Random questions

Jon Brisbin jon at jbrisbin.com
Mon Apr 18 18:12:09 EDT 2011


On Apr 18, 2011, at 2:47 PM, Ben Tilly wrote:

> The first is that I've heard that there is a limit to the size of the
> headers that can be sent using the http interface and therefore the
> number of links that you can use.  Is this true, and if so does anyone
> know what the limit is?

I suppose it depends on the length of your key. I've heard in the thousands. If you're using command-line clients or curl or something, you might find it gets untenable really quickly.

> 
> My second question is this.  It seems that if accesses are configured
> so that writes go to a quorum, and reads also comes from a quorum,
> then once a write is reported as complete, a read is close to
> guaranteed to come back with that write.  How reliable is this
> behavior?  I'm sure that there are boundary cases around a machine
> joining in the same window that the write/read is happening, but I'd
> like to know what they are.

This is completely unscientific, but in a large test suite with many hundreds of write and immediate read operations, a random number of them will fail for no apparent reason. I traced this down to the quorum thing. If you have a whole lot of writes and immediate reads that need to happen (e.g. in a test suite), you have to specify a full write (i.e. to every single vnode with no exceptions).

> 
> My third question is how bad an idea people think it is to try to get
> some form of atomic behavior from Riak by locking important read/write
> pairs.

There's some very interesting work being done lately to implement the Zookeeper protocol for doing atomic checkpointing (which would be necessary to achieve atomic operations in a distributed system) but it seems way too complicated for most of what people need this kind of thing for.

If true atomicity is really a concern, then use Redis and write a pub/sub handler to update your Riak documents whenever things change.

You know we could probably take the Riak RabbitMQ postcommit hook and adapt it to use Redis for something along these lines... :)

Thanks!

Jon Brisbin

http://jbrisbin.com
Twitter: @j_brisbin





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