failover to reading dbs.couch directly when ets is MIA

2 files changed, 29 insertions, 9 deletions

diff --git a/src/mem3.erl b/src/mem3.erl
index 5116f008..5610f085 100644
--- a/src/mem3.erl
+++ b/src/mem3.erl
@@ -52,12 +52,13 @@ nodes() ->
 shards(DbName) when is_list(DbName) ->
     shards(list_to_binary(DbName));
 shards(DbName) ->
-    case ets:lookup(partitions, DbName) of
+    try ets:lookup(partitions, DbName) of
     [] ->
-        % TODO fall back to checking dbs.couch directly
-        erlang:error(database_does_not_exist);
+        mem3_util:load_shards_from_disk(DbName);
     Else ->
         Else
+    catch error:badarg ->
+        mem3_util:load_shards_from_disk(DbName)
     end.
 
 -spec shards(DbName::iolist(), DocId::binary()) -> [#shard{}].
@@ -74,14 +75,14 @@ shards(DbName, DocId) ->
         range = ['$1','$2'],
         ref = '_'
     },
-    % TODO these conditions assume A < B, which we don't require
     Conditions = [{'<', '$1', HashKey}, {'=<', HashKey, '$2'}],
-    case ets:select(partitions, [{Head, Conditions, ['$_']}]) of
+    try ets:select(partitions, [{Head, Conditions, ['$_']}]) of
     [] ->
-        % TODO fall back to checking dbs.couch directly
-        erlang:error(database_does_not_exist);
+        mem3_util:load_shards_from_disk(DbName, DocId);
     Shards ->
         Shards
+    catch error:badarg ->
+        mem3_util:load_shards_from_disk(DbName, DocId)
     end.
 
 -spec choose_shards(DbName::iolist(), Options::list()) -> [#shard{}].
@@ -89,7 +90,7 @@ choose_shards(DbName, Options) when is_list(DbName) ->
     choose_shards(list_to_binary(DbName), Options);
 choose_shards(DbName, Options) ->
     try shards(DbName)
-    catch error:database_does_not_exist ->
+    catch error:E when E==database_does_not_exist; E==badarg ->
         Nodes = mem3:nodes(),
         NodeCount = length(Nodes),
         N = mem3_util:n_val(couch_util:get_value(n, Options), NodeCount),
diff --git a/src/mem3_util.erl b/src/mem3_util.erl
index b05faa15..e58bbd4c 100644
--- a/src/mem3_util.erl
+++ b/src/mem3_util.erl
@@ -2,7 +2,8 @@
 -author('brad@cloudant.com').
 
 -export([hash/1, name_shard/1, create_partition_map/4, build_shards/2,
-    n_val/2, to_atom/1, to_integer/1, write_db_doc/1, delete_db_doc/1]).
+    n_val/2, to_atom/1, to_integer/1, write_db_doc/1, delete_db_doc/1,
+    load_shards_from_disk/1, load_shards_from_disk/2]).
 
 -define(RINGTOP, 2 bsl 31).  % CRC32 space
 
@@ -114,3 +115,21 @@ n_val(N, NodeCount) when N > NodeCount ->
     NodeCount;
 n_val(N, _) ->
     N.
+
+load_shards_from_disk(DbName) when is_binary(DbName) ->
+    {ok, Db} = couch_db:open(<<"dbs">>, []),
+    try load_shards_from_db(Db, DbName) after couch_db:close(Db) end.
+
+load_shards_from_db(#db{} = ShardDb, DbName) ->
+    case couch_db:open_doc(ShardDb, DbName, []) of
+    {ok, #doc{body = {Props}}} ->
+        ?LOG_INFO("dbs cache miss for ~s", [DbName]),
+        build_shards(DbName, Props);
+    {not_found, _} ->
+        erlang:error(database_does_not_exist)
+    end.
+
+load_shards_from_disk(DbName, DocId)->
+    Shards = load_shards_from_disk(DbName),
+    HashKey = hash(DocId),
+    [S || #shard{range = [B,E]} = S <- Shards, B < HashKey, HashKey =< E].