Interpolation

Tyk Streams allows you to dynamically set config fields using [Bloblang]({{ ref “/product-stack/tyk-streaming/guides/bloblang/overview” }}) queries.

Bloblang Queries

output:
  kafka:
    addresses: [ "TODO:6379" ]
    topic: 'meow-${! json("topic") }'

A message with the contents {"topic":"foo","message":"hello world"} would be routed to the Kafka topic meow-foo.

If a literal string is required that matches this pattern (${!foo}) then you can escape it with double brackets. For example, the string ${{!foo}} would be read as the literal ${!foo}.

Bloblang supports arithmetic, boolean operators, coalesce and mapping expressions.

Examples

Reference Metadata

A common usecase for interpolated functions is dynamic routing at the output level using metadata:

output:
  kafka:
    addresses: [ TODO ]
    topic: ${! meta("output_topic") }
    key: ${! meta("key") }

Coalesce and Mapping

Bloblang supports coalesce and mapping, which makes it easy to extract values from slightly varying data structures:

pipeline:
  processors:
    - cache:
        resource: foocache
        operator: set
        key: '${! json().message.(foo | bar).id }'
        value: '${! content() }'

Here’s a map of inputs to resulting values:

{"foo":{"a":{"baz":"from_a"},"c":{"baz":"from_c"}}} -> from_a
{"foo":{"b":{"baz":"from_b"},"c":{"baz":"from_c"}}} -> from_b
{"foo":{"b":null,"c":{"baz":"from_c"}}}             -> from_c

Delayed Processing

We have a stream of JSON documents each with a unix timestamp field doc.received_at which is set when our platform receives it. We wish to only process messages an hour after they were received. We can achieve this by running the sleep processor using an interpolation function to calculate the seconds needed to wait for:

pipeline:
  processors:
  - sleep:
      duration: '${! 3600 - ( timestamp_unix() - json("doc.created_at").number() ) }s'

If the calculated result is less than or equal to zero the processor does not sleep at all. If the value of doc.created_at is a string then our method .number() will attempt to parse it into a number.