Support seamless replication

[je-ik]

Need support for multimaster-multislave replication schemes accross multiple proxima deployments with eventual consistency. Required features:

• the replication will work on commit logs only - all other consistency and replication will occur inside given proxima deployment
• easy configuration
• ability to observe non replicated items in commit log only (defaults to whole commit log in case there is no replication)

Draft of configuration:

 entities {
   original {
     attrA: { scheme: "bytes" }
     attrB: { scheme: "bytes" }
   }
 }
 replication {
   replicate-original-attrA {
     entity: original
     attributes: [ "attrA" ] # can be "*"
     # declare attribute families that will be written when update happens
     targets {
       first {
         // attribute family specifier
       }
       second {
         // attribute family specifier
       }
     }
     # declare single source family that will be read and written to local commit log
     source {
       // attribute family specifier
     }
   }
   replicate-original-all {
     entity: original
     attributes: [ "*" ] # replicate all attributes with some other settings
     targets { ... }
     source { ... }
   }
 }

In order to accomplish this we need to:

• create implicit attribute for each replicated one (prefixed with _<replication_name>_s_ for source attribute family, _<replication_name>_t_<target_name>_ for target attribute), _<replication_name>_ for direct writes and _<replication_name>_r_ for the replicated attribute containing all replicated data (which will be read instead of original attribute, which becames virtual)
• relax AttributeProxy so that it can be used only for writes (i.e. attribute is modified only on direct write, read happens on actual attribute) - this will be used to transform writes to attribute X into _<replication_name>_X
• start replication of _<replication_name>_X into X
• start replication of _<replication_name>_source_X into X
• start replication of _<replication_name>_X into each of _<replication_name>_target_<target_name>_X

[je-ik]

Closed via Closed via datadrivencz#105