Merge pull request #1432 from cloudant/support-callback-module-data-provider

Support callback module data provider

4 files changed, 104 insertions, 14 deletions

diff --git a/src/couch_epi/README.md b/src/couch_epi/README.md
index 66421ceb9..368ad9afd 100644
--- a/src/couch_epi/README.md
+++ b/src/couch_epi/README.md
@@ -39,7 +39,8 @@ could add an entry in its implementation of couch_epi_plugin behaviour:
                 {priv_file, "stats_descriptions.cfg"}, [{interval, 5000}]}
             {{couch_stats, descriptions},
                 {file, "/tmp/extra_stats.cfg"}, [{interval, 5000}]},
-            {{couch_stats, descriptions}, {module, my_stats}}
+            {{couch_stats, descriptions}, {static_module, my_stats}},
+            {{couch_stats, descriptions}, {callback_module, my_stats}}
         ].
 
 When service provider wants to learn about all the installed config data for it to use
@@ -65,6 +66,38 @@ There are also additional functions to get the same data in various formats
 - `couch_epi:keys(Handle)` - returns list of configured keys
 - `couch_epi:subscribers(Handle)` - return list of known subscribers
 
+The difference between `static_module` and `callback_module` providers is in how
+couch_epi detects the changes. `static_module` is designed for the cases when you
+have your data hardcoded in the module. For example you might have the following:
+
+```
+-export([data/0]).
+
+data() ->
+    [
+        {[complex, key, 2], [
+            {type, counter},
+            {desc, bar}
+        ]},
+        {[complex, key, 1], [
+            {type, counter},
+            {desc, updated_foo}
+        ]}
+    ].
+```
+
+The changes are detected by relying on `vsn` module attribute. Therefore we
+would notice the change only when data source module is recompiled.
+
+The `callback_module` provider uses the return value from `data/0` to detect
+changes and it is useful for cases when the data term is constructed dynamically.
+For example to cache values of CouchDB config one could use the following:
+
+```
+-export([data/0]).
+data() ->
+    config:get("dreyfus").
+```
 
 # Function dispatch example
 
diff --git a/src/couch_epi/src/couch_epi_data.erl b/src/couch_epi/src/couch_epi_data.erl
index bbed828bb..2bb09f6cf 100644
--- a/src/couch_epi/src/couch_epi_data.erl
+++ b/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(),
+    {ok, erlang:phash2(Data), Data}.
 
 hash_of_file(FilePath) ->
     {ok, Data} = file:read_file(FilePath),
diff --git a/src/couch_epi/src/couch_epi_sup.erl b/src/couch_epi/src/couch_epi_sup.erl
index 31f27d752..509f5a1c2 100644
--- a/src/couch_epi/src/couch_epi_sup.erl
+++ b/src/couch_epi/src/couch_epi_sup.erl
@@ -121,7 +121,8 @@ modules(#couch_epi_spec{kind = services, value = Module}) ->
     [Module];
 modules(#couch_epi_spec{kind = data_providers, value = Value}) ->
     case Value of
-        {module, Module} -> [Module];
+        {static_module, Module} -> [Module];
+        {callback_module, Module} -> [Module];
         _ -> []
     end;
 modules(#couch_epi_spec{kind = data_subscriptions, behaviour = Module}) ->
@@ -159,7 +160,7 @@ services() ->
 
 data_providers() ->
     [
-        {{test_app, descriptions}, {module, ?MODULE}, [{interval, 100}]}
+        {{test_app, descriptions}, {static_module, ?MODULE}, [{interval, 100}]}
     ].
 
 data_subscriptions() ->
diff --git a/src/couch_epi/test/couch_epi_tests.erl b/src/couch_epi/test/couch_epi_tests.erl
index 79122d75a..042753215 100644
--- a/src/couch_epi/test/couch_epi_tests.erl
+++ b/src/couch_epi/test/couch_epi_tests.erl
@@ -17,7 +17,7 @@
 -define(DATA_FILE1, ?ABS_PATH("test/fixtures/app_data1.cfg")).
 -define(DATA_FILE2, ?ABS_PATH("test/fixtures/app_data2.cfg")).
 
--export([notify_cb/4, save/3]).
+-export([notify_cb/4, save/3, get/2]).
 
 -record(ctx, {file, handle, pid, kv, key, modules = []}).
 
@@ -83,6 +83,14 @@
         ].
 ").
 
+-define(DATA_MODULE3(Name, Kv), "
+    -export([data/0]).
+
+data() ->
+    {ok, Data} = couch_epi_tests:get('" ++ atom_to_list(Kv) ++ "', data),
+    Data.
+").
+
 %% ------------------------------------------------------------------
 %% couch_epi_plugin behaviour
 %% ------------------------------------------------------------------
@@ -175,7 +183,7 @@ setup(data_file) ->
         handle = couch_epi:get_handle(Key),
         kv = KV,
         pid = Pid};
-setup(data_module) ->
+setup(static_data_module) ->
     error_logger:tty(false),
 
     Key = {test_app, descriptions},
@@ -183,7 +191,7 @@ setup(data_module) ->
     ok = generate_module(provider, ?DATA_MODULE1(provider)),
     KV = start_state_storage(),
 
-    ok = start_epi([{provider_epi, plugin_module([KV, {module, provider}])}]),
+    ok = start_epi([{provider_epi, plugin_module([KV, {static_module, provider}])}]),
 
     Pid = whereis(couch_epi:get_handle(Key)),
     Handle = couch_epi:get_handle(Key),
@@ -194,6 +202,33 @@ setup(data_module) ->
         modules = [Handle, provider],
         kv = KV,
         pid = Pid};
+setup(callback_data_module) ->
+    error_logger:tty(false),
+
+    Key = {test_app, descriptions},
+
+    KV = start_state_storage(),
+    Value = [
+        {[complex, key, 1], [
+            {type, counter},
+            {desc, foo}
+        ]}
+    ],
+    save(KV, data, Value),
+
+    ok = generate_module(provider, ?DATA_MODULE3(provider, KV)),
+
+    ok = start_epi([{provider_epi, plugin_module([KV, {callback_module, provider}])}]),
+
+    Pid = whereis(couch_epi:get_handle(Key)),
+    Handle = couch_epi:get_handle(Key),
+
+    #ctx{
+       key = Key,
+       handle = Handle,
+       modules = [Handle, provider],
+       kv = KV,
+       pid = Pid};
 setup(functions) ->
     Key = my_service,
     error_logger:tty(false),
@@ -244,7 +279,8 @@ epi_config_update_test_() ->
     ],
     Cases = [
         data_file,
-        data_module,
+        static_data_module,
+        callback_data_module,
         functions
     ],
     {
@@ -264,7 +300,8 @@ epi_data_source_test_() ->
     ],
     Cases = [
         data_file,
-        data_module
+        static_data_module,
+        callback_data_module
     ],
     {
         "epi data API tests",
@@ -305,7 +342,8 @@ epi_providers_order_test_() ->
 epi_reload_test_() ->
     Cases = [
         data_file,
-        data_module,
+        static_data_module,
+        callback_data_module,
         functions
     ],
     Funs = [
@@ -565,8 +603,21 @@ update(Case, #ctx{pid = Pid, modules = Modules} = Ctx) ->
 update_definitions(data_file, #ctx{file = File}) ->
     {ok, _} = file:copy(?DATA_FILE2, File),
     ok;
-update_definitions(data_module, #ctx{}) ->
+update_definitions(static_data_module, #ctx{}) ->
     ok = generate_module(provider, ?DATA_MODULE2(provider));
+update_definitions(callback_data_module, #ctx{kv = Kv}) ->
+    Value = [
+        {[complex, key, 2], [
+            {type, counter},
+            {desc, bar}
+        ]},
+        {[complex, key, 1], [
+            {type, counter},
+            {desc, updated_foo}
+        ]}
+    ],
+    save(Kv, data, Value),
+    ok;
 update_definitions(functions, #ctx{}) ->
     ok = generate_module(provider1, ?MODULE2(provider1)).
 
@@ -597,6 +648,8 @@ save(Kv, Key, Value) ->
     call(Kv, {set, Key, Value}).
 
 get(#ctx{kv = Kv}, Key) ->
+    call(Kv, {get, Key});
+get(Kv, Key) ->
     call(Kv, {get, Key}).
 
 call(Server, Msg) ->