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mockopampserver

README.md

@elastic/mockopampserver

A mock Open Agent Management Protocol (OpAMP, https://github.com/open-telemetry/opamp-spec) server for development and testing. The intent is that this is something useful to maintainers of OpAMP clients, especially for Elastic's Node.js OpAMP client (package @elastic/opamp-client-node).

Features:

  • It is a Node.js server (this may or may not be a feature to you :)
  • It supports the OpAMP HTTP transport.
  • Remote config (i.e. the OffersRemoteConfig server capability).
  • It logs the received AgentToServer and sent ServerToAgent protobuf messages in a somewhat readable format.
  • A way to use this in Node.js testing (see testMode: true and test* methods). See example usage in "packages/opamp-client-node/test/...".
  • "Bad mode": the MockOpAMPServer can be configured to be in one of a number of "bad modes" where it responds in an atypical way, to support testing error cases / failure modes. See badMode usages in "packages/opamp-client-node/test/client.test.js".

Limitations:

  • It only supports the HTTP transport of OpAMP, not the WebSocket Transport. (The spec says "Server implementations SHOULD accept both plain HTTP connections and WebSocket connections.")
  • Most of the optional server capabilities are not implemented: effective config, packages, connection settings, command, custom capabilities.

Usage

CLI usage with Docker

Releases of mockopampserver include published Docker images. You can start a server via:

docker run --rm -it -p 4320:4320 --name mockopampserver \
    ghcr.io/elastic/elastic-otel-node/mockopampserver:latest

CLI usage with npx

If you use npx, you can start a server via:

npx @elastic/mockopampserver

CLI usage from the repository

To build and use the server from this repository:

cd packages/mockopampserver
npm ci
npm start

Once the server is started, you can use it with an OpAMP client. For example:

cd ../packages/opamp-client-node
npm ci
npm run example

Or, if you don't have a particular OpAMP client to use, you can try sending a request via curl using the included simple AgentToServer protobuf file:

curl -si http://localhost:4320/v1/opamp -X POST \
    -H content-type:application/x-protobuf \
    --data-binary @./test/fixtures/AgentToServer.simple.bin \
    | ./scripts/ServerToAgent

(The ServerToAgent script will deserialize opamp.proto.ServerToAgent` binary content on stdin and dump a representation to stdout.)

Module usage

See the block comment for class MockOpAMPServer for docs on all config options.

Here is an example showing how MockOpAMPServer could be used in a JS test case:

const {MockOpAMPServer} = require('@elastic/mockopampserver');

test('some OpAMP client test', async (t) => {
    opampServer = new MockOpAMPServer({
        logLevel: 'warn', // use 'debug' for some debugging of the server
        hostname: '127.0.0.1',
        port: 0,
        testMode: true,
    });
    await opampServer.start();
    t.comment(`MockOpAMPServer started: ${opampServer.endpoint}`);

    // Reset test data in the OpAMP server for each test, if re-using the same
    // mock server for multiple tests.
    opampServer.testReset();

    // Use an OpAMP client with `opampServer.endpoint`.
    // ...

    // Get details of every request/response from the server's point of view.
    // Each entry can include the following fields:
    // - `req`: some fields from the incoming HTTP request
    // - `res`: some fields from the outgoing HTTP response
    // - `a2s`: the parsed AgentToServer protobuf object sent by the client
    // - `s2a`: the parsed ServerToAgent protobuf object sent by the server
    // - `err`: an Error instance, in some failure cases
    const reqs = opampServer.testGetRequests();
    // console.dir(reqs, {depth: 50});
});

An example "test request" object:

[
  {
    req: {
      method: 'POST',
      headers: {
        host: '127.0.0.1:51204',
        connection: 'keep-alive',
        'user-agent': '@elastic/opamp-client-node/0.1.0',
        'content-type': 'application/x-protobuf',
        'content-length': '39'
      }
    },
    res: {
      statusCode: 200,
      _header: 'HTTP/1.1 200 OK\r\n' +
        'Content-Type: application/x-protobuf\r\n' +
        'Content-Length: 20\r\n' +
        'Date: Tue, 27 May 2025 21:49:36 GMT\r\n' +
        'Connection: keep-alive\r\n' +
        'Keep-Alive: timeout=5\r\n' +
        '\r\n'
    },
    a2s: {
      '$typeName': 'opamp.proto.AgentToServer',
      instanceUid: Buffer(16) [Uint8Array] [
          1, 151,  19, 184, 240, 118,
        118, 159, 172,  57,  91,  52,
         29, 167,  17,  41
      ],
      sequenceNum: 1n,
      capabilities: 8193n,
      flags: 0n,
      agentDescription: {
        '$typeName': 'opamp.proto.AgentDescription',
        identifyingAttributes: [
          {
            '$typeName': 'opamp.proto.KeyValue',
            key: 'foo',
            value: {
              '$typeName': 'opamp.proto.AnyValue',
              value: { case: 'stringValue', value: 'bar' }
            }
          }
        ],
        nonIdentifyingAttributes: []
      }
    },
    s2a: {
      '$typeName': 'opamp.proto.ServerToAgent',
      instanceUid: Buffer(16) [Uint8Array] [
          1, 151,  19, 184, 240, 118,
        118, 159, 172,  57,  91,  52,
         29, 167,  17,  41
      ],
      flags: 0,
      capabilities: 3
    },
    err: undefined
  }
]

Remote config

Remote config is an important use case for OpAMP. Here is how it can be used with MockOpAMPServer.

The CLI supports a -F key=./some-file.json option. This will setup the mock server to offer that file as remote config to requesting clients/agents, resulting in a server response with:

      ...
      remoteConfig: {
        '$typeName': 'opamp.proto.AgentRemoteConfig',
        configHash: Uint8Array(32) [ ... ],
        config: {
          '$typeName': 'opamp.proto.AgentConfigMap',
          configMap: {
            'key': {                            // <--- given "key" is here
              '$typeName': 'opamp.proto.AgentConfigFile',
              body: Uint8Array(...) [ ... ],    // <--- content of ./some-file.json is here
              contentType: 'application/json'
            }
          }
        }
      }

To see an example:

# Run the server with config.
cd packages/mockopampserver
npm run example:remote-config

# Run the client.
cd packages/opamp-client-node
npm run example

The equivalent setup of the server in code is:

    const config = {foo: 42};
    const opampServer = new MockOpAMPServer({
        agentConfigMap: {
            configMap: {
                '': {
                    contentType: 'application/json',
                    body: Buffer.from(JSON.stringify(config), 'utf8'),
                },
            },
        },
        // ... other config options.
    });
    await opampServer.start();

Test mode API to live-update agent config

In addition to the agentConfigMap constructor option, the MockOpAMPServer supports a POST /api/agentConfigMap HTTP endpoint to update the Agent Config that the server will use. This endpoint is not part of the OpAMP spec. The endpoint is only enabled when testMode: true.

Here are some curl examples showing how to live-update the Agent Config offered by a running MockOpAMPServer

# Set `logging_level` for the "elastic" configMap key.
curl -i http://127.0.0.1:4320/api/agentConfigMap -F 'elastic={"logging_level":"debug"}'

# Set empty config for the "elastic" key.
curl -i http://127.0.0.1:4320/api/agentConfigMap -F 'elastic={}'

# Set the config to the contents of a local JSON file.
curl -i http://127.0.0.1:4320/api/agentConfigMap -F 'elastic=@./my-agent-config.json'

See SetAgentConfigMap in "lib/mockopampserver.js" for more examples.

mTLS example

This section shows how to use the mockopampserver and opamp-client-node for mTLS, where both client and server must have valid certs. In these examples we'll be using certificate data generated for the packages/opamp-client-node test suite.

Let's start the mockopampserver using CLI options to set TLS server options:

  • Custom CA certs (--cacert), a server certificate (--cert) and private key (--key). These option names match those from curl
  • Configure the TLS server to request client certificates from connecting clients (--request-client-cert).
cd packages/mockopampserver
npm ci
node lib/cli.js --cacert ../opamp-client-node/test/certs/ca.crt --cert ../opamp-client-node/test/certs/server.crt --key ../opamp-client-node/test/certs/server.key --request-client-cert

First, let's use curl as the client, as we did in a section above. The only differences are:

  • the server is now using HTTPS, and
  • we need to add the --cacert, --cert, and --key TLS options.
curl -v https://localhost:4320/v1/opamp -X POST \
    --cacert ../opamp-client-node/test/certs/ca.crt --cert ../opamp-client-node/test/certs/client.crt --key ../opamp-client-node/test/certs/client.key \
    -H content-type:application/x-protobuf \
    --data-binary @./test/fixtures/AgentToServer.simple.bin \
    | ./scripts/ServerToAgent

If that works you should see something like:

long curl output 
% curl -sv https://localhost:4320/v1/opamp -X POST \
    --cacert ../opamp-client-node/test/certs/ca.crt --cert ../opamp-client-node/test/certs/client.crt --key ../opamp-client-node/test/certs/client.key \
    -H content-type:application/x-protobuf \
    --data-binary @./test/fixtures/AgentToServer.simple.bin \
    | ./scripts/ServerToAgent
* Host localhost:4320 was resolved.
* IPv6: ::1
* IPv4: 127.0.0.1
*   Trying [::1]:4320...
* Connected to localhost (::1) port 4320
* ALPN: curl offers h2,http/1.1
* (304) (OUT), TLS handshake, Client hello (1):
} [314 bytes data]
*  CAfile: ../opamp-client-node/test/certs/ca.crt
*  CApath: none
* (304) (IN), TLS handshake, Server hello (2):
{ [122 bytes data]
* (304) (IN), TLS handshake, Unknown (8):
{ [21 bytes data]
* (304) (IN), TLS handshake, Request CERT (13):   <-- This is the TLS server requesting the client provide a TLS cert.
{ [143 bytes data]
* (304) (IN), TLS handshake, Certificate (11):
{ [2849 bytes data]
* (304) (IN), TLS handshake, CERT verify (15):
{ [520 bytes data]
* (304) (IN), TLS handshake, Finished (20):
{ [52 bytes data]
* (304) (OUT), TLS handshake, Certificate (11):
} [1415 bytes data]
* (304) (OUT), TLS handshake, CERT verify (15):
} [520 bytes data]
* (304) (OUT), TLS handshake, Finished (20):
} [52 bytes data]
* SSL connection using TLSv1.3 / AEAD-AES256-GCM-SHA384 / [blank] / UNDEF
* ALPN: server accepted http/1.1
* Server certificate:
*  subject: C=CL; ST=RM; L=EDOTTest; O=Test; OU=Server; CN=localhost
*  start date: Nov  3 23:18:29 2025 GMT
*  expire date: Nov  3 23:18:29 2026 GMT
*  common name: localhost (matched)
*  issuer: C=CL; ST=RM; L=EDOTTest; O=Root; OU=Test; CN=ca
*  SSL certificate verify ok.
* using HTTP/1.x                                <-- The TLS connection has successfully been established.
> POST /v1/opamp HTTP/1.1
> Host: localhost:4320
> User-Agent: curl/8.7.1
> Accept: */*
> content-type:application/x-protobuf
> Content-Length: 18
>
} [18 bytes data]
* upload completely sent off: 18 bytes
< HTTP/1.1 200 OK
< Content-Type: application/x-protobuf
< Content-Length: 22
< Date: Tue, 04 Nov 2025 00:46:07 GMT
< Connection: keep-alive
< Keep-Alive: timeout=5
<
{ [22 bytes data]
* Connection #0 to host localhost left intact
{                                               <-- The decoded ServerToAgent message starts here.
  '$typeName': 'opamp.proto.ServerToAgent',
  instanceUid: Buffer(16) [Uint8Array] [
     84, 108,  42, 240,  68,
    144,  68, 201, 175, 144,
     55, 194,  38,  98, 130,
    102
  ],
  flags: 1n,
  capabilities: 3n
}

Second, we can use @elastic/opamp-client-node as the client. See the opamp-client-node example using mTLS.

cd packages/opamp-client-node
npm run examples:mTLS