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281 lines
11 KiB
XML
281 lines
11 KiB
XML
<chapter xmlns="http://docbook.org/ns/docbook"
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xmlns:xlink="http://www.w3.org/1999/xlink"
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xmlns:xi="http://www.w3.org/2001/XInclude"
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version="5.0"
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xml:id="module-foundationdb">
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<title>FoundationDB</title>
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<para><emphasis>Source:</emphasis> <filename>modules/services/databases/foundationdb.nix</filename></para>
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<para><emphasis>Upstream documentation:</emphasis> <link xlink:href="https://apple.github.io/foundationdb/"/></para>
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<para><emphasis>Maintainer:</emphasis> Austin Seipp</para>
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<para><emphasis>Default version:</emphasis> 5.1.x</para>
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<para>FoundationDB (or "FDB") is a distributed, open source, high performance,
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transactional key-value store. It can store petabytes of data and deliver
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exceptional performance while maintaining consistency and ACID semantics
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(serializable transactions) over a large cluster.</para>
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<section><title>Configuring and basic setup</title>
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<para>To enable FoundationDB, add the following to your
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<filename>configuration.nix</filename>:
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<programlisting>
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services.foundationdb.enable = true;
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</programlisting>
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</para>
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<para>After running <command>nixos-rebuild</command>, you can verify whether
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FoundationDB is running by executing <command>fdbcli</command> (which is added
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to <option>environment.systemPackages</option>):
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<programlisting>
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$ sudo -u foundationdb fdbcli
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Using cluster file `/etc/foundationdb/fdb.cluster'.
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The database is available.
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Welcome to the fdbcli. For help, type `help'.
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fdb> status
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Using cluster file `/etc/foundationdb/fdb.cluster'.
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Configuration:
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Redundancy mode - single
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Storage engine - memory
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Coordinators - 1
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Cluster:
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FoundationDB processes - 1
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Machines - 1
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Memory availability - 5.4 GB per process on machine with least available
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Fault Tolerance - 0 machines
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Server time - 04/20/18 15:21:14
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...
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fdb>
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</programlisting>
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</para>
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<para>FoundationDB is run under the <command>foundationdb</command> user and
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group by default, but this may be changed in the NixOS configuration. The
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systemd unit <command>foundationdb.service</command> controls the
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<command>fdbmonitor</command> process.</para>
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<para>By default, the NixOS module for FoundationDB creates a single
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SSD-storage based database for development and basic usage. This storage engine
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is designed for SSDs and will perform poorly on HDDs; however it can handle far
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more data than the alternative "memory" engine and is a better default choice
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for most deployments. (Note that you can change the storage backend on-the-fly
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for a given FoundationDB cluster using <command>fdbcli</command>.)</para>
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<para>Furthermore, only 1 server process and 1 backup agent are started in the
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default configuration. See below for more on scaling to increase this.</para>
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<para>FoundationDB stores all data for all server processes under
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<filename>/var/lib/foundationdb</filename>. You can override this using
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<option>services.foundationdb.dataDir</option>, e.g.
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<programlisting>
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services.foundationdb.dataDir = "/data/fdb";
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</programlisting>
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</para>
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<para>Similarly, logs are stored under
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<filename>/var/log/foundationdb</filename> by default, and there is a
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corresponding <option>services.foundationdb.logDir</option> as well.</para>
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</section>
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<section><title>Scaling processes and backup agents</title>
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<para>Scaling the number of server processes is quite easy; simply specify
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<option>services.foundationdb.serverProcesses</option> to be the number of
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FoundationDB worker processes that should be started on the machine.</para>
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<para>FoundationDB worker processes typically require 4GB of RAM per-process at
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minimum for good performance, so this option is set to 1 by default since the
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maximum amount of RAM is unknown. You're advised to abide by this restriction,
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so pick a number of processes so that each has 4GB or more.</para>
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<para>A similar option exists in order to scale backup agent processes,
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<option>services.foundationdb.backupProcesses</option>. Backup agents are not
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as performance/RAM sensitive, so feel free to experiment with the number of
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available backup processes.</para>
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</section>
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<section><title>Clustering</title>
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<para>FoundationDB on NixOS works similarly to other Linux systems, so this
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section will be brief. Please refer to the full FoundationDB documentation for
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more on clustering.</para>
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<para>FoundationDB organizes clusters using a set of
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<emphasis>coordinators</emphasis>, which are just specially-designated worker
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processes. By default, every installation of FoundationDB on NixOS will start
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as its own individual cluster, with a single coordinator: the first worker
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process on <command>localhost</command>.</para>
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<para>Coordinators are specified globally using the
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<command>/etc/foundationdb/fdb.cluster</command> file, which all servers and
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client applications will use to find and join coordinators. Note that this file
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<emphasis>can not</emphasis> be managed by NixOS so easily: FoundationDB is
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designed so that it will rewrite the file at runtime for all clients and nodes
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when cluster coordinators change, with clients transparently handling this
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without intervention. It is fundamentally a mutable file, and you should not
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try to manage it in any way in NixOS.</para>
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<para>When dealing with a cluster, there are two main things you want to
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do:</para>
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<itemizedlist>
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<listitem><para>Add a node to the cluster for storage/compute.</para></listitem>
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<listitem><para>Promote an ordinary worker to a coordinator.</para></listitem>
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</itemizedlist>
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<para>A node must already be a member of the cluster in order to properly be
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promoted to a coordinator, so you must always add it first if you wish to
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promote it.</para>
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<para>To add a machine to a FoundationDB cluster:</para>
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<itemizedlist>
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<listitem><para>Choose one of the servers to start as the initial coordinator.
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</para></listitem>
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<listitem><para>Copy the <command>/etc/foundationdb/fdb.cluster</command> file
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from this server to all the other servers. Restart FoundationDB on all of
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these other servers, so they join the cluster.</para></listitem>
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<listitem><para>All of these servers are now connected and working together
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in the cluster, under the chosen coordinator.</para></listitem>
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</itemizedlist>
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<para>At this point, you can add as many nodes as you want by just repeating
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the above steps. By default there will still be a single coordinator: you can
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use <command>fdbcli</command> to change this and add new coordinators.</para>
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<para>As a convenience, FoundationDB can automatically assign coordinators
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based on the redundancy mode you wish to achieve for the cluster. Once all the
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nodes have been joined, simply set the replication policy, and then issue the
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<command>coordinators auto</command> command</para>
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<para>For example, assuming we have 3 nodes available, we can enable double
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redundancy mode, then auto-select coordinators. For double redundancy, 3
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coordinators is ideal: therefore FoundationDB will make
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<emphasis>every</emphasis> node a coordinator automatically:</para>
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<programlisting>
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fdbcli> configure double ssd
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fdbcli> coordinators auto
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</programlisting>
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<para>This will transparently update all the servers within seconds, and
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appropriately rewrite the <command>fdb.cluster</command> file, as well as
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informing all client processes to do the same.</para>
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</section>
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<section><title>Client connectivity</title>
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<para>By default, all clients must use the current
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<command>fdb.cluster</command> file to access a given FoundationDB cluster.
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This file is located by default in
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<command>/etc/foundationdb/fdb.cluster</command> on all machines with the
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FoundationDB service enabled, so you may copy the active one from your cluster
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to a new node in order to connect, if it is not part of the cluster.</para>
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</section>
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<section><title>Backups and Disaster Recovery</title>
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<para>The usual rules for doing FoundationDB backups apply on NixOS as written
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in the FoundationDB manual. However, one important difference is the security
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profile for NixOS: by default, the <command>foundationdb</command> systemd unit
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uses <emphasis>Linux namespaces</emphasis> to restrict write access to the
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system, except for the log directory, data directory, and the
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<command>/etc/foundationdb/</command> directory. This is enforced by default
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and cannot be disabled.</para>
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<para>However, a side effect of this is that the <command>fdbbackup</command>
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command doesn't work properly for local filesystem backups: FoundationDB uses a
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server process alongside the database processes to perform backups and copy the
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backups to the filesystem. As a result, this process is put under the
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restricted namespaces above: the backup process can only write to a limited
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number of paths.</para>
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<para>In order to allow flexible backup locations on local disks, the
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FoundationDB NixOS module supports a
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<option>services.foundationdb.extraReadWritePaths</option> option. This option
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takes a list of paths, and adds them to the systemd unit, allowing the
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processes inside the service to write (and read) the specified
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directories.</para>
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<para>For example, to create backups in <command>/opt/fdb-backups</command>,
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first set up the paths in the module options:</para>
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<programlisting>
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services.foundationdb.extraReadWritePaths = [ "/opt/fdb-backups" ];
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</programlisting>
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<para>Restart the FoundationDB service, and it will now be able to write to
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this directory (even if it does not yet exist.) Note: this path
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<emphasis>must</emphasis> exist before restarting the unit. Otherwise, systemd
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will not include it in the private FoundationDB namespace (and it will not add
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it dynamically at runtime).</para>
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<para>You can now perform a backup:</para>
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<programlisting>
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$ sudo -u foundationdb fdbbackup start -t default -d file:///opt/fdb-backups
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$ sudo -u foundationdb fdbbackup status -t default
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</programlisting>
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</section>
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<section><title>Known limitations</title>
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<para>The FoundationDB setup for NixOS should currently be considered beta.
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FoundationDB is not new software, but the NixOS compilation and integration has
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only undergone fairly basic testing of all the available functionality.</para>
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<itemizedlist>
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<listitem><para>TLS plugin support is compiled in, but it's currently not
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possible to specify the set of TLS certificate options in
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<command>services.foundationdb</command></para></listitem>
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<listitem><para>There is no way to specify individual parameters for
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individual <command>fdbserver</command> processes. Currently, all server
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processes inherit all the global <command>fdbmonitor</command> settings.
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</para></listitem>
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<listitem><para>Python bindings are not currently installed.</para></listitem>
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<listitem><para>Ruby bindings are not currently installed.</para></listitem>
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<listitem><para>Java bindings are not currently installed.</para></listitem>
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<listitem><para>Go bindings are not currently installed.</para></listitem>
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</itemizedlist>
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</section>
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<section><title>Options</title>
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<para>NixOS's FoundationDB module allows you to configure all of the most
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relevant configuration options for <command>fdbmonitor</command>, matching it
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quite closely. For a complete list of all options, check <command>man
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configuration.nix</command>.</para>
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</section>
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<section><title>Full documentation</title>
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<para>FoundationDB is a complex piece of software, and requires careful
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administration to properly use. Full documentation for administration can be
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found here: <link xlink:href="https://apple.github.io/foundationdb/"/>.</para>
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</section>
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</chapter>
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