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Doc update and clerk<->penciller changes

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Reviewing code to update comments revealed a weakness in the sequence of
events between penciller and clerk committing a manifest change wherby
an ill-timed crash could lead to files being deleted without the
manifest changing.

A different, and safer pattern now used between theses two actors.
This commit is contained in:
martinsumner 2016-10-09 22:33:45 +01:00
parent 4a8a2c1555
commit d2cc07a9eb
4 changed files with 119 additions and 66 deletions
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43
src/leveled_bookie.erl
View file

@ -18,9 +18,11 @@
%% -------- The actors ---------
%%
%% The store is fronted by a Bookie, who takes support from different actors:
%% - An Inker who persists new data into the jornal, and returns items from
%% - An Inker who persists new data into the journal, and returns items from
%% the journal based on sequence number
%% - A Penciller who periodically redraws the ledger
%% - A Penciller who periodically redraws the ledger, that associates keys with
%% sequence numbers and other metadata, as well as secondary keys (for index
%% queries)
%% - One or more Clerks, who may be used by either the inker or the penciller
%% to fulfill background tasks
%%
@ -61,11 +63,10 @@
%% well as the object size on disk within the Journal.
%%
%% Once the object has been persisted to the Journal, the Ledger can be updated.
%% The Ledger is updated by the Bookie applying a function (passed in at
%% startup) to the Value to return the Object Metadata, a function to generate
%% a hash of the Value and also taking the Primary Key, the IndexSpecs, the
%% Sequence Number in the Journal and the Object Size (returned from the
%% Inker).
%% The Ledger is updated by the Bookie applying a function (extract_metadata/4)
%% to the Value to return the Object Metadata, a function to generate a hash
%% of the Value and also taking the Primary Key, the IndexSpecs, the Sequence
%% Number in the Journal and the Object Size (returned from the Inker).
%%
%% The Bookie should generate a series of ledger key changes from this
%% information, using a function passed in at startup. For Riak this will be
@ -79,20 +80,20 @@
%% null,
%% {active, TS}|{tomb, TS}}
%%
%% Recent Ledger changes are retained initially in the Bookies' memory (in an
%% in-memory ets table). Periodically, the current table is pushed to the
%% Penciller for eventual persistence, and a new table is started.
%% Recent Ledger changes are retained initially in the Bookies' memory (in a
%% small generally balanced tree). Periodically, the current table is pushed to
%% the Penciller for eventual persistence, and a new table is started.
%%
%% This completes the non-deferrable work associated with a PUT
%%
%% -------- Snapshots (Key & Metadata Only) --------
%%
%% If there is a snapshot request (e.g. to iterate over the keys) the Bookie
%% must first produce a tree representing the results of the request which are
%% present in its in-memory view of the ledger. The Bookie then requests
%% a copy of the current Ledger manifest from the Penciller, and the Penciller
%5 should interest of the iterator at the manifest sequence number at the time
%% of the request.
%% may request a clone of the Penciller, or the Penciller and the Inker.
%%
%% The clone is seeded with the manifest. Teh clone should be registered with
%% the real Inker/Penciller, so that the real Inker/Penciller may prevent the
%% deletion of files still in use by a snapshot clone.
%%
%% Iterators should de-register themselves from the Penciller on completion.
%% Iterators should be automatically release after a timeout period. A file
@ -100,10 +101,6 @@
%% there are no registered iterators from before the point the file was
%% removed from the manifest.
%%
%% Snapshots may be non-recent, if recency is unimportant. Non-recent
%% snapshots do no require the Bookie to return the results of the in-memory
%% table, the Penciller alone cna be asked.
%%
%% -------- Special Ops --------
%%
%% e.g. Get all for SegmentID/Partition
@ -115,10 +112,11 @@
%% On startup the Bookie must restart both the Inker to load the Journal, and
%% the Penciller to load the Ledger. Once the Penciller has started, the
%% Bookie should request the highest sequence number in the Ledger, and then
%% and try and rebuild any missing information from the Journal
%% and try and rebuild any missing information from the Journal.
%%
%% To rebuild the Ledger it requests the Inker to scan over the files from
%% the sequence number and re-generate the Ledger changes.
%% the sequence number and re-generate the Ledger changes - pushing the changes
%% directly back into the Ledger.
@ -359,7 +357,6 @@ shutdown_wait([TopPause|Rest], Inker) ->
set_options(Opts) ->
%% TODO: Change the max size default, and allow setting through options
MaxJournalSize = case Opts#bookie_options.max_journalsize of
undefined ->
30000;
@ -497,8 +494,6 @@ load_fun(KeyInLedger, ValueInLedger, _Position, Acc0, ExtractFun) ->
{MinSQN, MaxSQN, Output} = Acc0,
{SQN, PK} = KeyInLedger,
% VBin may already be a term
% TODO: Should VSize include CRC?
% Using ExtractFun means we ignore simple way of getting size (from length)
{VBin, VSize} = ExtractFun(ValueInLedger),
{Obj, IndexSpecs} = case is_binary(VBin) of
true ->