.. _multi-data-center-deployments:

Multi data center deployments
-----------------------------

.. index:: web;web weight
.. index:: web;web weight list
.. index:: web;web weight add

Overview 
..........


Multi-node clusters can be deployed in both *Active/Active* or *Active/Standby* configurations:

.. tabularcolumns:: |p{7cm}|p{8cm}|

+-----------------------------+---------------------------------------------------------------------+
| Configuration               | Description                                                         | 
+=============================+=====================================================================+
| Active/Active               | Active/Active configurations have all the unified nodes enabled for |
|                             | transaction processing.                                             |
|                             |                                                                     |
|                             | In order to run an Active/Active configuration, the latency (RTT)   |
|                             | between data centers must not exceed 20ms.                          | 
+-----------------------------+---------------------------------------------------------------------+
| Active/Standby              | Active/Standby configurations have only the unified nodes in the    |
|                             | primary data center (the data center containing the unified node    |
|                             | with the primary database) enabled for transaction processing.      |
|                             |                                                                     |
|                             | Use an Active/Standby configuration for higher latencies.           |
+-----------------------------+---------------------------------------------------------------------+


Switch to an Active/Standby configuration 
.............................................

To switch to an *Active/Standby* configuration, do the following on all unified nodes in the secondary 
data center (the data center *not* containing the unified node with the primary database):

1. Log into platform: 

   ``ssh platform@<ip_address>``
   
#. Set the worker count on the nodes in the secondary data center to zero:

   ``voss workers 0``

   Setting the number of workers is persistent; that is, this setting will still apply after 
   upgrades and system restart.


Setting web weights for an Active/Standby configuration 
.........................................................

For an *Active/Standby* configuration, the proxy server web weights should be set to the 
unified nodes on the primary data center, using the ``web weight add`` command: 

The web weight specifies the routing and relative counts of the initial HTTP request from the web proxy to a 
unified node. The initial request could be a request such as a transaction or, for example, a *GET* 
request. 

Consider the following web weights configuration: 

::

   172.0.0.158:443: 1
   172.0.0.159:443: 1
   172.0.0.161:443: 1
   172.0.0.162:443: 1
   173.0.0.163:443: 0
   173.0.0.164:443: 0


This configuration means that the servers ``173.0.0.163`` and ``173.0.0.164`` serve as backup 
servers and requests are only routed to these if the other servers are not available.

While the other servers are available, an equal number of requests are routed to them in a 
round-robin manner.

.. note:: 

   Refer to the *Best Practices Guide* for more information on deployment models and web weight settings.



.. rubric:: Example: 
    
Consider an example where:

.. note::

   The examples below show system command output where
   the Phone Based Registration application is running.

1. Primary data center has unified nodes
   with IP addresses and ports:

   * 172.0.0.158:443
   * 172.0.0.159:443
   * 172.0.0.161:443
   * 172.0.0.162:443

   A *unified node* typically shows the following output for the ``web weight list`` command:

   ::
  
     $ web weight list
     Default service weights

      voss-deviceapi:
          phoneservices:
              172.0.0.158:8412: 1
          selfservice:
              172.0.0.158:5000: 1
          voss-deviceapi:
              172.0.0.158:9902: 1
          voss-portal:
              172.0.0.158:6001: 1
  

2. Secondary data center has unified nodes with IP addresses and ports:

   * 173.0.0.163:443
   * 173.0.0.164:443


The defaults of the ``web weight list`` command run on the *proxy servers* is as follows:

1. Primary data center proxy server:

::

   $ web weight list
   Default service weights

       phonebasedreg: 
           phoneservices:
               172.0.0.158:443: 1
               172.0.0.159:443: 1
               172.0.0.161:443: 1
               172.0.0.162:443: 1
               173.0.0.163:443: 0
               173.0.0.164:443: 0
       voss-deviceapi: 
           selfservice: 
               172.0.0.158:443: 1
               172.0.0.159:443: 1
               172.0.0.161:443: 1
               172.0.0.162:443: 1
               173.0.0.163:443: 0
               173.0.0.164:443: 0
           voss-deviceapi: 
               172.0.0.158:443: 1
               172.0.0.159:443: 1
               172.0.0.161:443: 1
               172.0.0.162:443: 1
               173.0.0.163:443: 0
               173.0.0.164:443: 0



2. Secondary data center proxy server:

::

   $ web weight list
   Default service weights

       phonebasedreg: 
           phoneservices:
               172.0.0.158:443: 0
               172.0.0.159:443: 0
               172.0.0.161:443: 0
               172.0.0.162:443: 0
               173.0.0.163:443: 1
               173.0.0.164:443: 1
       voss-deviceapi: 
           selfservice: 
               172.0.0.158:443: 0
               172.0.0.159:443: 0
               172.0.0.161:443: 0
               172.0.0.162:443: 0
               173.0.0.163:443: 1
               173.0.0.164:443: 1
           voss-deviceapi: 
               172.0.0.158:443: 0
               172.0.0.159:443: 0
               172.0.0.161:443: 0
               172.0.0.162:443: 0
               173.0.0.163:443: 1
               173.0.0.164:443: 1


To ensure that the secondary data center is configured for a *Standby* mode, change the web weights to 
show ``userweights``, as displayed in the output on the *secondary* data center proxy server below:

::

   $ web weight list
   Default service weights

       phonebasedreg: 
           phoneservices:
               172.0.0.158:443: 0
               172.0.0.159:443: 0
               172.0.0.161:443: 0
               172.0.0.162:443: 0
               173.0.0.163:443: 1
               173.0.0.164:443: 1
       voss-deviceapi: 
           selfservice: 
               172.0.0.158:443: 0
               172.0.0.159:443: 0
               172.0.0.161:443: 0
               172.0.0.162:443: 0
               173.0.0.163:443: 1
               173.0.0.164:443: 1
           voss-deviceapi: 
               172.0.0.158:443: 0
               172.0.0.159:443: 0
               172.0.0.161:443: 0
               172.0.0.162:443: 0
               173.0.0.163:443: 1
               173.0.0.164:443: 1

   Customized service weights
       userweights: 
           172.0.0.158:443: 1
           172.0.0.159:443: 1
           172.0.0.161:443: 1
           172.0.0.162:443: 1
           173.0.0.163:443: 0
           173.0.0.164:443: 0


The outcome is that the load balancing web weights have been changed to the unified nodes on the 
*primary* data center.