github.com/Jeffail/benthos/v3@v3.65.0/website/docs/configuration/metadata.md (about) 1 --- 2 title: Metadata 3 --- 4 5 In Benthos each message has raw contents and metadata, which is a map of key/value pairs representing an arbitrary amount of complementary data. 6 7 When an input protocol supports attributes or metadata they will automatically be added to your messages, refer to the respective input documentation for a list of metadata keys. When an output supports attributes or metadata any metadata key/value pairs in a message will be sent (subject to service limits). 8 9 ## Editing Metadata 10 11 Benthos allows you to add and remove metadata using the [`bloblang` processor][processors.bloblang]. For example, you can do something like this in your pipeline: 12 13 ```yaml 14 pipeline: 15 processors: 16 - bloblang: | 17 # Remove all existing metadata from messages 18 meta = deleted() 19 20 # Add a new metadata field `time` from the contents of a JSON 21 # field `event.timestamp` 22 meta time = event.timestamp 23 ``` 24 25 You can also use [Bloblang][guides.bloblang] to delete individual metadata keys with: 26 27 ```coffee 28 meta foo = deleted() 29 ``` 30 31 Or do more interesting things like remove all metadata keys with a certain prefix: 32 33 ```coffee 34 meta = meta().filter(!this.key.has_prefix("kafka_")) 35 ``` 36 37 ## Using Metadata 38 39 Metadata values can be referenced in any field that supports [interpolation functions][interpolation]. For example, you can route messages to Kafka topics using interpolation of metadata keys: 40 41 ```yaml 42 output: 43 kafka: 44 addresses: [ TODO ] 45 topic: ${! meta("target_topic") } 46 ``` 47 48 Benthos also allows you to conditionally process messages based on their metadata with the [`switch` processor][processors.switch]: 49 50 ```yaml 51 pipeline: 52 processors: 53 - switch: 54 - check: meta("doc_type") == "nested" 55 processors: 56 - sql_insert: 57 driver: mysql 58 dsn: foouser:foopassword@tcp(localhost:3306)/foodb 59 table: footable 60 columns: [ foo, bar, baz ] 61 args_mapping: | 62 root = [ 63 this.document.foo, 64 this.document.bar, 65 meta("kafka_topic"), 66 ] 67 ``` 68 69 ## Restricting Metadata 70 71 Outputs that support metadata, headers or some other variant of enriched fields on messages will attempt to send all metadata key/value pairs by default. However, sometimes it's useful to refer to metadata fields at the output level even though we do not wish to send them with our data. In this case it's possible to restrict the metadata keys that are sent with the field `metadata.exclude_prefixes` within the respective output config. 72 73 For example, if we were sending messages to kafka using a metadata key `target_topic` to determine the topic but we wished to prevent that metadata key from being sent as a header we could use the following configuration: 74 75 ```yaml 76 output: 77 kafka: 78 addresses: [ TODO ] 79 topic: ${! meta("target_topic") } 80 metadata: 81 exclude_prefixes: 82 - target_topic 83 ``` 84 85 And when the list of metadata keys that we do _not_ want to send is large it can be helpful to use a [Bloblang mapping][guides.bloblang] in order to give all of these "private" keys a common prefix: 86 87 ```yaml 88 pipeline: 89 processors: 90 # Has an explicit list of public metadata keys, and everything else is given 91 # an underscore prefix. 92 - bloblang: | 93 let allowed_meta = [ 94 "foo", 95 "bar", 96 "baz", 97 ] 98 meta = meta().map_each_key(key -> if !$allowed_meta.contains(key) { 99 "_" + key 100 }) 101 102 output: 103 kafka: 104 addresses: [ TODO ] 105 topic: ${! meta("_target_topic") 106 metadata: 107 exclude_prefixes: [ "_" ] 108 ``` 109 110 [interpolation]: /docs/configuration/interpolation 111 [processors.switch]: /docs/components/processors/switch 112 [processors.bloblang]: /docs/components/processors/bloblang 113 [guides.bloblang]: /docs/guides/bloblang/about