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Components
Components
Zuul is a distributed system consisting of several components, each of which is described below.
- graph {
-
node [shape=box] Gearman [shape=ellipse] Gerrit [fontcolor=grey] Zookeeper [shape=ellipse] Nodepool GitHub [fontcolor=grey]
Merger -- Gearman Executor -- Gearman Web -- Gearman
Gearman -- Scheduler; Scheduler -- Gerrit; Scheduler -- Zookeeper; Zookeeper -- Nodepool; Scheduler -- GitHub;
}
All Zuul processes read the /etc/zuul/zuul.conf
file (an
alternate location may be supplied on the command line) which uses an
INI file syntax. Each component may have its own configuration file,
though you may find it simpler to use the same file for all
components.
An example zuul.conf
:
[gearman]
server=localhost
[gearman_server]
start=true
log_config=/etc/zuul/gearman-logging.yaml
[zookeeper]
hosts=zk1.example.com,zk2.example.com,zk3.example.com
[webapp]
status_url=https://zuul.example.com/status
[scheduler]
log_config=/etc/zuul/scheduler-logging.yaml
A minimal Zuul system may consist of a scheduler
and executor
both running on the same host. Larger
installations should consider running multiple executors, each on a
dedicated host, and running mergers on dedicated hosts as well.
Common
The following applies to all Zuul components.
Configuration
The following sections of zuul.conf
are used by all Zuul
components:
gearman
Client connection information for Gearman.
server
Hostname or IP address of the Gearman server.
port
Port on which the Gearman server is listening.
ssl_ca
An openssl file containing a set of concatenated “certification authority” certificates in PEM formet.
ssl_cert
An openssl file containing the client public certificate in PEM format.
ssl_key
An openssl file containing the client private key in PEM format.
zookeeper
Client connection information for ZooKeeper
hosts
A list of zookeeper hosts for Zuul to use when communicating with Nodepool.
session_timeout
The ZooKeeper session timeout, in seconds.
Scheduler
The scheduler is the primary component of Zuul. The scheduler is not a scalable component; one, and only one, scheduler must be running at all times for Zuul to be operational. It receives events from any connections to remote systems which have been configured, enqueues items into pipelines, distributes jobs to executors, and reports results.
Configuration
The following sections of zuul.conf
are used by the
scheduler:
gearman_server
The builtin gearman server. Zuul can fork a gearman process from itself rather than connecting to an external one.
start
Whether to start the internal Gearman server.
listen_address
IP address or domain name on which to listen.
port
TCP port on which to listen.
log_config
Path to log config file for internal Gearman server.
ssl_ca
An openssl file containing a set of concatenated “certification authority” certificates in PEM formet.
ssl_cert
An openssl file containing the server public certificate in PEM format.
ssl_key
An openssl file containing the server private key in PEM format.
webapp
listen_address
IP address or domain name on which to listen.
port
Port on which the webapp is listening.
status_expiry
Zuul will cache the status.json file for this many seconds.
status_url
URL that will be posted in Zuul comments made to changes when starting jobs for a change.
scheduler
tenant_config
Path to tenant-config
file.
log_config
Path to log config file.
pidfile
Path to PID lock file.
state_dir
Path to directory in which Zuul should save its state.
Operation
To start the scheduler, run zuul-scheduler
. To stop it,
kill the PID which was saved in the pidfile specified in the
configuration.
Most of Zuul's configuration is automatically updated as changes to the repositories which contain it are merged. However, Zuul must be explicitly notified of changes to the tenant config file, since it is not read from a git repository. To do so, send the scheduler PID (saved in the pidfile specified in the configuration) a SIGHUP signal.
Merger
Mergers are an optional Zuul service; they are not required for Zuul to operate, but some high volume sites may benefit from running them. Zuul performs quite a lot of git operations in the course of its work. Each change that is to be tested must be speculatively merged with the current state of its target branch to ensure that it can merge, and to ensure that the tests that Zuul perform accurately represent the outcome of merging the change. Because Zuul's configuration is stored in the git repos it interacts with, and is dynamically evaluated, Zuul often needs to perform a speculative merge in order to determine whether it needs to perform any further actions.
All of these git operations add up, and while Zuul executors can also perform them, large numbers may impact their ability to run jobs. Therefore, administrators may wish to run standalone mergers in order to reduce the load on executors.
Configuration
The following section of zuul.conf
is used by the
merger:
merger
git_dir
Directory in which Zuul should clone git repositories.
git_user_email
Value to pass to git config user.email.
git_user_name
Value to pass to git config user.name.
log_config
Path to log config file for the merger process.
pidfile
Path to PID lock file for the merger process.
Operation
To start the merger, run zuul-merger
. To stop it, kill
the PID which was saved in the pidfile specified in the
configuration.
Executor
Executors are responsible for running jobs. At the start of each job, an executor prepares an environment in which to run Ansible which contains all of the git repositories specified by the job with all dependent changes merged into their appropriate branches. The branch corresponding to the proposed change will be checked out (in all projects, if it exists). Any roles specified by the job will also be present (also with dependent changes merged, if appropriate) and added to the Ansible role path. The executor also prepares an Ansible inventory file with all of the nodes requested by the job.
The executor also contains a merger. This is used by the executor to prepare the git repositories used by jobs, but is also available to perform any tasks normally performed by standalone mergers. Because the executor performs both roles, small Zuul installations may not need to run standalone mergers.
Trusted and Untrusted Playbooks
The executor runs playbooks in one of two execution contexts
depending on whether the project containing the playbook is a config-project
or an untrusted-project
. If the
playbook is in a config project, the executor runs the playbook in the
trusted execution context, otherwise, it is run in the
untrusted execution context.
Both execution contexts use bubblewrap1 to create a namespace to ensure that playbook executions are isolated and are unable to access files outside of a restricted environment. The administrator may configure additional local directories on the executor to be made available to the restricted environment.
The trusted execution context has access to all Ansible features, including the ability to load custom Ansible modules. Needless to say, extra scrutiny should be given to code that runs in a trusted context as it could be used to compromise other jobs running on the executor, or the executor itself, especially if the administrator has granted additional access through bubblewrap, or a method of escaping the restricted environment created by bubblewrap is found.
Playbooks run in the untrusted execution context are not permitted to load additional Ansible modules or access files outside of the restricted environment prepared for them by the executor. In addition to the bubblewrap environment applied to both execution contexts, in the untrusted context some standard Ansible modules are replaced with versions which prohibit some actions, including attempts to access files outside of the restricted execution context. These redundant protections are made as part of a defense-in-depth strategy.
Configuration
The following sections of zuul.conf
are used by the
executor:
executor
finger_port
Port to use for finger log streamer.
git_dir
Directory that Zuul should clone local git repositories to. The executor keeps a local copy of every git repository it works with to speed operations and perform speculative merging.
This should be on the same filesystem as executor.job_dir
so that
when git repos are cloned into the job workspaces, they can be
hard-linked to the local git cache.
job_dir
Directory that Zuul should use to hold temporary job directories. When each job is run, a new entry will be created under this directory to hold the configuration and scratch workspace for that job. It will be deleted at the end of the job (unless the --keep-jobdir command line option is specified).
This should be on the same filesystem as executor.git_dir
so that
when git repos are cloned into the job workspaces, they can be
hard-linked to the local git cache.
log_config
Path to log config file for the executor process.
pidfile
Path to PID lock file for the executor process.
private_key_file
SSH private key file to be used when logging into worker nodes.
user
User ID for the zuul-executor process. In normal operation as a
daemon, the executor should be started as the root
user,
but it will drop privileges to this user during startup.
variables
Path to an Ansible variables file to supply site-wide variables. This
should be a YAML-formatted file consisting of a single dictionary. The
contents will be made available to all jobs as Ansible variables. These
variables take precedence over all other forms (job variables and
secrets). Care should be taken when naming these variables to avoid
potential collisions with those used by jobs. Prefixing variable names
with a site-specific identifier is recommended. The default is not to
add any site-wide variables. See the User's Guide
<user_sitewide_variables>
for more information.
disk_limit_per_job
This integer is the maximum number of megabytes that any one job is allowed to consume on disk while it is running. If a job's scratch space has more than this much space consumed, it will be aborted.
trusted_ro_paths
List of paths, separated by :
to read-only bind mount
into trusted bubblewrap contexts.
trusted_rw_paths
List of paths, separated by :
to read-write bind mount
into trusted bubblewrap contexts.
untrusted_ro_paths
List of paths, separated by :
to read-only bind mount
into untrusted bubblewrap contexts.
untrusted_rw_paths
List of paths, separated by :
to read-write bind mount
into untrusted bubblewrap contexts.
execution_wrapper
Name of the execution wrapper to use when executing ansible-playbook. The default, bubblewrap is recommended for all installations.
There is also a nullwrap driver for situations where one wants to run Zuul without access to bubblewrap or in such a way that bubblewrap may interfere with the jobs themselves. However, nullwrap is considered unsafe, as bubblewrap provides significant protections against malicious users and accidental breakage in playbooks. As such, nullwrap is not recommended for use in production.
This option, and thus, nullwrap, may be removed in the future. bubblewrap has become integral to securely operating Zuul. If you have a valid use case for it, we encourage you to let us know.
merger
git_user_email
Value to pass to git config user.email.
git_user_name
Value to pass to git config user.name.
Operation
To start the executor, run zuul-executor
.
There are several commands which can be run to control the executor's behavior once it is running.
To stop the executor immediately, aborting all jobs (they may be
relaunched according to their retry policy), run
zuul-executor stop
.
To request that the executor stop executing new jobs and exit when
all currently running jobs have completed, run
zuul-executor graceful
.
To enable or disable running Ansible in verbose mode (with the
-vvv
argument to ansible-playbook) run
zuul-executor verbose
and
zuul-executor unverbose
.
Web Server
The Zuul web server currently acts as a websocket interface to live log streaming. Eventually, it will serve as the single process handling all HTTP interactions with Zuul.
Configuration
In addition to the common configuration sections, the following
sections of zuul.conf
are used by the web server:
web
listen_address
IP address or domain name on which to listen.
log_config
Path to log config file for the web server process.
pidfile
Path to PID lock file for the web server process.
port
Port to use for web server process.
websocket_url
Base URL on which the websocket service is exposed, if different than the base URL of the web app.
Operation
To start the web server, run zuul-web
. To stop it, kill
the PID which was saved in the pidfile specified in the
configuration.
Unless one has set execution_wrapper to nullwrap in the executor configuration.↩︎