Copy last uuid value in the configuration

[Badger]

Something like this should get you started.

input { generator { count => 1 lines => [ 'a1', 'b1', 'a2', 'b2' ] } }
filter {
    if [message] =~ /^a/ {
        #start
        mutate { add_tag => ["sessionStart"] }
        fingerprint { target => "uuid" }
        ruby { code => '@@uuid = event.get("uuid")' }
    } else {
        #end
        mutate { add_tag => ["sessionEnd"] }
        ruby { code => 'event.set("uuid", @@uuid)' }
    }
}
output { stdout { } }