What is XRay daemon？

Question

• When we are running a serverless app, say a beanstalk, a danymoDB and a SNS, there will be three XRay daemons in each JVM, right?
• If so,how could XRay trace a request from the beginning to the end? The trace ID will go with http request header or sth like that?

Answer 1

According to the docs, "The AWS X-Ray daemon is a software application that listens for traffic on UDP port 2000, gathers raw segment data, and relays it to the AWS X-Ray API. The daemon works in conjunction with the AWS X-Ray SDKs and must be running so that data sent by the SDKs can reach the X-Ray service."

It's important to note that the X-Ray SDK produces what's called a remote subsegment to mimic the result of the downstream in a client-sided manner. For a service like DynamoDB, you would see this. For something like SNS, the trace header information is propagated across through the Http headers. The X-Ray Daemon is used to forward segments generated when a service receives an upstream request; DynamoDB doesn't do this yet, and SNS forwards it through the trace header mentioned.

The Daemon isn't part of the JVM; it's an external process that's ran in an instance that forwards the trace data to the service. Technically, it can be run by a single instance, the same instance, or all instances.