Configuring Artifact Server

BuildStream caches the results of builds in a local artifact cache, and will avoid building an element if there is a suitable build already present in the local artifact cache.

In addition to the local artifact cache, you can configure one or more remote artifact caches and BuildStream will then try to pull a suitable build from one of the remotes, falling back to a local build if needed.

Configuring BuildStream to use remote caches

A project will often set up continuous build infrastructure that pushes built artifacts to a shared cache, so developers working on the project can make use of these pre-built artifacts instead of having to each build the whole project locally. The project can declare this cache in its project configuration file.

Users can declare additional remote caches in the user configuration. There are several use cases for this: your project may not define its own cache, it may be useful to have a local mirror of its cache, or you may have a reason to share artifacts privately.

Remote artifact caches are identified by their URL. There are currently two supported protocols:

  • http: Pull and push access, without transport-layer security
  • https: Pull and push access, with transport-layer security

BuildStream allows you to configure as many caches as you like, and will query them in a specific order:

  1. Project-specific overrides in the user config
  2. Project configuration
  3. User configuration

When an artifact is built locally, BuildStream will try to push it to all the caches which have the push: true flag set. You can also manually push artifacts to a specific cache using the bst artifact push command.

Artifacts are identified using the element’s cache key so the builds provided by a cache should be interchangable with those provided by any other cache.

Setting up a remote artifact cache

The rest of this page outlines how to set up a shared artifact cache.

Setting up the user

A specific user is not needed, however, a dedicated user to own the artifact cache is recommended.

useradd artifacts

The recommended approach is to run two instances on different ports. One instance has push disabled and doesn’t require client authentication. The other instance has push enabled and requires client authentication.

Alternatively, you can set up a reverse proxy and handle authentication and authorization there.

Installing the server

You will also need to install BuildStream on the artifact server in order to receive uploaded artifacts over ssh. Follow the instructions for installing BuildStream here.

When installing BuildStream on the artifact server, it must be installed in a system wide location, with pip3 install . in the BuildStream checkout directory.

Otherwise, some tinkering is required to ensure BuildStream is available in PATH when its companion bst-artifact-server program is run remotely.

You can install only the artifact server companion program without requiring BuildStream’s more exigent dependencies by setting the BST_ARTIFACTS_ONLY environment variable at install time, like so:

BST_ARTIFACTS_ONLY=1 pip3 install .

Command reference


CAS Artifact Server

bst-artifact-server [OPTIONS] REPO


-p, --port <port>

Port number [required]

--server-key <server_key>

Private server key for TLS (PEM-encoded)

--server-cert <server_cert>

Public server certificate for TLS (PEM-encoded)

--client-certs <client_certs>

Public client certificates for TLS (PEM-encoded)


Allow clients to upload blobs and update artifact cache

--head-room-min <head_room_min>

Disk head room minimum in bytes

--head-room-max <head_room_max>

Disk head room maximum in bytes



Required argument

Key pair for the server

For TLS you need a key pair for the server. The following example creates a self-signed key, which requires clients to have a copy of the server certificate (e.g., in the project directory). You can also use a key pair obtained from a trusted certificate authority instead.

openssl req -new -newkey rsa:4096 -x509 -sha256 -days 3650 -nodes -batch -subj "/" -out server.crt -keyout server.key


Note that in the -subj "/CN=<foo>" argument, /CN is the certificate common name, and as such <foo> should be the public hostname of the server. IP addresses will not provide you with working authentication.

In addition to this, ensure that the host server is recognised by the client. You may need to add the line: <ip address> <hostname> to your /etc/hosts file.

Authenticating users

In order to give permission to a given user to upload artifacts, create a TLS key pair on the client.

openssl req -new -newkey rsa:4096 -x509 -sha256 -days 3650 -nodes -batch -subj "/CN=client" -out client.crt -keyout client.key

Copy the public client certificate client.crt to the server and then add it to the authorized keys, like so:

cat client.crt >> /home/artifacts/authorized.crt

Serve the cache over https

Public instance without push:

bst-artifact-server --port 11001 --server-key server.key --server-cert server.crt /home/artifacts/artifacts

Instance with push and requiring client authentication:

bst-artifact-server --port 11002 --server-key server.key --server-cert server.crt --client-certs authorized.crt --enable-push /home/artifacts/artifacts

Managing the cache with systemd

We recommend running the cache as a systemd service, especially if it is running on a dedicated server, as this will allow systemd to manage the cache, in case the server encounters any issues.

Below are two examples of how to run the cache server as a systemd service. The first, is for pull only and the other is configured for push & pull. Notice that the two configurations use different ports.


# Pull
Description=Buildstream Artifact pull server

ExecStart=/usr/local/bin/bst-artifact-server --port 11001 --server-key {{certs_path}}/server.key --server-cert {{certs_path}}/server.crt {{artifacts_path}}



# Pull/Push
Description=Buildstream Artifact pull/push server

ExecStart=/usr/local/bin/bst-artifact-server --port 11002 --server-key {{certs_path}}/server.key --server-cert {{certs_path}}/server.crt --client-certs {{certs_path}}/authorized.crt --enable-push {{artifacts_path}}


Here we define when systemd should start the service, which is after the networking stack has been started, we then define how to run the cache with the desired configuration, under the artifacts user. The {{ }} are there to denote where you should change these files to point to your desired locations.


You may need to run some of the following commands as the superuser.

These files should be copied to /etc/systemd/system/. We can then start these services with:

systemctl enable bst-artifact-serve.service
systemctl enable bst-artifact-serve-receive.service

Then, to start these services:

systemctl start bst-artifact-serve.service
systemctl start bst-artifact-serve-receive.service

We can then check if the services are successfully running with:

journalctl -u bst-artifact-serve.service
journalctl -u bst-artifact-serve-receive.service

For more information on systemd services see: Creating Systemd Service Files.

Declaring remote artifact caches

Remote artifact caches can be declared within either:

  1. The project configuration, or
  2. The user configuration.

Please follow the above links to see examples showing how we declare remote caches in both the project configuration and the user configuration, respectively.