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Asynchronous Loggers for Low-Latency Logging (apache.org)
42 points by justin_hancock on Apr 26, 2013 | hide | past | web | favorite | 10 comments

Main LMAX Disruptor site: http://lmax-exchange.github.io/disruptor/

Related Google Group: http://groups.google.com/group/lmax-disruptor

API is not as easy as a queue, but the results speak for themselves...

Also, using disruptor (efficiently) forces a certain threading model onto your app (at least downstream of the ring).

The threading model is part of the "Mechanical Sympathy". Multiple threads can write into a ring buffer, without locks. On the other end, coming out of the ring buffer you can have one thread (which means that the biz logic never needs to lock anything and can be blazingly fast), or multiple threads doing something with the event independently (at LMAX they journal to disk and replicate to backup machine). These threads can process the event in parallel. And you can "pipeline" or stage threads, so after both journaling and replication are complete (and not earlier than that) the event is passed to the business logic.

Then if the business logic needs to do something slow, like network I/O, disk I/O, database access etc, you would set up one or more other Disruptors for that. Instead of doing the slow task in the biz logic thread, you'd post a task for that on the helper Disruptor. That way the business logic can stay single-threaded, uncontended and blazingly fast.

When the slow task (disk I/O or whatever) is done, it will post a new event onto the first Disruptor, so the business logic can pick up the result and process that.

So it's all asynchronous, all event-driven, but pretty flexible.

In Log4j 2.0 there is just one Disruptor that takes the log message, puts it in the ring buffer, and passes control back to the application. Coming out of the ring buffer the event is passed to the normal log4j loggers, parent loggers and appenders.

It is not completely zero-garbage, but all ring buffer slots are created up front and re-used during the life of the process. Coming into the ring buffer, log event parameters are copied into the ring buffer slot without creating new LogEvent objects. Slots in the ring buffer implement the LogEvent interface, so coming out of the ring buffer, the slot itself is passed downstream, again avoiding object creation. Other parts of log4j are not that garbage-conscious so there is still room for improvement in this area.

That's a great description of how it works - looking forward to reading more of your comments on HN in the future.

I wish "Mechanical Sympathy" was defined/explored more in computer science. Googling mechanical sympathy just leads me to a blog...although it's an extremely interesting blog and tied to the idea of mechanical sympathy.

Also, LMax Disruptor looks really cool on the surface (can't wait to dive in). However, I haven't found too much literature that talks about its possible tradeoffs. Glad this story was posted.

Check Martins talks on the topic too - http://www.infoq.com/author/Martin-Thompson. Very interesting indeed.

There's the related Google Group as well: https://groups.google.com/forum/#!forum/mechanical-sympathy

It just means understanding what's happening at the lowest-level, whilst not necessarily working at that level of abstraction.

Using LMax seems really interesting for this case, I'll definitely take a look at the source and hot it was done (that's the best way to learn something :)).

i don't think that so-called 'Disruptor' is something more than a trivial implementation of 'pipeline parallelism'. i'm sure this keyword typed in google scholar would lead you to far more interesting things in real computer science

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