Database Reference
In-Depth Information
Figure 7. Structure of tables peer, constraints and partners in 6P2P database
8. A
propagation
is a relationship between a peer (the
target peer
) and another peer (the
source peer
)
where the query has been sent (propagated) to. While propagating queries, the following three
objectives are taken into account: (1) avoiding cycles, (2) deciding about propagation modes (
P2P
or
local
), and (3) deciding about merging modes (
full
or
partial
).
6P2P Database
A peer's database consists of five tables:
Peer
,
Constraints
,
Partners
(Figure 7),
Queries
, and
Propaga-
tions
(Figure 8).
1.
Peer(myPeer, myPatt, myData, xfdXQuery, keyXQuery)
- has exactly one row, where:
myPeer
- the
URL of the peer owning the database;
myPatt
- the schema (tree-pattern formula) of the data source;
myData
- the peer's data source, i.e. an XML document or an XML view over some data reposi-
tories.
xfdXQuery
and
keyXQuery
are XQuery programs obtained by the translation of constraints
specifications, TP-XFDs and keys, respectively.
2.
Constraints(constrId, constrType, constrExp)
- stores information about the local data constraints
(in this chapter we discuss only TP-XFDs).
3.
Partners(partPeer, partPatt, mapXQuery)
- stores information about all peer's partners (acquain-
tances), where:
partPeer
- the URL of the partner;
partPatt
- the right-hand side of the schema
mapping to the partner (variable names reflect correspondences between paths in the source and
in the target schema).
mapXQuery
is an XQuery program obtained by translation(by means of
Algorithm 1) of the mapping determined by the
Peer.myPatt
(left-hand side of the mapping) and
Partners.partPatt
(right-hand side of the mapping).
4.
Queries
and
Propagations
(Figure 8) maintain information about queries,
qryId
, and their threads,
qryThreadId
, managed in the 6P2P system. The user specifies a qualifier of the query,
myQualif
,
as well as propagation (
propagMode
) and merging (
mergeMode
) modes.
Query Propagation in 6P2P
Symbolic values in tables (Figure 8) indicate relationships between tuples in tables maintained by peers
@
P
and @
P'
. Algorithm 3 describes propagation of a query (more precisely, a thread of the query) stored
in @
P
:
Queries
. The propagation mode can be either
P2P
(a query is to be propagated to all partners
with
P2P
mode), or
local
(a query is to be answered in the peer's data source without propagation).
In Algorithm 3, the tuple
q
1 contains a query and its context (e.g.
q
1.
myPatt
is the schema against
which
q
1.
myQualif
has been formulated).
LeadsToCycle
(
q
1, @
P'
) returns
true
if the propagation causes
