Merge pull request #128 from chanchiem/master

Update Readme: Disabling the SDK

Author: chanchiem

README.md

@@ -191,6 +191,34 @@ if xray_recorder.is_sampled():
     xray_recorder.put_metadata('mykey', metadata)
 ```
 
+### Disabling X-Ray
+Often times, it may be useful to be able to disable X-Ray for specific use cases, whether to stop X-Ray from sending traces at any moment, or to test code functionality that originally depended on X-Ray instrumented packages to begin segments prior to the code call. For example, if your application relied on an XRayMiddleware to instrument incoming web requests, and you have a method which begins subsegments based on the segment generated by that middleware, it would be useful to be able to disable X-Ray for your unit tests so that `SegmentNotFound` exceptions are not thrown when you need to test your method.
+
+There are two ways to disable X-Ray, one is through environment variables, and the other is through the SDKConfig module.
+
+**Disabling through the environment variable:**
+
+Prior to running your application, make sure to have the environment variable `AWS_XRAY_SDK_ENABLED` set to `false`. 
+
+**Disabling through the SDKConfig module:**
+```
+from aws_xray_sdk import global_sdk_config
+
+global_sdk_config.set_sdk_enabled(False)
+```
+
+**Important Notes:**
+* Environment Variables always take precedence over the SDKConfig module when disabling/enabling. If your environment variable is set to `false` while your code calls `global_sdk_config.set_sdk_enabled(True)`, X-Ray will still be disabled.
+
+* If you need to re-enable X-Ray again during runtime and acknowledge disabling/enabling through the SDKConfig module, you may run the following in your application:
+```
+import os
+from aws_xray_sdk import global_sdk_config
+
+del os.environ['AWS_XRAY_SDK_ENABLED']
+global_sdk_config.set_sdk_enabled(True)
+```
+
 ### Trace AWS Lambda functions
 
 ```python