最近在看paxos协议，感觉很复杂，一步步来吧，学习的过程就是这样。

Mike Burrows, inventor of the Chubby service at Google, says that “there is only one consensus protocol, and that’s Paxos” – all other approaches are just broken versions of Paxos. 

摘自：

相对来说，raft使用状态机来简化paxos协议，然后riak_ensemble实现的是Multi-Paxos协议。理论上说，Multi-Paxos比raft协议要并发强。

今天看到riak_ensemble中的文件检验，其中使用了CRC32检验。

%% 在riak_ensemble_save.erl文件中write(File, Data) ->    CRC = erlang:crc32(Data),    Size = byte_size(Data),    Meta = <
    
     >,    Out = [Meta, Data,  %% copy 1           Data, Meta], %% copy 2    ok = filelib:ensure_dir(File),    try        _ = Out,        ok = riak_ensemble_util:replace_file(File, Out),        ok = riak_ensemble_util:replace_file(File ++ ".backup", Out),        ok    catch        _:Err ->            {error, Err}    end.%% 在riak_ensemble_util.erl文件中replace_file(FN, Data) ->    TmpFN = FN ++ ".tmp",    {ok, FH} = file:open(TmpFN, [write, raw]),    try        ok = file:write(FH, Data),        ok = file:sync(FH),        ok = file:close(FH),        ok = file:rename(TmpFN, FN),        {ok, Contents} = read_file(FN),        true = (Contents == iolist_to_binary(Data)),        ok    catch _:Err ->            {error, Err}    end.read_file(FName) ->    case file:open(FName, [read, raw, binary]) of        {ok, FD} ->            Result = read_file(FD, []),            ok = file:close(FD),            case Result of                {ok, IOList} ->                    {ok, iolist_to_binary(IOList)};                {error, _}=Err ->                    Err            end;        {error,_}=Err ->            Err    end.read_file(FD, Acc) ->    case file:read(FD, 4096) of        {ok, Data} ->            read_file(FD, [Data|Acc]);        eof ->            {ok, lists:reverse(Acc)};        {error, _}=Err ->            Err    end.
    

可以看到，写文件前，先检验文件内容的crc32值以及长度，然后同时写入文件和文件备份，写入后，再以4k对齐方式读入文件内容，确保文件写入没有问题。使用try catch 确保文件写入、同步、检验是在一个事务里面。然后在写入文件和文件备份时，使用try catch确保文件和文件备份是同时完成的。

然后是文件读入，这时也要检验：

%% 在riak_ensemble_save.erl 文件中do_read(File) ->    case riak_ensemble_util:read_file(File) of        {ok, Binary} ->            safe_read(Binary);        {error, _} ->            not_found    end.safe_read(<
    
     >) ->    case erlang:crc32(Data) of        CRC ->            {ok, Data};        _ ->            safe_read_backup(Rest)    end;safe_read(Binary) ->    safe_read_backup(Binary).safe_read_backup(Binary) when byte_size(Binary) =< 8 ->    not_found;safe_read_backup(Binary) ->    BinSize = byte_size(Binary),    Skip = BinSize - 8,    <<_:Skip/binary, CRC:32/integer, Size:32/integer>> = Binary,    Skip2 = Skip - Size,    case Binary of        <<_:Skip2/binary, Data:Size/binary, _:8/binary>> ->            case erlang:crc32(Data) of                CRC ->                    {ok, Data};                _ ->                    not_found            end;        _ ->            not_found    end.
    

可以看到，读入文件时，会按照上面写入的规则通过crc32值来检验一下文件内容。如果检验失败，则尝试去掉内容头部，然后对内容按照规格检验。