Hi Danny,

as Eugene pointed out, there are essentially two "modes of operation" of CheckpointMark. It can:

 a) be used to somehow restore state of a reader (in call to UnboundedSource#createReader)

 b) confirm processed elements in CheckpointMark#finalizeCheckpoint

If your source doesn't provide a persistent position in data stream that can be referred to (and serialized - example of this would be kafka offsets), then what you actually need to serialize is not the channel, but a way how to restore it - e.g. by opening a new channel with a given 'consumer group name'. Then you just use this checkpoint to commit your processed data in finalizeCheckpoint.

Note that the finalizeCheckpoint is not guaranteed to be called - that can happen in cases when an error occurs and the source has to be rewind back - that is what direct runner emulates with the probability of 'readerReuseChance'.

I'm reading the documentation of RabbitMQ very quickly, but if I understand it correctly, then you have to create a subscription to the broker, serialize identifier of the subscription into the checkpointmark and then just recover the subscription in call to UnboundedSource#createReader. That should do the trick.

Hope this helps, sorry if I'm not using 100% correct RabbitMQ terminology as I said, I'm not quite familiar with it.

Best,

 Jan

On 11/14/19 5:26 AM, Daniel Robert wrote:

I believe I've nailed down a situation that happens in practice that causes Beam and Rabbit to be incompatible. It seems that runners can and do make assumptions about the serializability (via Coder) of a CheckpointMark.

To start, these are the semantics of RabbitMQ:

- the client establishes a connection to the server
- client opens a channel on the connection
- messages are either pulled or pushed to the client from the server along this channel - when messages are done processing, they are acknowledged *client-side* and must be acknowledged on the *same channel* that originally received the message.

Since a channel (or any open connection) is non-serializable, it means that a CheckpointMark that has been serialized cannot ever be used to acknowledge these messages and correctly 'finalize' the checkpoint. It also, as previously discussed in this thread, implies a rabbit Reader cannot accept an existing CheckpointMark at all; the Reader and the CheckpointMark must share the same connection to the rabbit server ("channel").

Next, I've found how DirectRunner (and presumably others) can attempt to serialize a CheckpointMark that has not been finalized. In https://github.com/apache/beam/blob/master/runners/direct-java/src/main/java/org/apache/beam/runners/direct/UnboundedReadEvaluatorFactory.java#L150, the DirectRunner applies a probability and if it hits, it sets the current reader to 'null' but retains the existing CheckpointMark, which it then attempts to pass to a new reader via a Coder.

This puts the shard, the runner, and the reader with differing views of the world. In UnboundedReadEvaluatorFactory's processElement function, a call to getReader(shard) ( https://github.com/apache/beam/blob/master/runners/direct-java/src/main/java/org/apache/beam/runners/direct/UnboundedReadEvaluatorFactory.java#L132 ) clones the shard's checkpoint mark and passes that to the new reader. The reader ignores it, creating its own, but even if it accepted it, it would be accepting a serialized CheckpointMark, which wouldn't work. Later, the runner calls finishRead ( https://github.com/apache/beam/blob/master/runners/direct-java/src/main/java/org/apache/beam/runners/direct/UnboundedReadEvaluatorFactory.java#L246 ). The shard's CheckpointMark (unserialized; which should still be valid) is finalized. The reader's CheckpointMark (which may be a different instance) becomes the return value, which is referred to as "finishedCheckpoint" in the calling code, which is misleading at best and problematic at worst as *this* checkpoint has not been finalized.

So, tl;dr: I cannot find any means of maintaining a persistent connection to the server for finalizing checkpoints that is safe across runners. If there's a guarantee all of the shards are on the same JVM instance, I could rely on global, static collections/instances as a workaround, but if other runners might serialize this across the wire, I'm stumped. The only workable situation I can think of right now is to proactively acknowledge messages as they are received and effectively no-op in finalizeCheckpoint. This is very different, semantically, and can lead to dropped messages if a pipeline doesn't finish processing the given message.

Any help would be much appreciated.

Thanks,
-Danny

On 11/7/19 10:27 PM, Eugene Kirpichov wrote:
Hi Daniel,

This is probably insufficiently well documented. The CheckpointMark is used for two purposes: 1) To persistently store some notion of how much of the stream has been consumed, so that if something fails we can tell the underlying streaming system where to start reading when we re-create the reader. This is why CheckpointMark is Serializable. E.g. this makes sense for Kafka. 2) To do acks - to let the underlying streaming system know that the Beam pipeline will never need data up to this CheckpointMark. Acking does not require serializability - runners call ack() on the same in-memory instance of CheckpointMark that was produced by the reader. E.g. this makes sense for RabbitMq or Pubsub.

In practice, these two capabilities tend to be mutually exclusive: some streaming systems can provide a serializable CheckpointMark, some can do acks, some can do neither - but very few (or none) can do both, and it's debatable whether it even makes sense for a system to provide both capabilities: usually acking is an implicit form of streaming-system-side checkpointing, i.e. when you re-create the reader you don't actually need to carry over any information from an old CheckpointMark - the necessary state (which records should be delivered) is maintained on the streaming system side.

These two are lumped together into one API simply because that was the best design option we came up with (not for lack of trying, but suggestions very much welcome - AFAIK nobody is happy with it).

RabbitMQ is under #2 - it can't do serializable checkpoint marks, but it can do acks. So you can simply ignore the non-serializability.

On Thu, Nov 7, 2019 at 12:07 PM Daniel Robert <daniel.rob...@acm.org <mailto:daniel.rob...@acm.org>> wrote:

    (Background: I recently upgraded RabbitMqIO from the 4.x to 5.x
    library.
    As part of this I switched to a pull-based API rather than the
    previously-used push-based. This has caused some nebulous
    problems so
    put up a correction PR that I think needs some eyes fairly
    quickly as
    I'd consider master to be broken for rabbitmq right now. The PR
    keeps
    the upgrade but reverts to the same push-based implementation as
    in 4.x:
    https://github.com/apache/beam/pull/9977 )

    Regardless, in trying to get the pull-based API to work, I'm
    finding the
    interactions between rabbitmq and beam with CheckpointMark to be
    fundamentally impossible to implement so I'm hoping for some
    input here.

    CheckointMark itself must be Serializable, presumably this means
    it gets
    shuffled around between nodes. However 'Channel', the tunnel through
    which it communicates with Rabbit to ack messages and finalize the
    checkpoint, is non-Serializable. Like most other CheckpointMark
    implementations, Channel is 'transient'. When a new
    CheckpointMark is
    instantiated, it's given a Channel. If an existing one is
    supplied to
    the Reader's constructor (part of the 'startReader()' interface),
    the
    channel is overwritten.

    *However*, Rabbit does not support 'ack'ing messages on a channel
    other
    than the one that consumed them in the first place. Attempting to
    do so
    results in a '406 (PRECONDITION-FAILED) - unknown delivery tag'.
    (See
    https://www.grzegorowski.com/rabbitmq-406-channel-closed-precondition-failed

    ).

    Truthfully, I don't really understand how the current
    implementation is
    working; it seems like a happy accident. But I'm curious if someone
    could help me debug and implement how to bridge the
    re-usable/serializable CheckpointMark requirement in Beam with this
    limitation of Rabbit.

    Thanks,
    -Daniel Robert

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