Metacat

Replication

===========

.. Note:: 

  Note that much of the functionality provided by the replication subsystem in Metacat

  has now been generalized and standardized by DataONE, so consider utilizing the

  DataONE services for replication as it is a more general and standardized approach

  than this Metacat-specific replication system.  The Metacat replication system

  will be supported for a while longer, but will likely be deprecated in a future

  release in favor of using the DataONE replication approach. 

Metacat has a built-in replication feature that allows different Metacat servers 

to share data (both XML documents and data files) between each other. Metacat 

can replicate not only its home server's original documents, but also those 

that were replicated from partner Metacat servers. When changes are made to 

one server in a replication network, the changes are automatically propogated 

to the network, even if the network is down.

Replication allows users to manage their data locally and (by replicating them 

to a shared Metacat repository) to make those data available to the greater 

scientific community via a centralized search. In other words, your Metacat can 

be part of a broader network, but you retain control over the local repository 

and how it is managed.

For example, the KNB Network (Figure 6.1), which currently consists of ten 

different Metacat servers from around the world, uses replication to "join" 

the disperate servers to form a single robust and searchable data 

repository--facilitating data discovery, while leaving the data ownership and 

management with the local administrators.

.. figure:: images/screenshots/image059.jpg

   :align: center

   A map of the KNB Metacat network.

When properly configured, Metacat's replication mechanism can be triggered by 

several types of events that occur on either the home or partner server: a 

document insertion, an update, or an automatic replication (i.e., Delta-T 

monitoring), which is set at a user-specified time interval.

+----------------------+----------------------------------------------------------+

| Replication Triggers | Description                                              |

+======================+==========================================================+

| Insert               | Whenever a document is inserted into Metacat, the server |

|                      | notifies each server in its replication list             |

|                      | that it has a new file available.                        |

+----------------------+----------------------------------------------------------+

| Update               | Whenever a document is updated, the server notifies      |

|                      | each server in its replication list of the update.       |

+----------------------+----------------------------------------------------------+

| Delta-T monitoring   | At a user-specified time interval, Metacat checks each   |

|                      | of the servers in its replication list                   |

|                      | for updated documents.                                   |

+----------------------+----------------------------------------------------------+

Configuring Replication

-----------------------

To configure replication, you must configure both the home and partner servers:

1. Create a list of partner servers on your home server using the Replication Control Panel

2. Create certificate files for the home server

3. Create certificate files for the partner server

4. Import partner certificate files to the home server

5. Import home certificate to the partner server

6. Update your Metacat database 

Each step is discussed in more detail in the following sections.

Using the Replication Control Panel

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

To add, remove, or alter servers on your home server's Replication list, or to 

activate and customize the Delta-T handler, use the Replication control panel, 

which is accessed via the Metacat Administration interface at the following URL::

   http://somehost.somelocation.edu/context/admin

"http://somehost.somelocation.edu/context" should be replaced with the name 

of your Metacat server and context (e.g., http://knb.ecoinformatics.org/knb/). 

You must be logged in to Metacat as an administrator.

.. figure:: images/screenshots/image061.jpg

   :align: center

   Replication control panel.

Note that currently, you cannot use the Replication Control Panel to remove a 

server after a replication has occurred. At this point in time, the only way to 

remove a replication server after replication has occurred is to remove the 

certificates. 

Generating and Exchanging Security Certificates

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Before you can take advantage of Metacat's replication feature, you must 

generate security certificates on both the replication partner and home servers. 

Depending on how the certificates are generated, the certificates may need to be

exchanged so that each machine "trusts" that the other has replication access.

Certificates that are purchased from a commercial and well-recognized 

Certificate Authority do not need to be exchanged with the other replication

partner before replication takes place.  Metacat replication relies on SSL with

client certificate authentication enabled.  When a replication partner server 

communicates with another replication partner, it presents a certificate that

serves to verify and authenticate that the server is trusted.

If you must generate a self-signed certificate, the partner replication server 

will need the public certificate added to its existing Certificate Authorities. 

Generate Certificates for Metacat running under Apache/Tomcat

.............................................................

Note: Instructions are for Ubuntu/Debian systems.

1. Generate a private key using openssl. The key will be named 

   ``<hostname>-apache.key``, where ``<hostname>`` is the name of your Metacat 

   server. Example values for the individual key fields are included in the

   table below.

   ::

     openssl req -new -out REQ.pem -keyout <hostname>-apache.key

   +--------------------------+-------------------------------------------------------------------------+

   | Key Field                | Description and Example Value                                           |

   +==========================+=========================================================================+

   | Country Name             | Two letter country code  (e.g., US)                                     |

   +--------------------------+-------------------------------------------------------------------------+

   | State or Province Name   | The name of your state or province spelled in full (e.g., California)   |

   +--------------------------+-------------------------------------------------------------------------+

   | Locality Name            | The name of your city (e.g., Santa Barbara)                             |

   +--------------------------+-------------------------------------------------------------------------+

   | Organization Name        | The company or organization name (e.g., UCSB)                           |

   +--------------------------+-------------------------------------------------------------------------+

   | Organizational Unit Name | The department or section name (e.g., NCEAS)                            |

   +--------------------------+-------------------------------------------------------------------------+

   | Common Name              | The host server name without port numbers (e.g., myserver.mydomain.edu) |

   +--------------------------+-------------------------------------------------------------------------+

   | Email Address            | Administrator's contact email (e.g., administrator@mydomain.edu)        |

   +--------------------------+-------------------------------------------------------------------------+

   | A challenge password     | --leave this field blank--                                              |

   +--------------------------+-------------------------------------------------------------------------+

   | An optional company name | --leave this field blank--                                              |

   +--------------------------+-------------------------------------------------------------------------+

2. Create the local certificate file by running the command:

   ::

     openssl req -x509 -days 800 -in REQ.pem -key <hostname>-apache.key -out <hostname>-apache.crt

   Use the same ``<hostname>`` you used when you generated the key. A file named 

   ``<hostname>-apache.crt`` will be created in the directory from which you 

   ran the openssl command. Note: You can name the certificate file anything 

   you'd like, but keep in mind that the file will be sent to the partner 

   machine used for replication. The certificate name should have enough 

   meaning that someone who sees it on that machine can figure out where it 

   came from and for what purpose it should be used. 

3. Enter the certificate into Apache's security configuration. This will

   be used to identify your server to a replication partner. You must 

   register the certificate in the local Apache instance. Note that the 

   security files may be in a different directory from the one used in the 

   instructions depending on how you installed Apache. Copy the certificate and 

   key file using the following commands:

   ::

     sudo cp <hostname>-apache.crt /etc/ssl/certs 

     sudo cp <hostname>-apache.key /etc/ssl/private 

4. Apache needs to be configured to request a “client certificate” when the 

   replication API is utilized. The helper file named "knb-ssl" has default 

   rules that configure Apache for SSL and client certificate authentication. 

   Set up these SSL settings by copying the knb-ssl file into the ``sites-available`` 

   directory, editing pertinent values to match your system and running 

   ``a2ensite`` to enable the site. (Note: some settings in knb-ssl need to be 

   changed to match the specifics of your system.) 

   ::

     sudo cp <metacat_helper_dir>/knb-ssl <apache_install_dir>/sites-available

     sudo a2ensite knb-ssl

5. Restart Apache to bring in changes by typing: 

   ::

     sudo /etc/init.d/apache2 restart

6. If using a self-signed certificate, SCP ``<hostname>-apache.crt`` to the 

   replication partner machine where it will be added as an additional 

   Certificate Authority.

If using self-signed certificates, after you have created and SCP'd a 

certificate file to each replication partner, and received a certificate file 

from each partner in return, both home and partner servers must add the 

respective partner certificates as Certificate Authorities.

To import a certificate

.......................

1. Copy it into the Apache directory

   ::

     sudo cp <remotehostfilename> /etc/ssl/certs/

2. Rehash the certificates for Apache by running: 

   ::

     cd /etc/ssl/certs

     sudo c_rehash

   where the ``<remotehostfilename>`` is the name of the certificate file 

   created on the remote partner machine and SCP'd to the home machine. 

Update your Metacat database

~~~~~~~~~~~~~~~~~~~~~~~~~~~~

The simplest way to update the Metacat database to use replication is to use 

the Replication Control Panel. You can also update the database u

sing SQL. Instructions for both options are included in this section.

.. figure:: images/screenshots/image063.jpg

   :align: center

   Using the Replication Control Panel to update the Metacat database.

To update your Metacat database to use replication, select the "Add this server" 

radio button from the Replication Control Panel, enter the partner server name, 

and specify how the replication should occur (whether to replicate xml, data, 

or use the local machine as a hub).

To update the database using SQL

................................

1. Log in to the database

   ::

     psql -U metacat -W -h localhost metacat

2. Select all rows from the replication table

   ::

     select * from xml_replication;  

3. Insert the partner server. 

   ::

     INSERT INTO xml_replication (server,last_checked,replicate,datareplicate,hub) VALUES ('<partner.server/context>/servlet/replication',NULL,1,1,0);

   Where ``<partner.server/context>`` is the name of the partner server and 

   context. The values 'NULL, 1,1,0' indicate (respectively) the last time 

   replication occurred, that XML docs should be replicated to the partner 

   server, that data files should be replicated to the partner server, and 

   that the local server should not act as a hub. Set a value of 'NULL,0,0,0' 

   if your Metacat is only receiving documents from the partner site and not 

   replicating to that site.

4. Exit the database 

5. Restart Apache and Tomcat on both home and partner replication machines