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+# Generating Chaos in a test GitLab instance
+
+As [Werner Vogels](https://twitter.com/Werner), the CTO at Amazon Web Services, famously put it, **Everything fails, all the time**.
+
+As a developer, it's as important to consider the failure modes in which your software will operate as much as normal operation. Doing so can mean the difference between a minor hiccup leading to a scattering of 500 errors experienced by a tiny fraction of users and a full site outage affect all users for an extended period.
+
+To paraphrase [Tolstoy](https://en.wikipedia.org/wiki/Anna_Karenina_principle), _all happy servers are alike, but all failing servers are failing in their own way_. Luckily, there are ways we can attempt to simulate these failure modes, and the chaos endpoints are tools for assisting in this process.
+
+Currently, there are four endpoints for simulating the following conditions: slow requests, cpu-bound requests, memory leaks and unexpected process crashes.
+
+## Enabling Chaos Endpoints
+
+For obvious reasons, these endpoints are not enabled by default. They can be enabled by setting the `GITLAB_ENABLE_CHAOS_ENDPOINTS` environment variable.
+
+For example, if you're using the [GDK](https://gitlab.com/gitlab-org/gitlab-development-kit) this can be done with the following command:
+
+```shell
+GITLAB_ENABLE_CHAOS_ENDPOINTS=1 gdk run
+```
+
+### Securing the Chaos Endpoints
+
+**It is highly recommended that you secure access to the Chaos endpoints using a secret token**. This is recommended when enabling these endpoints locally, and essential when running in a staging or other shared environment. _It goes without saying that you should not enable them in production unless you absolutely know what you're doing._
+
+A secret can be set through the `GITLAB_CHAOS_SECRET` environment variable. For example, when using the [GDK](https://gitlab.com/gitlab-org/gitlab-development-kit) this can be done with the following command line:
+
+```shell
+GITLAB_ENABLE_CHAOS_ENDPOINTS=1 GITLAB_CHAOS_SECRET=secret gdk run
+```
+
+Replace `secret` with your own secret token.
+
+## Invoking Chaos
+
+Once you have enabled the chaos endpoints and restarted the application you can start testing using the endpoints.
+
+### Memory Leaks
+
+To simulate a memory leak in your application, use the `/-/chaos/leakmem` endpoint.
+
+For example, if your GitLab instance is listening at `localhost:3000`, you could `curl` the endpoint as follows:
+
+```shell
+curl http://localhost:3000/-/chaos/leakmem?memory_mb=1024 -H 'X-Chaos-Secret: secret'
+```
+
+The `memory_mb` parameter tells the application how much memory it should leak.
+
+Note: the memory is not retained after the request, so once its completed, the Ruby garbage collector will attempt to recover the memory.
+
+### CPU Spin
+
+This endpoint attempts to fully utilise a single core, at 100%, for the given period.
+
+```shell
+curl http://localhost:3000/-/chaos/cpuspin?duration_s=60 -H 'X-Chaos-Secret: secret'
+```
+
+The `duration_s` parameter will configure how long the core is utilised.
+
+Depending on your rack server setup, your request may timeout after a predermined period (normally 60 seconds). If you're using Unicorn, this is done by killing the worker process.
+
+### Sleep
+
+This endpoint is similar to the CPU Spin endpoint but simulates off-processor activity, such backend services of IO. It will sleep for a given duration.
+
+```shell
+curl http://localhost:3000/-/chaos/sleep?duration_s=60 -H 'X-Chaos-Secret: secret'
+```
+
+The `duration_s` parameter will configure how long the request will sleep for.
+
+As with the CPU Spin endpoint, this may lead to your request timing out if duration exceeds the configured limit.
+
+### Kill
+
+This endpoint will simulate the unexpected death of a worker process using a `kill` signal.
+
+```shell
+curl http://localhost:3000/-/chaos/kill -H 'X-Chaos-Secret: secret'
+```
+
+Note: since this endpoint uses the `KILL` signal, the worker is not given a chance to cleanup or shutdown.