tahoe-lafs/docs/historical/peer-selection-tahoe3.txt

= THIS PAGE DESCRIBES HISTORICAL ARCHITECTURE CHOICES: THE CURRENT CODE DOES NOT WORK AS DESCRIBED HERE. =

When a file is uploaded, the encoded shares are sent to other peers. But to
which ones? The PeerSelection algorithm is used to make this choice.

In the old (May 2007) version, the verifierid is used to consistently-permute
the set of all peers (by sorting the peers by HASH(verifierid+peerid)). Each
file gets a different permutation, which (on average) will evenly distribute
shares among the grid and avoid hotspots.

This permutation places the peers around a 2^256^-sized ring, like the rim of
a big clock. The 100-or-so shares are then placed around the same ring (at 0,
1/100*2^256^, 2/100*2^256^, ... 99/100*2^256^). Imagine that we start at 0 with
an empty basket in hand and proceed clockwise. When we come to a share, we
pick it up and put it in the basket. When we come to a peer, we ask that peer
if they will give us a lease for every share in our basket.

The peer will grant us leases for some of those shares and reject others (if
they are full or almost full). If they reject all our requests, we remove
them from the ring, because they are full and thus unhelpful. Each share they
accept is removed from the basket. The remainder stay in the basket as we
continue walking clockwise.

We keep walking, accumulating shares and distributing them to peers, until
either we find a home for all shares, or there are no peers left in the ring
(because they are all full). If we run out of peers before we run out of
shares, the upload may be considered a failure, depending upon how many
shares we were able to place. The current parameters try to place 100 shares,
of which 25 must be retrievable to recover the file, and the peer selection
algorithm is happy if it was able to place at least 75 shares. These numbers
are adjustable: 25-out-of-100 means an expansion factor of 4x (every file in
the grid consumes four times as much space when totalled across all
StorageServers), but is highly reliable (the actual reliability is a binomial
distribution function of the expected availability of the individual peers,
but in general it goes up very quickly with the expansion factor).

If the file has been uploaded before (or if two uploads are happening at the
same time), a peer might already have shares for the same file we are
proposing to send to them. In this case, those shares are removed from the
list and assumed to be available (or will be soon). This reduces the number
of uploads that must be performed.

When downloading a file, the current release just asks all known peers for
any shares they might have, chooses the minimal necessary subset, then starts
downloading and processing those shares. A later release will use the full
algorithm to reduce the number of queries that must be sent out. This
algorithm uses the same consistent-hashing permutation as on upload, but
instead of one walker with one basket, we have 100 walkers (one per share).
They each proceed clockwise in parallel until they find a peer, and put that
one on the "A" list: out of all peers, this one is the most likely to be the
same one to which the share was originally uploaded. The next peer that each
walker encounters is put on the "B" list, etc.

All the "A" list peers are asked for any shares they might have. If enough of
them can provide a share, the download phase begins and those shares are
retrieved and decoded. If not, the "B" list peers are contacted, etc. This
routine will eventually find all the peers that have shares, and will find
them quickly if there is significant overlap between the set of peers that
were present when the file was uploaded and the set of peers that are present
as it is downloaded (i.e. if the "peerlist stability" is high). Some limits
may be imposed in large grids to avoid querying a million peers; this
provides a tradeoff between the work spent to discover that a file is
unrecoverable and the probability that a retrieval will fail when it could
have succeeded if we had just tried a little bit harder. The appropriate
value of this tradeoff will depend upon the size of the grid, and will change
over time.