How do I select a random element from an ets set in Erlang/Elixir?

Question

I have a large number of processes that I need to keep track of in an ets set, and then randomly select single processes. So I created the set like this:

:ets.new(:pid_lookup, [:set, :protected, :named_table])

then for argument's sake let's just stick self() in it 1000 times:

Enum.map 1..1000, fn x -> :ets.insert(:pid_lookup, {x, self()}) end

Now I need to select one at random. I know I could just select a random one using :ets.lookup(:pid_lookup, :rand.uniform(1000)), but what if I don't know the size of the set (in the above case, 1000) in advance?

How do I find out the size of an ets set? And/or is there a better way to choose a random pid from an ets data structure?

You could use :ets.info(tab, :size) to get the size of the table. I don't really know the best option of grabbing a random element from it though. What is the usecase for grabbing a random pid from the set? — Justin Wood
@JustinWood I'm just benchmarking genservers. I'm creating 1000 of them then I want each to send messages to another random one a bunch of times and see how long it takes with various message sizes. But as I create the Genservers they start so the other genservers might not be up yet when they start sending. So I need to know how many have been created in the initial stages of the benchmark. — Thomas Browne
Well the 1000 is arbitrary of course. I'll check out the performance based on various numbers. I essentially want to get a mental map of how fast message passing is. — Thomas Browne
If you just want to test message passing, why not spin up all the gen servers and then start sending messages? — Justin Wood
Well I do want to know how to select stuff at random in an ets table for a bunch of statistics work I do anyway. But to answer your question directly, as I spin up the 1000 Genservers, the first ones will start sending before the last ones have spun up. Therefore, I can't just select a random number out of 1000. I need to know how many have been spun up, and for that I need to know what the size of the ETS table. — Thomas Browne

HelloWorld HelloWorld · Accepted Answer · 2020-01-07T18:08:32

If keys are sequential number

tab = :ets.new(:tab, [])
Enum.each(1..1000, & :ets.insert(tab, {&1, :value}))
size = :ets.info(tab,  :size) 
# size = 1000
value_picked_randomly = :ets.lookup(tab, Enum.random(1..1000))

:ets.info(tab, :size) returns a size of a table; which is a number of records inserted on given table.

If you don't know that the keys are

first = :ets.first(tab)
:ets.lookup(tab, first)
func = fn key->
    if function_that_may_return_true() do
        key = case :ets.next(tab, key) do
         :'$end_of_table' -> throw :reached_end_of_table
         key -> func.(key)
        end
    else 
        :ets.lookup(tab, key)
    end
end
func.()

func will iterate over the ets table and returns a random value. This will be time consuming, so it will not be an ideal solution for tables with large number of records.

How do I select a random element from an ets set in Erlang/Elixir?

2 Answers