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From Josh Elser <>
Subject Re: BatchWriter Improvements - An end user's perspective
Date Tue, 30 Aug 2016 18:02:53 GMT
tl;dr These are very good points, Mike. If you have the time, I think it 
would be great to start scratching out requirements, stub out what the 
API would look like, and box the work (as that would make it easier for 
you or another dev to pick it up and implement it).

Michael Moss wrote:
> Hello, Folks.
> As I look at the following tickets, I thought it might be useful to share
> how we are using the BatchWriter, some of the challenges we've had, some
> thoughts about it's redesign and how we might get involved.
> One of our primary use cases of the BatchWriter is from within a Storm
> topology, reading from Kafka. Generally speaking, storm might be persisting
> a a single or small set of mutations at a time (low latency), or in larger
> batches with Trident (higher throughput). In addition to ACCUMULO-2990 (any
> TimedOutException, which then throws MutationsRejectedException and
> requires a new connection to be made), one of our requirements is to ensure
> that any given thread's mutations are the ones which are flushed and none
> others (pseudo transactions). Otherwise, we might get a failure for a
> mutation which belongs to another thread (and already ACKed by Storm) which
> means we don't have a 'handle' on that offset anymore in Kafka to replay
> the failure - i.e. the message could be 'lost'.
> Despite being threadsafe, we end up using a single BatchWriter per thread
> to make reasoning about the above simpler, but this creates a resource
> issue - number of connections to accumulo and zk.
> This all makes me wonder what the design goals might have been for the
> current version of the driver and if the efforts to rewrite it might
> benefit from incorporating elements to address some of these use cases
> above.
> What can we learn from how drivers for other "NoSQL" databases are
> implemented? Would it make sense to remove all the global variables
> ("somethingFailed"), thread sleep/notify, frequent calls to
> "checkForFailures()" and consider using a 'connection pool' model where
> writes are single-threaded, linearized and isolated during the connection
> lease?

The MultiTableBatchWriter was an attempt in this direction for bounded 
resources. In the case where you had a single client writing to multiple 
tables, you wanted to be able to say "I want all ingest from this client 
to my tables to use X resources".

I think your point about resource management across multiple 
BatchWriters is a big problem presently when looked at with the 
concurrency problems you outline.

Being unable to determine which mutations succeeded/failed in a batch is 
a big pain. However, making this have exactly-once semantics would be 
extremely difficult

 > Could we make the client non-blocking and with optional pipelining,
 > so multiple writes could share a connection and allow interleaving of
 > operations (with individual acks)?

Right now, I don't think so. Multiplexing one connection isn't something 
that Thrift is capable of AFAIK (whereas this is something that Hadoop 
RPC can do). Presently, connections will remain open/cached to a 
tserver, but they cannot be concurrently shared.

> Looking forward to hearing everyone's thoughts.
> -Mike

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