[erlang-questions] ets:safe_fixtable/2 & ets:tab2file/{2, 3} question

Fred Hebert <>
Thu Dec 17 15:24:22 CET 2015


On 12/17, Benoit Chesneau wrote:
>But what happen when I use `ets:tab2file/2` while keys are continuously
>added at the end? When does it stop?
>

I'm not sure what answer you expect to the question "how can I keep an 
infinitely growing table from taking an infinite amount of time to dump 
to disk" that doesn't require locking it to prevent the growth from 
showing up.

Do note that safe_fixtable/2 does *not* prevent new inserted elements 
from showing up in your table -- it only prevents objects from being 
taken out or being iterated over twice. While it's easier to create a 
pathological case with an ordered_set table (keeping adding +1 as keys 
near the end), it is not beyond the realm of possibility to do so with 
other table types (probably with lots of insertions and playing with 
process priorities, or predictable hash sequences).

I don't believe there's any way to lock a public table (other than 
implicit blocking in match and select functions). If I were to give a 
wild guess, I'd say to look at ets:info(Tab,size), and have your 
table-dumping process stop when it reaches the predetermined size or 
meets an earlier exit. This would let you bound the time it takes you to 
dump the table, at the cost of possibly neglecting to add information 
(which you would do anyway -- you would just favor older info before 
newer info).  This would however imply reimplementing your own tab2file 
functionality.

Regards,
Fred.



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