diff --git a/.rubocop_todo.yml b/.rubocop_todo.yml
index 5dcda35b..0273023d 100644
--- a/.rubocop_todo.yml
+++ b/.rubocop_todo.yml
@@ -292,7 +292,6 @@ Rails/FilePath:
 # Include: **/test/**/*
 Rails/RefuteMethods:
   Exclude:
-    - 'test/models/hiera_data/hierarchy_test.rb'
     - 'test/models/hiera_data/yaml_file_test.rb'
     - 'test/models/hierarchy_test.rb'
     - 'test/models/user_test.rb'
@@ -336,7 +335,6 @@ Style/ClassAndModuleChildren:
     - 'test/models/hiera_data/config_test.rb'
     - 'test/models/hiera_data/data_file_test.rb'
     - 'test/models/hiera_data/git_repo_test.rb'
-    - 'test/models/hiera_data/hierarchy_test.rb'
     - 'test/models/hiera_data/interpolation_test.rb'
     - 'test/models/hiera_data/yaml_file_test.rb'
     - 'test/test_helper.rb'
@@ -375,7 +373,6 @@ Style/GuardClause:
     - 'app/controllers/page_controller.rb'
     - 'app/controllers/sessions_controller.rb'
     - 'app/models/hiera_data/data_file.rb'
-    - 'app/models/hiera_data/hierarchy.rb'
 
 # Offense count: 1
 # Configuration parameters: MinBranchesCount.
@@ -399,7 +396,6 @@ Style/IfUnlessModifier:
     - 'app/controllers/sessions_controller.rb'
     - 'app/controllers/users_controller.rb'
     - 'app/models/hiera_data/config.rb'
-    - 'app/models/hiera_data/hierarchy.rb'
 
 # Offense count: 1
 # This cop supports safe auto-correction (--auto-correct).
@@ -416,7 +412,6 @@ Style/MutableConstant:
 Style/NumericLiteralPrefix:
   Exclude:
     - 'app/models/hiera_data/yaml_file.rb'
-    - 'test/models/hiera_data/hierarchy_test.rb'
     - 'test/models/hiera_data/yaml_file_test.rb'
 
 # Offense count: 1
@@ -431,10 +426,8 @@ Style/NumericLiterals:
 # Configuration parameters: PreferredDelimiters.
 Style/PercentLiteralDelimiters:
   Exclude:
-    - 'app/models/hiera_data/hierarchy.rb'
     - 'config/initializers/friendly_id.rb'
     - 'test/models/environment_test.rb'
-    - 'test/models/hiera_data/hierarchy_test.rb'
     - 'test/models/key_test.rb'
 
 # Offense count: 5
@@ -444,7 +437,6 @@ Style/PercentLiteralDelimiters:
 Style/PreferredHashMethods:
   Exclude:
     - 'app/models/hiera_data/data_file.rb'
-    - 'app/models/hiera_data/hierarchy.rb'
     - 'app/models/hiera_data/yaml_file.rb'
 
 # Offense count: 3
diff --git a/app/models/hiera_data/data_file.rb b/app/models/hiera_data/data_file.rb
index a5528c61..8f341661 100644
--- a/app/models/hiera_data/data_file.rb
+++ b/app/models/hiera_data/data_file.rb
@@ -61,7 +61,7 @@ def create_file(type)
       when :yaml
         YamlFile.new(path: @path)
       else
-        raise HDM::Error, "unsupported data file type #{type}"
+        raise Hdm::Error, "unsupported data file type #{type}"
       end
     end
   end
diff --git a/app/models/hiera_data/hierarchy.rb b/app/models/hiera_data/hierarchy.rb
index 003215ad..004a8e03 100644
--- a/app/models/hiera_data/hierarchy.rb
+++ b/app/models/hiera_data/hierarchy.rb
@@ -1,6 +1,16 @@
 class HieraData
   class Hierarchy
-    LOOKUP_FUNCTIONS = %w(lookup_key data_hash data_dig hiera3_backend).freeze
+    LOOKUP_FUNCTIONS = %w[lookup_key data_hash data_dig hiera3_backend].freeze
+    BACKENDS = {
+      "data_hash" => {
+        "json_data" => :json,
+        "yaml_data" => :yaml
+      },
+      "lookup_key" => {
+        "eyaml_lookup_key" => :eyaml
+      }
+    }.freeze
+
     attr_reader :raw_hash
 
     def initialize(raw_hash:, base_path:)
@@ -17,17 +27,7 @@ def lookup_function
     end
 
     def backend
-      @backend ||=
-        case [lookup_function, raw_hash[lookup_function]]
-        when ["data_hash", "yaml_data"]
-          :yaml
-        when ["data_hash", "json_data"]
-          :json
-        when ["lookup_key", "eyaml_lookup_key"]
-          :eyaml
-        else
-          raise HDM::Error, "unknown backend #{raw_hash[lookup_function]}"
-        end
+      @backend ||= determine_backend
     end
 
     def datadir(facts: nil)
@@ -44,15 +44,15 @@ def datadir(facts: nil)
     end
 
     def private_key
-      if backend == :eyaml
-        @base_path.join(raw_hash.dig("options", "pkcs7_private_key"))
-      end
+      return unless backend == :eyaml
+
+      @base_path.join(raw_hash.dig("options", "pkcs7_private_key"))
     end
 
     def public_key
-      if backend == :eyaml
-        @base_path.join(raw_hash.dig("options", "pkcs7_public_key"))
-      end
+      return unless backend == :eyaml
+
+      @base_path.join(raw_hash.dig("options", "pkcs7_public_key"))
     end
 
     def encryptable?
@@ -62,7 +62,7 @@ def encryptable?
     end
 
     def uses_globs?
-      raw_hash.has_key?("glob") || raw_hash.has_key?("globs")
+      raw_hash.key?("glob") || raw_hash.key?("globs")
     end
 
     def paths
@@ -93,5 +93,16 @@ def setup_paths
       base_key = uses_globs? ? "glob" : "path"
       Array(raw_hash[base_key] || raw_hash.fetch("#{base_key}s", []))
     end
+
+    def determine_backend
+      key = lookup_function
+      value = raw_hash[lookup_function]
+      backends = BACKENDS
+      custom_mappings = Rails.configuration.hdm[:custom_lookup_function_mapping]
+      backends = backends.deep_merge({ "lookup_key" => custom_mappings }) if custom_mappings.present?
+      backends.fetch(key).fetch(value).to_sym
+    rescue KeyError
+      raise Hdm::Error, "unknown backend #{value}"
+    end
   end
 end
diff --git a/config/hdm.yml.template b/config/hdm.yml.template
index cc95923c..0ea3cef9 100644
--- a/config/hdm.yml.template
+++ b/config/hdm.yml.template
@@ -79,3 +79,12 @@ production:
 #     idp_cert_fingerprint: "aaa"
 #     idp_cert: "cert" # use either fingerprint _or_ cert but not both
 
+# Example for a custom lookup function, called `my_custom_function`,
+# mapped to an existing backend, `eyaml`
+# production:
+#   read_only: false
+#   allow_encryption: true
+#   puppet_db:
+#     server: "https://localhost:8081"
+#   custom_lookup_function_mapping:
+#     my_custom_function: eyaml
diff --git a/test/models/hiera_data/hierarchy_test.rb b/test/models/hiera_data/hierarchy_test.rb
index 374ce568..84ce0cd6 100644
--- a/test/models/hiera_data/hierarchy_test.rb
+++ b/test/models/hiera_data/hierarchy_test.rb
@@ -1,225 +1,252 @@
 require 'test_helper'
 
-class HieraData::HierarchyTest < ActiveSupport::TestCase
-  test "#uses_globs? returns true if `glob` key present" do
-    glob = "/*/singular/glob"
-    raw_hash = { "name" => "Singular path", "glob" => glob }
-    hierarchy = HieraData::Hierarchy.new(raw_hash:, base_path: ".")
-    assert hierarchy.uses_globs?
-  end
-
-  test "#uses_globs? returns true if `globs` key present" do
-    globs = ["/*/array/globs", "/test/**/globs"]
-    raw_hash = { "name" => "Singular path", "globs" => globs }
-    hierarchy = HieraData::Hierarchy.new(raw_hash:, base_path: ".")
-    assert hierarchy.uses_globs?
-  end
-
-  test "#paths supports the singular `path` setting" do
-    path = "/test/singular/path"
-    raw_hash = { "name" => "Singular path", "path" => path }
-    hierarchy = HieraData::Hierarchy.new(raw_hash:, base_path: ".")
-    assert_equal [path], hierarchy.paths
-  end
-
-  test "#paths returns all non-interpolated path names" do
-    hierarchy = HieraData::Hierarchy.new(raw_hash:, base_path: ".")
-    expected_paths = [
-      "nodes/%{::facts.fqdn}.yaml",
-      "role/%{::facts.role}-%{::facts.env}.yaml",
-      "role/%{::facts.role}.yaml",
-      "zone/%{::facts.zone}.yaml",
-      "common.yaml"
-    ]
-    assert_equal expected_paths, hierarchy.paths
-  end
-
-  test "#paths returns an empty array if no data is available" do
-    hierarchy = HieraData::Hierarchy.new(raw_hash: {}, base_path: ".")
-    assert hierarchy.paths.empty?
-  end
-
-  test "#paths supports the singular `glob` setting" do
-    glob = "/*/singular/glob"
-    raw_hash = { "name" => "Singular path", "glob" => glob }
-    hierarchy = HieraData::Hierarchy.new(raw_hash:, base_path: ".")
-    assert_equal [glob], hierarchy.paths
-  end
-
-  test "#paths supports the `globs` array setting" do
-    globs = ["/*/array/globs", "/test/**/globs"]
-    raw_hash = { "name" => "Singular path", "globs" => globs }
-    hierarchy = HieraData::Hierarchy.new(raw_hash:, base_path: ".")
-    assert_equal globs, hierarchy.paths
-  end
-
-  test "#resolved_paths uses facts to resolve paths" do
-    hierarchy = HieraData::Hierarchy.new(raw_hash:, base_path: ".")
-    facts = {
-      "fqdn" => "testhost",
-      "role" => "hdm_test",
-      "env" => "development",
-      "zone" => "internal"
-    }
-    expected_resolved_paths = [
-      "nodes/testhost.yaml",
-      "role/hdm_test-development.yaml",
-      "role/hdm_test.yaml",
-      "zone/internal.yaml",
-      "common.yaml"
-    ]
-    assert_equal expected_resolved_paths, hierarchy.resolved_paths(facts:)
-  end
-
-  test "#resolved_paths resolves globs" do
-    base_path = Rails.root.join("test/fixtures/files/puppet/environments/globs")
-    globs = ["common/*.yaml"]
-    raw_hash = { "name" => "Common", "datadir" => "data", "globs" => globs }
-    hierarchy = HieraData::Hierarchy.new(raw_hash:, base_path:)
-    facts = { "fqdn" => "testhost" }
-    expected_resolved_paths = [
-      "common/foobar.yaml",
-      "common/hdm.yaml"
-    ]
-    assert_equal expected_resolved_paths, hierarchy.resolved_paths(facts:)
-  end
-
-  test "#name returns the existing name" do
-    hierarchy = HieraData::Hierarchy.new(raw_hash:, base_path: ".")
-    assert_equal "Yaml hierarchy", hierarchy.name
-  end
-
-  test "#candidate_files returns files matching paths with variables replaced by globs" do
-    base_path = Rails.root.join("test/fixtures/files/puppet/environments/multiple_hierarchies")
-    hierarchy = HieraData::Hierarchy.new(
-      raw_hash: raw_hash.merge("datadir" => "data"),
-      base_path:
-    )
-    expected_candidate_files = [
-      "nodes/4msusyei.betadots.training.yaml",
-      "nodes/60wxmaw5.betadots.training.yaml",
-      "nodes/test.host.yaml",
-      "role/hdm_test.yaml",
-      "zone/internal.yaml",
-      "common.yaml"
-    ]
-    assert_equal expected_candidate_files, hierarchy.candidate_files
-  end
+class HieraData
+  class HierarchyTest < ActiveSupport::TestCase
+    test "#uses_globs? returns true if `glob` key present" do
+      glob = "/*/singular/glob"
+      raw_hash = { "name" => "Singular path", "glob" => glob }
+      hierarchy = HieraData::Hierarchy.new(raw_hash:, base_path: ".")
+      assert hierarchy.uses_globs?
+    end
 
-  test "#datadir uses facts to resolve datadir" do
-    raw_hash = {
-      "name" => "dynamic datadir",
-      "datadir" => "%{facts.datadir}"
-    }
-    facts = { "datadir" => "data1" }
-    base_path = Rails.root.join("test/fixtures/files/puppet/environments/dynamic_datadir")
-    hierarchy = HieraData::Hierarchy.new(raw_hash:, base_path:)
+    test "#uses_globs? returns true if `globs` key present" do
+      globs = ["/*/array/globs", "/test/**/globs"]
+      raw_hash = { "name" => "Singular path", "globs" => globs }
+      hierarchy = HieraData::Hierarchy.new(raw_hash:, base_path: ".")
+      assert hierarchy.uses_globs?
+    end
 
-    assert_equal base_path.join("data1"), hierarchy.datadir(facts:)
-  end
+    test "#paths supports the singular `path` setting" do
+      path = "/test/singular/path"
+      raw_hash = { "name" => "Singular path", "path" => path }
+      hierarchy = HieraData::Hierarchy.new(raw_hash:, base_path: ".")
+      assert_equal [path], hierarchy.paths
+    end
 
-  def raw_hash
-    {
-      "name" => "Yaml hierarchy",
-      "paths" => [
+    test "#paths returns all non-interpolated path names" do
+      hierarchy = HieraData::Hierarchy.new(raw_hash:, base_path: ".")
+      expected_paths = [
         "nodes/%{::facts.fqdn}.yaml",
         "role/%{::facts.role}-%{::facts.env}.yaml",
         "role/%{::facts.role}.yaml",
         "zone/%{::facts.zone}.yaml",
         "common.yaml"
       ]
-    }
-  end
+      assert_equal expected_paths, hierarchy.paths
+    end
 
-  class HieraData::HierarchyForYamlDataTest < ActiveSupport::TestCase
-    test "data_hash specified and yaml" do
-      hierarchy = HieraData::Hierarchy.new(raw_hash:, base_path: ".")
-      assert_equal :yaml, hierarchy.backend
+    test "#paths returns an empty array if no data is available" do
+      hierarchy = HieraData::Hierarchy.new(raw_hash: {}, base_path: ".")
+      assert hierarchy.paths.empty?
     end
 
-    def raw_hash
-      {
-        "name" => "Yaml hierarchy",
-        "data_hash" => "yaml_data"
-      }
+    test "#paths supports the singular `glob` setting" do
+      glob = "/*/singular/glob"
+      raw_hash = { "name" => "Singular path", "glob" => glob }
+      hierarchy = HieraData::Hierarchy.new(raw_hash:, base_path: ".")
+      assert_equal [glob], hierarchy.paths
     end
-  end
 
-  class HieraData::HierarchyForJSONDataTest < ActiveSupport::TestCase
-    test "data_hash specified and json" do
+    test "#paths supports the `globs` array setting" do
+      globs = ["/*/array/globs", "/test/**/globs"]
+      raw_hash = { "name" => "Singular path", "globs" => globs }
       hierarchy = HieraData::Hierarchy.new(raw_hash:, base_path: ".")
-      assert_equal :json, hierarchy.backend
+      assert_equal globs, hierarchy.paths
     end
 
-    def raw_hash
-      {
-        "name" => "JSON hierarchy",
-        "data_hash" => "json_data"
+    test "#resolved_paths uses facts to resolve paths" do
+      hierarchy = HieraData::Hierarchy.new(raw_hash:, base_path: ".")
+      facts = {
+        "fqdn" => "testhost",
+        "role" => "hdm_test",
+        "env" => "development",
+        "zone" => "internal"
       }
+      expected_resolved_paths = [
+        "nodes/testhost.yaml",
+        "role/hdm_test-development.yaml",
+        "role/hdm_test.yaml",
+        "zone/internal.yaml",
+        "common.yaml"
+      ]
+      assert_equal expected_resolved_paths, hierarchy.resolved_paths(facts:)
     end
-  end
 
-  class HieraData::HierarchyForEyamlDataTest < ActiveSupport::TestCase
-    setup do
-      @tmpdir = Dir.mktmpdir
+    test "#resolved_paths resolves globs" do
+      base_path = Rails.root.join("test/fixtures/files/puppet/environments/globs")
+      globs = ["common/*.yaml"]
+      raw_hash = { "name" => "Common", "datadir" => "data", "globs" => globs }
+      hierarchy = HieraData::Hierarchy.new(raw_hash:, base_path:)
+      facts = { "fqdn" => "testhost" }
+      expected_resolved_paths = [
+        "common/foobar.yaml",
+        "common/hdm.yaml"
+      ]
+      assert_equal expected_resolved_paths, hierarchy.resolved_paths(facts:)
     end
 
-    teardown do
-      FileUtils.remove_entry @tmpdir
+    test "#name returns the existing name" do
+      hierarchy = HieraData::Hierarchy.new(raw_hash:, base_path: ".")
+      assert_equal "Yaml hierarchy", hierarchy.name
     end
 
-    test "lookup function is not data_hash" do
-      hierarchy = HieraData::Hierarchy.new(raw_hash:, base_path: ".")
-      assert_equal :eyaml, hierarchy.backend
+    test "#candidate_files returns files matching paths with variables replaced by globs" do
+      base_path = Rails.root.join("test/fixtures/files/puppet/environments/multiple_hierarchies")
+      hierarchy = HieraData::Hierarchy.new(
+        raw_hash: raw_hash.merge("datadir" => "data"),
+        base_path:
+      )
+      expected_candidate_files = [
+        "nodes/4msusyei.betadots.training.yaml",
+        "nodes/60wxmaw5.betadots.training.yaml",
+        "nodes/test.host.yaml",
+        "role/hdm_test.yaml",
+        "zone/internal.yaml",
+        "common.yaml"
+      ]
+      assert_equal expected_candidate_files, hierarchy.candidate_files
     end
 
-    test "#private_key returns path from options" do
-      hierarchy = HieraData::Hierarchy.new(raw_hash:, base_path: ".")
-      assert_equal "private.key", hierarchy.private_key.to_s
+    test "#datadir uses facts to resolve datadir" do
+      raw_hash = {
+        "name" => "dynamic datadir",
+        "datadir" => "%{facts.datadir}"
+      }
+      facts = { "datadir" => "data1" }
+      base_path = Rails.root.join("test/fixtures/files/puppet/environments/dynamic_datadir")
+      hierarchy = HieraData::Hierarchy.new(raw_hash:, base_path:)
+
+      assert_equal base_path.join("data1"), hierarchy.datadir(facts:)
     end
 
-    test "#public_key returns path from options" do
-      hierarchy = HieraData::Hierarchy.new(raw_hash:, base_path: ".")
-      assert_equal "public.key", hierarchy.public_key.to_s
+    def raw_hash
+      {
+        "name" => "Yaml hierarchy",
+        "paths" => [
+          "nodes/%{::facts.fqdn}.yaml",
+          "role/%{::facts.role}-%{::facts.env}.yaml",
+          "role/%{::facts.role}.yaml",
+          "zone/%{::facts.zone}.yaml",
+          "common.yaml"
+        ]
+      }
     end
 
-    test "#encryptable? is true if all keys present and readable" do
-      tmpdir_path = Pathname.new(@tmpdir)
-      %w(private.key public.key).each { |f| FileUtils.touch(tmpdir_path.join(f)) }
-      hierarchy = HieraData::Hierarchy.new(raw_hash:, base_path: @tmpdir)
+    class HierarchyForYamlDataTest < ActiveSupport::TestCase
+      test "data_hash specified and yaml" do
+        hierarchy = HieraData::Hierarchy.new(raw_hash:, base_path: ".")
+        assert_equal :yaml, hierarchy.backend
+      end
 
-      assert hierarchy.encryptable?
+      def raw_hash
+        {
+          "name" => "Yaml hierarchy",
+          "data_hash" => "yaml_data"
+        }
+      end
     end
 
-    test "#encryptable? is false if a key is missing" do
-      hierarchy = HieraData::Hierarchy.new(raw_hash:, base_path: @tmpdir)
+    class HierarchyForJSONDataTest < ActiveSupport::TestCase
+      test "data_hash specified and json" do
+        hierarchy = HieraData::Hierarchy.new(raw_hash:, base_path: ".")
+        assert_equal :json, hierarchy.backend
+      end
 
-      refute hierarchy.encryptable?
+      def raw_hash
+        {
+          "name" => "JSON hierarchy",
+          "data_hash" => "json_data"
+        }
+      end
     end
 
-    test "#encryptable? is false if a key is not readable" do
-      tmpdir_path = Pathname.new(@tmpdir)
-      files = %w(private.key public.key).map { |f| tmpdir_path.join(f) }
-      files.each do |path|
-        FileUtils.touch(path)
-        File.chmod(0000, path)
+    class HierarchyForEyamlDataTest < ActiveSupport::TestCase
+      setup do
+        @tmpdir = Dir.mktmpdir
       end
-      hierarchy = HieraData::Hierarchy.new(raw_hash:, base_path: @tmpdir)
 
-      refute hierarchy.encryptable?
+      teardown do
+        FileUtils.remove_entry @tmpdir
+      end
 
-      File.chmod(0600, *files)
+      test "lookup function is not data_hash" do
+        hierarchy = HieraData::Hierarchy.new(raw_hash:, base_path: ".")
+        assert_equal :eyaml, hierarchy.backend
+      end
+
+      test "#private_key returns path from options" do
+        hierarchy = HieraData::Hierarchy.new(raw_hash:, base_path: ".")
+        assert_equal "private.key", hierarchy.private_key.to_s
+      end
+
+      test "#public_key returns path from options" do
+        hierarchy = HieraData::Hierarchy.new(raw_hash:, base_path: ".")
+        assert_equal "public.key", hierarchy.public_key.to_s
+      end
+
+      test "#encryptable? is true if all keys present and readable" do
+        tmpdir_path = Pathname.new(@tmpdir)
+        %w[private.key public.key].each { |f| FileUtils.touch(tmpdir_path.join(f)) }
+        hierarchy = HieraData::Hierarchy.new(raw_hash:, base_path: @tmpdir)
+
+        assert hierarchy.encryptable?
+      end
+
+      test "#encryptable? is false if a key is missing" do
+        hierarchy = HieraData::Hierarchy.new(raw_hash:, base_path: @tmpdir)
+
+        assert_not hierarchy.encryptable?
+      end
+
+      test "#encryptable? is false if a key is not readable" do
+        tmpdir_path = Pathname.new(@tmpdir)
+        files = %w[private.key public.key].map { |f| tmpdir_path.join(f) }
+        files.each do |path|
+          FileUtils.touch(path)
+          File.chmod(0o000, path)
+        end
+        hierarchy = HieraData::Hierarchy.new(raw_hash:, base_path: @tmpdir)
+
+        assert_not hierarchy.encryptable?
+
+        File.chmod(0o600, *files)
+      end
+
+      def raw_hash
+        {
+          "name" => "EYaml hierarchy",
+          "lookup_key" => "eyaml_lookup_key",
+          "options" => {
+            "pkcs7_private_key" => "private.key",
+            "pkcs7_public_key" => "public.key"
+          }
+        }
+      end
     end
 
-    def raw_hash
-      {
-        "name" => "EYaml hierarchy",
-        "lookup_key" => "eyaml_lookup_key",
-        "options" => {
-          "pkcs7_private_key" => "private.key",
-          "pkcs7_public_key" => "public.key"
+    class HierarchyForCustomBackend < ActiveSupport::TestCase
+      test "custom lookup function specified but no mapping present" do
+        hierarchy = HieraData::Hierarchy.new(raw_hash:, base_path: ".")
+        assert_raises Hdm::Error do
+          hierarchy.backend
+        end
+      end
+
+      test "custom lookup function mapped to eyaml" do
+        Rails.configuration.hdm[:custom_lookup_function_mapping] = {
+          "custom_eyaml_function" => "eyaml"
         }
-      }
+        hierarchy = HieraData::Hierarchy.new(raw_hash:, base_path: ".")
+        assert_equal :eyaml, hierarchy.backend
+        Rails.configuration.hdm.delete(:custom_lookup_function_mapping)
+      end
+
+      def raw_hash
+        {
+          "name" => "JSON hierarchy",
+          "lookup_key" => "custom_eyaml_function"
+        }
+      end
     end
   end
 end