12. 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.

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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.

12.1. 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.

12.1.1. 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.

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Replication control panel.

Note that currently, you cannot use the Replication Control Panel to remove a server after a replication has occurred. To stop replication between two servers, update the flags that control whether metadata and/or data are replicated.

12.1.2. 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 that public certificate (or the certificate of the signing CA) added to its existing Certificate Authorities.

12.1.2.1. 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 “metacat-site-ssl” has default rules that configure Apache for SSL and client certificate authentication. Set up these SSL settings by copying the metacat-site-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 metacat-site-ssl need to be changed to match the specifics of your system and Metacat deployment.)

    sudo cp <metacat_helper_dir>/metacat-site-ssl <apache_install_dir>/sites-available
    sudo a2ensite metacat-site-ssl
  5. Enable the ssl module:

    sudo a2enmod ssl
  6. Restart Apache to bring in changes by typing:

    sudo /etc/init.d/apache2 restart
  7. 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.

12.1.2.2. 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.

12.1.2.3. To import a certificate into Java keystore (for self-signed certificates)

  1. Use Java’s keytool to import to the default Java keystore

    sudo keytool -import -alias <remotehostname_alias> -file <remotehostfilename> -keystore $JAVA_HOME/lib/security/cacerts
  2. Restart Tomcat

    sudo /etc/init.d/tomcat6 restart

    where the <remotehostfilename> is the name of the certificate file created on the remote partner machine and SCP’d to the home machine and <remotehostname_alias> is a short memorable alias for this certificate and $JAVA_HOME is the same as configured for running Tomcat. NOTE: the cacerts path may be different depending on your exact Java installation.

12.1.2.4. Update Metacat properties

Metacat needs to be configured with the path to both the server certificate and the private key. 1. Edit metacat.properties, modifying these properties to match your specific deployment.

replication.certificate.file=/etc/ssl/certs/<hostname>-apache.crt
replication.privatekey.file=/etc/ssl/private/<hostname>-apache.key
replication.privatekey.password=<password, or blank if not protected>

12.1.3. 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 using SQL. Instructions for both options are included in this section.

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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).

12.1.3.1. 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