func addPosting(b *flatbuffers.Builder, p types.Posting) flatbuffers.UOffsetT {
so := b.CreateByteString(p.Source()) // Do this before posting start.
var bo flatbuffers.UOffsetT
if p.ValueLength() > 0 {
bo = b.CreateByteVector(p.ValueBytes())
}
types.PostingStart(b)
types.PostingAddUid(b, p.Uid())
if bo > 0 {
types.PostingAddValue(b, bo)
}
types.PostingAddSource(b, so)
types.PostingAddTs(b, p.Ts())
types.PostingAddOp(b, p.Op())
return types.PostingEnd(b)
}
// mutationIndex (mindex) is useful to avoid having to parse the entire
// postinglist upto idx, for every Get(*types.Posting, idx), which has a
// complexity of O(idx). Iteration over N size posting list would this push
// us into O(N^2) territory, without this technique.
//
// Using this technique,
// we can overlay mutation layers over immutable posting list, to allow for
// O(m) lookups, where m = size of mutation list. Obviously, the size of
// mutation list should be much smaller than the size of posting list, except
// in tiny posting lists, where performance wouldn't be such a concern anyways.
//
// Say we have this data:
// Posting List (plist, immutable):
// idx: 0 1 2 3 4 5
// value: 2 5 9 10 13 15
//
// Mutation List (mlist):
// idx: 0 1 2
// value: 7 10 13' // posting uid is 13 but other values vary.
// Op: SET DEL SET
// Effective: ADD DEL REP (REP = replace)
//
// ----------------------------------------------------------------------------
// regenerateIndex would generate these:
// mlayer (layer just above posting list contains only replace instructions)
// idx: 4
// value: 13'
// Op: SET
// Effective: REP (REP = replace)
//
// mindex:
// idx: 2 4
// value: 7 10
// moveidx: -1 +1
// Effective: ADD DEL
//
// Now, let's see how the access would work:
// idx: get --> calculation [idx, served from, value]
// idx: 0 --> 0 [0, plist, 2]
// idx: 1 --> 1 [1, plist, 5]
// idx: 2 --> ADD from mindex
// --> [2, mindex, 7] // also has moveidx = -1
// idx: 3 --> 3 + moveidx=-1 = 2 [2, plist, 9]
// idx: 4 --> DEL from mindex
// --> 4 + moveidx=-1 + moveidx=+1 = 4 [4, mlayer, 13']
// idx: 5 --> 5 + moveidx=-1 + moveidx=+1 = 5 [5, plist, 15]
//
// Thus we can provide mutation layers over immutable posting list, while
// still ensuring fast lookup access.
//
// NOTE: This function expects the caller to hold a RW Lock.
// Update: With mergeMutation function, we're adding mutations with a cost
// of O(log M + log N), where M = number of previous mutations, and N =
// number of postings in the immutable posting list.
func (l *List) mergeMutation(mp *types.Posting) {
curUid := mp.Uid()
pi, puid := l.lePostingIndex(curUid) // O(log N)
mi, muid := l.leMutationIndex(curUid) // O(log M)
inPlist := puid == curUid
// O(1) follows, but any additions or deletions from mindex would
// be O(M) due to element shifting. In terms of benchmarks, this performs
// a LOT better than when I was running O(N + M), re-generating mutation
// flatbuffers, linked lists etc.
mlink := new(MutationLink)
mlink.posting = mp
if mp.Op() == Del {
if muid == curUid { // curUid found in mindex.
if inPlist { // In plist, so replace previous instruction in mindex.
mlink.moveidx = 1
mlink.idx = pi + mi
l.mindex[mi] = mlink
} else { // Not in plist, so delete previous instruction in mindex.
l.mdelta -= 1
l.mindexDeleteAt(mi)
}
} else { // curUid not found in mindex.
if inPlist { // In plist, so insert in mindex.
mlink.moveidx = 1
l.mdelta -= 1
mlink.idx = pi + mi + 1
l.mindexInsertAt(mlink, mi+1)
} else {
// Not found in plist, and not found in mindex. So, ignore.
}
}
} else if mp.Op() == Set {
if muid == curUid { // curUid found in mindex.
if inPlist { // In plist, so delete previous instruction, set in mlayer.
l.mindexDeleteAt(mi)
l.mlayer[pi] = *mp
} else { // Not in plist, so replace previous set instruction in mindex.
// NOTE: This prev instruction couldn't have been a Del instruction.
mlink.idx = pi + 1 + mi
mlink.moveidx = -1
l.mindex[mi] = mlink
}
} else { // curUid not found in mindex.
if inPlist { // In plist, so just set it in mlayer.
// If this posting matches what we already have in posting list,
// we don't need to `dirty` this by adding to mlayer.
plist := l.getPostingList()
var cp types.Posting
if ok := plist.Postings(&cp, pi); ok {
if samePosting(&cp, mp) {
return // do nothing.
}
}
l.mlayer[pi] = *mp
} else { // not in plist, not in mindex, so insert in mindex.
mlink.moveidx = -1
l.mdelta += 1
mlink.idx = pi + 1 + mi + 1 // right of pi, and right of mi.
l.mindexInsertAt(mlink, mi+1)
}
}
} else {
glog.WithField("op", mp.Op()).Fatal("Invalid operation.")
}
}
func (l *List) updateMutationLayer(mpost *types.Posting) bool {
l.AssertLock()
x.AssertTrue(mpost.Op == Set || mpost.Op == Del)
// First check the mutable layer.
midx := sort.Search(len(l.mlayer), func(idx int) bool {
mp := l.mlayer[idx]
return mpost.Uid <= mp.Uid
})
// This block handles the case where mpost.UID is found in mutation layer.
if midx < len(l.mlayer) && l.mlayer[midx].Uid == mpost.Uid {
// mp is the posting found in mlayer.
oldPost := l.mlayer[midx]
// Note that mpost.Op is either Set or Del, whereas oldPost.Op can be
// either Set or Del or Add.
msame := samePosting(oldPost, mpost)
if msame && ((mpost.Op == Del) == (oldPost.Op == Del)) {
// This posting has similar content as what is found in mlayer. If the
// ops are similar, then we do nothing. Note that Add and Set are
// considered similar, and the second clause is true also when
// mpost.Op==Add and oldPost.Op==Set.
return false
}
if !msame && mpost.Op == Del {
// Invalid Del as contents do not match.
return false
}
// Here are the remaining cases.
// Del, Set: Replace with new post.
// Del, Del: Replace with new post.
// Set, Del: Replace with new post.
// Set, Set: Replace with new post.
// Add, Del: Undo by removing oldPost.
// Add, Set: Replace with new post. Need to set mpost.Op to Add.
if oldPost.Op == Add {
if mpost.Op == Del {
// Undo old post.
copy(l.mlayer[midx:], l.mlayer[midx+1:])
l.mlayer[len(l.mlayer)-1] = nil
l.mlayer = l.mlayer[:len(l.mlayer)-1]
return true
}
// Add followed by Set is considered an Add. Hence, mutate mpost.Op.
mpost.Op = Add
}
l.mlayer[midx] = mpost
return true
}
// Didn't find it in mutable layer. Now check the immutable layer.
pl := l.getPostingList(0)
pidx := sort.Search(len(pl.Postings), func(idx int) bool {
p := pl.Postings[idx]
return mpost.Uid <= p.Uid
})
var uidFound, psame bool
if pidx < len(pl.Postings) {
p := pl.Postings[pidx]
uidFound = mpost.Uid == p.Uid
if uidFound {
psame = samePosting(p, mpost)
}
}
if mpost.Op == Set {
if psame {
return false
}
if !uidFound {
// Posting not found in PL. This is considered an Add operation.
mpost.Op = Add
}
} else if !psame { // mpost.Op==Del
// Either we fail to find UID in immutable PL or contents don't match.
return false
}
// Doesn't match what we already have in immutable layer. So, add to mutable layer.
if midx >= len(l.mlayer) {
// Add it at the end.
l.mlayer = append(l.mlayer, mpost)
return true
}
// Otherwise, add it where midx is pointing to.
l.mlayer = append(l.mlayer, nil)
copy(l.mlayer[midx+1:], l.mlayer[midx:])
l.mlayer[midx] = mpost
return true
}