Connectivity

This section outlines and troubleshoots connectivity requirements.

Hostname

Each node in the cluster must have a unique hostname, resolvable forward and backward with every other node with both normal and reverse DNS name lookup.

Run hostname -f to check the node's hostname. For example:

$ hostname -f
node125

If hostname -f returns a name, run getent hosts `hostname` to return the node's IP address and fully-qualified domain name (FQDN).

$ getent hosts `hostname`
10.250.1.53     node125.corp.example.com

To troubleshoot hostname problems, edit the /etc/hosts file as root. A simple /etc/hosts might contain:

127.0.0.1     localhost
10.10.5.10    mapr-hadoopn.maprtech.prv mapr-hadoopn

A common problem is an incorrect loopback entry (127.0.x.x) that prevents the IP address from being assigned to the hostname. For example, on Ubuntu, the default /etc/hosts file might contain:

127.0.0.1     localhost
127.0.1.1     node125.corp.example.com

A loopback (127.0.x.x) entry with the node's hostname will confuse the installer and other programs. Edit the /etc/hosts file and delete any entries that associate the hostname with a loopback IP. Only associate the hostname with the actual IP address.

Note: For more information about Ubuntu's default /etc/hosts file, see https://bugs.launchpad.net/ubuntu/+source/cloud-init/+bug/871966.

Use the ping command to verify that each node can reach the others using each node's hostname. For more information, see the hosts(5) man page.

Common Users

A user that accesses the cluster must have the same credentials and user ID (uid) on each node in the cluster. Every person or department that runs MapR jobs must have an account and must also belong to a common group ID (gid). The uid for each user, and the gid for each group, must be consistent across all nodes.

A 'mapr' user must exist. The 'mapr' user has full privileges to administer the cluster. If you create the 'mapr' user before you install MapR, you can test for connectivity issues. If you do not create the 'mapr' user, installing MapR automatically creates the user for you. The 'mapr' user ID is automatically created on each node if you do not use a directory service, such as LDAP.

To create a group, add a user to the group, or create the 'mapr' user, run the following command as root substituting a uid for m and a gid for n. (The error "cannot lock /etc/passwd" suggests that the command was not run as root.)

$ useradd mapr --gid n --uid m

Example: $ groupadd -g 5000 mapr $ useradd -g 5000 -u 5000 mapr

To verify that the users or groups were created, su mapr. Verify that a home directory was created (usually /home/mapr) and that the users or groups have read-write access to it. The users or groups must have write access to the /tmp directory, or the warden will fail to start services.

Optional: Passwordless ssh

If you plan to use the scripted rolling upgrade procedure to upgrade the cluster in the future, it is very helpful for the common user to be able to ssh from each webserver node to any other node without providing a password. Otherwise, passwordless ssh between nodes is optional because MapR will run without it.

Setting up passwordless ssh is straightforward. On each webserver node, generate a key pair and append the key to an authorization file. Then copy this authorization file to each node, so that every node is available from the webserver node.

su mapr (if you are not already logged in as mapr) ssh-keygen -t rsa -P '' -f ~/ filename

The ssh-keygen command creates filename , containing the private key, and filename .pub, containing the public key. For convenience, you may want to name the file for the hostname of the node. For example, on the node with hostname "node10.10.1.1,"

ssh-keygen -t rsa -P '' -f ~/node10.10.1.1

In this example, append the file /home/mapr/node10.10.1.1.pub to the authorized_keys file.

Append each webserver node's public key to a single file, using a command like cat filename .pub >> authorized_keys. (The key file is simple text, so you can append the file in several ways, including a text editor.) When every webserver node's empty passphrase public key has been generated, and the public key file has been appended to the master "authorized_keys" file, copy this master keys file to each node as ~/.ssh/authorized_keys, where ~ refers to the mapr user's home directory (typically /home/mapr).