Merge branch 'ci_triggers_docs' into 'master'

Documentation on CI triggers

Closes #3432

## Notes

* Implement build trigger API - https://gitlab.com/gitlab-org/gitlab-ci/merge_requests/229
* Build trigger API - https://gitlab.com/gitlab-org/gitlab-ci/issues/257
* Build pipeline - https://dev.gitlab.org/gitlab/gitlab-ci/issues/282 and https://gitlab.com/gitlab-org/gitlab-ce/issues/3743
* Dependent builds - https://dev.gitlab.org/gitlab/gitlab-ci/issues/328
* Travis docs - https://docs.travis-ci.com/user/triggering-builds/
* Custom variables Circle CI example - https://circleci.com/docs/nightly-builds
* Triggers API (CI) will be done in a separate MR
* Notify https://gitlab.com/gitlab-org/gitlab-ci/issues/368 once done

## Docs needed to change

- [x] `doc/ci/README.md`
- [x] `doc/README.md`

See merge request !2161

7 files changed, 197 insertions, 20 deletions

diff --git a/doc/README.md b/doc/README.md
index 8bac00f2f23..f40a257b42f 100644
--- a/doc/README.md
+++ b/doc/README.md
@@ -28,17 +28,18 @@
 - [Using SSH keys](ci/ssh_keys/README.md)
 - [User permissions](ci/permissions/README.md)
 - [API](ci/api/README.md)
+- [Triggering builds through the API](ci/triggers/README.md)
 
 ### CI Languages
 
-+ [Testing PHP](ci/languages/php.md)
+- [Testing PHP](ci/languages/php.md)
 
 ### CI Services
 
-+ [Using MySQL](ci/services/mysql.md)
-+ [Using PostgreSQL](ci/services/postgres.md)
-+ [Using Redis](ci/services/redis.md)
-+ [Using Other Services](ci/docker/using_docker_images.md#how-to-use-other-images-as-services)
+- [Using MySQL](ci/services/mysql.md)
+- [Using PostgreSQL](ci/services/postgres.md)
+- [Using Redis](ci/services/redis.md)
+- [Using Other Services](ci/docker/using_docker_images.md#how-to-use-other-images-as-services)
 
 ### CI Examples
 
diff --git a/doc/ci/README.md b/doc/ci/README.md
index 965b007bedb..a1f5513d88e 100644
--- a/doc/ci/README.md
+++ b/doc/ci/README.md
@@ -2,25 +2,26 @@
 
 ### User documentation
 
-+ [Quick Start](quick_start/README.md)
-+ [Configuring project (.gitlab-ci.yml)](yaml/README.md)
-+ [Configuring runner](runners/README.md)
-+ [Configuring deployment](deployment/README.md)
-+ [Using Docker Images](docker/using_docker_images.md)
-+ [Using Docker Build](docker/using_docker_build.md)
-+ [Using Variables](variables/README.md)
-+ [Using SSH keys](ssh_keys/README.md)
+* [Quick Start](quick_start/README.md)
+* [Configuring project (.gitlab-ci.yml)](yaml/README.md)
+* [Configuring runner](runners/README.md)
+* [Configuring deployment](deployment/README.md)
+* [Using Docker Images](docker/using_docker_images.md)
+* [Using Docker Build](docker/using_docker_build.md)
+* [Using Variables](variables/README.md)
+* [Using SSH keys](ssh_keys/README.md)
+* [Triggering builds through the API](triggers/README.md)
 
 ### Languages
 
-+ [Testing PHP](languages/php.md)
+* [Testing PHP](languages/php.md)
 
 ### Services
 
-+ [Using MySQL](services/mysql.md)
-+ [Using PostgreSQL](services/postgres.md)
-+ [Using Redis](services/redis.md)
-+ [Using Other Services](docker/using_docker_images.md#how-to-use-other-images-as-services)
+* [Using MySQL](services/mysql.md)
+* [Using PostgreSQL](services/postgres.md)
+* [Using Redis](services/redis.md)
+* [Using Other Services](docker/using_docker_images.md#how-to-use-other-images-as-services)
 
 ### Examples
 
@@ -32,5 +33,5 @@
 
 ### Administrator documentation
 
-+ [User permissions](permissions/README.md)
-+ [API](api/README.md)
+* [User permissions](permissions/README.md)
+* [API](api/README.md)
diff --git a/doc/ci/triggers/README.md b/doc/ci/triggers/README.md
new file mode 100644
index 00000000000..2c1de5859f8
--- /dev/null
+++ b/doc/ci/triggers/README.md
@@ -0,0 +1,175 @@
+# Triggering Builds through the API
+
+_**Note:** This feature was [introduced][ci-229] in GitLab CE 7.14_
+
+Triggers can be used to force a rebuild of a specific branch, tag or commit,
+with an API call.
+
+## Add a trigger
+
+You can add a new trigger by going to your project's **Settings > Triggers**.
+The **Add trigger** button will create a new token which you can then use to
+trigger a rebuild of this particular project.
+
+Once at least one trigger is created, on the **Triggers** page you will find
+some descriptive information on how you can
+
+Every new trigger you create, gets assigned a different token which you can
+then use inside your scripts or `.gitlab-ci.yml`. You also have a nice
+overview of the time the triggers were last used.
+
+![Triggers page overview](img/triggers_page.png)
+
+## Revoke a trigger
+
+You can revoke a trigger any time by going at your project's
+**Settings > Triggers** and hitting the **Revoke** button. The action is
+irreversible.
+
+## Trigger a build
+
+To trigger a build you need to send a `POST` request to GitLab's API endpoint:
+
+```
+POST /projects/:id/trigger/builds
+```
+
+The required parameters are the trigger's `token` and the Git `ref` on which
+the trigger will be performed. Valid refs are the branch, the tag or the commit
+SHA. The `:id` of a project can be found by [querying the API](../api/projects.md)
+or by visiting the **Triggers** page which provides self-explanatory examples.
+
+When a rebuild is triggered, the information is exposed in GitLab's UI under
+the **Builds** page and the builds are marked as `triggered`.
+
+![Marked rebuilds as triggered on builds page](img/builds_page.png)
+
+---
+
+You can see which trigger caused the rebuild by visiting the single build page.
+The token of the trigger is exposed in the UI as you can see from the image
+below.
+
+![Marked rebuilds as triggered on a single build page](img/trigger_single_build.png)
+
+---
+
+See the [Examples](#examples) section for more details on how to actually
+trigger a rebuild.
+
+## Pass build variables to a trigger
+
+You can pass any number of arbitrary variables in the trigger API call and they
+will be available in GitLab CI so that they can be used in your `.gitlab-ci.yml`
+file. The parameter is of the form:
+
+```
+variables[key]=value
+```
+
+This information is also exposed in the UI.
+
+![Build variables in UI](img/trigger_variables.png)
+
+---
+
+See the [Examples](#examples) section below for more details.
+
+## Examples
+
+Using cURL you can trigger a rebuild with minimal effort, for example:
+
+```bash
+curl -X POST \
+     -F token=TOKEN \
+     -F ref=master \
+     https://gitlab.example.com/api/v3/projects/9/trigger/builds
+```
+
+In this case, the project with ID `9` will get rebuilt on `master` branch.
+
+
+### Triggering a build within `.gitlab-ci.yml`
+
+You can also benefit by using triggers in your `.gitlab-ci.yml`. Let's say that
+you have two projects, A and B, and you want to trigger a rebuild on the `master`
+branch of project B whenever a tag on project A is created. This is the job you
+need to add in project's A `.gitlab-ci.yml`:
+
+```yaml
+build_docs:
+  stage: deploy
+  script:
+  - "curl -X POST -F token=TOKEN -F ref=master https://gitlab.example.com/api/v3/projects/9/trigger/builds"
+  only:
+  - tags
+```
+
+Now, whenever a new tag is pushed on project A, the build will run and the
+`build_docs` job will be executed, triggering a rebuild of project B. The
+`stage: deploy` ensures that this job will run only after all jobs with
+`stage: test` complete successfully.
+
+_**Note:** If your project is public, passing the token in plain text is
+probably not the wiser idea, so you might want to use a
+[secure variable](../variables/README.md#user-defined-variables-secure-variables)
+for that purpose._
+
+### Making use of trigger variables
+
+Using trigger variables can be proven useful for a variety of reasons.
+
+* Identifiable jobs. Since the variable is exposed in the UI you can know
+  why the rebuild was triggered if you pass a variable that explains the
+  purpose.
+* Conditional job processing. You can have conditional jobs that run whenever
+  a certain variable is present.
+
+Consider the following `.gitlab-ci.yml` where we set three
+[stages](../yaml/README.md#stages) and the `upload_package` job is run only
+when all jobs from the test and build stages pass. When the `UPLOAD_TO_S3`
+variable is non-zero, `make upload` is run.
+
+```yaml
+stages:
+- test
+- build
+- package
+
+run_tests:
+  script:
+  - make test
+
+build_package:
+  stage: build
+  script:
+  - make build
+
+upload_package:
+  stage: package
+  script:
+  - if [ -n "${UPLOAD_TO_S3}" ]; then make upload; fi
+```
+
+You can then trigger a rebuild while you pass the `UPLOAD_TO_S3` variable
+and the script of the `upload_package` job will run:
+
+```bash
+curl -X POST \
+  -F token=TOKEN \
+  -F ref=master \
+  -F "variables[UPLOAD_TO_S3]=true" \
+  https://gitlab.example.com/api/v3/projects/9/trigger/builds
+```
+
+### Using cron to trigger nightly builds
+
+Whether you craft a script or just run cURL directly, you can trigger builds
+in conjunction with cron. The example below triggers a build on the `master`
+branch of project with ID `9` every night at `00:30`:
+
+```bash
+30 0 * * * curl -X POST -F token=TOKEN -F ref=master https://gitlab.example.com/api/v3/projects/9/trigger/builds
+```
+
+[ci-229]: https://gitlab.com/gitlab-org/gitlab-ci/merge_requests/229
diff --git a/doc/ci/triggers/img/builds_page.png b/doc/ci/triggers/img/builds_page.png
new file mode 100644
index 00000000000..e78794fbee7
--- /dev/null
+++ b/doc/ci/triggers/img/builds_page.png
diff --git a/doc/ci/triggers/img/trigger_single_build.png b/doc/ci/triggers/img/trigger_single_build.png
new file mode 100644
index 00000000000..c25f27409d6
--- /dev/null
+++ b/doc/ci/triggers/img/trigger_single_build.png
diff --git a/doc/ci/triggers/img/trigger_variables.png b/doc/ci/triggers/img/trigger_variables.png
new file mode 100644
index 00000000000..2207e8b34cb
--- /dev/null
+++ b/doc/ci/triggers/img/trigger_variables.png
diff --git a/doc/ci/triggers/img/triggers_page.png b/doc/ci/triggers/img/triggers_page.png
new file mode 100644
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--- /dev/null
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