iilyak commented on a change in pull request #1432: Support callback module
data provider
URL: https://github.com/apache/couchdb/pull/1432#discussion_r202649851
##########
File path: src/couch_epi/src/couch_epi_data.erl
##########
@@ -102,12 +102,15 @@ definitions({file, FilePath}) ->
{error, Reason} ->
{error, {FilePath, Reason}}
end;
-definitions({module, Module}) when is_atom(Module) ->
- definitions({module, [Module]});
-definitions({module, Modules}) ->
+definitions({static_module, Module}) when is_atom(Module) ->
+ definitions({static_module, [Module]});
+definitions({static_module, Modules}) ->
Data = lists:append([M:data() || M <- Modules]),
Hash = couch_epi_functions_gen:hash(Modules),
- {ok, Hash, Data}.
+ {ok, Hash, Data};
+definitions({callback_module, Module}) ->
+ Data = Module:data(),
Review comment:
There is one use case when `static_module` is useful. That's why after
thinking some more I think we should keep it.
You can define same data provider in multiple applications. This is helpful
for cases similar to use of `stats`. Currently stats definitions are loaded
from plain configuration files in priv directory of every application. It can
be rewritten using couch_epi as follows:
```
cat dreyfus_epi.erl
data_providers() ->
[
{{couch_stats, descriptions}, {static_module, dreyfus_stats}}
].
```
```
cat global_changes_epi.erl
data_providers() ->
[
{{couch_stats, descriptions}, {static_module, global_changes_stats}}
].
```
```
cat dreyfus_stats.erl
-module(dreyfus_stats).
-export([data/0]).
data() -> [
{[dreyfus, httpd, search], [
{type, histogram},
{desc, <<"Distribution of overall search request latency as
experienced by the end user">>}
]},
{[dreyfus, rpc, search], [
{type, histogram},
{desc, <<"length of a search RPC worker">>}
]}
].
```
```
cat global_changes_stats.erl
-module(global_changes_stats).
-export([data/0]).
data() ->
[
{[global_changes, db_writes], [
{type, counter},
{desc, <<"number of db writes performed by global changes">>}
]},
{[global_changes, event_doc_conflict], [
{type, counter},
{desc, <<"number of conflicted event docs encountered by global
changes">>}
]}
].
```
In this case one single module would be generated which would include data
from all applications. It would be more efficient to detect changes using `vsn`
attribute of a module.
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