istio.io/_docs/concepts/policy-and-control/mixer.md

---
title: Mixer
overview: Architectural deep-dive into the design of Mixer, which provides the policy and control mechanisms within the service mesh.
              
order: 20

layout: docs
type: markdown
---

The page explains Mixer's role and general architecture.

## Background

Infrastructure backends are designed to provide support functionality that is used to build services.
They include such things as access control systems, telemetry capturing systems, quota enforcement
systems, billing systems, and so forth. Services traditionally directly integrate with these
backend systems, creating a hard coupling and baking-in specific semantics and usage options.

Mixer provides a generic intermediation layer between application code and infrastructure backends. 
Its design moves policy decisions out of the app layer and into configuration instead, under operator control.
Instead of having application code integrate with specific backends, the app code instead does a fairly simple
integration with Mixer, and Mixer takes responsibility for interfacing with the backend systems.

Mixer is *not* designed to create a _portability layer_ on top of infrastructure backends. It's not about trying to
just define a universal logging API, universal metric API, universal billing API, and so forth. Instead, Mixer is designed to
change the boundaries between layers in order to reduce systemic complexity, eliminating policy logic from service code and giving control
to operators instead.

<img style="display:block;width:60%;margin:auto;" src="./img/mixer/traffic.svg" title="Flow of traffic." />
<p style="text-align:center;">Mixer Traffic Flow</p>

Mixer provides three core features:

- **Precondition Checking**. Enables callers to verify a number of preconditions before responding to an incoming request from a service consumer. 
Preconditions can include whether the service consumer is properly authenticated, is on the service's whitelist, passes ACL checks, and more.

- **Quota Management**. Enables services to allocate and free quota on a number of dimensions, Quotas are used as a relatively simple resource
management tool to provide some fairness between service consumers when contending for limited resources. Rate limits are
examples of quotas.

- **Telemetry Reporting**. Enables services to report logging and monitoring. In the future, it will also enable tracing and billing
streams intended for both the service operator as well as for service consumers.

These mechanisms are applied based on a set of [attributes](./attributes.html) that are
materialized for every request into Mixer. Within Istio, Envoy depends heavily on Mixer. Services running within the mesh
can also use Mixer to report telemetry or manage quotas. (Note: as of Istio {{ site.data.istio.version }}, only Envoy can call Mixer.)

## Adapters

Mixer is a highly modular and extensible component. One of it's key functions is to abstract
away the details of different policy and telemetry backend systems, allowing Envoy and Istio-based
services to be agnostic of those backends, which keeps them portable.


Mixer's flexibility in dealing with different infrastructure backends is achieved by having a general-purpose
plug-in model. Individual plug-ins are known as *adapters* and they allow
Mixer to interface to different infrastructure backends that deliver core functionality, such as logging, monitoring, quotas, ACL
checking, and more. Adapters enable Mixer to expose a single consistent API, independent of the backends in use.
The exact set of adapters used at runtime is determined through configuration and can easily be extended
to target new or custom infrastructure backends.

<img style="width:35%;display:block;margin:auto;" src="./img/mixer/adapters.svg" title="Mixer and its adapters." />

## Configuration state

Mixer's core runtime methods (`Check`, `Report`, and `Quota`) all accept a set of attributes on input and
produce a set of attributes on output. The work that the individual methods perform is dictated by the set of input
attributes, as well as by Mixer's current configuration. To that end, the service operator is responsible
for:

- Configuring the set of *aspects* that the deployment uses. An aspect is essentially a chunk of configuration
state that configures an adapter (adapters being binary plugins as described [below](#adapters)).

- Establishing the types of adapter parameters that Mixer can manipulate. These
types are described in configuration through a set of *descriptors* (as described [here](./mixer-config#descriptors))

- Creating rules to map the attributes of every incoming request into a 
specific set of aspects and adapter parameters.

The above configuration state is required to have Mixer know what to do with incoming attributes
and dispatch to the appropriate infrastructure backends.

Refer [here](./mixer-config.html) for detailed information on Mixer's configuration model.

## Request phases

When a request comes in to Mixer, it goes through a number of distinct handling phases:

- **Supplementary Attribute Production**. The first thing that happens in Mixer is to run a globally configured
set of adapters that are responsible for introducing new attributes. These attributes are combined with the attributes
from the request to form the total set of attributes for the operation.

- **Resolution**. The second phase is to evaluate the set of attributes to determine the effective 
configuration to apply for the request. See [here](./mixer-config.html#resolution) for information on how resolution works. The effective
configuration determines the set of aspects and descriptors available to handle the request in the
subsequent phases.

- **Attribute Processing**. The third phase takes the total set of attributes
and produces a set of *adapter parameters*. Attribute processing is initially
configured through a simple declarative form as described [here](./mixer-config.html).

- **Adapter Dispatching**. The Resolution phase establishes the set of available aspects and the Attribute
Processing phase creates a set of adapter parameters. The Adapter Dispatching phase invokes the adapters
associated with each aspect and passes them those parameters.

<img style="display:block;width:50%;margin:auto;" src="./img/mixer/phases.svg" title="Phases of mixer request processing" />
<p style="text-align:center;">Request Phases</p>

## Scripting

> This section is preliminary and subject to change. We're still experimenting with the concept of scripting in Mixer.

Mixer's attribute processing phase is implemented via a scripting language (exact language *TBD*). 
The scripts are provided a set of attributes and are responsible for producing the adapter parameters and dispatching
control to individual configured adapters.

For common uses, the operator authors adapter parameter production rules via a relatively simple declarative format
and expression syntax. Mixer ingests such rules and produces a script that performs the necessary runtime work
of accessing the request's incoming attributes and producing the requisite adapter parameters.

For advanced uses, the operator can bypass the declarative format and author directly in the scripting
language. This is more complex, but provides ultimate flexibility.