Merge Multiline json into single line json using codec multiline plugin

Elastic Stack Logstash
2

If you want to read the entire file as a single event I would use this

input {
    file {
        path => "/tmp/iaas-install.output.json"
        codec => multiline { pattern => "^Spalanzani" negate => true what => "previous" auto_flush_interval => 2 }
        start_position => "beginning"
        sincedb_path => "/dev/null"
    }
}

That is, use a pattern that never matches and then auto_flush after reading the whole file.