Merge pull request #1 from OddDotNet/tkenna/feature/cache-timeouts

Updated docs for cache expiration and background service

_pages/documentation/concepts/test-harness.md

@@ -1,6 +1,7 @@
 ---
 layout: single
 title: "OpenTelemetry Test Harness"
+toc: true
 permalink: /documentation/concepts/test-harness/
 sidebar:
   nav: "docs"
@@ -20,4 +21,34 @@ the container.
 
 ## Health Check
 The container provides a health check endpoint on port 4318 at `/healthz` and can be
-used to confirm the application is up first before continuing. 
+used to confirm the application is up first before continuing. 
+
+# Environment Variables
+The following environment variables are recognized by OddDotNet and can be used to 
+configure various things such as cache expiration and cleanup intervals.
+
+All variables begin with `ODD`. Underscores can be single `_` or double `__`, eg.
+`ODD_CACHE_EXPIRATION` and `ODD__CACHE__EXPIRATION` will both work. Single underscores
+take precedence.
+
+You can pass environment variables to the container using the `-e` flag. eg. 
+
+`docker run -p 4317:4317 -e ODD_CACHE_EXPIRATION=5000 ghcr.io/odddotnet/odddotnet:latest`
+
+## ODD_CACHE_CLEANUP_INTERVAL (ms)
+The container includes a `BackgroundService` responsible for pruning the signal lists
+of any expired signals. This background service runs once per second (`default = 1000`),
+but can be configured to run at a different interval if needed.
+
+## ODD_CACHE_EXPIRATION (ms)
+Determines how long a telemetry signal remains in cache before being pruned. Signals may
+be received by the container before a query request has been started. As a result, it is
+possible to "miss" some of the signals because they came in before the query began. The
+container will cache signals, and the query will check for previously received signals
+first to find a match. 
+
+The default value is 30,000 milliseconds.
+
+If OddDotNet is receiving a large number of signals within a 30 second period, memory
+pressure may become an issue. If a full 30 seconds of signal history is not necessary,
+lowering this value can help reduce the container's memory footprint.