web node is responsible for running the web UI, API, and as well as performing all pipeline scheduling. It's basically the brain of Concourse.
Nothing special - the
web node is a pretty simple Go application that can be run like a 12-factor app.
concourse CLI can run as a
web node via the
Before running it, let's configure a local user so we can log in:
This will configure a single user,
myuser, with the password
mypass. You'll probably want to change those to sensible values, and later you may want to configure a proper auth provider - check out Auth & Teams whenever you're ready.
Next, you'll need to configure the session signing key, the SSH key for the worker gateway, and the authorized worker key. Check Generating Keys to learn what these are and how they are created.
CONCOURSE_SESSION_SIGNING_KEY=path/to/session_signing_key CONCOURSE_TSA_HOST_KEY=path/to/tsa_host_key CONCOURSE_TSA_AUTHORIZED_KEYS=path/to/authorized_worker_keys
web needs to know how to reach your Postgres database. This can be set like so:
CONCOURSE_POSTGRES_HOST=127.0.0.1 # default CONCOURSE_POSTGRES_PORT=5432 # default CONCOURSE_POSTGRES_DATABASE=atc # default CONCOURSE_POSTGRES_USER=my-user CONCOURSE_POSTGRES_PASSWORD=my-password
If you're running PostgreSQL locally, you can probably just point it to the socket and rely on the
Now that everything's set, run:
All logs will be emitted to
stdout, with any panics or lower-level errors being emitted to
CPU usage: peaks during pipeline scheduling, primarily when scheduling
jobs. Mitigated by adding more
web nodes. In this regard,
web nodes can be considered compute-heavy more than anything else at large scale.
Memory usage: not very well classified at the moment as it's not generally a concern. Give it a few gigabytes and keep an eye on it.
Disk usage: none
Bandwidth usage: aside from handling external traffic, the
web node will at times have to stream bits out from one worker and into another while executing Steps.
Highly available: yes;
web nodes can all be configured the same (aside from
--peer-url) and placed behind a load balancer. Periodic tasks like garbage-collection will not be duplicated for each node.
Horizontally scalable: yes; they will coordinate workloads using the database, resulting in less work for each node and thus lower CPU usage.
dbon its configured port for persistence
dbon its configured port for locking and coordinating in a multi-
workernodes on ports
resources, executing builds, and performing garbage-collection
webnodes (possibly itself) on an ephemeral port when a worker is forwarded through the web node's TSA
web nodes themselves are stateless - they don't store anything on disk, and coordinate entirely using the database.
web node can be scaled up for high availability. They'll also roughly share their scheduling workloads, using the database to synchronize. This is done by just running more
web commands on different machines, and optionally putting them behind a load balancer.
To run a cluster of
web nodes, you'll first need to ensure they're all pointing to the same PostgreSQL server.
Next, you'll need to configure a peer URL. This is a URL that can be used to reach this
web node's web server from other
web nodes. Typically this uses a private IP, like so:
Finally, if all of these nodes are going to be accessed through a load balancer, you'll need to configure the external URL that will be used to reach your Concourse cluster:
Aside from the peer URL, all configuration must be consistent across all
web nodes in the cluster to ensure consistent results.
web nodes can be killed and restarted willy-nilly. No draining is necessary; if the
web node was orchestrating a build it will just continue where it left off when it comes back or, or the build will be picked up by one of the other
To upgrade a
web node, stop its process and start a new one using the newly installed
concourse. Any migrations will be run automatically on start. If
web nodes are started in parallel, only one will run the migrations.
Note that we don't currently guarantee a lack of funny-business if you're running mixed Concourse versions - database migrations can perform modiciations that confuse other
web nodes. So there may be some turbulence during a rolling upgrade, but everything should stabilize once all
web nodes are running the latest version.
If you're stuck in a pinch and need to downgrade from one version of Concourse to another, you can use the
concourse migrate command.
Note: support for down migrations is a fairly recent addition to Concourse; it is not supported for downgrading to v3.6.0 and below.
First, grab the desired migration version by running the following:
# make sure this is the *old* Concourse binary $ concourse migrate --supported-db-version 1551110547
That number (yours will be different) is the expected migration version for that version of Concourse.
Next, run the following with the new Concourse binary:
$ concourse migrate --migrate-db-to-version=1551110547
This will need the same
CONCOURSE_POSTGRES_* configuration described in Running.
Once this completes, switch all
web nodes back to the older
concourse binary and you should be good to go.