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+# RFC 0135 - Clients and Environment Variables
+* Comments: [#135](https://api.github.com/repos/taskcluster/taskcluster-rfcs/pulls/135)
+* Proposed by: @djmitche
+
+# Summary
+
+Client libraries should not automatically read environment variables.
+
+## Motivation
+
+Some client libraries currently automatically use environment variables
+ * `TASKCLUSTER_CLIENT_ID`
+ * `TASKCLUSTER_ACCESS_TOKEN`
+ * `TASKCLUSTER_CERTIFICATE`
+ * `TASKCLUSTER_ROOT_URL`
+if they are set, and if no explicit configuration is given when constructing the client.
+But clients differ in their use of these variables.
+
+This is problematic because not all code will want to configure settings based on
+environment variables, for example workers that take their configuration from
+configuration files.
+
+This has also caused issues with testing and deploying software, where the
+"hidden" context of environment variables can cause tests to pass or fail, or a
+service to fail when deployed in a different context.
+
+Finally, we should strive for some similarity in usage of the client libraries.
+
+# Details
+
+The proposed change is that client libraries do not *automatically* look to
+these four environment variables for configuration.  The libraries would still have
+options to explicitly "opt in" to using the environment variables, e.g.,
+
+```go
+queue := queue.NewFromEnvVars()
+```
+
+or
+
+```js
+queue = new taskcluster.Queue({fromEnvVars: true});
+```
+
+## Standardized Variables
+
+This RFC also serves to standardize the meanings of the following environment variables.
+These meanings capture the current *de facto* meanings.
+
+ * `TASKCLUSTER_CLIENT_ID` - the `clientId` for API requests
+ * `TASKCLUSTER_ACCESS_TOKEN` - the corresponding `accessToken` for API requests
+ * `TASKCLUSTER_CERTIFICATE` - if present, a JSON-formatted temporary credential certificate.  In this case, `TASKCLUSTER_CLIENT_ID` and `TASKCLUSTER_ACCESS_TOKEN` are defined as described in [the temporary credentials documentation](https://docs.taskcluster.net/docs/manual/design/apis/hawk/temporary-credentials).  NOTE: users are not expected to decode this string.
+ * `TASKCLUSTER_ROOT_URL` - the root URL of the cluster to which API requests should be made
+ * `TASKCLUSTER_PROXY_URL` - if present, the root URL of a `taskcluster-proxy` instance suitable for accessing the Taskcluster deployment in which the current task is running (defined in [RFC 0128](https://github.com/taskcluster/taskcluster-rfcs/blob/master/rfcs/0128-redeployable-clients.md))
+
+## Taskcluster-CLI
+
+The `taskcluster-cli` application is often referred to as a "shell client library".
+However, environment variables are a common and accepted way of sending configuration to command-line tools, so `taskcluster-cli` will continue to use these environment variables for its configuration.
+
+# Current Status
+
+## Python
+
+The Python client currently accepts all four environment variables mentioned above, as well as a `rootUrl` option to the class constructor.
+This was released in version 5.0.0.
+
+## JS
+
+The JS client currently accepts all four environment variables mentioned above, as well as a `rootUrl` option to the class constructor.
+This was released in version 10.0.0.
+
+## Go
+
+The Go client does not read environment variables directly and does not [yet](https://bugzilla.mozilla.org/show_bug.cgi?id=1428422) support root URLs at all.
+(That is, it is already compliant with this RFC)
+
+## Java
+
+The Java client does not read environment variables directly and does not support root URLs at all.
+(That is, it is already compliant with this RFC)
+
+## Web
+
+The Web client runs in a browser, and therefore does not accept environment variables at all.
+It does accept a `rootUrl` option similar to the JS client.
+
+# Compatibility
+
+This is a breaking change, necessitating a major version bump for the Python and JS clients.
+
+RFC 0128 adds the requirement to specify a root URL, with no default available.
+Because failing to specify a root URL is a fatal error, any use of clients that are not upgraded to supply explicit configuration or use `NewFromEnvVars` will likely fail clearly in testing.
+Such clear failures should ease the task of upgrading client consumers, assuming good test coverage.
+
+However, many consumers have already been updated to use the newest versions of these clients.
+Such upgraded consumers may encounter subtle issues where a client continues to work for read-only operations, but fails (perhaps only in production) for operations requiring credentials.
+Thus, the upgrade from 5.x (for Python) or 10.x (for JS) will require some caution.
+
+# Implementation
+
+https://bugzilla.mozilla.org/show_bug.cgi?id=1508333