Introducing the Istio v1alpha3 routing API

Up until now, Istio has provided a simple API for traffic management using four configuration resources: RouteRule, DestinationPolicy, EgressRule, and (Kubernetes) Ingress. With this API, users have been able to easily manage the flow of traffic in an Istio service mesh. The API has allowed users to route requests to specific versions of services, inject delays and failures for resilience testing, add timeouts and circuit breakers, and more, all without changing the application code itself.

While this functionality has proven to be a very compelling part of Istio, user feedback has also shown that this API does have some shortcomings, specifically when using it to manage very large applications containing thousands of services, and when working with protocols other than HTTP. Furthermore, the use of Kubernetes Ingress resources to configure external traffic has proven to be woefully insufficient for our needs.

To address these, and other concerns, a new traffic management API, a.k.a. v1alpha3, is being introduced, which will completely replace the previous API going forward. Although the v1alpha3 model is fundamentally the same, it is not backward compatible and will require manual conversion from the old API. A conversion tool is included in the next few releases of Istio to help with the transition.

To justify this disruption, the v1alpha3 API has gone through a long and painstaking community review process that has hopefully resulted in a greatly improved API that will stand the test of time. In this article, we will introduce the new configuration model and attempt to explain some of the motivation and design principles that influenced it.

Design principles

A few key design principles played a role in the routing model redesign:

  • Explicitly model infrastructure as well as intent. For example, in addition to configuring an ingress gateway, the component (controller) implementing it can also be specified.
  • The authoring model should be “producer oriented” and “host centric” as opposed to compositional. For example, all rules associated with a particular host are configured together, instead of individually.
  • Clear separation of routing from post-routing behaviors.

Configuration resources in v1alpha3

A typical mesh will have one or more load balancers (we call them gateways) that terminate TLS from external networks and allow traffic into the mesh. Traffic then flows through internal services via sidecar gateways. It is also common for applications to consume external services (e.g., Google Maps API). These may be called directly or, in certain deployments, all traffic exiting the mesh may be forced through dedicated egress gateways. The following diagram depicts this mental model.

Role of gateways in the mesh
Gateways in an Istio service mesh

With the above setup in mind, v1alpha3 introduces the following new configuration resources to control traffic routing into, within, and out of the mesh.

  1. Gateway
  2. VirtualService
  3. DestinationRule
  4. ServiceEntry

VirtualService, DestinationRule, and ServiceEntry replace RouteRule, DestinationPolicy, and EgressRule respectively. The Gateway is a platform independent abstraction to model the traffic flowing into dedicated middleboxes.

The figure below depicts the flow of control across configuration resources.

Relationship between different v1alpha3 elements
Relationship between different v1alpha3 elements

Gateway

A Gateway configures a load balancer for HTTP/TCP traffic, regardless of where it will be running. Any number of gateways can exist within the mesh and multiple different gateway implementations can co-exist. In fact, a gateway configuration can be bound to a particular workload by specifying the set of workload (pod) labels as part of the configuration, allowing users to reuse off the shelf network appliances by writing a simple gateway controller.

For ingress traffic management, you might ask: Why not reuse Kubernetes Ingress APIs? The Ingress APIs proved to be incapable of expressing Istio’s routing needs. By trying to draw a common denominator across different HTTP proxies, the Ingress is only able to support the most basic HTTP routing and ends up pushing every other feature of modern proxies into non-portable annotations.

Istio Gateway overcomes the Ingress shortcomings by separating the L4-L6 spec from L7. It only configures the L4-L6 functions (e.g., ports to expose, TLS configuration) that are uniformly implemented by all good L7 proxies. Users can then use standard Istio rules to control HTTP requests as well as TCP traffic entering a Gateway by binding a VirtualService to it.

For example, the following simple Gateway configures a load balancer to allow external https traffic for host bookinfo.com into the mesh:

apiVersion: networking.istio.io/v1alpha3
kind: Gateway
metadata:
  name: bookinfo-gateway
spec:
  servers:
  - port:
      number: 443
      name: https
      protocol: HTTPS
    hosts:
    - bookinfo.com
    tls:
      mode: SIMPLE
      serverCertificate: /tmp/tls.crt
      privateKey: /tmp/tls.key

To configure the corresponding routes, a VirtualService (described in the following section) must be defined for the same host and bound to the Gateway using the gateways field in the configuration:

apiVersion: networking.istio.io/v1alpha3
kind: VirtualService
metadata:
  name: bookinfo
spec:
  hosts:
    - bookinfo.com
  gateways:
  - bookinfo-gateway # <---- bind to gateway
  http:
  - match:
    - uri:
        prefix: /reviews
    route:
    ...

The Gateway can be used to model an edge-proxy or a purely internal proxy as shown in the first figure. Irrespective of the location, all gateways can be configured and controlled in the same way.

VirtualService

Replacing route rules with something called “virtual services” might seem peculiar at first, but in reality it’s fundamentally a much better name for what is being configured, especially after redesigning the API to address the scalability issues with the previous model.

In effect, what has changed is that instead of configuring routing using a set of individual configuration resources (rules) for a particular destination service, each containing a precedence field to control the order of evaluation, we now configure the (virtual) destination itself, with all of its rules in an ordered list within a corresponding VirtualService resource. For example, where previously we had two RouteRule resources for the Bookinfo application’s reviews service, like this:

apiVersion: config.istio.io/v1alpha2
kind: RouteRule
metadata:
  name: reviews-default
spec:
  destination:
    name: reviews
  precedence: 1
  route:
  - labels:
      version: v1
---
apiVersion: config.istio.io/v1alpha2
kind: RouteRule
metadata:
  name: reviews-test-v2
spec:
  destination:
    name: reviews
  precedence: 2
  match:
    request:
      headers:
        cookie:
          regex: "^(.*?;)?(user=jason)(;.*)?$"
  route:
  - labels:
      version: v2

In v1alph3, we provide the same configuration in a single VirtualService resource:

apiVersion: networking.istio.io/v1alpha3
kind: VirtualService
metadata:
  name: reviews
spec:
  hosts:
    - reviews
  http:
  - match:
    - headers:
        cookie:
          regex: "^(.*?;)?(user=jason)(;.*)?$"
    route:
    - destination:
        host: reviews
        subset: v2
  - route:
    - destination:
        host: reviews
        subset: v1

As you can see, both of the rules for the reviews service are consolidated in one place, which at first may or may not seem preferable. However, if you look closer at this new model, you’ll see there are fundamental differences that make v1alpha3 vastly more functional.

First of all, notice that the destination service for the VirtualService is specified using a hosts field (repeated field, in fact) and is then again specified in a destination field of each of the route specifications. This is a very important difference from the previous model.

A VirtualService describes the mapping between one or more user-addressable destinations to the actual destination workloads inside the mesh. In our example, they are the same, however, the user-addressed hosts can be any DNS names with optional wildcard prefix or CIDR prefix that will be used to address the service. This can be particularly useful in facilitating turning monoliths into a composite service built out of distinct microservices without requiring the consumers of the service to adapt to the transition.

For example, the following rule allows users to address both the reviews and ratings services of the Bookinfo application as if they are parts of a bigger (virtual) service at http://bookinfo.com/:

apiVersion: networking.istio.io/v1alpha3
kind: VirtualService
metadata:
  name: bookinfo
spec:
  hosts:
    - bookinfo.com
  http:
  - match:
    - uri:
        prefix: /reviews
    route:
    - destination:
        host: reviews
  - match:
    - uri:
        prefix: /ratings
    route:
    - destination:
        host: ratings
  ...

The hosts of a VirtualService do not actually have to be part of the service registry, they are simply virtual destinations. This allows users to model traffic for virtual hosts that do not have routable entries inside the mesh. These hosts can be exposed outside the mesh by binding the VirtualService to a Gateway configuration for the same host (as described in the previous section).

In addition to this fundamental restructuring, VirtualService includes several other important changes:

  1. Multiple match conditions can be expressed inside the VirtualService configuration, reducing the need for redundant rules.

  2. Each service version has a name (called a service subset). The set of pods/VMs belonging to a subset is defined in a DestinationRule, described in the following section.

  3. VirtualService hosts can be specified using wildcard DNS prefixes to create a single rule for all matching services. For example, in Kubernetes, to apply the same rewrite rule for all services in the foo namespace, the VirtualService would use *.foo.svc.cluster.local as the host.

DestinationRule

A DestinationRule configures the set of policies to be applied while forwarding traffic to a service. They are intended to be authored by service owners, describing the circuit breakers, load balancer settings, TLS settings, etc.. DestinationRule is more or less the same as its predecessor, DestinationPolicy, with the following exceptions:

  1. The host of a DestinationRule can include wildcard prefixes, allowing a single rule to be specified for many actual services.
  2. A DestinationRule defines addressable subsets (i.e., named versions) of the corresponding destination host. These subsets are used in VirtualService route specifications when sending traffic to specific versions of the service. Naming versions this way allows us to cleanly refer to them across different virtual services, simplify the stats that Istio proxies emit, and to encode subsets in SNI headers.

A DestinationRule that configures policies and subsets for the reviews service might look something like this:

apiVersion: networking.istio.io/v1alpha3
kind: DestinationRule
metadata:
  name: reviews
spec:
  host: reviews
  trafficPolicy:
    loadBalancer:
      simple: RANDOM
  subsets:
  - name: v1
    labels:
      version: v1
  - name: v2
    labels:
      version: v2
    trafficPolicy:
      loadBalancer:
        simple: ROUND_ROBIN
  - name: v3
    labels:
      version: v3

Notice that, unlike DestinationPolicy, multiple policies (e.g., default and v2-specific) are specified in a single DestinationRule configuration.

ServiceEntry

ServiceEntry is used to add additional entries into the service registry that Istio maintains internally. It is most commonly used to allow one to model traffic to external dependencies of the mesh such as APIs consumed from the web or traffic to services in legacy infrastructure.

Everything you could previously configure using an EgressRule can just as easily be done with a ServiceEntry. For example, access to a simple external service from inside the mesh can be enabled using a configuration something like this:

apiVersion: networking.istio.io/v1alpha3
kind: ServiceEntry
metadata:
  name: foo-ext
spec:
  hosts:
  - foo.com
  ports:
  - number: 80
    name: http
    protocol: HTTP

That said, ServiceEntry has significantly more functionality than its predecessor. First of all, a ServiceEntry is not limited to external service configuration, it can be of two types: mesh-internal or mesh-external. Mesh-internal entries are like all other internal services but are used to explicitly add services to the mesh. They can be used to add services as part of expanding the service mesh to include unmanaged infrastructure (e.g., VMs added to a Kubernetes-based service mesh). Mesh-external entries represent services external to the mesh. For them, mTLS authentication is disabled and policy enforcement is performed on the client-side, instead of on the usual server-side for internal service requests.

Because a ServiceEntry configuration simply adds a destination to the internal service registry, it can be used in conjunction with a VirtualService and/or DestinationRule, just like any other service in the registry. The following DestinationRule, for example, can be used to initiate mTLS connections for an external service:

apiVersion: networking.istio.io/v1alpha3
kind: DestinationRule
metadata:
  name: foo-ext
spec:
  host: foo.com
  trafficPolicy:
    tls:
      mode: MUTUAL
      clientCertificate: /etc/certs/myclientcert.pem
      privateKey: /etc/certs/client_private_key.pem
      caCertificates: /etc/certs/rootcacerts.pem

In addition to its expanded generality, ServiceEntry provides several other improvements over EgressRule including the following:

  1. A single ServiceEntry can configure multiple service endpoints, which previously would have required multiple EgressRules.
  2. The resolution mode for the endpoints is now configurable (NONE, STATIC, or DNS).
  3. Additionally, we are working on addressing another pain point: the need to access secure external services over plain text ports (e.g., http://google.com:443). This should be fixed in the coming weeks, allowing you to directly access https://google.com from your application. Stay tuned for an Istio patch release (0.8.x) that addresses this limitation.

Creating and deleting v1alpha3 route rules

Because all route rules for a given destination are now stored together as an ordered list in a single VirtualService resource, adding a second and subsequent rules for a particular destination is no longer done by creating a new (RouteRule) resource, but instead by updating the one-and-only VirtualService resource for the destination.

old routing rules:

$ istioctl create -f my-second-rule-for-destination-abc.yaml

v1alpha3 routing rules:

$ istioctl replace -f my-updated-rules-for-destination-abc.yaml

Deleting route rules other than the last one for a particular destination is also done using istioctl replace.

When adding or removing routes that refer to service versions, the subsets will need to be updated in the service’s corresponding DestinationRule. As you might have guessed, this is also done using istioctl replace.

Summary

The Istio v1alpha3 routing API has significantly more functionality than its predecessor, but unfortunately is not backwards compatible, requiring a one time manual conversion. The previous configuration resources, RouteRule, DesintationPolicy, and EgressRule, will not be supported from Istio 0.9 onwards. Kubernetes users can continue to use Ingress to configure their edge load balancers for basic routing. However, advanced routing features (e.g., traffic split across two versions) will require use of Gateway, a significantly more functional and highly recommended Ingress replacement.

Acknowledgments

Credit for the routing model redesign and implementation work goes to the following people (in alphabetical order):

  • Frank Budinsky (IBM)
  • Zack Butcher (Google)
  • Greg Hanson (IBM)
  • Costin Manolache (Google)
  • Martin Ostrowski (Google)
  • Shriram Rajagopalan (VMware)
  • Louis Ryan (Google)
  • Isaiah Snell-Feikema (IBM)
  • Kuat Yessenov (Google)