[GitHub] nickva commented on a change in pull request #496: Couchdb 3287 pluggable storage engines

[gi...@git.apache.org]

nickva commented on a change in pull request #496: Couchdb 3287 pluggable storage engines
URL: https://github.com/apache/couchdb/pull/496#discussion_r118552367
 
 

 ##########
 File path: src/couch/src/couch_db_engine.erl
 ##########
 @@ -0,0 +1,877 @@
+% Licensed under the Apache License, Version 2.0 (the "License"); you may not
+% use this file except in compliance with the License. You may obtain a copy of
+% the License at
+%
+%   http://www.apache.org/licenses/LICENSE-2.0
+%
+% Unless required by applicable law or agreed to in writing, software
+% distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
+% WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
+% License for the specific language governing permissions and limitations under
+% the License.
+
+-module(couch_db_engine).
+
+
+-include("couch_db.hrl").
+-include("couch_db_int.hrl").
+
+
+-type filepath() :: iolist().
+-type docid() :: binary().
+-type rev() :: {non_neg_integer(), binary()}.
+-type revs() :: [rev()].
+-type json() :: any().
+
+-type doc_pair() :: {
+        #full_doc_info{} | not_found,
+        #full_doc_info{} | not_found
+    }.
+
+-type doc_pairs() :: [doc_pair()].
+
+-type db_open_options() :: [
+        create
+    ].
+
+-type delete_options() :: [
+        {context, delete | compaction} |
+        sync
+    ].
+
+-type purge_info() :: [{docid(), revs()}].
+-type epochs() :: [{Node::atom(), UpdateSeq::non_neg_integer()}].
+-type size_info() :: [{Name::atom(), Size::non_neg_integer()}].
+
+-type write_stream_options() :: [
+        {buffer_size, Size::pos_integer()} |
+        {encoding, atom()} |
+        {compression_level, pos_integer()}
+    ].
+
+-type doc_fold_options() :: [
+        {start_key, Key::any()} |
+        {end_key, Key::any()} |
+        {end_key_gt, Key::any()} |
+        {dir, fwd | rev} |
+        include_reductions |
+        include_deleted
+    ].
+
+-type changes_fold_options() :: [
+        {dir, fwd | rev}
+    ].
+
+-type db_handle() :: any().
+
+-type doc_fold_fun() :: fun((#full_doc_info{}, UserAcc::any()) ->
+        {ok, NewUserAcc::any()} |
+        {stop, NewUserAcc::any()}).
+
+-type local_doc_fold_fun() :: fun((#doc{}, UserAcc::any()) ->
+        {ok, NewUserAcc::any()} |
+        {stop, NewUserAcc::any()}).
+
+-type changes_fold_fun() :: fun((#doc_info{}, UserAcc::any()) ->
+        {ok, NewUserAcc::any()} |
+        {stop, NewUserAcc::any()}).
+
+
+% This is called by couch_server to determine which
+% engine should be used for the given database. DbPath
+% is calculated based on the DbName and the configured
+% extension for a given engine. The first engine to
+% return true is the engine that will be used for the
+% database.
+-callback exists(DbPath::filepath()) -> boolean().
+
+
+% This is called by couch_server to delete a database. It
+% is called from inside the couch_server process which
+% means that the storage engine does not have to guarantee
+% its own consistency checks when executing in this
+% context. Although since this is executed in the context
+% of couch_server it should return relatively quickly.
+-callback delete(
+            RootDir::filepath(),
+            DbPath::filepath(),
+            DelOpts::delete_options()) ->
+        ok | {error, Reason::atom()} | any().
+
+
+% This function can be called from multiple contexts. It
+% will either be called just before a call to delete/3 above
+% or when a compaction is cancelled which executes in the
+% context of a couch_db_updater process. It is intended to
+% remove any temporary files used during compaction that
+% may be used to recover from a failed compaction swap.
+-callback delete_compaction_files(
+            RootDir::filepath(),
+            DbPath::filepath(),
+            DelOpts::delete_options()) ->
+        ok.
+
+
+% This is called from the couch_db_updater:init/1 context. As
+% such this means that it is guaranteed to only have one process
+% executing for a given DbPath argument (ie, opening a given
+% database is guaranteed to only happen in a single process).
+% However, multiple process may be trying to open different
+% databases concurrently so if a database requires a shared
+% resource that will require concurrency control at the storage
+% engine layer.
+%
+% The returned DbHandle should be a term that can be freely
+% copied between processes and accessed concurrently. However
+% its guaranteed that the handle will only ever be mutated
+% in a single threaded context (ie, within the couch_db_updater
 
 Review comment:
   Just to clarify, the handle itself is not mutated, just the database. Handle is just an opaque context and once returned from `init` it is not mutated outside the engine module implementation?
 
----------------------------------------------------------------
This is an automated message from the Apache Git Service.
To respond to the message, please log on GitHub and use the
URL above to go to the specific comment.
 
For queries about this service, please contact Infrastructure at:
users@infra.apache.org


With regards,
Apache Git Services