Istio provides a simple Domain-specific language (DSL) to control how API calls and layer-4 traffic flow across various services in the application deployment. The DSL allows the operator to configure service-level properties such as circuit breakers, timeouts, retries, as well as set up common continuous deployment tasks such as canary rollouts, A/B testing, staged rollouts with %-based traffic splits, etc. See routing rules reference for detailed information.
For example, a simple rule to send 100% of incoming traffic for a “reviews” service to version “v1” can be described using the Rules DSL as follows:
apiVersion: config.istio.io/v1alpha2
kind: RouteRule
metadata:
name: reviews-default
spec:
destination:
name: reviews
route:
- labels:
version: v1
weight: 100
The destination is the name of the service to which the traffic is being routed. The route labels identify the specific service instances that will recieve traffic. For example, in a Kubernetes deployment of Istio, the route label “version: v1” indicates that only pods containing the label “version: v1” will receive traffic.
Rules can be configured using the istioctl CLI, or in a Kubernetes deployment using the kubectl command instead. See the configuring request routing task for examples.
There are three kinds of traffic management rules in Istio: Route Rules, Destination Policies (these are not the same as Mixer policies), and Egress Rules. All three kinds of rules control how requests are routed to a destination service.
Route rules control how requests are routed within an Istio service mesh. For example, a route rule could route requests to different versions of a service. Requests can be routed based on the source and destination, HTTP header fields, and weights associated with individual service versions. The following important aspects must be kept in mind while writing route rules:
Every rule corresponds to some destination service identified by a destination field in the rule. For example, rules that apply to calls to the “reviews” service will typically include at least the following.
destination:
name: reviews
The destination value specifies, implicitly or explicitly, a fully qualified domain name (FQDN). It is used by Istio Pilot for matching rules to services.
Normally, the FQDN of the service is composed from three components: name, namespace, and domain:
FQDN = name + "." + namespace + "." + domain
These fields can be explicitly specified as follows.
destination:
name: reviews
namespace: default
domain: svc.cluster.local
More commonly, to simplify and maximize reuse of the rule (for example, to use the same rule in more than one namespace or domain), the rule destination specifies only the name field, relying on defaults for the other two.
The default value for the namespace is the namespace of the rule itself, which can be specified in the metadata field of the rule, or during rule install using the istioctl -n <namespace> create or kubectl -n <namespace> create command. The default value of the domain field is implementation specific. In Kubernetes, for example, the default value is svc.cluster.local.
In some cases, such as when referring to external services in egress rules or on platforms where namespace and domain are not meaningful, an alternative service field can be used to explicitly specify the destination:
destination:
service: my-service.com
When the service field is specified, all other implicit or explicit values of the other fields are ignored.
Rules can optionally be qualified to only apply to requests that match some specific criteria such as the following:
1. Restrict to a specific caller. For example, the following rule only applies to calls from the “reviews” service.
apiVersion: config.istio.io/v1alpha2
kind: RouteRule
metadata:
name: reviews-to-ratings
spec:
destination:
name: ratings
match:
source:
name: reviews
...
The source value, just like destination, specifies a FQDN of a service, either implicitly or explicitly.
2. Restrict to specific versions of the caller. For example, the following rule refines the previous example to only apply to calls from version “v2” of the “reviews” service.
apiVersion: config.istio.io/v1alpha2
kind: RouteRule
metadata:
name: reviews-v2-to-ratings
spec:
destination:
name: ratings
match:
source:
name: reviews
labels:
version: v2
...
3. Select rule based on HTTP headers. For example, the following rule will only apply to an incoming request if it includes a “cookie” header that contains the substring “user=jason”.
apiVersion: config.istio.io/v1alpha2
kind: RouteRule
metadata:
name: ratings-jason
spec:
destination:
name: reviews
match:
request:
headers:
cookie:
regex: "^(.*?;)?(user=jason)(;.*)?$"
...
If more than one header is provided, then all of the corresponding headers must match for the rule to apply.
Multiple criteria can be set simultaneously. In such a case, AND semantics apply. For example, the following rule only applies if the source of the request is “reviews:v2” AND the “cookie” header containing “user=jason” is present.
apiVersion: config.istio.io/v1alpha2
kind: RouteRule
metadata:
name: ratings-reviews-jason
spec:
destination:
name: ratings
match:
source:
name: reviews
labels:
version: v2
request:
headers:
cookie:
regex: "^(.*?;)?(user=jason)(;.*)?$"
...
Each route rule identifies one or more weighted backends to call when the rule is activated. Each backend corresponds to a specific version of the destination service, where versions can be expressed using labels.
If there are multiple registered instances with the specified tag(s), they will be routed to based on the load balancing policy configured for the service, or round-robin by default.
For example, the following rule will route 25% of traffic for the “reviews” service to instances with the “v2” tag and the remaining traffic (i.e., 75%) to “v1”.
apiVersion: config.istio.io/v1alpha2
kind: RouteRule
metadata:
name: reviews-v2-rollout
spec:
destination:
name: reviews
route:
- labels:
version: v2
weight: 25
- labels:
version: v1
weight: 75
By default, the timeout for http requests is 15 seconds, but this can be overridden in a route rule as follows:
apiVersion: config.istio.io/v1alpha2
kind: RouteRule
metadata:
name: ratings-timeout
spec:
destination:
name: ratings
route:
- labels:
version: v1
httpReqTimeout:
simpleTimeout:
timeout: 10s
The number of retries for a given http request can also be specified in a route rule. The maximum number of attempts, or as many as possible within the default or overridden timeout period, can be set as follows:
apiVersion: config.istio.io/v1alpha2
kind: RouteRule
metadata:
name: ratings-retry
spec:
destination:
name: ratings
route:
- labels:
version: v1
httpReqRetries:
simpleRetry:
attempts: 3
Note that request timeouts and retries can also be overridden on a per-request basis.
See the request timeouts task for a demonstration of timeout control.
A route rule can specify one or more faults to inject while forwarding http requests to the rule’s corresponding request destination. The faults can be either delays or aborts.
The following example will introduce a 5 second delay in 10% of the requests to the “v1” version of the “reviews” microservice.
apiVersion: config.istio.io/v1alpha2
kind: RouteRule
metadata:
name: ratings-delay
spec:
destination:
name: reviews
route:
- labels:
version: v1
httpFault:
delay:
percent: 10
fixedDelay: 5s
The other kind of fault, abort, can be used to prematurely terminate a request, for example, to simulate a failure.
The following example will return an HTTP 400 error code for 10% of the requests to the “ratings” service “v1”.
apiVersion: config.istio.io/v1alpha2
kind: RouteRule
metadata:
name: ratings-abort
spec:
destination:
name: ratings
route:
- labels:
version: v1
httpFault:
abort:
percent: 10
httpStatus: 400
Sometimes delays and abort faults are used together. For example, the following rule will delay by 5 seconds all requests from the “reviews” service “v2” to the “ratings” service “v1” and then abort 10 percent of them:
apiVersion: config.istio.io/v1alpha2
kind: RouteRule
metadata:
name: ratings-delay-abort
spec:
destination:
name: ratings
match:
source:
name: reviews
labels:
version: v2
route:
- labels:
version: v1
httpFault:
delay:
fixedDelay: 5s
abort:
percent: 10
httpStatus: 400
To see fault injection in action, see the fault injection task.
Multiple route rules could be applied to the same destination. The order of evaluation of rules corresponding to a given destination, when there is more than one, can be specified by setting the precedence field of the rule.
destination:
name: reviews
precedence: 1
The precedence field is an optional integer value, 0 by default. Rules with higher precedence values are evaluated first. If there is more than one rule with the same precedence value the order of evaluation is undefined.
When is precedence useful? Whenever the routing story for a particular service is purely weight based, it can be specified in a single rule, as shown in the earlier example. When, on the other hand, other criteria (e.g., requests from a specific user) are being used to route traffic, more than one rule will be needed to specify the routing. This is where the rule precedence field must be set to make sure that the rules are evaluated in the right order.
A common pattern for generalized route specification is to provide one or more higher priority rules that qualify rules by source/headers to specific destinations, and then provide a single weight-based rule with no match criteria at the lowest priority to provide the weighted distribution of traffic for all other cases.
For example, the following 2 rules, together, specify that all requests for the “reviews” service that includes a header named “Foo” with the value “bar” will be sent to the “v2” instances. All remaining requests will be sent to “v1”.
apiVersion: config.istio.io/v1alpha2
kind: RouteRule
metadata:
name: reviews-foo-bar
spec:
destination:
name: reviews
precedence: 2
match:
request:
headers:
Foo: bar
route:
- labels:
version: v2
---
apiVersion: config.istio.io/v1alpha2
kind: RouteRule
metadata:
name: reviews-default
spec:
destination:
name: reviews
precedence: 1
route:
- labels:
version: v1
weight: 100
Notice that the header-based rule has the higher precedence (2 vs. 1). If it was lower, these rules wouldn’t work as expected since the weight-based rule, with no specific match criteria, would be evaluated first which would then simply route all traffic to “v1”, even requests that include the matching “Foo” header. Once a rule is found that applies to the incoming request, it will be executed and the rule-evaluation process will terminate. That’s why it’s very important to carefully consider the priorities of each rule when there is more than one.
Destination policies describe various routing related policies associated with a particular service or version, such as the load balancing algorithm, the configuration of circuit breakers, health checks, etc.
Unlike route rules, destination policies cannot be qualified based on attributes of a request other than the calling service, but they can be restricted to apply to requests that are routed to destination backends with specific labels. For example, the following load balancing policy will only apply to requests targeting the “v1” version of the “ratings” microservice that are called from version “v2” of the “reviews” service.
apiVersion: config.istio.io/v1alpha2
metadata:
name: ratings-lb-policy
spec:
source:
name: reviews
labels:
version: v2
destination:
name: ratings
labels:
version: v1
loadBalancing:
name: ROUND_ROBIN
A simple circuit breaker can be set based on a number of criteria such as connection and request limits.
For example, the following destination policy sets a limit of 100 connections to “reviews” service version “v1” backends.
apiVersion: config.istio.io/v1alpha2
metadata:
name: reviews-v1-cb
spec:
destination:
name: reviews
labels:
version: v1
circuitBreaker:
simpleCb:
maxConnections: 100
The complete set of simple circuit breaker fields can be found here.
Similar to route rules, destination policies are associated with a particular destination however if they also include labels their activation depends on route rule evaluation results.
The first step in the rule evaluation process evaluates the route rules for a destination, if any are defined, to determine the labels (i.e., specific version) of the destination service that the current request will be routed to. Next, the set of destination policies, if any, are evaluated to determine if they apply.
NOTE: One subtlety of the algorithm to keep in mind is that policies that are defined for specific tagged destinations will only be applied if the corresponding tagged instances are explicitly routed to. For example, consider the following rule, as the one and only rule defined for the “reviews” service.
apiVersion: config.istio.io/v1alpha2
metadata:
name: reviews-v1-cb
spec:
destination:
name: reviews
labels:
version: v1
circuitBreaker:
simpleCb:
maxConnections: 100
Since there is no specific route rule defined for the “reviews” service, default round-robin routing behavior will apply, which will presumably call “v1” instances on occasion, maybe even always if “v1” is the only running version. Nevertheless, the above policy will never be invoked since the default routing is done at a lower level. The rule evaluation engine will be unaware of the final destination and therefore unable to match the destination policy to the request.
You can fix the above example in one of two ways. You can either remove the labels: from the rule, if “v1” is the only instance anyway, or, better yet, define proper route rules for the service. For example, you can add a simple route rule for “reviews:v1”.
apiVersion: config.istio.io/v1alpha2
kind: RouteRule
metadata:
name: reviews-default
spec:
destination:
name: reviews
route:
- labels:
version: v1
Although the default Istio behavior conveniently sends traffic from all versions of a source service to all versions of a destination service without any rules being set, as soon as version discrimination is desired rules are going to be needed. Therefore, setting a default rule for every service, right from the start, is generally considered a best practice in Istio.
Egress rules are used to enable requests to services outside of an Istio service mesh. For example, the following rule can be used to allow external calls to services hosted under the *.foo.com domain.
apiVersion: config.istio.io/v1alpha2
kind: EgressRule
metadata:
name: foo-egress-rule
spec:
destination:
service: *.foo.com
ports:
- port: 80
protocol: http
- port: 443
protocol: https
The destination of an egress rule is specified using the service field, which can be either a fully qualified or wildcard domain name. It represents a white listed set of one or more external services that services in the mesh are allowed to access. The supported wildcard syntax can be found here.
Currently, only HTTP-based services can be expressed using an egress rule, however, TLS origination from the sidecar can be achieved by setting the protocol of the associated service port to “https”, as shown in the above example. The service must be accessed over HTTP (e.g., http://secure-service.foo.com:443, instead of https://secure-service.foo.com), however, the sidecar will upgrade the connection to TLS in this case.
Egress rules work well in conjunction with route rules and destination policies as long as they refer to the external services using the exact same specification for the destination service as the corresponding egress rule. For example, the following rule can be used in conjunction with the above egress rule to set a 10s timeout for calls to the external services.
apiVersion: config.istio.io/v1alpha2
kind: RouteRule
metadata:
name: foo-timeout-rule
spec:
destination:
service: *.foo.com
httpReqTimeout:
simpleTimeout:
timeout: 10s
Destination policies and route rules to redirect and forward traffic, to define retry, timeout and fault injection policies are all supported for external destinations. Weighted (version-based) routing is not possible, however, since there is no notion of multiple versions of an external service.