Distributing Queries

(currently work-in-progress)

If OdysseusNet is activated, the node has the possibility to distribute own queries to other remote OdysseusNodes. For this, the user has to specify that he wish to distribute:

#CONFIG DISTRIBUTE true

This #CONFIG DISTRIBUTE statement in Odysseus Script indicates that the queries which follows should be distributed in the network. 

General process of query distribution

Query distribution in OdysseusNet is divided in three core phases: Partition, Modification and Allocation. Due to implementation reasons, two additional phases were integrated: Preprocess before Partition and Postprocess after Allocation. The picture below shows the three phases mentioned earlier (and the transmission phase).

At the beginning, Odysseus translates the query in a logical graph. This logical graph is preprocessed and then partitioned. The result of the Partition phase is a collection of query parts, which are individually modified in the Modification phase. However, the Modification-phase is optional an can be omitted. After that, the Allocation phase decides, which query part should be executed on which OdysseusNode. Finally, the query parts are transmitted to the selected nodes. Preprocess, Partition, Modification, Allocation and Postprocess provide multiple strategies the user can select from (individually for each query, if needed). For this, the user has to use specific OdysseusScript keywords. One example for query distribution is as follows:

/// Indication that we want to distribute queries
#CONFIG DISTRIBUTE true 

#NODE_PARTITION OPERATORCLOUD			/// Select OperatorCloud strategy for Partition phase
#NODE_MODIFICATION REPLICATION			/// Select Replication-strategy for Modification phase
#NODE_ALLOCATION USER					/// Select User-strategy for Allocation phase
#NODE_POSTPROCESSOR LOCALSOURCES		/// Select multiple strategies for post processing
#NODE_POSTPROCESSOR LOCALSINKS
#NODE_POSTPROCESSOR MERGE

/// "Typical" query definition here
#METADATA TimeInterval
#PARSER PQL
#ADDQUERY
<Query>

The strategies are explained in the next sections. Partition and Allocation allow only one selected strategy. The other phases can be used multiple times per query. If the user had not specified strategies for some phases, OdysseusNet will use specific strategies as default (configurable):

The Transmission phase cannot be altered.

Preprocess

Partition

Modification

Allocation

Postprocess