Setting up a Replica Server (Stratum 1)

While a CernVM-FS Stratum 0 repository server is able to serve clients directly, when having many clients it is better to serve them by a set of Stratum 1 replica servers. Multiple Stratum 1 servers improve the reliability, reduce the load, and protect the Stratum 0 master copy of the repository from direct accesses. Stratum 0 server, Stratum 1 servers and the site-local proxy servers can be seen as content distribution network. The figure below shows the situation for the repositories hosted in the cern.ch domain.

CernVM-FS content distribution network for the cern.ch domain: Stratum1 replica servers are located in Europe, the U.S. and Asia. One protected read/write instance (Stratum 0) is feeding up the public, distributed mirror servers. A distributed hierarchy of proxy servers fetches content from the closest public mirror server.

A Stratum 1 server is a standard web server that uses the CernVM-FS server toolkit to create and maintain a mirror of a CernVM-FS repository served by a Stratum 0 server. To this end, the cvmfs_server utility provides the add-replica command. This command will register the Stratum 0 URL and prepare the local web server. Periodical synchronization has to be scheduled, for instance with cron, using the cvmfs_server snapshot -a command. The advantage over general purpose mirroring tools such as rSync is that all CernVM-FS file integrity verifications mechanisms from the Fuse client are reused. Additionally, by the aid of the CernVM-FS file catalogs, the cvmfs_server utility knows beforehand (without remote listing) which files to transfer.

In order to prevent accidental synchronization from a repository, the Stratum 0 repository maintainer has to create a .cvmfs_master_replica file in the HTTP root directory. This file is created by default when a new repository is created. Note that replication can thrash caches that might exist between Stratum 1 and Stratum 0. A direct connection is therefore preferable.

Recommended Setup

The vast majority of HTTP requests will be served by the site’s local proxy servers. Being a publicly available service, however, we recommend installing a Squid frontend in front of the Stratum 1 web server.

We suggest the following key parameters:

Storage

RAID-protected storage. The cvmfs_server utility should have low latency to the storage because it runs lots of system calls (stat()) against it.

Web server

A standard Apache server. Directory listing is not required. In addition, it is a good practice to exclude search engines from the replica web server by an appropriate robots.txt. The web server should be close to the storage in terms of latency.

Squid frontend

Squid should be used as a frontend to Apache, configured as a reverse proxy. It is recommended to run it on the same machine as Apache instead of a separate machine, to reduce the number of points of failure. In that case caching can be disabled for the data (since there’s no need to store it again on the same disk), but caching is helpful for the responses to geo api calls. Using a squid is also helpful for participating in shared monitoring such as the WLCG Squid Monitor.

Alternatively, separate Squid server machines may be configured in a round-robin DNS and each forward to the Apache server, but note that if any of them are down the entire service will be considered down by CernVM-FS clients. A front end hardware load balancer that quickly takes a machine that is down out of service would help reduce the impact.

High availability

On the subject of availability, note that it is not advised to use two separate complete Stratum 1 servers in a single round-robin service because they will be updated at different rates. That would cause errors when a client sees an updated catalog from one Stratum 1 but tries to read corresponding data files from the other that does not yet have the files. Different Stratum 1s should either be separately configured on the clients, or a pair can be configured as a high availability active/standby pair using the cvmfs-contrib cvmfs-hastratum1 package. An active/standby pair can also be managed by switching a DNS name between two different servers.

DNS cache

The geo api on a Stratum 1 does DNS lookups. It caches lookups for 5 minutes so the DNS server load does not tend to be severe, but we still recommend installing a DNS caching mechanism on the machine such as dnsmasq or bind. We do not recommend nscd since it does not honor the DNS Time-To-Live protocol.

Apache Configuration

In general the cvmfs_server utility automatically manages the Apache configuration. However, for systems based on Red Hat Enterprise Linux 7 it is recommended that heavily used Stratum 1s disable the “prefork” Multi-Process Module (MPM) and instead use the “worker” or “event” MPM which perform much better under heavy load because they work with multiple threads per process. That can be done by changing which module is uncommented in /etc/httpd/conf.modules.d/00-mpm.conf. The “event” MPM is the default on Red Hat Enterprise Linux 8.

Squid Configuration

If you participate in the Open Science Grid (OSG) or the European Grid Infrastructure (EGI), you are encouraged to use their distribution of squid called frontier-squid. It is kept up to date with the latest squid bug fixes and has features for easier upgrading and monitoring. Step-by-step instructions for setting it up with a Stratum 1 is available in the OSG documentation.

Otherwise, a squid package is available in most Linux operating systems. The Squid configuration differs from the site-local Squids because the Stratum 1 Squid servers are transparent to the clients (reverse proxy). As the expiry rules are set by the web server, Squid cache expiry rules remain unchanged.

The following lines should appear accordingly in /etc/squid/squid.conf:

http_port 8000 accel
http_access allow all
cache_peer <APACHE_HOSTNAME> parent <APACHE_PORT> 0 no-query originserver

cache_mem <MEM_CACHE_SIZE> MB
cache_dir ufs /var/spool/squid <DISK_CACHE_SIZE in MB> 16 256
maximum_object_size 1024 MB
maximum_object_size_in_memory 128 KB

Note that http_access allow all has to be inserted before (or instead of) the line http_access deny all. If Apache is running on the same host, the APACHE_HOSTNAME will be localhost. Also, in that case there is not a performance advantage for squid to cache files that came from the same machine, so you can configure squid to not cache files. Do that with the following lines:

acl CVMFSAPI urlpath_regex ^/cvmfs/[^/]*/api/
cache deny !CVMFSAPI

Then the squid will only cache API calls. You can then set MEM_CACHE_SIZE and DISK_CACHE_SIZE quite small. Even if squid is configured to cache everything it is best to keep MEM_CACHE_SIZE small, because it is generally better to leave as much RAM to the operating system for file system caching as possible.

Check the configuration syntax by squid -k parse. Create the hard disk cache area with squid -z. In order to make the increased number of file descriptors effective for Squid, execute ulimit -n 8192 prior to starting the squid service.

The Squid also needs to respond to port 80, but Squid might not have the ability to directly listen there if it is run unprivileged, plus Apache listens on port 80 by default. Direct external port 80 traffic to port 8000 with the following command:

iptables -t nat -A PREROUTING -p tcp -m tcp --dport 80 -j REDIRECT --to-ports 8000

If IPv6 is supported, do the same command with ip6tables. This will leave localhost traffic to port 80 going directly to Apache, which is good because cvmfs_server uses that, and it doesn’t need to go through squid.

Note

Port 8000 might be assigned to soundd. On SElinux systems, this assignment must be changed to the HTTP service by semanage port -m -t http_port_t -p tcp 8000. The cvmfs-server RPM for EL7 executes this command as a post-installation script.

Geo API Setup

One of the essential services supplied by Stratum 1s to CernVM-FS clients is the Geo API. This enables clients to share configurations worldwide while automatically sorting Stratum 1s geographically to prioritize connecting to the closest ones. This makes use of a GeoIP database from Maxmind that translates IP addresses of clients to longitude and latitude.

The database is free, but the Maxmind End User License Agreement requires that each user of the database sign up for an account and promise to update the database to the latest version within 30 days of when they issue a new version. The signup process will end with giving you a License Key. The cvmfs_server add-replica and snapshot commands will take care of automatically updating the database if you put a line like the following in /etc/cvmfs/server.local, replacing <license key> with the key you get from the signup process:

CVMFS_GEO_LICENSE_KEY=<license key>

To keep the key secret, set the mode of /etc/cvmfs/server.local to 600. You can test that it works by running cvmfs_server update-geodb.

Alternatively, if you have a separate mechanism of installing and updating the Geolite2 City database file, you can instead set CVMFS_GEO_DB_FILE to the full path where you have installed it. If the path is NONE, then no database will be required, but note that this will break the client Geo API so only use it for testing, when the server is not used by production clients. If the database is installed in the default directory used by Maxmind’s own geoipupdate tool, /usr/share/GeoIP, then cvmfs_server will use it from there and neither variable needs to be set.

Normally repositories on Stratum 1s are created owned by root, and the cvmfs_server snapshot command is run by root. If you want to use a different user ID while still using the built-in mechanism for updating the geo database, change the owner of /var/lib/cvmfs-server/geo and /etc/cvmfs/server.local to the user ID.

The built-in geo database update mechanism normally checks for updates once a week on Tuesdays but can be controlled through a set of variables defined in cvmfs_server beginning with CVMFS_UPDATEGEO_. Look in the cvmfs_server script for the details. An update can also be forced at any time by running cvmfs_server update-geodb.

Monitoring

The cvmfs_server utility reports status and problems to stdout and stderr.

For the web server infrastructure, we recommend cvmfs-servermon which watches for problems in every repository’s .cvmfs_status.json status file.

In order to tune the hardware and cache sizes, keep an eye on the Squid server’s CPU and I/O load.

Keep an eye on HTTP 404 errors. For normal CernVM-FS traffic, such failures should not occur. Traffic from CernVM-FS clients is marked by an X-CVMFS2 header.

Maintenance processes

If any replicated repositories have Garbage Collection enabled, the Stratum 1 also needs to run garbage collection in order to prevent the disk space usage from growing rapidly. Run cvmfs_server gc -af periodically (e.g. daily or weekly) from cron to run garbage collection on all repositories that have garbage collection enabled. Logs will go into /var/log/cvmfs/gc.log.

In addition, over time problems can show up with a small percentage of files stored on a large Stratum 1. Run cvmfs_server check -a daily from cron to start a check process. On a large Stratum 1 it will run for many days, but only with a single thread, so it is not very intrusive. If another check is still in process a new one will not start. Each repository by default will only be checked at most once every 30 days. Logs will go into /var/log/cvmfs/checks.log and problems will be recorded in a repository’s .cvmfs_status.json.