Setting up a copy of the web service nova.astrometry.net

These are instructions for how to set up a web service like our http://nova.astrometry.net . This requires a bit of sysadmin savvy.

Roadmap

The code for the web service lives in the “net” subdirectory of the git repository; https://github.com/dstndstn/astrometry.net/tree/master/net . It is not included in the source code releases, so you’ll need to git clone the code.

The web service has several parts:
  • the web front-end. This is a Django app that we run in Apache via WSGI. Other deployment options are possible but untested.
  • the database. The Django app uses a database. We use postgres. Other databases (sqlite3, mysql) would work but are, you guessed it, untested.
  • front-end processing. The front-end server has to do some asynchronous processing. That is, the web-facing part of it queues submissions that are processed in a separate process, called *process-submissions.py*. On nova, we run this inside a screen process on the web server.
  • the solve server. On nova, we have the web front-end running on one machine, and the “engine” running on another machine; the web server runs ssh to connect to the solve server.

Setup – web front-end

I would recommend creating a new user on your system, running the Apache server as that user, and creating a database account for that user. On nova that user is called (you guessed it), “nova”.

As that user, you’ll want to check out the code, eg:

cd
git clone https://github.com/dstndstn/astrometry.net.git nova

See Running on a local machine (your laptop, say) for how to run the web server using sqlite3 and Django’s development web server.

For “real” use, you may want to set up a postgres database and run the web service via Apache.

Notice that in the net/ directory we have a bunch of settings_XXX.py files. Each of these describes the setup of a deployment of the web site. We use symlinks to determine which one runs, eg, on the nova server we have:

ln -s settings_nova.py settings.py

Note also that we store the database secrets in a separate, secret SVN repository, which we check out into the directory net/secrets; ie, on the nova server:

$ ls -1 net/secrets/
django_db.py
__init__.py

where __init__.py is empty, and django_db.py contains:

DATABASE_USER = 'nova'
DATABASE_PASSWORD = 'SOSECRET'
DATABASE_HOST = 'localhost'
DATABASE_PORT = ''

Setting up your database is sort of beyond the scope of this document. The django docs should have some material to get you started.

The following may work to set up a postgres database:

# as root, run the "psql" program, and enter:
create role nova;
alter role nova createdb;

# as user "nova", run "psql" and
create database "an-nova";

Then, to initialize the database, cd into the astrometry/net directory and run the Django setup scripts:

python manage.py syncdb
python manage.py migrate

and test it out:

python manage.py runserver

You probably want to run the web app under Apache. The Apache configuration files for nova are not public, but your apache2.conf file might end up containing entries such as:

User nova
Group nova
Include /etc/apache2/mods-available/wsgi.load
Include /etc/apache2/mods-available/wsgi.conf
WSGIScriptAlias / /home/nova/nova/net/nova.wsgi

See the Django deployment docs for much more detailed setup help.

Setup – front-end processing

You need to run the process_submissions.py script on the web server to actually process user submissions. On nova, we run this inside a screen session; unfortunately this means we have to manually start it whenever the web server is rebooted. cd into the astrometry/net subdirectory and run, eg:

python -u process_submissions.py --jobthreads=8 --subthreads=4 < /dev/null >> proc.log 2>&1 &

Setup – solve-server processing

For actually running the astrometry engine, the web server uses ssh to connect to a solve server. One could probably use a local version (running on the web server) without too many changes to the code, but that has not been implemented yet.

When the web server wants to run the astrometry engine, it executes the following crazy code (https://github.com/dstndstn/astrometry.net/blob/master/net/process_submissions.py#L288):

cmd = ('(echo %(jobid)s; '
       'tar cf - --ignore-failed-read -C %(jobdir)s %(axyfile)s) | '
       'ssh -x -T %(sshconfig)s 2>>%(logfile)s | '
       'tar xf - --atime-preserve -m --exclude=%(axyfile)s -C %(jobdir)s '
       '>>%(logfile)s 2>&1' %
       dict(jobid='job-%s-%i' % (settings.sitename, job.id),
            axyfile=axyfn, jobdir=jobdir,
            sshconfig=settings.ssh_solver_config,
            logfile=logfn))

So it first sends the job id, then a tar stream of the required input files, and pipes that to ssh. It streams the error output to the logfile, and pipes the standard out to tar to receive the results. It’s sweet.

Notice that sshconfig string there, which come from the settings.py file. For nova, for example, ssh_solver_config = ‘an-nova’. We then have an entry in the nova user’s ~/.ssh/config file:

Host an-nova
Hostname solveserver.domain.org
User solve
IdentityFile ~/.ssh/id_nova_backend

And, naturally, we use SSH keys to automate the login.

On the solve server, the solve user has an entry in ~/.ssh/authorized_keys for the id_nova_backend.pub public key, that tells the ssh server what should be run when that key is used to log in:

# id_nova_backend
#
command="cd /home/solve/nova/blind; ../net/testscript-astro",no-port-forwarding,no-X11-forwarding,no-agent-forwarding,no-pty ssh-rsa AAAA[.....] nova@webserver

That script (https://github.com/dstndstn/astrometry.net/blob/master/net/testscript-astro) first reads the job id, creates a working directory for the job, uses tar to receive the input files, and then runs the astrometry-engine program to actually run the requested job. Finally, it uses tar to bundle up and send back the results.

(Note that, at present, the testscript-astro script still tries to run the astrometry-engine by its old name, backend … we haven’t updated that script in a while. That script also includes hard-coded paths, so you will have to edit for your site.)