Status

Current state: Draft
Proof of concept demo available here: https://github.com/yukim/cassandra-opentelemetry-demo

Discussion thread: -

JIRA: -

Released: -

Please keep the discussion on the mailing list rather than commenting on the wiki (wiki discussions get unwieldy fast).

Presentation at Cassandra Summit 2023 here:

Scope

• Provide unified way of exporting tracing, metrics, and logging to external monitoring system with ease of configuration
  • Exporting repair tracing is out-of-scope in this proposal.

Goals

The goal of this proposal is to integrate OpenTelemetry into Apache Cassandra so that exporting telemetries and setting up the monitoring system is much easier.

This feature is opt-in and does not remove the currently available telemetries and the way to export.

Approach

Using OpenTelemetry SDK, manually instrument Apache Cassandra for tracing, metrics and logs, using the current telemetry implementation as much as possible.

Timeline

todo

Mailing list / Slack channels

Mailing list: 

• here

Slack channel: 

• here

Discussion threads: 

• here

Related JIRA tickets

JIRA(s): 

• -

Motivation

Troubleshooting Apache Cassandra can be time-consuming and challenging when faced with failures or performance issues. Without a proper observability system in place, it becomes difficult to identify the root cause of these problems.

Apache Cassandra already implements its own methods or relies on external libraries to provide operators and administrators with deep insights into the complex distributed database using the three pillars of observability: Tracing, Metrics, and Logging.

• For tracing, Apache Cassandra has its own Tracing API.
• For metrics collection and reporting, Apache Cassandra uses Codahale’s Metrics library to collect and expose through JMX.
• For the logging, Apache Cassandra uses Slf4J/Logback logging library.

However, these features become significantly more valuable when available within an observability system that can correlate these telemetries together. Otherwise, operators and admins are left manually pulling out individual telemetries and assembling information by hand to make assumptions about the root cause of a problem.

To implement observability in Apache Cassandra, operators must devise their own methods to extract these telemetries and establish a monitoring stack. This often involves using open-source software like Prometheus/Grafana or commercial services like Datadog. The process of setting up the stack varies depending on the software used, making it complex and oftentimes overlooked by operators.

OpenTelemetry is the project hosted at CNCF to provide "an Observability framework and toolkit designed to create and manage telemetry data such as traces, metrics, and logs."(https://opentelemetry.io/docs/concepts/what-is-opentelemetry/). It specifies APIs to collect tracing, metrics, and logging, and protocols to export to the external observation software.

Introducing OpenTelemetry makes exporting telemetries and integrating observability software much easier, requiring fewer configurations. It also enables observability software projects, vendors, and service providers to develop Cassandra-specific solutions based on the standardized semantics provided by this CEP and OpenTelemetry spec.

Audience

• Cassandra contributors / DevOps / DBAs
• APM, Observation OSS projects / software venders / service providers

Proposed Changes

This CEP aims to integrate OpenTelemetry Tracing, Metrics and Logging with the existing implementations (Cassandra’s own Tracing API, Dropwizard metrics, and Slf4J/Logback), while maintaining the backward compatibility.

Configuration

Apache Cassandra uses OpenTelemetry SDK to manually instrument itself to export tracing, metrics and logs. Operators can configure how these telemetry are exported to the external system by changing the configuration in cassandra.yaml file and either jvm-server.options file or environmental variables.

Apache Cassandra uses OpenTelemetry SDK Autoconfigure to configure OpenTelemetry exporters.

By default, only the OTLP exporter jars are included in the Apache Cassandra distribution, and when enabled, Apache Cassandra starts exporting telemetries to OTLP collector running in localhost:4317 through grpc.

Operators can provide the necessary jars and configuration to use other exporters as well. For example, if you want to export tracing to Jaeger, you need to add opentelemetry-exporter-jaeger.jar file in the classpath, and configure through jvm-server.options:

# jvm-server.options

# Configure tracing export to Jaeger
-Dotel.traces.exporter=jaeger
-Dotel.exporter.jaeger.endpoint=http://jaeger:14250

OpenTelemetry can be configured using environmental variables as well. This is useful for containerized environment like Kubernetes.

Exporting Trace through OpenTelemetry

OpenTelemetry Tracing root Span will be created when a coordinator node receives the message from the client (the same timing when Cassandra Tracing starts).

In order to avoid re-instrumenting code with OpenTelemetry API, Tracing object will produce OpenTelemetry tracing event when its trace method is called. Note that OpenTelemetry Tracing will not produce anything when it is not enabled.

Context propagation

From the client to Cassandra node through Native Protocol

The CEP proposes standardized way to propagate OpenTelemetry Context from the applications through Native Protocol’s custom payload, rather than introducing new header definitions.

By using custom payload, the clients can easily start propagating Context without upgrading the driver they are using.

Right now, only text propagator based on W3C Trace Context is officially defined and provided. Binary propagator will be added in the future (https://github.com/open-telemetry/opentelemetry-specification/issues/437).

By following the W3C standard, the CEP proposes to use the following custom payload key value pairs to propagate OpenTelemetry Context :

In the client side, we can use the standard class to inject the Context into custom payload. For java, it would be like:

Map<String, ByteBuffer> payload = new HashMap<>();
W3CTraceContextPropagator.getInstance().inject(Context.current(), payload, (carrier, key, value) -> {
    if (carrier != null) {
        carrier.put(key, ByteBuffer.wrap(value.getBytes(StandardCharsets.UTF_8)));
    }
});
Statement<?> injected = statement.setCustomPayload(payload);
session.execute(injected);

Between nodes through inter-node messaging protocol

To propagate OpenTelemetry Context between the nodes, the sender adds the new ParamType to the message only when:

• The message is a request message (not response message).
• Current Span is recording.
  • This is to prevent creating unnecessary root Spans for every Message Verbs.

The new ParamType Context takes the following values from the current Context.

• Trace parent info
  • trace id
  • Span Id
  • Flags
• Trace state

Upon receiving the message containing Context ParamType, the receiver constructs the remote span context, which becomes the parent of the span in this node.

Between threads through ExecutorLocals

Like Cassandra TracingState, OpenTelemetry Context will be held in ExecutorLocals to propagate Context between threads. Alternatively, Context can be directly passed to Runnables to propagate context between threads.

List of Attributes associated in Tracing

The following Attributes are associated to Spans and events. Attribute names should follow the Attribute Naming rule.

Exporting Metrics through OpenTelemetry

Cassandra is already instrumented by Dropwizard Metrics library. Instead of rewriting every metrics with OpenTelemetry SDK, create OpenTelemetry adapter using MetricRegistryListener.

OpenTelemetry MetricRegistryListener is registered only when OpenTelemetry feature is enabled in cassandra.yaml.

This adapter uses OpenTelemetry Metrics SDK's asynchronous instrument variants to record metrics from the original Dropwizard metrics counterpart upon collection.

https://opentelemetry.io/docs/instrumentation/java/manual/#metrics

Exporting Logging through OpenTelemetry

Using Logback Adapter library, Apache Cassandra can directly export log to the collector without changing the current logging code in Apache Cassandra.

https://opentelemetry.io/docs/instrumentation/java/manual/#logs

Operators need to activate OpenTelemetry appender in logback.xml 

New or Changed Public Interfaces

New org.apache.cassandra.telemetry.Telemetry class

• Holds the global io.opentelemtry.api.OpenTelemetry instance.
• Initialize OpenTelemetry instance using OpenTelemetry SDK Autoconfigure when opentelemetry configuration in cassandra.yaml has enabled: true.
  • By using OpenTelemetry SDK Autoconfigure, users can configure OpenTelemetry exporters through Java system properties and environmental variables.
• By default, OpenTelemetry is disabled.
  • When OpenTelemetry is disabled, No-Op OpenTelemetry related instances are created through OpenTelemetry SDK.
• Sets up OpenTelemetry tracing, metrics and logs providers.

New cassandra.yaml entry

# Export Apache Cassandra telemetries(tracing, metrics and logs)
# through OpenTelemetry.
#
# Use jvm-server.options file to configure OpenTelemetry SDK with
# system properties.
# <https://github.com/open-telemetry/opentelemetry-java/tree/main/sdk-extensions/autoconfigure>
# Apache Cassandra only comes with the libraries to export telemetry
# through OLTP.
# You need appropriate jar files if you plan to use other exporters.
opentelemetry_enabled: false # default: false

Necessary config change

-Xss needs to be increased (from 256k to 512k) for OLTP export (Okhttp3 based gRPC call) to work

New library dependencies

The following dependencies will be added to the distribution.

OpenTelemetry instrumentation and configuration

• io.opentelemetry:opentelemetry-api
• io.opentelemetry:opentelemetry-sdk
• io.opentelemetry:opentelemetry-sdk-extension-autoconfigure

Runtime dependencies for exporting via OTLP

• io.opentelemetry:opentelemetry-exporter-otlp

Extra dependencies to consider

• io.opentelemetry.instrumentation:opentelemetry-runtime-metrics
  • The library to enable JVM metrics instrumentation
• io.opentelemetry:opentelemetry-exporter-otlp-logs
  • Necessary if Apache Cassandra decides to adapt unstable OpenTelemetry Log export. The library is separated at the moment, with -alpha in its version.
• io.opentelemetry.instrumentation::opentelemetry-logback-appender-1.0
  • The library to enable exporting logs through Logback appender

Semantic convention

OpenTelemetry Resources are immutable attributes (key-value pairs) that describes the entity producing telemetries, in this case, Apache Cassandra server itself. Resources are created once when configuring OpenTelemetry, and those are associated to every telemetries produced by Apache Cassandra.

See Attribute Naming for the specification of naming of Resources.

Namespacing

Every Attribute names (including Resource names) begin with cassandra. For example, to describe Cassandra version, the Resource name will be cassandra.version.

Note that this namespace is used by OpenTelemetry’s JMX metrics gatherer already, however, since the source entity of telemetries is Apache Cassandra, conflicting with JMX metric gatherer shouldn’t be a problem.

List of Resources provided by Apache Cassandra

The following Resources are proposed to be configured when OpenTelemetry is enabled. OpenTelemetry standard Resources as well as Cassandra specific Resources are configured.

TODO

• Should we use Cassandra specific Resources such as cassandra.cluster.name over standard convention like service.namespace ?
  • Using standard may be better to use existing telemetry services.
• Do we need anything else?

Compatibility, Deprecation, and Migration Plan

OpenTelemetry integration adds capability to export existing Tracing/Metrics/Logs through OpenTelemetry Protocol. This feature will not prevent existing telemetry collection. Users should be able to use existing Cassandra tracing, export Metrics through JMX, and output logs to files.

Test Plan

OpenTelemetry Java library provides in-memory telemetry sinks for testing. Use it to verify the exported telemetries.

Rejected Alternatives

• Apache Skywalking project provides similar protocol to export observability, however, there seems no APMs that support the Apache Skywalking protocol other than Apache Skywalking itself.