Jaeger libraries implement consistent upfront (or head-based) sampling. For example, assume we have a simple call graph where service A calls service B, and B calls service C: A -> B -> C. When service A receives a request that contains no tracing information, Jaeger tracer will start a new trace, assign it a random trace ID, and make a sampling decision based on the currently installed sampling strategy. The sampling decision will be propagated with the requests to B and to C, so those services will not be making the sampling decision again but instead will respect the decision made by the top service A. This approach guarantees that if a trace is sampled, all its spans will be recorded in the backend. If each service was making its own sampling decision we would rarely get complete traces in the backend.

Client Sampling Configuration

When using configuration object to instantiate the tracer, the type of sampling can be selected via sampler.type and sampler.param properties. Jaeger libraries support the following samplers:

• Constant (sampler.type=const) sampler always makes the same decision for all traces. It either samples all traces (sampler.param=1) or none of them (sampler.param=0).
• Probabilistic (sampler.type=probabilistic) sampler makes a random sampling decision with the probability of sampling equal to the value of sampler.param property. For example, with sampler.param=0.1 approximately 1 in 10 traces will be sampled.
• Rate Limiting (sampler.type=ratelimiting) sampler uses a leaky bucket rate limiter to ensure that traces are sampled with a certain constant rate. For example, when sampler.param=2.0 it will sample requests with the rate of 2 traces per second.
• Remote (sampler.type=remote, which is also the default) sampler consults jaeger-agent for the appropriate sampling strategy to use in the current service. This allows controlling the sampling strategies in the services from a central configuration in Jaeger backend (see Remote Sampling), or even dynamically (see Adaptive Sampling).

Remote Sampling

If your client SDKs are configured to use remote sampling configuration (see Remote Sampling API) then sampling rates can be centrally controlled via jaeger-collectors. In this setup a sampling strategy configuration is served to the client SDK that describes endpoints and their sampling probabilities. This configuration can be generated by jaeger-collector in two different ways: periodically loaded from a file or dynamically calculated based on traffic. The method of generation is controlled by the environment variable SAMPLING_CONFIG_TYPE which can be set to either file (default) or adaptive.

File-based Sampling Configuration

jaeger-collectors can be instantiated with the --sampling.strategies-file option that points to a file containing sampling strategies to be served to Jaeger clients. The option’s value can contain a path to a JSON file, which will be automatically reloaded if its contents change, or an HTTP URL from where the file will be periodically retrieved, with reload frequency controlled by the --sampling.strategies-reload-interval option.

If no configuration is provided, jaeger-collectors will return the default probabilistic sampling policy with probability 0.001 (0.1%) for all services.

Example strategies.json:

{
  "service_strategies": [
    {
      "service": "foo",
      "type": "probabilistic",
      "param": 0.8,
      "operation_strategies": [
        {
          "operation": "op1",
          "type": "probabilistic",
          "param": 0.2
        },
        {
          "operation": "op2",
          "type": "probabilistic",
          "param": 0.4
        }
      ]
    },
    {
      "service": "bar",
      "type": "ratelimiting",
      "param": 5
    }
  ],
  "default_strategy": {
    "type": "probabilistic",
    "param": 0.5,
    "operation_strategies": [
      {
        "operation": "/health",
        "type": "probabilistic",
        "param": 0.0
      },
      {
        "operation": "/metrics",
        "type": "probabilistic",
        "param": 0.0
      }
    ]
  }
}

service_strategies element defines service specific sampling strategies and operation_strategies defines operation specific sampling strategies. There are 2 types of strategies possible: probabilistic and ratelimiting which are described above (NOTE: ratelimiting is not supported for operation_strategies). default_strategy defines the catch-all sampling strategy that is propagated if the service is not included as part of service_strategies.

In the above example:

• All operations of service foo are sampled with probability 0.8 except for operations op1 and op2 which are probabilistically sampled with probabilities 0.2 and 0.4 respectively.
• All operations for service bar are rate-limited at 5 traces per second.
• Any other service will be sampled with probability 0.5 defined by the default_strategy.
• The default_strategy also includes shared per-operation strategies. In this example we disable tracing on /health and /metrics endpoints for all services by using probability 0. These per-operation strategies will apply to any new service not listed in the config, as well as to the foo and bar services unless they define their own strategies for these two operations.

Adaptive Sampling

Since Jaeger v1.27.

Adaptive sampling works in jaeger-collector by observing the spans received from services and recalculating sampling probabilities for each service/endpoint combination to ensure that the volume of collected traces matches --sampling.target-samples-per-second. When a new service or endpoint is detected, it is initially sampled with --sampling.initial-sampling-probability until enough data is collected to calculate the rate appropriate for the traffic going through the endpoint.

Adaptive sampling requires a storage backend to store the observed traffic data and computed probabilities. At the moment memory (for all-in-one deployment), cassandra, badger, elasticsearch and opensearch are supported as sampling storage backends.

By default adaptive sampling will attempt to use the backend specified by SPAN_STORAGE_TYPE to store data. However, a second type of backend can also be specified by using SAMPLING_STORAGE_TYPE. For instance, SPAN_STORAGE_TYPE=elasticsearch SAMPLING_STORAGE_TYPE=cassandra ./jaeger-collector will run jaeger-collector in a mode where it attempts to store its span data in the configured elasticsearch cluster and its adaptive sampling data in the configured cassandra cluster. Note that this feature can not be used to store span and adaptive sampling data in two different backends of the same type.

Read this blog post  for more details on adaptive sampling engine.