Yoj transactions at ydb topics

[ansirotenko]

Transaction support for ydb partitions is expected to be available in the fall. In our project, we use yoj to work with our tables everywhere. It is required to marry yoj transactions and ydb-java-sdk transactions to be able to write topic messages (also commit offset at read) at the same transaction for the table. Currently it is possible to access current transaction of the txManager, but it is not possible to create ydb-java-sdk transaction from it. 2 main issues are statusFuture and sessionId could not be extracted from the yoj transaction. Also it would be great to have txId generated upfront, not lazily (if this is ok, im not sure).

Overall if yoj transaction would provide method like getYdbTransaction that would return instance of tech.ydb.common.transaction.YdbTransaction, it would be the most preferrable way

[nvamelichev]

Currently YOJ does not use YdbTransaction internally, at all. YdbTransaction has been introduced quite recently and is an experimental API (subject to sudden change) in the YDB SDK:

public interface Session implements AutoCloseable {
    // <...>

    @ExperimentalApi("New table transaction interfaces are experimental and may change without notice")
    TableTransaction createNewTransaction(TxMode var1);

    @ExperimentalApi("New table transaction interfaces are experimental and may change without notice")
    CompletableFuture<Result<TableTransaction>> beginTransaction(TxMode var1, BeginTxSettings var2);

    // <...>
}

We can try to implement YdbTransaction interface in YdbRepositoryTransaction (e.g. YdbRepositoryTransaction.getYdbTransactionWrapper(), but this comes with limitations:

• getStatusFuture() will throw UnsupportedOperationException. Also, implementing TableTransaction is out of the question: we don't want to give users abilities to circumvent YOJ in performing queries in YDB transaction
• getSessionId() semantics if YDB session is not yet acquired or has been invalidated: it's complicated. Some possibilities:
  • return null
  • throw IllegalStateException
  • try to acquire new session, then return its ID
    • But what do you do if the session has been invalidated (typically by an error, like transaction locks invalidated, that has destroyed the session in YDB)? Likely return an error...
• getId() will return null transaction ID if YDB transaction is not yet established. This seems to be the way it is implemented in YDB SDK, so this should be fine.

[ansirotenko]

If you implement any solution that would allow to reuse yoj transactions to work with ydb topics, that would be good enough

[nvamelichev]

@ansirotenko Please try out v2.5.7, it has YdbRepositoryTransaction.toSdkTransaction() which returns a (very limited, very simple) YdbTransaction implementation:
https://github.com/ydb-platform/yoj-project/releases/tag/v2.5.7

If this wrapper is sufficient to reuse YOJ transaction to also work with YDB topics, that's great!
(Otherwise, it will be a large rework of the YOJ core to use YdbTransaction inside instead of earlier YDB SDK methods that take e.g. TxControl.)

[ansirotenko]

Unfortunately it doesnt work. got following exception.

java.lang.IllegalArgumentException: Transaction is not active. Can only write topic messages in already running transactions from other services.

My code looks like this.

        MyWriter writer = writers.get(partitionId);

        var transaction = (YdbRepositoryTransaction<?>)Tx.Current.get().getRepositoryTransaction();
        var sdkTransaction = transaction.toSdkTransaction();

        byte[] data = SerializationUtils.serialize(message);
        writer.write(data, sdkTransaction).join();

    public CompletableFuture<WriteAck> write(byte[] data, YdbTransaction transaction) {
        Message message = Message.of(data);

        try {
            return asyncWriter.send(message, SendSettings.newBuilder()
                    .setTransaction(transaction)
                    .build())
                .whenComplete((result, ex) -> {
                    if (ex != null) {
                        log.error("Exception on writing to topic: ", ex);
                    } else {
                        switch (result.getState()) {
                            case WRITTEN:
                                WriteAck.Details details = result.getDetails();
                                StringBuilder str = new StringBuilder("Success");
                                if (details != null) {
                                    str.append(", offset: ").append(details.getOffset());
                                }
                                log.debug(str.toString());
                                break;
                            case ALREADY_WRITTEN:
                                log.warn("Topic message has already been written");
                                break;
                            default:
                                break;
                        }
                    }
                });
        } catch (QueueOverflowException e) {
            throw new RuntimeException(e);
        }
    }

[nvamelichev]

Oh. It seems that the YDB transaction has not been initiated by YOJ, probably because no statements (neither reads nor writes) have been executed. (IllegalStateExcception is thrown if YdbTransaction.isActive() returns false, which it does if YdbTransaction.getId() == null, see https://github.com/ydb-platform/ydb-java-sdk/blob/29ff1aded14d81bc7fc291583b82494e4fb702cc/topic/src/main/java/tech/ydb/topic/settings/ReceiveSettings.java#L70).

YOJ transactions are intentionally lazy (to e.g. execute some data preparation before allocating database resources). Please run any statement in transaction (do anything with a YOJ Table, for example) before invoking writer.write(data, sdkTransaction).join();. This should help. You can check the sdkTransaction.isActive() and sdkTransaction.getId() to see whether a YDB transaction has been allocated by YOJ.

[ansirotenko]

@nvamelichev I have followed your suggestion. Indeed, If I add any statement before writing to topic in same transaction, as a result no exception will be thrown. But apparently operation hangs forever. Im executing this experiment at unit test.

Unit test code

txManager.tx(
    () -> txManager.tx(() -> {
        db.myTable().save(myItem);
        
        MyWriter writer = writers.get(partitionId);

        var transaction = (YdbRepositoryTransaction<?>)Tx.Current.get().getRepositoryTransaction();
        var sdkTransaction = transaction.toSdkTransaction();

        byte[] data = SerializationUtils.serialize(message);
        writer.write(data, sdkTransaction).join();
    })
);

...

 public CompletableFuture<WriteAck> write(byte[] data, YdbTransaction transaction) {
      Message message = Message.of(data);

      try {
          return asyncWriter.send(message, SendSettings.newBuilder()
                  .setTransaction(transaction)
                  .build())
              .whenComplete((result, ex) -> {
                  if (ex != null) {
                      log.error("Exception on writing to topic: ", ex);
                  } else {
                      switch (result.getState()) {
                          case WRITTEN:
                              WriteAck.Details details = result.getDetails();
                              StringBuilder str = new StringBuilder("Success");
                              if (details != null) {
                                  str.append(", offset: ").append(details.getOffset());
                              }
                              log.debug(str.toString());
                              break;
                          case ALREADY_WRITTEN:
                              log.warn("Topic message has already been written");
                              break;
                          default:
                              break;
                      }
                  }
              });
      } catch (QueueOverflowException e) {
          throw new RuntimeException(e);
      }
  }

ydb docker image is latest. tried to remove and pull image again before experiment. Image describe gives following

        "Id": "sha256:41209b686d7bec78434ccb3dfbb7f1ae949f0fdbdbcf8bd88bda6faca114dbd9",
        "RepoTags": [
            "cr.yandex/yc/yandex-docker-local-ydb:latest"
        ],
        "RepoDigests": [
            "cr.yandex/yc/yandex-docker-local-ydb@sha256:f39237e6bab018c2635af765faa893061fc359892e53f7b51d40ab503b90846f"
        ],

[nvamelichev]

@ansirotenko Going back to this issue... I could not reproduce the "operation hangs forever" problem with just writing to a topic inside a transaction.

You can see my tests here, I've added them to YOJ test suite:
https://github.com/ydb-platform/yoj-project/blob/main/repository-ydb-v2/src/test/java/tech/ydb/yoj/repository/ydb/YdbRepositoryIntegrationTest.java#L1063-L1198

The tests use SyncWriter and SyncReader, but AsyncWriter worked equally as well for writing to topic inside transaction. I set the transaction to use immediate writes (execute write statements right away and not buffer them until transaction commit) so that I could just call db.projects().save(...) and have an active YDB transaction after that. Otherwise, my tests are pretty similar to your example in your previous comment.

The only time I got some similar problem was when I tried to read from the topic, while still being in transaction, and I set transaction in SyncReader's read settings. But such reads won't work anyway: *Reader implementations in YDB Java SDK need a real YdbTransaction.getStatusFuture() but YOJ YdbTransaction implementation does not provide it.