Frequently Asked Questions
Answers to some frequently asked questions about Jaeger.
Why is the Dependencies page empty?
The Dependencies page shows a graph of services traced by Jaeger and connections between them. When you are using
all-in-one binary with in-memory storage, the graph is calculated on-demand from all the traces stored in memory. However, if you are using a real distributed storage like Cassandra or Elasticsearch, it is too expensive to scan all the data in the database to build the service graph. Instead, the Jaeger project provides “big data” jobs that can be used to extract the service graph data from traces:
- https://github.com/jaegertracing/spark-dependencies - the older Spark job that can be run periodically
- https://github.com/jaegertracing/jaeger-analytics - the new (experimental) streaming Flink jobs that run continuously and builds the service graph in smaller time intervals
Why do I not see any spans in Jaeger?
Please refer to the Troubleshooting guide.
Do I need to run jaeger-agent?
jaeger-agent is not always necessary. Jaeger client libraries can be configured to export trace data directly to
jaeger-collector. However, the following are the reasons why running
jaeger-agent is recommended:
- If we want Jaeger client libraries to send trace data directly to collectors, we must provide them with a URL of the HTTP endpoint. It means that our applications require additional configuration containing this parameter, especially if we are running multiple Jaeger installations (e.g. in different availability zones or regions) and want the data sent to a nearby installation. In contrast, when using the agent, the libraries require no additional configuration because the agent is always accessible via
localhost. It acts as a sidecar and proxies the requests to the appropriate collectors.
- The agent can be configured to enrich the tracing data with infrastructure-specific metadata by adding extra tags to the spans, such as the current zone, region, etc. If the agent is running as a host daemon, it will be shared by all applications running on the same host. If the agent is running as a true sidecar, i.e. one per application, it can provide additional functionality such as strong authentication, multi-tenancy (see this blog post), pod name, etc.
- Agents allow implementing traffic control to the collectors. If we have thousands of hosts in the data center, each running many applications, and each application sending data directly to the collectors, there may be too many open connections for each collector to handle. The agents can load balance this traffic with fewer connections.
What is the recommended storage backend?
The Jaeger team recommends Elasticsearch as the storage backend over Cassandra, for the following reasons:
Cassandra is a key-value database, so it is more efficient for retrieving traces by trace ID, but it does not provide the same powerful search capabilities as Elasticsearch. Effectively, the Jaeger backend implements the search functionality on the client side, on top of k-v storage, which is limited and may produce inconsistent results (see issue-166 for more details). Elasticsearch does not suffer from these issues, resulting in better usability. Elasticsearch can also be queried directly, e.g. from Kibana dashboards, and provide useful analytics and aggregations.
Based on past performance experiments we observed single writes to be much faster in Cassandra than Elasticsearch, which might suggest that it may sustain higher write throughput. However, because the Jaeger backend needs to implement search capability on top of k-v storage, writing spans to Cassandra is actually subject to large write amplification: in addition to writing a record for the span itself, Jaeger performs extra writes for service name and operation name indexing, as well as extra index writes for every tag. In contrast, saving a span to Elasticsearch is a single write, and all indexing takes place inside the ES node. As a result, the overall throughput to Cassandra is comparable with Elasticsearch.
One benefit of Cassandra backend is simplified maintenance due to its native support for data TTL. In Elasticsearch the data expiration is managed through index rotation, which requires additional setup (see Elasticsearch Rollover).