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OUTLIER-DETECTION-EDS-RELATIONSHIP.md

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Outlier Detection and EDS Relationship in Consul

Overview

This document explains the relationship between Outlier Detection (passive health checking) and EDS (Endpoint Discovery Service) in Consul's service mesh implementation.

What is EDS?

EDS (Endpoint Discovery Service) is part of Envoy's xDS (discovery service) protocol. It dynamically provides Envoy with the list of healthy endpoints (IP addresses and ports) for upstream services.

Key Points:

  • EDS is used when service instances are addressed by IP addresses (not hostnames)
  • Endpoints are discovered and updated dynamically through the xDS protocol
  • EDS updates must arrive after CDS (Cluster Discovery Service) updates

The Critical Relationship

1. Outlier Detection REQUIRES EDS

From agent/xds/clusters.go:480-482:

// Endpoints are managed separately by EDS
// Having an empty config enables outlier detection with default config.
OutlierDetection: &envoy_cluster_v3.OutlierDetection{},

Why? Outlier detection monitors the health of individual endpoints. It needs:

  • Multiple endpoints to monitor
  • Dynamic endpoint updates to track health status
  • The ability to temporarily remove unhealthy endpoints from the pool

2. When EDS is Used

From agent/xds/clusters.go:1344-1365:

useEDS := true
if _, ok := cfgSnap.ConnectProxy.PeerUpstreamEndpointsUseHostnames[uid]; ok {
    // If we're using local mesh gw, the fact that upstreams use hostnames don't matter.
    // If we're not using local mesh gw, then resort to CDS.
    if upstreamConfig.MeshGateway.Mode != structs.MeshGatewayModeLocal {
        useEDS = false
    }
}

// If none of the service instances are addressed by a hostname we
// provide the endpoint IP addresses via EDS
if useEDS {
    c.ClusterDiscoveryType = &envoy_cluster_v3.Cluster_Type{Type: envoy_cluster_v3.Cluster_EDS}
    c.EdsClusterConfig = &envoy_cluster_v3.Cluster_EdsClusterConfig{
        EdsConfig: &envoy_core_v3.ConfigSource{
            InitialFetchTimeout: cfgSnap.GetXDSCommonConfig(s.Logger).GetXDSFetchTimeout(),
            ResourceApiVersion:  envoy_core_v3.ApiVersion_V3,
            ConfigSourceSpecifier: &envoy_core_v3.ConfigSource_Ads{
                Ads: &envoy_core_v3.AggregatedConfigSource{},
            },
        },
    }
}

EDS is used when:

  • Service instances are addressed by IP addresses (not hostnames)
  • Using local mesh gateway mode (even with hostnames)
  • For peered services in certain configurations

EDS is NOT used when:

  • Service instances use hostnames (DNS-based discovery)
  • Using remote mesh gateway mode with hostname-based services

3. Why Hostnames Don't Work with EDS

From agent/xds/endpoints.go:223-224:

// Also skip gateways with a hostname as their address. EDS cannot resolve hostnames,
// so we provide them through CDS instead.

Reason: EDS expects IP addresses, not DNS names. For hostname-based services, Consul embeds the endpoints directly in the CDS (Cluster Discovery Service) configuration.

The Flow: Outlier Detection + EDS

Step-by-Step Process:

  1. Configuration Phase

    • User defines PassiveHealthCheck in service-defaults config entry
    • Consul stores this in UpstreamConfig.PassiveHealthCheck

  2. Cluster Generation (CDS)

    • Consul's XDS server generates Envoy cluster configuration
    • Determines if EDS should be used (IP-based vs hostname-based)
    • If using EDS: 
      ClusterDiscoveryType: &envoy_cluster_v3.Cluster_Type{Type: envoy_cluster_v3.Cluster_EDS}
OutlierDetection: config.ToOutlierDetection(passiveHealthCheck, override, allowZero)
    • Cluster is sent to Envoy via CDS

  3. Endpoint Generation (EDS)

    • Consul generates endpoint list with IP addresses
    • Endpoints are sent to Envoy via EDS
    • Must arrive AFTER CDS updates (ordering requirement)

  4. Runtime Monitoring

    • Envoy monitors traffic to each endpoint
    • Outlier detection tracks failures (5xx errors, timeouts, etc.)
    • Unhealthy endpoints are temporarily ejected from load balancing
    • Ejected endpoints are periodically re-tested and restored when healthy

xDS Update Ordering

From agent/xds/delta.go:583-589:

// 3. EDS updates (if any) must arrive after CDS updates for the respective clusters.
{TypeUrl: xdscommon.EndpointType, Upsert: true},
// 4. LDS updates must arrive after corresponding CDS/EDS updates.
{TypeUrl: xdscommon.ListenerType, Upsert: true, Remove: true},

Critical: EDS updates must come after CDS to ensure Envoy has the cluster configuration (including outlier detection settings) before receiving endpoints.

Special Cases

1. Peered Services

From agent/xds/clusters.go:1316-1320:

outlierDetection := config.ToOutlierDetection(upstreamConfig.PassiveHealthCheck, nil, true)
// We can't rely on health checks for services on cluster peers because they
// don't take into account service resolvers, splitters and routers. Setting
// MaxEjectionPercent to 100% gives outlier detection the power to eject the
// entire cluster.

Special behavior: For peered services, MaxEjectionPercent is set to 100% because:

  • Traditional health checks don't work across cluster peers
  • Outlier detection becomes the primary health mechanism
  • Need ability to eject entire cluster if all instances are unhealthy

2. Ingress Gateways

From agent/xds/clusters.go:1198-1205:

// Configure the outlier detector for upstream service
var override *structs.PassiveHealthCheck
if svc != nil {
    override = svc.PassiveHealthCheck
}
outlierDetection := config.ToOutlierDetection(cfgSnap.IngressGateway.Defaults.PassiveHealthCheck, override, false)

Two-level configuration:

  • Gateway-level defaults apply to all upstreams
  • Per-service overrides can customize behavior for specific services

Debugging Tips

1. Enable Debug Logging

consul agent -dev -log-level=debug

2. Check Envoy Admin Interface

# View cluster configuration (includes outlier detection)
curl http://localhost:19000/config_dump | jq '.configs[1].dynamic_active_clusters'

# View endpoint health status
curl http://localhost:19000/clusters | grep -A 5 "outlier_detection"

3. Verify EDS is Being Used

Look for ClusterDiscoveryType: EDS in cluster config:

curl http://localhost:19000/config_dump | jq '.configs[1].dynamic_active_clusters[] | select(.cluster.name=="db") | .cluster.type'

4. Monitor Outlier Detection Events

# Check Envoy stats for ejections
curl http://localhost:19000/stats | grep outlier_detection

5. Use VS Code Debugger

Set breakpoints in:

Summary

Outlier Detection and EDS are tightly coupled:

  1. Outlier detection monitors individual endpoints → Requires EDS to provide dynamic endpoint list
  2. EDS provides IP-based endpoints → Enables outlier detection to track per-endpoint health
  3. Hostname-based services use CDS → Cannot use outlier detection effectively (no dynamic endpoint tracking)
  4. xDS ordering matters → CDS (with outlier config) must arrive before EDS (with endpoints)

Key Takeaway: For outlier detection to work properly, services must use IP-based discovery (EDS), not hostname-based discovery (CDS with embedded endpoints).