From: Brian Warner Date: Wed, 28 Mar 2007 03:14:45 +0000 (-0700) Subject: ICodecEncoder: resolve some questions about the API, still more to examine X-Git-Url: https://git.rkrishnan.org/components/com_hotproperty/css/?a=commitdiff_plain;h=2b2e0fc0d447efaa8bcccf18a40785e4c9eaa788;p=tahoe-lafs%2Ftahoe-lafs.git ICodecEncoder: resolve some questions about the API, still more to examine --- diff --git a/src/allmydata/encode_new.py b/src/allmydata/encode_new.py index 1f03e364..75384572 100644 --- a/src/allmydata/encode_new.py +++ b/src/allmydata/encode_new.py @@ -139,7 +139,10 @@ class Encoder(object): def _encoded_segment(self, (shares, shareids)): dl = [] - for shareid,subshare in zip(shareids, shares): + for i in range(len(shares)): + subshare = shares[i] + shareid = shareids[i] + #for shareid,subshare in zip(shareids, shares): d = self.send_subshare(shareid, self.segment_num, subshare) dl.append(d) self.subshare_hashes[shareid].append(hash(subshare)) diff --git a/src/allmydata/interfaces.py b/src/allmydata/interfaces.py index f34bd0d4..63bb6c53 100644 --- a/src/allmydata/interfaces.py +++ b/src/allmydata/interfaces.py @@ -99,7 +99,22 @@ class ICodecEncoder(Interface): data. For example, set_params(1000, 5, 5) offers no redundancy at all, whereas set_params(1000, 1, 10) provides 10x redundancy. + Numerical Restrictions: 'data_size' is required to be an integral + multiple of 'required_shares'. In general, the caller should choose + required_shares and max_shares based upon their reliability + requirements and the number of peers available (the total storage + space used is roughly equal to max_shares*data_size/required_shares), + then choose data_size to achieve the memory footprint desired (larger + data_size means more efficient operation, smaller data_size means + smaller memory footprint). + + In addition, 'max_shares' must be equal to or greater than + 'required_shares'. Of course, setting them to be equal causes + encode() to degenerate into a particularly slow form of the 'split' + utility. + See encode() for more details about how these parameters are used. + set_params() must be called before any other ICodecEncoder methods may be invoked. """ @@ -133,6 +148,36 @@ class ICodecEncoder(Interface): """Return the length of the shares that encode() will produce. """ + def encode_proposal(data, desired_share_ids=None): + """Encode some data. + + 'data' must be a string (or other buffer object), and len(data) must + be equal to the 'data_size' value passed earlier to set_params(). + + This will return a Deferred that will fire with two lists. The first + is a list of shares, each of which is a string (or other buffer + object) such that len(share) is the same as what get_share_size() + returned earlier. The second is a list of shareids, in which each is + an integer. The lengths of the two lists will always be equal to each + other. The user should take care to keep each share closely + associated with its shareid, as one is useless without the other. + + The length of this output list will normally be the same as the value + provided to the 'max_shares' parameter of set_params(). This may be + different if 'desired_share_ids' is provided. + + 'desired_share_ids', if provided, is required to be a sequence of + ints, each of which is required to be >= 0 and < max_shares. If not + provided, encode() will produce 'max_shares' shares, as if + 'desired_share_ids' were set to range(max_shares). You might use this + if you initially thought you were going to use 10 peers, started + encoding, and then two of the peers dropped out: you could use + desired_share_ids= to skip the work (both memory and CPU) of + producing shares for the peers which are no longer available. + + """ + pass + def encode(inshares, desired_share_ids=None): """Encode some data. This may be called multiple times. Each call is independent. @@ -146,14 +191,6 @@ class ICodecEncoder(Interface): encode(), unless of course it already happens to be an even multiple of required_shares in length.) - QUESTION for zooko: that implies that 'data_size' must be an - integral multiple of 'required_shares', right? Which means these - restrictions should be documented in set_params() rather than (or in - addition to) encode(), since that's where they must really be - honored. This restriction feels like an abstraction leak, but maybe - it is cleaner to enforce constraints on 'data_size' rather than - quietly implement internal padding. I dunno. - ALSO: the requirement to break up your data into 'required_shares' chunks before calling encode() feels a bit surprising, at least from the point of view of a user who doesn't know how FEC works. It feels @@ -169,11 +206,6 @@ class ICodecEncoder(Interface): word 'data' for that-which-is-not-shares.. maybe we should use that term? - ALSO*3: given that we need to keep share0+shareid0 attached from - encode() to the eventual decode(), would it be better to return and - accept a zip() of these two lists? i.e. [(share0,shareid0), - (share1,shareid1),...] - 'desired_share_ids', if provided, is required to be a sequence of ints, each of which is required to be >= 0 and < max_shares. If not provided, encode() will produce 'max_shares' shares, as if @@ -198,6 +230,22 @@ class ICodecEncoder(Interface): (max_shares - required_shares) * get_share_size(). """ + # rejected ideas: + # + # returning a list of (shareidN,shareN) tuples instead of a pair of + # lists (shareids..,shares..). Brian thought the tuples would + # encourage users to keep the share and shareid together throughout + # later processing, Zooko pointed out that the code to iterate + # through two lists is not really more complicated than using a list + # of tuples and there's also a performance improvement + # + # having 'data_size' not required to be an integral multiple of + # 'required_shares'. Doing this would require encode() to perform + # padding internally, and we'd prefer to have any padding be done + # explicitly by the caller. Yes, it is an abstraction leak, but + # hopefully not an onerous one. + + class ICodecDecoder(Interface): def set_serialized_params(params): """Set up the parameters of this encoder, from a string returned by