextValue{name=image, value=Optional[{"image":["image1binarydata","image2binarydata"]}]}, answer=TextValue{name=answer, value=Optional[{"answer":[{"filename":"c.js","answer":"some answer"}]}]}, creation_year=BigIntValue{name=creation_year, value=2019}, creation_month=BigIntValue{name=creation_month, value=12}, notes=TextValue{name=notes, value=Optional[**some notesupdated**]}, tx_version=IntValue{name=tx_version, value=1}}, consistency=LINEARIZABLE, condition=Optional[com.scalar.db.api.**PutIfNotExists**#21bf308]}

Answer 1

Scalar DB doesn't allow a blind write for the existing record. It looks there is no get before the update.
I think this process should check the current values and update the values in a transaction. In this code, there is no guarantee for atomicity between the get and the update.

Answer 2

Since the protocol is based on Snapshot Isolation, it needs to get a record in a snapshot first to update the record.

Answer 3

After Yuji's comment below, I changed the update method to add a get before put and the operation is successful now.
def update(transaction:DistributedTransaction, answer:AnswerOfAPracticeQuestion) = {
logger.trace(s"updating answer value ${answer}")
val key = AnswerKeys(answer.answeredBy.get.answerer_id,answer.question_id,answer.answer_id)
val result = get(transaction,key)
if(result.isLeft) throw result.left.get else add(transaction,answer, new PutIfExists())
}
I however don't understand why a get is required before update or delete in Scalardb