On Friday, May 22, 2015 at 7:33:20 AM UTC-7, Joachim Nolten wrote:
>
> Following the example in
> http://sequel.jeremyevans.net/rdoc/files/doc/sharding_rdoc.html does not
> seem to work for me.
>
> First:
>
> DB=Sequel.connect('postgres://master_server/database', :servers=>{:
> read_only=>proc{|db| {:host=>db.get_slave_host}}})
> def DB.get_slave_host
> @current_host ||= -1
> "slave_server#{(@current_host+=1)%4}"
> end
>
>
This should only be an issue if the adapter is creating a new connection
right away, before the method is defined. I can't replicate that in my
local setup with the code you posted, but depending on the other extensions
you are using, it is possible. You could always just do:
DB=Sequel.connect('postgres://master_server/database', :servers=>{:
read_only=>proc{|db| db.respond_to?(:get_slave_host) ? {:host=>db.
get_slave_host} : {}}})
>
> Results in a 'method not defined' error (this is in Ruby 2.1.3). I have
> changed my code to define the method inside the proc:
>
> DB=Sequel.connect('postgres://master_server/database', :servers=>{:
> read_only=>proc{ |db|
> unless db.respond_to?(:get_slave_host)
> def db.get_slave_host
> @current_host ||= -1
> "slave_server#{(@current_host+=1)%4}"
> end
> end
> {:host=>db.get_slave_host}}})
>
>
> But the method only appears to be called twice: Once at load time and once
> for the first query.
> After that, the last return value is used, which does not result in a
> round robin selection of the DB servers.
>
Are you running multiple queries concurrently? If not, then the behavior
is expected. Sequel will only create new connections (to other slave
servers) if they are actually needed, i.e. a request for a connection comes
in, and there aren't any available connections in the pool. If you are
using a single-threaded web server with multiple processes and want to use
multiple slaves, use @current_host ||= rand(4). That's actually a better
default in general, I'll update the documentation to reflect it.
Thanks,
Jeremy
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