Re: global singleton gen_server

Maybe you can accomplish what you want using the distributed configuration
options in the Kernel app.  I use this myself to make sure that certain
gen_servers are only ever run on one node in my cluster.  Have a look at
the config options for Kernel. I use the 'distributed' option with the
'sync_nodes_*' options.

https://www.erlang.org/docs/21/man/kernel_app.html#configuration


On Thu, Dec 16, 2021 at 13:09 Vyacheslav Levytskyy <[email protected]>
wrote:

> I have similar problem, and didn't find an ultimate solution yet. My
> current approach is to register the singleton gen_server using {via,
> my_global_module, my_regname(...)} rather than {global, ?MODULE}. In
> "my_global_module" I control how a conflict is resolved between running
> server and a new one. In the "resolve" callback of global:register_name I'm
> able to query for each of gen_server's version and decide which of two
> gen_server lives and which is to shut down. This helps also in case of
> rolling updates.
>
> Best regards,
> Vyacheslav
> On 16.12.2021 19:34, Roberto Ostinelli wrote:
>
> Not necessarily. By the time your first gen_server start you might already
> have more than one node in your cluster.
> I'm ok having other nodes joining later on, I'm trying to avoid a clash
> where multiple nodes execute the "cluster init" code
>
>
> On Thu, Dec 16, 2021 at 3:48 PM Roger Lipscombe <[email protected]>
> wrote:
>
>> Define "cluster start". In normal operation, there's nothing stopping
>> another node from joining the cluster at some arbitrary later time,
>> even days later. The first node to come up is a degenerate, single
>> node cluster, no?
>>
>> On Thu, 16 Dec 2021 at 11:00, Roberto Ostinelli <[email protected]>
>> wrote:
>> >
>> > Dear all,
>> > I'm trying to run an operation only once on cluster start but there's
>> clearly something that I'm missing in the global module (see other related
>> question here [1]).
>> >
>> > Therefore this time I'm trying to have a single global gen_server which
>> I start directly from application start/2 callback with:
>> > gen_server:start_link({global, ?MODULE}, ?MODULE, [], Options).
>> >
>> > Nodes are already connected at that point (I can see them with
>> nodes()), however for some reason this registration works on all the nodes
>> and I find myself with multiple gen_server registered globally.
>> >
>> > What is the proper way to register a global singleton gen_server at
>> cluster level on application start? mY need is to run an operation only
>> _once_ a cluster boots up.
>> >
>> > [1]
>> http://erlang.org/pipermail/erlang-questions/2021-December/101781.html
>>
>