136 lines
7.6 KiB
Plaintext
136 lines
7.6 KiB
Plaintext
---
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layout: docs
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page_title: Service Mesh - How it Works
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description: >-
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Consul's service mesh enforces secure service communication using mutual TLS (mTLS) encryption and explicit authorization. Learn how the service mesh certificate authorities, intentions, and agents work together in the ""Connect"" subsystem to provide Consul’s service mesh capabilities.
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---
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# How Service Mesh Works
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This topic describes how many of the core features of Consul's service mesh functionality works.
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It is not a prerequisite,
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but this information will help you understand how Consul service mesh behaves in more complex scenarios.
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Consul Connect is the component shipped with Consul that enables service mesh functionality. The terms _Consul Connect_ and _Consul service mesh_ are used interchangeably throughout this documentation.
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To try service mesh locally, complete the [Getting Started with Consul service
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mesh](https://learn.hashicorp.com/tutorials/consul/service-mesh?utm_source=docs)
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tutorial.
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## Mutual Transport Layer Security (mTLS)
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The core of Connect is based on [mutual TLS](https://en.wikipedia.org/wiki/Mutual_authentication).
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Connect provides each service with an identity encoded as a TLS certificate.
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This certificate is used to establish and accept connections to and from other
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services. The identity is encoded in the TLS certificate in compliance with
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the [SPIFFE X.509 Identity Document](https://github.com/spiffe/spiffe/blob/master/standards/X509-SVID.md).
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This enables Connect services to establish and accept connections with
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other SPIFFE-compliant systems.
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The client service verifies the destination service certificate
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against the [public CA bundle](/api-docs/connect/ca#list-ca-root-certificates).
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This is very similar to a typical HTTPS web browser connection. In addition
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to this, the client provides its own client certificate to show its
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identity to the destination service. If the connection handshake succeeds,
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the connection is encrypted and authorized.
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The destination service verifies the client certificate against the [public CA
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bundle](/api-docs/connect/ca#list-ca-root-certificates). After verifying the
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certificate, the next step depends upon the configured application protocol of
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the destination service. TCP (L4) services must authorize incoming _connections_
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against the configured set of Consul [intentions](/docs/connect/intentions),
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whereas HTTP (L7) services must authorize incoming _requests_ against those same
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intentions. If the intention check responds successfully, the
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connection/request is established. Otherwise the connection/request is
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rejected.
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To generate and distribute certificates, Consul has a built-in CA that
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requires no other dependencies, and
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also ships with built-in support for [Vault](/docs/connect/ca/vault). The PKI system is designed to be pluggable
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and can be extended to support any system by adding additional CA providers.
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All APIs required for Connect typically respond in microseconds and impose
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minimal overhead to existing services. To ensure this, Connect-related API calls
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are all made to the local Consul agent over a loopback interface, and all [agent
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Connect endpoints](/api-docs/agent/connect) implement local caching, background
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updating, and support blocking queries. Most API calls operate on purely local
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in-memory data.
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## Agent Caching and Performance
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To enable fast responses on endpoints such as the [agent Connect
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API](/api-docs/agent/connect), the Consul agent locally caches most Connect-related
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data and sets up background [blocking queries](/api-docs/features/blocking) against
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the server to update the cache in the background. This allows most API calls
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such as retrieving certificates or authorizing connections to use in-memory
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data and respond very quickly.
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All data cached locally by the agent is populated on demand. Therefore, if
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Connect is not used at all, the cache does not store any data. On first request,
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the data is loaded from the server and cached. The set of data cached is: public
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CA root certificates, leaf certificates, intentions, and service discovery
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results for upstreams. For leaf certificates and intentions, only data related
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to the service requested is cached, not the full set of data.
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Further, the cache is partitioned by ACL token and datacenters. This is done
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to minimize the complexity of the cache and prevent bugs where an ACL token
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may see data it shouldn't from the cache. This results in higher memory usage
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for cached data since it is duplicated per ACL token, but with the benefit
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of simplicity and security.
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With Connect enabled, you'll likely see increased memory usage by the
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local Consul agent. The total memory is dependent on the number of intentions
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related to the services registered with the agent accepting Connect-based
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connections. The other data (leaf certificates and public CA certificates)
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is a relatively fixed size per service. In most cases, the overhead per
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service should be relatively small: single digit kilobytes at most.
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The cache does not evict entries due to memory pressure. If memory capacity
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is reached, the process will attempt to swap. If swap is disabled, the Consul
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agent may begin failing and eventually crash. Cache entries do have TTLs
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associated with them and will evict their entries if they're not used. Given
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a long period of inactivity (3 days by default), the cache will empty itself.
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## Connections Across Datacenters
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A sidecar proxy's [upstream configuration](/docs/connect/registration/service-registration#upstream-configuration-reference)
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may specify an alternative datacenter or a prepared query that can address services
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in multiple datacenters (such as the [geo failover](https://learn.hashicorp.com/tutorials/consul/automate-geo-failover) pattern).
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[Intentions](/docs/connect/intentions) verify connections between services by
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source and destination name seamlessly across datacenters.
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Connections can be made via gateways to enable communicating across network
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topologies, allowing connections between services in each datacenter without
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externally routable IPs at the service level.
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## Intention Replication
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Intention replication happens automatically but requires the
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[`primary_datacenter`](/docs/agent/config/config-files#primary_datacenter)
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configuration to be set to specify a datacenter that is authoritative
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for intentions. In production setups with ACLs enabled, the
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[replication token](/docs/agent/config/config-files#acl_tokens_replication) must also
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be set in the secondary datacenter server's configuration.
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## Certificate Authority Federation
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The primary datacenter also acts as the root Certificate Authority (CA) for Connect.
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The primary datacenter generates a trust-domain UUID and obtains a root certificate
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from the configured CA provider which defaults to the built-in one.
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Secondary datacenters fetch the root CA public key and trust-domain ID from the
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primary and generate their own key and Certificate Signing Request (CSR) for an
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intermediate CA certificate. This CSR is signed by the root in the primary
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datacenter and the certificate is returned. The secondary datacenter can now use
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this intermediate to sign new Connect certificates in the secondary datacenter
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without WAN communication. CA keys are never replicated between datacenters.
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The secondary maintains watches on the root CA certificate in the primary. If the
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CA root changes for any reason such as rotation or migration to a new CA, the
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secondary automatically generates new keys and has them signed by the primary
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datacenter's new root before initiating an automatic rotation of all issued
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certificates in use throughout the secondary datacenter. This makes CA root key
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rotation fully automatic and with zero downtime across multiple datacenters.
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