Gateway

• Used for 
• Authentication,
• Authorisation
• Security
• Limit the request.
• Fault tolerance
• Service aggregation
• To call a micro service, we need to aggregate response of many other Micro services.
• Gateway pattern provides a unified proxy interface to the client That delegates request to various micro services based on URI pattern.
• JVM based Router and server side load balancer
• We have multiple micro services and based on URL pattern request goes to corresponding micro service.
• This filter and routing logic is taken care by Zuul proxy Which is also an API gateway.
• Spring cloud has a nice integration with Embed ZUUL PROXY.
• We provide a unified proxy interface that will delegate the request or forward the request to various micro services based on URL pattern.
• To trace our request flow, there is a filter component. there are four types of filters
• PreFilter
• Prefilter is called before our request route to microservices.
• Before our request route to Micro services.
• RouteFilter
• Once request routes route filter is called.
• After request routes to micro services. 
• PostFilter
• Once our micro service end point execution is done, Post filter is called.
• After response comes, Post filter is called.
• ErrorFilter
• If there is any error We trace that using error-filter.
• If we get error in response error filter is called.
• Based on URL pattern, the request needs to Forward to which micro service this filter and routing logic Is taken care by API Gateway.
• Zuul
• Zuul is JVM based router and server side load balancer.
• Zuul is decommissioned as of spring boot 3 And spring cloud Gateway is fully used and supported now.
• Zuul is an ad service that provides dynamic Routing, monitoring, resiliency, Security and more.
• Internal architecture of Zuul
• HTTP request hits Zuul servlet Which passes request to Zuul filter runner.
• The Zuul Filter runner Then possesses request through different pre routing Filters, routing filters.
• These filters are based on groovy.
• The Zuul filter runner Decides Which filter will execute and in what order.
• There are some predefined filters And there may be some custom filters.
• Then the request goes to request context for processing.
• The Post filters are applied then.
• If we want to pass anything from Gateway to service or runner, then we said that inside the request context, which passes with the request.
• Filter persistence is used To inject filters inside our API Gateway engine
• Filter poller Execute the filter in the defined order.
• Filter publisher comprises of filter persistence And filter poller.
• When the filter loader gets loaded, it is going to execute the filter runner.
• The filter polar uses filter directories, File filter manager And filter loader, To load the Concerned filter.

• Components of Zuul
• Filters
• Types
• PreFilter
• PostFilter
• RouteFilter
• ErrorFilter
• Execution order
• Decides the order in which custom filters should execute.
• Criteria
• Criteria which decides on which a particular filter should be picked.
• Action
• Action to be performed in a filter
• This is defined in run method

• API Gateway monitoring
• Management endpoints
• Routes
• /routes
• Give the details of end points of all the services we have considered
• Filters
• /filters
• Gives us all the filters that we have configured in our application.