记录一次filebeat 8.5.0 输出多个topic报错：“Dropping event: no topic could be selected”

报错：

报错的filebeat-xxx.yml文件：

filebeat.inputs:
################ Web logs ################
- type: log
  enabled: true
  tail_files: true
  scan_frequency: 5s
  backoff: 1s
  max_backoff: 10s
  paths:
    - /data/log/nginx/xxx.xxx.com*.log
  tags: ["web", "xxx"]                      
  fields_under_root: true
  fields:
    from: "xxx.xxx.com"
    log_topic: "xxx-xxx-web"


################ App logs ################
- type: log
  enabled: true
  paths:
    - /data/service/xxx/xxx/nohup.out
  tags: ["xxxx"]
  fields_under_root: true
  fields:
    from: "xxx-xxx"
    log_topic: "xxx-xxx-app"
  multiline.pattern: '^\d{4}\-\d{2}\-\d{2}\s\d+\:\d+\:\d+\.\d{2,12}\s'
  multiline.negate: true
  multiline.match: after


output.kafka:
  enabled: true
  hosts: ["x.x.x.x:xxxx"]
  topic: '%{[fields.log_topic]}'
  required_acks: 1
  compression: gzip
  keep_alive: 10s

报错原因：

因为output.kafka配置中的topic 取不到变量值，从而导致出现报错。

解决参考：

1、当配置fields_under_root: true时，output.kafka中的topic配置为topic: '%{[log_topic]}'

......
fields_under_root: true
  fields:
    from: "xxx-xxx"
    log_topic: "xxx-xxx-app"


output.kafka
......
  topic: '%{[log_topic]}'
......

 2、当配置未fields_under_root: true时，output.kafka中的topic配置为topic: '%{[fileds.log_topic]}'

......
  fields:
    from: "xxx-xxx"
    log_topic: "xxx-xxx-app"


output.kafka
......
  topic: '%{[fileds.log_topic]}'
......