consul/agent/proxycfg/proxycfg.go

// Package proxycfg contains components for sourcing the data required to
// configure Connect proxies. The Manager provides an API with which proxy
// services can be registered, and coordinates the fetching (and refreshing)
// of intentions, upstreams, discovery chain, certificates etc. Consumers
// such as the xDS server can then subscribe to receive snapshots of this
// data whenever it changes.
//
// Consul client agents support the configuration of proxies locally
// registered to them, whereas Consul servers support both this and proxies
// in the catalog.
//
// The following diagram depicts the component relationships on a server, as
// this is the more complex mode of operation:
//
//                +-------+       1.       +------------+
//                | Local | ◀------------▶ | Local      |
//                | State |                | State Sync |
//                +-------+                +-----+------+
//                  ▲                            |
//                  |     +---------------+      | 2.
//               4. | 4a. | Local         |      |
//                  | +-▶ | Config Source +-+    |
//                  | |   +---------------+ |    |
//                  | |                     ▼    ▼
// +--------+ 3.  +-+-+-----------+ 6.    +----------+ 2a.  +----------+
// | xDS    +---▶ | Catalog       +-----▶ | proxycfg +----▶ | proxycfg |
// | Server | ◀---+ Config Source +-----▶ | Manager  +--+   | State    |
// +--------+  8. +----+----------+ 7.    +----------+  |   +----------+
//                  5. |                                |
//                     ▼                            7a. |   +----------+
//                   +-------+                          +-▶ | proxycfg |
//                   | State |                              | State    |
//                   | Store |                              +----------+
//                   +-------+
//
//  1. local.Sync watches the agent's local state for changes.
//  2. If any sidecar proxy or gateway services are registered to the local agent
//     they are sync'd to the proxycfg.Manager.
// 2a. proxycfg.Manager creates a state object for the service and begins
//     pre-fetching data (go to 8).
//  3. Client begins a stream and the xDS server calls Watch on its ConfigSource -
//     on a client agent this would be a local config source, on a server it would
//     be a catalog config source.
//  4. The catalog config source will check if service is registered locally.
// 4a. If the service *is* registered locally it hands off the the local config
//     source, which calls Watch on the proxycfg manager (and serves the pre-
//     fetched data).
//  5. Otherwise, it fetches the service from the state store.
//  6. It calls Watch on the proxycfg manager.
//  7. It registers the service with the proxycfg manager.
// 7a. See: 2a.
//  8. xDS server receives snapshots of configuration data whenever it changes.
package proxycfg