Composite / Stereotype Filters

[benmadore-glassdoor]

Let's say I have a different types of routes in my application, routes of Type A and Type B.

And my application has filters: Filter1, Filter2 ... Filter10

For Type A routes I want to always apply Filters 1,2,3,4,5 and for Type B routes I always want to apply Filters 6,7,8.

Filter9 and Filter10 may by applied to either type of route.

Rather than having to copy/paste and specify each of the individual filters on every single route in my application, I'd love some mechanism to create Composite sets of filters that should be applied.

So for a route of Type A instead of:

- filters:
  - Filter1
  - Filter2
  - Filter3
  - Filter4
  - Filter5
  - Filter10
  ...

I could write something like:

-filters:
  - TypeA
  - Filter10
  ...

I attempted to do this via creating an TypeAGatewayFilterFactory that simply invokes 1,2,3,4,5 in turn, but this is not ideal as it doesn't respect the global ordering of the OrderedFilters the filters will always be applied immediately after the ordering of the TypeA OrderedFilter in the specific sequence they're applied.

I also attempted to use a yaml anchor reference e.g.

type-a-filters: &type-a-filters
  - Filter1
  - Filter2
  - Filter3
 ...
- filters
  - Filter10
  - *common-filters

But i got a errors like:

Binding to target [Bindable@1e8add86 type = java.util.List<org.springframework.cloud.gateway.filter.FilterDefinition>, value = 'provided', annotations = array<Annotation>[@jakarta.validation.Valid()], bindMethod = [null]] failed:

  Property: spring.cloud.gateway.routes[0].filters[1][0]
  Value: "Filter1"
  Origin: "spring.cloud.gateway.routes[0].filters[1][0]" from property source "bootstrapProperties-devConfigTest/gateway-app/"
  Reason: The elements [spring.cloud.gateway.routes[0].filters[1][0],spring.cloud.gateway.routes[0].filters[1][1],spring.cloud.gateway.routes[0].filters[1][2]] were left unbound.
  Property: spring.cloud.gateway.routes[0].filters[1][1]
  Value: "Filter2"
  Origin: "spring.cloud.gateway.routes[0].filters[1][1]" from property source "bootstrapProperties-devConfigTest/gateway-app/"
  Reason: The elements [spring.cloud.gateway.routes[0].filters[1][0],spring.cloud.gateway.routes[0].filters[1][1],spring.cloud.gateway.routes[0].filters[1][2]] were left unbound.
  Property: spring.cloud.gateway.routes[0].filters[1][2]
  Value: "Filter3"
  Origin: "spring.cloud.gateway.routes[0].filters[1][2]" from property source "bootstrapProperties-devConfigTest/gateway-app/"
  Reason: The elements [spring.cloud.gateway.routes[0].filters[1][0],spring.cloud.gateway.routes[0].filters[1][1],spring.cloud.gateway.routes[0].filters[1][2]] were left unbound.

Is there any alternate mechanism I should be considering to enable these types of Composite Filters?

[spencergibb]

I don't know if boot supports yaml anchor references. @philwebb do you know?

[philwebb]

I'm pretty sure we don't have any tests for yaml anchor references, so they're certainly not officially supported. We rely on SnakeYAML to do the actual parsing, and I don't know how that deals with them. I've opened spring-projects/spring-boot#43125 to investigate.

[benmadore-glassdoor]

@philwebb @spencergibb So it turns out that anchors do "just work", i just had some syntactical issues.

But they unfortunately specifically don't support my desired use case (merging elements of a sequence) yaml/yaml#35

So it appears I'm back to square 0. Any suggestions on a mechanism to perhaps programmatically trigger adding a set of Filters to given yaml-defined route (at startup) such that the actual filter execution order follows the order defined by the OrderedGatewayFilter?

[philwebb]

You could let Spring Boot do the list conversion then try something like this:

type-a-filters: "Filter1, Filter2, Filter3"
filters: "${type-a-filter}, Filter10"

When filters is resolved it should create a String containing "Filter1, Filter2, Filter3, Filter10" which we would convert to a List by splitting on the comma.

[spencergibb]

Unless the filters have no arguments that's not going to work either.

[benmadore-glassdoor]

@spencergibb That's a good point - a single shortcut configuration param works e.g. this seems fine:

type-a-filters: "Filter1, Filter2, Filter3=foo"
filters: "${type-a-filter}, Filter10=bar"

but if i actually have to specify out a nested configuration structure, or provide multiple comma delimited shortcut params... that doesn't work.

:(