Spring Cloud Gateway Global Filters

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Filters act as the place where you can modify the incoming requests before sending the requests to the internal microservices or before responding back to the client.The GlobalFilter interface has the same signature as GatewayFilter. These are special filters that are conditionally applied to all routes.

Forward Routing Filter

The ForwardRoutingFilter looks for a URI in the exchange attribute ServerWebExchangeUtils.GATEWAY_REQUEST_URL_ATTR. If the URL has a forward scheme (such as forward:///localendpoint), it uses the Spring DispatcherHandler to handle the request. The path part of the request URL is overridden with the path in the forward URL. The unmodified original URL is appended to the list in the ServerWebExchangeUtils.GATEWAY_ORIGINAL_REQUEST_URL_ATTR attribute.

The LoadBalancerClient Filter

The LoadBalancerClientFilter looks for a URI in the exchange attribute named ServerWebExchangeUtils.GATEWAY_REQUEST_URL_ATTR. If the URL has a scheme of lb (such as lb://myname), it uses the Spring Cloud LoadBalancerClient to resolve the name (myname in this case) to an actual host and port and replaces the URI in the same attribute. The unmodified original URL is appended to the list in the ServerWebExchangeUtils.GATEWAY_ORIGINAL_REQUEST_URL_ATTR attribute. The filter also looks in the ServerWebExchangeUtils.GATEWAY_SCHEME_PREFIX_ATTR attribute to see if it equals lb. If so, the same rules apply.

spring:
  cloud:
    gateway:
      routes:
      - id: myRoute
        uri: lb://myname
        predicates:
        - Path=/myname/**

Netty Routing Filter

The Netty routing filter runs if the URL located in the ServerWebExchangeUtils.GATEWAY_REQUEST_URL_ATTR exchange attribute has a http or https scheme. It uses the Netty HttpClient to make the downstream proxy request. The response is put in the ServerWebExchangeUtils.CLIENT_RESPONSE_ATTR exchange attribute for use in a later filter

Netty Write Response Filter

The NettyWriteResponseFilter runs if there is a Netty HttpClientResponse in the ServerWebExchangeUtils.CLIENT_RESPONSE_ATTR exchange attribute. It runs after all other filters have completed and writes the proxy response back to the gateway client response. (There is also an experimental WebClientWriteResponseFilter that performs the same function but does not require Netty.)

The Gateway Metrics Filter

To enable gateway metrics, add spring-boot-starter-actuator as a project dependency. Then, by default, the gateway metrics filter runs as long as the property spring.cloud.gateway.metrics.enabled is not set to false. This filter adds a timer metric named gateway.requests with the following tags:

routeId: The route ID.

routeUri: The URI to which the API is routed.

outcome: The outcome, as classified by HttpStatus.Series.

status: The HTTP status of the request returned to the client.

httpStatusCode: The HTTP Status of the request returned to the client.

httpMethod: The HTTP method used for the request.

Websocket Routing Filter

If the URL located in the ServerWebExchangeUtils.GATEWAY_REQUEST_URL_ATTR exchange attribute has a ws or wss scheme, the websocket routing filter runs. It uses the Spring WebSocket infrastructure to forward the websocket request downstream.

You can load-balance websockets by prefixing the URI with lb, such as lb:ws://serviceid.

spring:
  cloud:
    gateway:
      routes:
      # SockJS route
      - id: websocket_sockjs_route
        uri: http://localhost:3001
        predicates:
        - Path=/websocket/info/**
      # Normal Websocket route
      - id: websocket_route
        uri: ws://localhost:3001
        predicates:
        - Path=/websocket/*

RouteToRequestUrl Filter

If there is a Route object in the ServerWebExchangeUtils.GATEWAY_ROUTE_ATTR exchange attribute, the RouteToRequestUrlFilter runs. It creates a new URI, based off of the request URI but updated with the URI attribute of the Route object. The new URI is placed in the ServerWebExchangeUtils.GATEWAY_REQUEST_URL_ATTR exchange attribute`.If the URI has a scheme prefix, such as lb:ws://serviceid, the lb scheme is stripped from the URI and placed in the ServerWebExchangeUtils.GATEWAY_SCHEME_PREFIX_ATTR for use later in the filter chain.

The Netty Write Response Filter

Marking An Exchange As Routed

After the gateway has routed a ServerWebExchange, it marks that exchange as “routed” by adding gatewayAlreadyRouted to the exchange attributes. Once a request has been marked as routed, other routing filters will not route the request again, essentially skipping the filter. There are convenience methods that you can use to mark an exchange as routed or check if an exchange has already been routed.

Conclusion

Spring Cloud Gateway determines which route should get called. Once decided the request is routed to the intended microservice. Before routing this request we can apply some filters to the request. These filters are known as pre filters. After applying the filters the intended microservice call is made and the response is returned back to the Spring Cloud Gateway which returns this response back to the caller. Before returning the response we can again apply some filters to this response. Such filters are called post filters.