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.. _using_backup_servers:
Using backup servers
********************
The :code:`ecflow_client` can be configured to contact alternate backup servers in case the primary server is not available -- this typically most useful in Operational environments.
The use of backup servers is, by default, enabled only for :term:`Task commands <child command>`. This behaviour can be customized by setting the environment variable
:code:`ECF_HOSTFILE_POLICY`. This variable can take the following values:
- :code:`task` (default): backup servers are used only for Task commands.
- :code:`all`: backup servers are used for all commands, including Task and User commands.
The list of backup servers can be specified by defining the environment variable :code:`ECF_HOSTFILE`, indicating the location of a file, by convention located at :code:`$HOME/.ecf_hostfile`, with the following format:
.. code-block:: shell
# This is a comment
host1 # port 3141 is used by default, when not specified
host2:port2
host3:port3
To enable the :code:`ecflow_client` to read the file and use the listed backup servers, the environment variable :code:`ECF_HOSTFILE` must be set before running the :code:`ecflow_client` command:
.. code-block:: shell
export ECF_HOSTFILE=$HOME/.ecf_hostfile
.. important::
The maximum retry period is defined by :code:`ECF_TIMEOUT`, which by default is set to 24 hours.
This means that the :code:`ecflow_client` will continue to loop over the list and retry primary host followed by alternate hosts for up to :code:`ECF_TIMEOUT`, before giving up and reporting a failure.
.. warning::
When executing a command, the :code:`ecflow_client` will always first try to connect to the primary host, as defined by command line options or :code:`ECF_HOST`::code:`ECF_PORT`.
If the first attemp to contact the primary host fails, the client will automatically retry contacting the primary server after waiting for a retry period of 10 seconds.
Only after this second attempt has failed, will the :code:`ecflow_client` then immediatelly try to connect to the backup servers listed in the :code:`ECF_HOSTFILE`.
This implies that the :code:`ecflow_client` will not try to connect to the backup servers immediately, and thus contacting the backup server incurs in a minimum 10 seconds delay.
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