src/couch/src/couch_db_engine.erl

% 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, non_neg_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()}.


% 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
% process).
-callback init(DbPath::filepath(), db_open_options()) ->
    {ok, DbHandle::db_handle()}.


% This is called in the context of couch_db_updater:terminate/2
% and as such has the same properties for init/2. It's guaranteed
% to be consistent for a given database but may be called by many
% databases concurrently.
-callback terminate(Reason::any(), DbHandle::db_handle()) -> Ignored::any().


% This is called in the context of couch_db_updater:handle_call/3
% for any message that is unknown. It can be used to handle messages
% from asynchronous processes like the engine's compactor if it has one.
-callback handle_db_updater_call(Msg::any(), DbHandle::db_handle()) ->
        {reply, Resp::any(), NewDbHandle::db_handle()} |
        {stop, Reason::any(), Resp::any(), NewDbHandle::db_handle()}.


% This is called in the context of couch_db_updater:handle_info/2
% and has the same properties as handle_call/3.
-callback handle_db_updater_info(Msg::any(), DbHandle::db_handle()) ->
    {noreply, NewDbHandle::db_handle()} |
    {noreply, NewDbHandle::db_handle(), Timeout::timeout()} |
    {stop, Reason::any(), NewDbHandle::db_handle()}.


% These functions are called by any process opening or closing
% a database. As such they need to be able to handle being
% called concurrently. For example, the legacy engine uses these
% to add monitors to the main engine process.
-callback incref(DbHandle::db_handle()) -> {ok, NewDbHandle::db_handle()}.
-callback decref(DbHandle::db_handle()) -> ok.
-callback monitored_by(DbHande::db_handle()) -> [pid()].


% This is called in the context of couch_db_updater:handle_info/2
% and should return the timestamp of the last activity of
% the database. If a storage has no notion of activity or the
% value would be hard to report its ok to just return the
% result of os:timestamp/0 as this will just disable idle
% databases from automatically closing.
-callback last_activity(DbHandle::db_handle()) -> elrang:timestamp().


% All of the get_* functions may be called from many
% processes concurrently.

% The database should make a note of the update sequence when it
% was last compacted. If the database doesn't need compacting it
% can just hard code a return value of 0.
-callback get_compacted_seq(DbHandle::db_handle()) ->
            CompactedSeq::non_neg_integer().


% The number of documents in the database which have all leaf
% revisions marked as deleted.
-callback get_del_doc_count(DbHandle::db_handle()) ->
            DelDocCount::non_neg_integer().


% This number is reported in the database info properties and
% as such can be any JSON value.
-callback get_disk_version(DbHandle::db_handle()) -> Version::json().


% The number of documents in the database that have one or more
% leaf revisions not marked as deleted.
-callback get_doc_count(DbHandle::db_handle()) -> DocCount::non_neg_integer().


% The epochs track which node owned the database starting at
% a given update sequence. Each time a database is opened it
% should look at the epochs. If the most recent entry is not
% for the current node it should add an entry that will be
% written the next time a write is performed. An entry is
% simply a {node(), CurrentUpdateSeq} tuple.
-callback get_epochs(DbHandle::db_handle()) -> Epochs::epochs().


% Get the last purge request performed.
-callback get_last_purged(DbHandle::db_handle()) -> LastPurged::purge_info().


% Get the current purge sequence. This should be incremented
% for every purge operation.
-callback get_purge_seq(DbHandle::db_handle()) -> PurgeSeq::non_neg_integer().


% Get the revision limit. This should just return the last
% value that was passed to set_revs_limit/2.
-callback get_revs_limit(DbHandle::db_handle()) -> RevsLimit::pos_integer().


% Get the current security properties. This should just return
% the last value that was passed to set_security/2.
-callback get_security(DbHandle::db_handle()) -> SecProps::any().


% This information is displayed in the database info poperties. It
% should just be a list of {Name::atom(), Size::non_neg_integer()}
% tuples that will then be combined across shards. Currently,
% various modules expect there to at least be values for:
%
%   file     - Number of bytes on disk
%
%   active   - Theoretical minimum number of bytes to store this db on disk
%              which is used to guide decisions on compaction
%
%   external - Number of bytes that would be required to represent the
%              contents outside of the database (for capacity and backup
%              planning)
-callback get_size_info(DbHandle::db_handle()) -> SizeInfo::size_info().


% The current update sequence of the database. The update
% sequence should be incrememnted for every revision added to
% the database.
-callback get_update_seq(DbHandle::db_handle()) -> UpdateSeq::non_neg_integer().


% Whenever a database is created it should generate a
% persistent UUID for identification in case the shard should
% ever need to be moved between nodes in a cluster.
-callback get_uuid(DbHandle::db_handle()) -> UUID::binary().


% These functions are only called by couch_db_updater and
% as such are guaranteed to be single threaded calls. The
% database should simply store these values somewhere so
% they can be returned by the corresponding get_* calls.

-callback set_revs_limit(DbHandle::db_handle(), RevsLimit::pos_integer()) ->
        {ok, NewDbHandle::db_handle()}.

-callback set_security(DbHandle::db_handle(), SecProps::any()) ->
        {ok, NewDbHandle::db_handle()}.


% This function will be called by many processes concurrently.
% It should return a #full_doc_info{} record or not_found for
% every provided DocId in the order those DocId's appear in
% the input.
%
% Traditionally this function will only return documents that
% were present in the database when the DbHandle was retrieved
% from couch_server. It is currently unknown what would break
% if a storage engine deviated from that property.
-callback open_docs(DbHandle::db_handle(), DocIds::[docid()]) ->
        [#full_doc_info{} | not_found].


% This function will be called by many processes concurrently.
% It should return a #doc{} record or not_found for every
% provided DocId in the order they appear in the input.
%
% The same caveats around database snapshots from open_docs
% apply to this function (although this function is called
% rather less frequently so it may not be as big of an
% issue).
-callback open_local_docs(DbHandle::db_handle(), DocIds::[docid()]) ->
        [#doc{} | not_found].


% This function will be called from many contexts concurrently.
% The provided RawDoc is a #doc{} record that has its body
% value set to the body value returned from write_doc_body/2.
%
% This API exists so that storage engines can store document
% bodies externally from the #full_doc_info{} record (which
% is the traditional approach and is recommended).
-callback read_doc_body(DbHandle::db_handle(), RawDoc::doc()) ->
        doc().


% This function is called concurrently by any client process
% that is writing a document. It should accept a #doc{}
% record and return a #doc{} record with a mutated body it
% wishes to have written to disk by write_doc_body/2.
%
% This API exists so that storage engines can compress
% document bodies in parallel by client processes rather
% than forcing all compression to occur single threaded
% in the context of the couch_db_updater process.
-callback serialize_doc(DbHandle::db_handle(), Doc::doc()) ->
        doc().


% This function is called in the context of a couch_db_updater
% which means its single threaded for the given DbHandle.
%
% The returned #doc{} record should have its Body set to a value
% that will be stored in the #full_doc_info{} record's revision
% tree leaves which is passed to read_doc_body/2 above when
% a client wishes to read a document.
%
% The BytesWritten return value is used to determine the number
% of active bytes in the database which can is used to make
% a determination of when to compact this database.
-callback write_doc_body(DbHandle::db_handle(), Doc::doc()) ->
        {ok, FlushedDoc::doc(), BytesWritten::non_neg_integer()}.


% This function is called from the context of couch_db_updater
% and as such is guaranteed single threaded for the given
% DbHandle.
%
% This is probably the most complicated function in the entire
% API due to a few subtle behavior requirements required by
% CouchDB's storage model.
%
% The Pairs argument is a list of pairs (2-tuples) of
% #full_doc_info{} records. The first element of the pair is
% the #full_doc_info{} that exists on disk. The second element
% is the new version that should be written to disk. There are
% three basic cases that should be followed:
%
%     1. {not_found, #full_doc_info{}} - A new document was created
%     2. {#full_doc_info{}, #full_doc_info{}} - A document was updated
%     3. {#full_doc_info{}, not_found} - A document was purged completely
%
% Number one and two are fairly straight forward as long as proper
% accounting for moving entries in the udpate sequence are accounted
% for. However, case 3 you'll notice is "purged completely" which
% means it needs to be removed from the database including the
% update sequence. Also, for engines that are not using append
% only storage like the legacy engine, case 2 can be the result of
% a purge so special care will be needed to see which revisions
% should be removed.
%
% The LocalDocs variable is applied separately. Its important to
% note for new storage engine authors that these documents are
% separate because they should *not* be included as part of the
% changes index for the database.
%
% The PurgedDocIdRevs is the list of Ids and Revisions that were
% purged during this update. While its not guaranteed by the API,
% currently there will never be purge changes comingled with
% standard updates.
%
% Traditionally an invocation of write_doc_infos should be all
% or nothing in so much that if an error occurs (or the VM dies)
% then the database doesn't retain any of the changes. However
% as long as a storage engine maintains consistency this should
% not be an issue as it has never been a guarantee and the
% batches are non-deterministic (from the point of view of the
% client).
-callback write_doc_infos(
    DbHandle::db_handle(),
    Pairs::doc_pairs(),
    LocalDocs::[#doc{}],
    PurgedDocIdRevs::[{docid(), revs()}]) ->
        {ok, NewDbHandle::db_handle()}.


% This function is called in the context of couch_db_udpater and
% as such is single threaded for any given DbHandle.
%
% This call is made periodically to ensure that the database has
% stored all updates on stable storage. (ie, here is where you fsync).
-callback commit_data(DbHandle::db_handle()) ->
        {ok, NewDbHande::db_handle()}.


% This function is called by multiple processes concurrently.
%
% This function along with open_read_stream are part of the
% attachments API. For the time being I'm leaving these mostly
% undocumented. There are implementations of this in both the
% legacy btree engine as well as the alternative engine
% implementations for the curious, however this is a part of the
% API for which I'd like feed back.
%
% Currently an engine can elect to not implement these API's
% by throwing the atom not_supported.
-callback open_write_stream(
    DbHandle::db_handle(),
    Options::write_stream_options()) ->
        {ok, pid()}.


% See the documentation for open_write_stream
-callback open_read_stream(DbHandle::db_handle(), StreamDiskInfo::any()) ->
        {ok, {Module::atom(), ReadStreamState::any()}}.


% See the documentation for open_write_stream
-callback is_active_stream(DbHandle::db_handle(), ReadStreamState::any()) ->
        boolean().


% This funciton is called by many processes concurrently.
%
% This function is called to fold over the documents in
% the database sorted by the raw byte collation order of
% the document id. For each document id, the supplied user
% function should be invoked with the first argument set
% to the #full_doc_info{} record and the second argument
% set to the current user supplied accumulator. The return
% value of the user function is a 2-tuple of {Go, NewUserAcc}.
% The NewUserAcc value should then replace the current
% user accumulator. If Go is the atom ok, iteration over
% documents should continue. If Go is the atom stop, then
% iteration should halt and the return value should be
% {ok, NewUserAcc}.
%
% Possible options to this function include:
%
%     1. start_key - Start iteration at the provided key or
%        or just after if the key doesn't exist
%     2. end_key - Stop iteration prior to visiting the provided
%        key
%     3. end_key_gt - Stop iteration just after the provided key
%     4. dir - The atom fwd or rev. This is to be able to iterate
%        over documents in reverse order. The logic for comparing
%        start_key, end_key, and end_key_gt are then reversed (ie,
%        when rev, start_key should be greater than end_key if the
%        user wishes to see results)
%     5. include_reductions - This is a hack for _all_docs since
%        it currently relies on reductions to count an offset. This
%        is a terrible hack that will need to be addressed by the
%        API in the future. If this option is present the supplied
%        user function expects three arguments, where the first
%        argument is a #full_doc_info{} record, the second argument
%        is the current list of reductions to the left of the current
%        document, and the third argument is the current user
%        accumulator. The return value from the user function is
%        unaffected. However the final return value of the function
%        should include the final total reductions as the second
%        element of a 3-tuple. Like I said, this is a hack.
%     6. include_deleted - By default deleted documents are not
%        included in fold_docs calls. However in some special
%        cases we do want to see them (as of now, just in couch_changes
%        during the design document changes optimization)
%
% Historically, if a process calls this function repeatedly it
% would see the same results returned even if there were concurrent
% updates happening. However there doesn't seem to be any instance of
% that actually happening so a storage engine that includes new results
% between invocations shouldn't have any issues.
-callback fold_docs(
    DbHandle::db_handle(),
    UserFold::doc_fold_fun(),
    UserAcc::any(),
    doc_fold_options()) ->
        {ok, LastUserAcc::any()}.


% This function may be called by many processes concurrently.
%
% This should behave exactly the same as fold_docs/4 except that it
% should only return local documents and the first argument to the
% user function is a #doc{} record, not a #full_doc_info{}.
-callback fold_local_docs(
    DbHandle::db_handle(),
    UserFold::local_doc_fold_fun(),
    UserAcc::any(),
    doc_fold_options()) ->
        {ok, LastUserAcc::any()}.


% This function may be called by many processes concurrently.
%
% This function is called to fold over the documents (not local
% documents) in order of their most recent update. Each document
% in the database should have exactly one entry in this sequence.
% If a document is updated during a call to this funciton it should
% not be included twice as that will probably lead to Very Bad Things.
%
% This should behave similarly to fold_docs/4 in that the supplied
% user function should be invoked with a #full_doc_info{} record
% as the first arugment and the current user accumulator as the
% second argument. The same semantics for the return value from the
% user function should be handled as in fold_docs/4.
%
% The StartSeq parameter indicates where the fold should start
% *after*. As in, if a change with a value of StartSeq exists in the
% database it should not be included in the fold.
%
% The only option currently supported by the API is the `dir`
% option that should behave the same as for fold_docs.
-callback fold_changes(
    DbHandle::db_handle(),
    StartSeq::non_neg_integer(),
    UserFold::changes_fold_fun(),
    UserAcc::any(),
    changes_fold_options()) ->
        {ok, LastUserAcc::any()}.


% This function may be called by many processes concurrently.
%
% This function is called to count the number of documents changed
% since the given UpdateSeq (ie, not including the possible change
% at exactly UpdateSeq). It is currently only used internally to
% provide a status update in a replication's _active_tasks entry
% to indicate how many documents are left to be processed.
%
% This is a fairly difficult thing to support in engine's that don't
% behave exactly like a tree with efficient support for counting rows
% between keys. As such returning 0 or even just the difference between
% the current update sequence is possibly the best some storage engines
% can provide. This may lead to some confusion when interpreting the
% _active_tasks entry if the storage engine isn't accounted for by the
% client.
-callback count_changes_since(
    DbHandle::db_handle(),
    UpdateSeq::non_neg_integer()) ->
        TotalChanges::non_neg_integer().


% This function is called in the context of couch_db_updater and as
% such is guaranteed to be single threaded for the given DbHandle.
%
% If a storage engine requires compaction this is a trigger to start
% it off. However a storage engine can do whatever it wants here. As
% this is fairly engine specific there's not a lot guidance that is
% generally applicable.
%
% When compaction is finished the compactor should use
% gen_server:cast/2 to send a {compact_done, CompactEngine, CompactInfo}
% message to the Parent pid provided. Currently CompactEngine
% must be the same engine that started the compaction and CompactInfo
% is an arbitrary term that's passed to finish_compaction/4.
-callback start_compaction(
    DbHandle::db_handle(),
    DbName::binary(),
    Options::db_open_options(),
    Parent::pid()) ->
        {ok, NewDbHandle::db_handle(), CompactorPid::pid()}.


% This function is called in the context of couch_db_udpater and as
% such is guarnateed to be single threaded for the given DbHandle.
%
% Same as for start_compaction, this will be extremely specific to
% any given storage engine.
%
% The split in the API here is so that if the storage engine needs
% to update the DbHandle state of the couch_db_updater it can as
% finish_compaction/4 is called in the context of the couch_db_updater.
-callback finish_compaction(
    OldDbHandle::db_handle(),
    DbName::binary(),
    Options::db_open_options(),
    CompactInfo::any()) ->
        {ok, CompactedDbHandle::db_handle(), CompactorPid::pid() | undefined}.


-export([
    exists/2,
    delete/4,
    delete_compaction_files/4,

    init/3,
    terminate/2,
    handle_db_updater_call/3,
    handle_db_updater_info/2,

    incref/1,
    decref/1,
    monitored_by/1,

    last_activity/1,

    get_engine/1,
    get_compacted_seq/1,
    get_del_doc_count/1,
    get_disk_version/1,
    get_doc_count/1,
    get_epochs/1,
    get_last_purged/1,
    get_purge_seq/1,
    get_revs_limit/1,
    get_security/1,
    get_size_info/1,
    get_update_seq/1,
    get_uuid/1,

    set_revs_limit/2,
    set_security/2,

    open_docs/2,
    open_local_docs/2,
    read_doc_body/2,

    serialize_doc/2,
    write_doc_body/2,
    write_doc_infos/4,
    commit_data/1,

    open_write_stream/2,
    open_read_stream/2,
    is_active_stream/2,

    fold_docs/4,
    fold_local_docs/4,
    fold_changes/5,
    count_changes_since/2,

    start_compaction/1,
    finish_compaction/2
]).


exists(Engine, DbPath) ->
    Engine:exists(DbPath).


delete(Engine, RootDir, DbPath, DelOpts) when is_list(DelOpts) ->
    Engine:delete(RootDir, DbPath, DelOpts).


delete_compaction_files(Engine, RootDir, DbPath, DelOpts)
        when is_list(DelOpts) ->
    Engine:delete_compaction_files(RootDir, DbPath, DelOpts).


init(Engine, DbPath, Options) ->
    case Engine:init(DbPath, Options) of
         {ok, EngineState} ->
             {ok, {Engine, EngineState}};
         Error ->
             throw(Error)
    end.


terminate(Reason, #db{} = Db) ->
    #db{engine = {Engine, EngineState}} = Db,
    Engine:terminate(Reason, EngineState).


handle_db_updater_call(Msg, _From, #db{} = Db) ->
    #db{
        engine = {Engine, EngineState}
    } = Db,
    case Engine:handle_db_updater_call(Msg, EngineState) of
        {reply, Resp, NewState} ->
            {reply, Resp, Db#db{engine = {Engine, NewState}}};
        {stop, Reason, Resp, NewState} ->
            {stop, Reason, Resp, Db#db{engine = {Engine, NewState}}}
    end.


handle_db_updater_info(Msg, #db{} = Db) ->
    #db{
        name = Name,
        engine = {Engine, EngineState}
    } = Db,
    case Engine:handle_db_updater_info(Msg, EngineState) of
        {noreply, NewState} ->
            {noreply, Db#db{engine = {Engine, NewState}}};
        {noreply, NewState, Timeout} ->
            {noreply, Db#db{engine = {Engine, NewState}}, Timeout};
        {stop, Reason, NewState} ->
            couch_log:error("DB ~s shutting down: ~p", [Name, Msg]),
            {stop, Reason, Db#db{engine = {Engine, NewState}}}
    end.


incref(#db{} = Db) ->
    #db{engine = {Engine, EngineState}} = Db,
    {ok, NewState} = Engine:incref(EngineState),
    {ok, Db#db{engine = {Engine, NewState}}}.


decref(#db{} = Db) ->
    #db{engine = {Engine, EngineState}} = Db,
    Engine:decref(EngineState).


monitored_by(#db{} = Db) ->
    #db{engine = {Engine, EngineState}} = Db,
    Engine:monitored_by(EngineState).


last_activity(#db{} = Db) ->
    #db{engine = {Engine, EngineState}} = Db,
    Engine:last_activity(EngineState).


get_engine(#db{} = Db) ->
    #db{engine = {Engine, _}} = Db,
    Engine.


get_compacted_seq(#db{} = Db) ->
    #db{engine = {Engine, EngineState}} = Db,
    Engine:get_compacted_seq(EngineState).


get_del_doc_count(#db{} = Db) ->
    #db{engine = {Engine, EngineState}} = Db,
    Engine:get_del_doc_count(EngineState).


get_disk_version(#db{} = Db) ->
    #db{engine = {Engine, EngineState}} = Db,
    Engine:get_disk_version(EngineState).


get_doc_count(#db{} = Db) ->
    #db{engine = {Engine, EngineState}} = Db,
    Engine:get_doc_count(EngineState).


get_epochs(#db{} = Db) ->
    #db{engine = {Engine, EngineState}} = Db,
    Engine:get_epochs(EngineState).


get_last_purged(#db{} = Db) ->
    #db{engine = {Engine, EngineState}} = Db,
    Engine:get_last_purged(EngineState).


get_purge_seq(#db{} = Db) ->
    #db{engine = {Engine, EngineState}} = Db,
    Engine:get_purge_seq(EngineState).


get_revs_limit(#db{} = Db) ->
    #db{engine = {Engine, EngineState}} = Db,
    Engine:get_revs_limit(EngineState).


get_security(#db{} = Db) ->
    #db{engine = {Engine, EngineState}} = Db,
    Engine:get_security(EngineState).


get_size_info(#db{} = Db) ->
    #db{engine = {Engine, EngineState}} = Db,
    Engine:get_size_info(EngineState).


get_update_seq(#db{} = Db) ->
    #db{engine = {Engine, EngineState}} = Db,
    Engine:get_update_seq(EngineState).

get_uuid(#db{} = Db) ->
    #db{engine = {Engine, EngineState}} = Db,
    Engine:get_uuid(EngineState).


set_revs_limit(#db{} = Db, RevsLimit) ->
    #db{engine = {Engine, EngineState}} = Db,
    {ok, NewSt} = Engine:set_revs_limit(EngineState, RevsLimit),
    {ok, Db#db{engine = {Engine, NewSt}}}.


set_security(#db{} = Db, SecProps) ->
    #db{engine = {Engine, EngineState}} = Db,
    {ok, NewSt} = Engine:set_security(EngineState, SecProps),
    {ok, Db#db{engine = {Engine, NewSt}}}.


open_docs(#db{} = Db, DocIds) ->
    #db{engine = {Engine, EngineState}} = Db,
    Engine:open_docs(EngineState, DocIds).


open_local_docs(#db{} = Db, DocIds) ->
    #db{engine = {Engine, EngineState}} = Db,
    Engine:open_local_docs(EngineState, DocIds).


read_doc_body(#db{} = Db, RawDoc) ->
    #db{engine = {Engine, EngineState}} = Db,
    Engine:read_doc_body(EngineState, RawDoc).


serialize_doc(#db{} = Db, #doc{} = Doc) ->
    #db{engine = {Engine, EngineState}} = Db,
    Engine:serialize_doc(EngineState, Doc).


write_doc_body(#db{} = Db, #doc{} = Doc) ->
    #db{engine = {Engine, EngineState}} = Db,
    Engine:write_doc_body(EngineState, Doc).


write_doc_infos(#db{} = Db, DocUpdates, LocalDocs, PurgedDocIdRevs) ->
    #db{engine = {Engine, EngineState}} = Db,
    {ok, NewSt} = Engine:write_doc_infos(
            EngineState, DocUpdates, LocalDocs, PurgedDocIdRevs),
    {ok, Db#db{engine = {Engine, NewSt}}}.


commit_data(#db{} = Db) ->
    #db{engine = {Engine, EngineState}} = Db,
    {ok, NewSt} = Engine:commit_data(EngineState),
    {ok, Db#db{engine = {Engine, NewSt}}}.


open_write_stream(#db{} = Db, Options) ->
    #db{engine = {Engine, EngineState}} = Db,
    Engine:open_write_stream(EngineState, Options).


open_read_stream(#db{} = Db, StreamDiskInfo) ->
    #db{engine = {Engine, EngineState}} = Db,
    Engine:open_read_stream(EngineState, StreamDiskInfo).


is_active_stream(#db{} = Db, ReadStreamState) ->
    #db{engine = {Engine, EngineState}} = Db,
    Engine:is_active_stream(EngineState, ReadStreamState).


fold_docs(#db{} = Db, UserFun, UserAcc, Options) ->
    #db{engine = {Engine, EngineState}} = Db,
    Engine:fold_docs(EngineState, UserFun, UserAcc, Options).


fold_local_docs(#db{} = Db, UserFun, UserAcc, Options) ->
    #db{engine = {Engine, EngineState}} = Db,
    Engine:fold_local_docs(EngineState, UserFun, UserAcc, Options).


fold_changes(#db{} = Db, StartSeq, UserFun, UserAcc, Options) ->
    #db{engine = {Engine, EngineState}} = Db,
    Engine:fold_changes(EngineState, StartSeq, UserFun, UserAcc, Options).


count_changes_since(#db{} = Db, StartSeq) ->
    #db{engine = {Engine, EngineState}} = Db,
    Engine:count_changes_since(EngineState, StartSeq).


start_compaction(#db{} = Db) ->
    #db{
        engine = {Engine, EngineState},
        name = DbName,
        options = Options
    } = Db,
    {ok, NewEngineState, Pid} = Engine:start_compaction(
            EngineState, DbName, Options, self()),
    {ok, Db#db{
        engine = {Engine, NewEngineState},
        compactor_pid = Pid
    }}.


finish_compaction(Db, CompactInfo) ->
    #db{
        engine = {Engine, St},
        name = DbName,
        options = Options
    } = Db,
    NewDb = case Engine:finish_compaction(St, DbName, Options, CompactInfo) of
        {ok, NewState, undefined} ->
            couch_event:notify(DbName, compacted),
            Db#db{
                engine = {Engine, NewState},
                compactor_pid = nil
            };
        {ok, NewState, CompactorPid} when is_pid(CompactorPid) ->
            Db#db{
                engine = {Engine, NewState},
                compactor_pid = CompactorPid
            }
    end,
    ok = gen_server:call(couch_server, {db_updated, NewDb}, infinity),
    {ok, NewDb}.