gocept Link Monitoring Server
The Link Monitoring Server (LMS) is a coherent set of components to regularly check whether given URLs are reachable. It currently supports HTTP, HTTPS and FTP URLs.
gocept.lms is the third version of LMS (hence the major version number of 3.x).
The first version was directly integrated into CMFLinkChecker and ran as a thread. This was not very stable.
The second version used the Twisted framework and a MySQL database. This was not very stable either.
The third version now is written in Zope 3/Grok, uses the ZODB as data storage and separates concerns wherever possible. This is stable.
The LMS consists of small components that only communicate via storing data in the ZODB.
The XML-RPC server provides the API to talk to the LMS from applications/clients that want to have their URLs checked. It provides operations to register and unregister clients and URLs.
A management system can register clients. A client can register URLs it wants to be checked.
The scheduler looks over the age of the data from URLs that where added by the clients. It decides when a URL needs to be checked and inserts it into the check queue accordingly.
The checker pulls URLs from the check queue and performs the actual check. It has measures in place that limit the number of checks performed in parallel and to avoid pounding too hard on external systems. (Per default it checks up to 20 URLs at the same time and assures that URLs that belong to the same host are checked at most once per second).
The notifier keeps the registered clients updated about the state information for the URLs they subscribed to. Clients are notified via XML-RPC.
The syncer is responsible for keeping the URL databases of the LMS and its clients synchronized. It does this by regularly getting a full snapshot of the clients’ databases and provides them with the current status information for all URLs they are interested in.
The installation works using a buildout. An example can be found at our SVN:
% svn co https://code.gocept.com/svn/gocept/gocept.lms/deployment/profiles
The buildout currently defines two profiles:
- prod.cfg for a production setup
- test.cfg for a test setup
The profiles are not really different right now, though. They’re there to illustrate the possibilities. The test profile looks like this:
[buildout] extends = base.cfg [app] admin-password = admin appname = test mail-server-host = localhost [zeo] address = localhost:8100 [lms] address = localhost:8080
To select the test profile create a buildout.cfg which includes it:
[buildout] extends = profiles/test.cfg
Next, bootstrap the buildout with Python 2.5:
% python2.5 bootstrap.py
This creates the actual buildout script as bin/buildout:
Running the buildout created a set of scripts, corresponding to the various components. The test profile uses a deployment sandbox, so all the scripts are contained in parts/deployment/etc/init.d:
% ls parts/deployment/etc/init.d/ lms-checker lms-notifier lms-scheduler lms-syncer lms-web lms-zeo
To get started, first start the ZEO and the LMS web interface:
% parts/deployment/etc/init.d/lms-zeo start % parts/deployment/etc/init.d/lms-web start
Point your browser to http://localhost:8080. This opens the grok admin UI. Create an LMS with the id test. Once created, the LMS will display “Congratulations”.
Start the other services now:
% parts/deployment/etc/init.d/lms-checker start % parts/deployment/etc/init.d/lms-notifier start % parts/deployment/etc/init.d/lms-scheduler start % parts/deployment/etc/init.d/lms-syncer start
The next step is to register a client. The LMS welcome page has a link to a very rudimentary form. There you have to enter the following data:
- Client Id
- This is an identifier for the client. It is used in combination with the password to authenticate XML-RPC requests.
- The password to authenticate the client.
- Contact name
- Emails sent by the LMS will contain the name to address the recipient.
- Contact email address
- Emails sent by the LMS regarding this client will be sent to this address.
- Callback URL
- The callback URL is the XML-RPC point where the callback methods are invoked. For a gocept.linkchecker installation this would be http://example.com/portal_linkchecker/database/.
After registering the client, you can configure gocept.linkchecker (or any other client) to talk to the LMS.
- Update reference to SVN.
- Fix bug while accepting multiple URLs: don’t break if a single URL isn’t addable (ConstraintNotSatisfied).
- Correctly handle InvalidURL exceptions from urllib.
- Actually mark URLs that do not have handlers available as ‘uncheckable’.
- Process at most 1000 items from the queue at once to decrease latency in the checker.
- Make registration and sync of URLs resilient towards malformed URLs. We now require URLs to be ASCII only. Please make your client encode the URL before sending it to us.
- Increased debug output for client notification a lot. We show all URLs that the client gets notified about now.
- Mark URLs that do not have handlers associated or are forbidden by the policy as checked so they will only reappar after the same period a successful check would.
- Provide a minimal UI for inspecting the URL database and allow purging selected URLs from the database.
- Fix problem in conflict resolution: if an object relied on the default of a class then the state dict doesn’t contain that value.
- Improved error handling for URLs which have an empty string (or bad statust line) as response.
- Remove dependency on twisted.
- Provide a separate state (recommended color: blue) for URLs that the LMS can not or refuses to check.
- Don’t classify mailto links (optimization).
- Updated documentation a bit.
- Declared namespace package.
- Updated used egg versions.
- Added missing dependency to pytz.
- first public release.