, 2002). The first query asks
for data with a degree of freshness of at least 0.9. Only the first OLAP
node has a freshness index that meets this limit and hence is the only
possible target node. In contrast, Query 2 is asking only for data with a
freshness of at least 0.5. This freshness limit is met by all cluster nodes.
Figure 3. Principle of freshness-aware scheduling
update updatem Query
limit 0.
Query
limit 0.
Query
limit 0.
freshness
.0
freshness
0.
freshness
0.8
freshness
0. refresh
transactions
node
node node nodem
OLTP node OLAP nodes
246 R?¶hm
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Idea Group Inc. is prohibited.
Hence, FAS is free to route Query 2 to any of the OLAP nodes. In Figure,
the last node is chosen to serve the query. Note that it actually could have
been evaluated by any node of the cluster. Query 2 sees indeed a cluster
of size n, while for Query 1 only one node is usable.
The implications to correctness are that queries contended with stale data are serialised
before update transactions which have already committed but have not been
propagated to the query??™s target node so far. This requires that refresh and readonly
transactions are interleaved correctly on the various nodes, and that read-only
transactions access only one version of the data.
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