docs: add QuotaPolicy capability guide

site/docs/capabilities/index.md

@@ -37,6 +37,7 @@ Comprehensive traffic handling and routing capabilities:
 
 - **[Model Virtualization](./traffic/model-virtualization.md)**: Abstract and virtualize AI models
 - **[Provider Fallback](./traffic/provider-fallback.md)**: Automatic failover between AI providers
+- **[Quota Policy](./traffic/quota-policy.md)**: Token-based quota management for controlling total consumption budgets
 - **[Usage-based Rate Limiting](./traffic/usage-based-ratelimiting.md)**: Token-aware rate limiting for AI workloads
 - **[Header and Body Mutations](./traffic/header-body-mutations.md)**: Customize HTTP headers and JSON body fields per backend or route
 

site/docs/capabilities/traffic/index.md

@@ -3,4 +3,4 @@ id: traffic
 title: Traffic Handling
 ---
 
-This section provides information about traffic routing related capabilities in Envoy AI Gateway.
+This section provides information about traffic routing related capabilities in Envoy AI Gateway, including [Quota Policy](./quota-policy.md) for controlling total token consumption budgets across time windows.

site/docs/capabilities/traffic/quota-policy.md

@@ -0,0 +1,353 @@
+---
+id: quota-policy
+title: Quota Policy
+sidebar_position: 6
+---
+
+# Quota Policy
+
+`QuotaPolicy` enables token-based quota management for AI inference services in Envoy AI Gateway.
+When a backend's quota is exceeded, requests are rejected with a `429 Too Many Requests` status code.
+If the `AIGatewayRoute` rule lists multiple backends, traffic automatically fails over to the next
+backend whose quota has not been exhausted.
+
+:::note QuotaPolicy vs. Rate Limiting
+QuotaPolicy manages **total consumption budgets** (for example, 100,000 tokens per hour). This is
+distinct from [usage-based rate limiting](./usage-based-ratelimiting.md), which controls **request
+velocity** (for example, requests per second). Use QuotaPolicy when you need to cap cumulative token
+spend across a time window.
+:::
+
+## Overview
+
+Key features of QuotaPolicy:
+
+- **Per-model token quotas** — assign token budgets to individual models served by an `AIServiceBackend`.
+- **CEL cost expressions** — weight input, output, cached, and reasoning tokens differently when
+  computing how much a request burns down a quota.
+- **Client-selector bucket rules** — carve out per-tenant or per-header quotas using request attributes.
+- **Shadow mode** — evaluate quota rules without enforcing them, for safe rollout.
+- **Automatic failover** — when a quota is exhausted, route to the next available backend on the rule.
+
+## How It Works
+
+1. A `QuotaPolicy` is attached to one or more `AIServiceBackend` resources via `targetRefs`.
+2. For each completed request, the token cost is computed using the configured cost expression
+   (defaults to `total_tokens`).
+3. The cost is charged against the matching quota bucket (the per-model default bucket, or a matching
+   bucket rule).
+4. When a bucket's quota is exceeded, subsequent matching requests receive `429 Too Many Requests`.
+5. If the `AIGatewayRoute` rule lists multiple `backendRefs`, traffic fails over to the next backend
+   whose quota is still available.
+
+:::tip Prerequisites
+Quota enforcement uses the same infrastructure as usage-based rate limiting:
+
+1. **Redis Deployment**: A Redis instance for storing quota counters. See the [redis.yaml example](https://github.com/envoyproxy/ai-gateway/blob/main/examples/token_ratelimit/redis.yaml) for a simple deployment.
+2. **Envoy Gateway Configuration**: Envoy Gateway must be configured at installation time to enable rate limiting and point to your Redis instance. See the [Envoy Gateway Installation Guide](../../getting-started/prerequisites.md#additional-features-rate-limiting-inferencepool-etc).
+
+See [Usage-based Rate Limiting](./usage-based-ratelimiting.md) for more detail on the rate limit infrastructure that QuotaPolicy builds on.
+:::
+
+## Configuration
+
+### Per-Model Quotas
+
+Use `perModelQuotas` to apply a token budget to a specific model served by the targeted backend(s).
+
+:::warning The model name must match the route
+A `perModelQuotas` entry only applies when its `modelName` matches the `modelNameOverride` set on the
+`AIGatewayRoute` rule's `backendRefs` for the targeted backend. If they do not match, the quota is
+silently **not** applied.
+:::
+
+Given an `AIGatewayRoute` that routes a model to the backend:
+
+```yaml
+apiVersion: aigateway.envoyproxy.io/v1alpha1
+kind: AIGatewayRoute
+metadata:
+  name: my-route
+spec:
+  rules:
+    - backendRefs:
+        - name: my-backend
+          modelNameOverride: my-model # <-- the QuotaPolicy modelName must match this
+```
+
+attach a `QuotaPolicy` to the backend:
+
+```yaml
+apiVersion: aigateway.envoyproxy.io/v1alpha1
+kind: QuotaPolicy
+metadata:
+  name: my-quota-policy
+spec:
+  targetRefs:
+    - group: aigateway.envoyproxy.io
+      kind: AIServiceBackend
+      name: my-backend
+  perModelQuotas:
+    - modelName: "my-model"
+      quota:
+        mode: Shared
+        defaultBucket:
+          limit: 10000 # Maximum tokens allowed in the window.
+          duration: "1h" # Sliding window.
+```
+
+You can attach quotas for multiple models, each with its own budget:
+
+```yaml
+perModelQuotas:
+  - modelName: gpt-4
+    quota:
+      defaultBucket:
+        limit: 10000 # Strict limit for the expensive model.
+        duration: "1h"
+  - modelName: gpt-3.5-turbo
+    quota:
+      defaultBucket:
+        limit: 100000 # Higher limit for the cost-effective model.
+        duration: "1h"
+```
+
+:::note
+When multiple `QuotaPolicy` resources define the same model for the same `AIServiceBackend`, the
+policy whose namespace/name sorts alphabetically first takes precedence.
+:::
+
+### Custom Cost Expression
+
+By default, a request's cost is its `total_tokens`. You can override this with a
+[CEL](https://github.com/google/cel-spec) expression that weights token types differently. The
+following variables are available in a `costExpression`:
+
+| Variable                      | Type   | Description                                      |
+| ----------------------------- | ------ | ------------------------------------------------ |
+| `input_tokens`                | uint   | Prompt / input tokens.                           |
+| `output_tokens`               | uint   | Completion / output tokens.                      |
+| `total_tokens`                | uint   | Total tokens (the default cost).                 |
+| `cached_input_tokens`         | uint   | Input tokens served from the provider's cache.   |
+| `cache_creation_input_tokens` | uint   | Input tokens charged for writing to the cache.   |
+| `reasoning_tokens`            | uint   | Reasoning tokens (for reasoning-capable models). |
+| `model`                       | string | The resolved model name.                         |
+| `backend`                     | string | The serving backend name.                        |
+| `route_name`                  | string | The route name.                                  |
+
+```yaml
+perModelQuotas:
+  - modelName: gpt-4
+    quota:
+      # Cached input tokens cost 10% of regular input tokens;
+      # output tokens cost 6x regular input tokens.
+      costExpression: "input_tokens + cached_input_tokens * 0.1 + output_tokens * 6"
+      defaultBucket:
+        limit: 50000
+        duration: "1h"
+```
+
+:::tip
+Use a custom cost expression when token types have significantly different costs with your provider —
+for example, output tokens are typically more expensive than input tokens. The expression must
+evaluate to a non-negative integer.
+:::
+
+### Bucket Mode
+
+The `mode` field on a per-model `quota` controls how the `defaultBucket` and matching `bucketRules`
+interact when a request matches one or more rules.
+
+Currently only **`Shared`** mode is supported (it is also the default, so the field can be omitted):
+
+- The request is charged to **all** matching `bucketRules` **and** the `defaultBucket`.
+- The request is allowed only if quota is available in **every** matching bucket.
+
+```yaml
+perModelQuotas:
+  - modelName: gpt-4
+    quota:
+      mode: Shared # Default — may be omitted.
+      defaultBucket:
+        limit: 10000
+        duration: "1h"
+```
+
+:::note
+An exclusive bucket mode (charging matching rules **or** the default bucket, but not both) is planned
+but not yet available. The only accepted value today is `Shared`.
+:::
+
+### Client Selectors with Bucket Rules
+
+Bucket rules let you carve out dedicated quotas for specific clients identified by request attributes
+such as headers. This is useful for multi-tenant deployments where each tenant needs its own budget.
+Because the mode is `Shared`, a request that matches a bucket rule is charged against **both** that
+rule's bucket **and** the `defaultBucket`.
+
+```yaml
+perModelQuotas:
+  - modelName: gpt-4
+    quota:
+      defaultBucket:
+        limit: 10000 # Shared budget across all tenants.
+        duration: "1h"
+      bucketRules:
+        # Premium tenant gets a dedicated, higher per-tenant budget.
+        - clientSelectors:
+            - headers:
+                - name: x-tenant-id
+                  type: Exact
+                  value: premium-tenant
+          quota:
+            limit: 50000
+            duration: "1h"
+```
+
+Use `type: Distinct` to create a separate bucket for every unique value of a header — for example, a
+per-tenant budget keyed on the tenant ID:
+
+```yaml
+bucketRules:
+  - clientSelectors:
+      - headers:
+          - name: x-tenant-id
+            type: Distinct # One bucket per unique tenant ID.
+    quota:
+      limit: 5000
+      duration: "1h"
+```
+
+Supported header match types are `Exact`, `Distinct`, and `RegularExpression`.
+
+`clientSelectors` are Envoy Gateway [`RateLimitSelectCondition`](https://gateway.envoyproxy.io/docs/api/extension_types/#ratelimitselectcondition) values. Besides `headers`, a selector can also match on `sourceCIDR` (client IP range), `methods` (HTTP method), `path`, and `queryParams`. Refer to the Envoy Gateway rate limit API reference for the full set of selector options.
+
+### Shadow Mode
+
+Shadow mode lets you test a bucket rule without rejecting traffic. When `shadowMode` is enabled on a
+bucket rule, all quota checks are performed (cache lookups, counter updates, telemetry), but the
+outcome is never enforced — the request always succeeds even if the quota is exceeded.
+
+```yaml
+bucketRules:
+  - clientSelectors:
+      - headers:
+          - name: x-tenant-id
+            type: Distinct
+    quota:
+      limit: 5000
+      duration: "1h"
+    shadowMode: true # Evaluate but do not enforce.
+```
+
+:::tip
+Use shadow mode when rolling out a new quota rule. Monitor the telemetry to confirm the limit is set
+correctly, then enable enforcement by removing `shadowMode` (or setting it to `false`).
+:::
+
+Shadow mode is configured per bucket rule. It cannot be set on the `defaultBucket`.
+
+## Duration Format
+
+The `duration` field selects the sliding-window size. It must be exactly one of the following values:
+
+| Value  | Window     |
+| ------ | ---------- |
+| `"1s"` | One second |
+| `"1m"` | One minute |
+| `"1h"` | One hour   |
+| `"1d"` | One day    |
+
+The window is fixed-size — arbitrary multiples such as `"30s"` or `"15m"` are **not** valid and will
+be rejected by the CRD schema. Choose the `limit` to express your budget within one of these windows.
+
+## Service-Wide Quota
+
+The `serviceQuota` field applies a single budget to **all** models served by the targeted backend,
+without naming individual models. Unlike `perModelQuotas`, it accepts only an optional
+`costExpression` plus a flat `limit` and `duration` — bucket modes, `bucketRules`, and client
+selectors are **not** available at the service-wide level (the underlying type is
+`ServiceQuotaDefinition`, versus `QuotaDefinition` for per-model quotas). For per-tenant rules or
+client selectors, use `perModelQuotas` instead.
+
+```yaml
+apiVersion: aigateway.envoyproxy.io/v1alpha1
+kind: QuotaPolicy
+metadata:
+  name: my-quota-policy
+spec:
+  targetRefs:
+    - group: aigateway.envoyproxy.io
+      kind: AIServiceBackend
+      name: my-backend
+  serviceQuota:
+    # Optional custom cost expression (defaults to total_tokens).
+    costExpression: "input_tokens + output_tokens * 3"
+    quota:
+      limit: 200000
+      duration: "1h"
+```
+
+:::warning Not yet enforced
+In the current release, `serviceQuota` is **not applied by the data plane**. The gateway builds rate
+limit configuration for it, but no request-time enforcement is wired up, so on its own it does not
+reject or count traffic. Use `perModelQuotas` for enforced token quotas until service-wide
+enforcement lands.
+:::
+
+## Full Example
+
+The following `QuotaPolicy` combines per-model quotas, custom cost expressions, bucket rules with
+client selectors, and shadow mode.
+
+```yaml
+apiVersion: aigateway.envoyproxy.io/v1alpha1
+kind: QuotaPolicy
+metadata:
+  name: full-quota-policy
+spec:
+  targetRefs:
+    - group: aigateway.envoyproxy.io
+      kind: AIServiceBackend
+      name: my-backend
+
+  perModelQuotas:
+    - modelName: gpt-4
+      quota:
+        costExpression: "input_tokens + cached_input_tokens * 0.1 + output_tokens * 6"
+        mode: Shared
+        defaultBucket:
+          limit: 10000
+          duration: "1h"
+        bucketRules:
+          # Premium tenant gets a dedicated, higher quota.
+          - clientSelectors:
+              - headers:
+                  - name: x-tenant-id
+                    type: Exact
+                    value: premium-tenant
+            quota:
+              limit: 50000
+              duration: "1h"
+          # Track per-tenant usage in shadow mode (no enforcement).
+          - clientSelectors:
+              - headers:
+                  - name: x-tenant-id
+                    type: Distinct
+            quota:
+              limit: 5000
+              duration: "1h"
+            shadowMode: true
+
+    - modelName: gpt-3.5-turbo
+      quota:
+        defaultBucket:
+          limit: 100000
+          duration: "1h"
+```
+
+## References
+
+- [API Reference](../../api/api.mdx)
+- [Usage-Based Rate Limiting](./usage-based-ratelimiting.md)
+- [Provider Fallback](./provider-fallback.md)

site/docs/capabilities/traffic/usage-based-ratelimiting.md

@@ -9,6 +9,10 @@ import TabItem from '@theme/TabItem';
 
 This guide focuses on AI Gateway's specific capabilities for token-based rate limiting in LLM requests. For general rate limiting concepts and configurations, refer to [Envoy Gateway's Rate Limiting documentation](https://gateway.envoyproxy.io/docs/tasks/traffic/global-rate-limit/).
 
+:::info Quota Policy vs. Rate Limiting
+AI Gateway also provides [Quota Policy](./quota-policy.md) for managing **total consumption budgets** (for example, 100,000 tokens per hour). Use QuotaPolicy when you need to cap cumulative token spend, and usage-based rate limiting (this page) when you need to control **request velocity**.
+:::
+
 ## Overview
 
 AI Gateway leverages Envoy Gateway's Global Rate Limit API to provide token-based rate limiting for LLM requests. Key features include: