kdb/q: Query multiple handles with hopen - handle

Question

I would like to be able to query several handles at once, where the tables have the same format like:
handles: 8000,8001,8003
tables: foo
Want to do something like:
x:hopen `8000`8001`8003
x select from foo col1,col2
So i get rows from each foo table on each handle.
Is there a way to achieve this?
Thank you

Answer 1

Use 'each' to hopen each handle
q)h:hopen each 8000 8001 8002
q)h
476 480 484i
Use apply each-left to send the same query to each server
q)r:h#\:"select col1,col2 from foo"
q)r
+`col1`col2!(1 2;2 3)
+`col1`col2!(1 2;2 3)
+`col1`col2!(1 2;2 3)
Then you'll have to raze the result:
q)show res:raze r
col1 col2
---------
1 2
2 3
1 2
2 3
1 2
2 3

Answer 2

If you are not planning to reuse the handles, you can do
q)raze`::8000`::8001`::8003#\:"select from foo col1,col2"

Answer 3

Same as other answers, but more complicated using set (neg h) rather than get (h)
The cookbook/load-balancing helps in this example, too.
q)h:hopen each 8000 8001 8002
q)h
476 480 484i
q)r:(0#0i)!() /dictionary of handles and results
Set the callback for the response from the servers
q).z.ps:{#[`r;.z.w;:;x]}
Send a "set" query to each handle
q)(neg h)#\:({(neg .z.w)value"select col1,col2 from foo"};`)
Wait until all messages have a response
q)h .\:()
Finally, putting the result together
q)raze r h
The only advantage is concurrency.
As #AlexanderBelopolsky pointed out, don't forget
q)hclose each h