fix: added support for read preference specified by the URL

README.md

@@ -788,15 +788,15 @@ a simple map, supporting the following keys:
 
 * `:mode`, possible values: `:primary`, `:primary_preferred`, `:secondary`, `:secondary_preferred` and `:nearest`
 * `:max_staleness_ms`, the maxStaleness value in milliseconds
-* `:tag_sets`, the set of tags, for example: `[dc: "west", usage: "production"]`
+* `:tags`, the set of tags, for example: `[dc: "west", usage: "production"]`
 
 The driver selects the server using the read preference. 
 
 ```elixr 
 prefs = %{
     mode: :secondary,
     max_staleness_ms: 120_000,
-    tag_sets: [dc: "west", usage: "production"]
+    tags: [dc: "west", usage: "production"]
 }
 
 Mongo.find_one(top, "dogs", %{name: "Oskar"}, read_preference: prefs)

lib/mongo/read_preference.ex

@@ -4,134 +4,141 @@ defmodule Mongo.ReadPreference do
   @moduledoc ~S"""
   Determines which servers are considered suitable for read operations
 
-  A read preference consists of a mode and optional `tag_sets`, max_staleness_ms, and `hedge`.
+  A read preference consists of a mode and optional `tags`, max_staleness_ms, and `hedge`.
   The mode prioritizes between primaries and secondaries to produce either a single suitable server or a list of candidate servers.
-  If tag_sets and maxStalenessSeconds are set, they determine which candidate servers are eligible for selection.
+  If tags and maxStalenessSeconds are set, they determine which candidate servers are eligible for selection.
   If hedge is set, it configures how server hedged reads are used.
 
   The default mode is `:primary`.
-  The default tag_sets is a list with an empty tag set: [{}].
+  The default tags is a list with an empty tag set: [{}].
   The default max_staleness_ms is unset.
   The default hedge is unset.
 
   ## mode
 
   * `:primary` Only an available primary is suitable.
-  * `:secondary` All secondaries (and only secondaries) are candidates, but only eligible candidates (i.e. after applying tag_sets and maxStalenessSeconds) are suitable.
+  * `:secondary` All secondaries (and only secondaries) are candidates, but only eligible candidates (i.e. after applying tags and maxStalenessSeconds) are suitable.
   * `:primary_preferred` If a primary is available, only the primary is suitable. Otherwise, all secondaries are candidates,
        but only eligible secondaries are suitable.
   * `:secondary_preferred` All secondaries are candidates. If there is at least one eligible secondary, only eligible secondaries are suitable.
        Otherwise, when there are no eligible secondaries, the primary is suitable.
   * `:nearest` The primary and all secondaries are candidates, but only eligible candidates are suitable.
 
   """
-  @type t :: %{
-          mode:
-            :primary
-            | :secondary
-            | :primary_preferred
-            | :secondary_preferred
-            | :nearest,
-          tag_sets: [%{String.t() => String.t()}],
-          max_staleness_ms: non_neg_integer,
-          hedge: BSON.document()
-        }
 
   @primary %{
     mode: :primary,
-    tag_sets: [],
+    tags: [],
     max_staleness_ms: 0
   }
 
-  def primary(map \\ nil)
+  @doc """
+  Merge default values to the read preferences and converts deprecated tag_sets to tags
+  """
+  def merge_defaults(%{tag_sets: tags} = map) do
+    map =
+      map
+      |> Map.delete(:tag_sets)
+      |> Map.put(:tags, tags)
+
+    Map.merge(@primary, map)
+  end
 
-  def primary(map) when is_map(map) do
+  def merge_defaults(map) when is_map(map) do
     Map.merge(@primary, map)
   end
 
-  def primary(_), do: @primary
+  def merge_defaults(_other) do
+    @primary
+  end
 
   @doc """
   Add read preference to the cmd
   """
   def add_read_preference(cmd, opts) do
     case Keyword.get(opts, :read_preference) do
-      nil -> cmd
-      pref -> cmd ++ ["$readPreference": pref]
+      nil ->
+        cmd
+
+      pref ->
+        cmd ++ ["$readPreference": pref]
     end
   end
 
   @doc """
-  From the specs:
-
-  Use of slaveOk
-
-  There are two usages of slaveOK:
-
-  * A driver query parameter that predated read preference modes and tag set lists.
-  * A wire protocol flag on OP_QUERY operations
-
+  Converts the preference to the mongodb format for replica sets
   """
-  def slave_ok(%{:mode => :primary}) do
-    %{:mode => :primary}
+  def to_replica_set(%{:mode => :primary}) do
+    %{mode: :primary}
   end
 
-  def slave_ok(config) do
+  def to_replica_set(config) do
     mode =
       case config[:mode] do
-        :primary_preferred -> :primaryPreferred
-        :secondary_preferred -> :secondaryPreferred
-        other -> other
-      end
+        :primary_preferred ->
+          :primaryPreferred
 
-    filter_nils(mode: mode, tag_sets: config[:tag_sets])
-  end
+        :secondary_preferred ->
+          :secondaryPreferred
 
-  ##
-  # Therefore, when sending queries to a mongos, the following rules apply:
-  #
-  # For mode 'primary', drivers MUST NOT set the slaveOK wire protocol flag and MUST NOT use $readPreference
-  def mongos(%{mode: :primary}) do
-    nil
-  end
+        other ->
+          other
+      end
 
-  # For mode 'secondary', drivers MUST set the slaveOK wire protocol flag and MUST also use $readPreference
-  def mongos(%{mode: :secondary} = config) do
-    transform(config)
-  end
+    case config[:tags] do
+      [] ->
+        %{mode: mode}
 
-  # For mode 'primaryPreferred', drivers MUST set the slaveOK wire protocol flag and MUST also use $readPreference
-  def mongos(%{mode: :primary_preferred} = config) do
-    transform(config)
-  end
+      nil ->
+        %{mode: mode}
 
-  # For mode 'secondaryPreferred', drivers MUST set the slaveOK wire protocol flag. If the read preference contains a
-  # non-empty tag_sets parameter, maxStalenessSeconds is a positive integer, or the hedge parameter is non-empty,
-  # drivers MUST use $readPreference; otherwise, drivers MUST NOT use $readPreference
-  def mongos(%{mode: :secondary_preferred} = config) do
-    transform(config)
+      tags ->
+        %{mode: mode, tags: [tags]}
+    end
   end
 
-  # For mode 'nearest', drivers MUST set the slaveOK wire protocol flag and MUST also use $readPreference
-  def mongos(%{mode: :nearest} = config) do
-    transform(config)
+  @doc """
+  Converts the preference to the mongodb format for mongos
+  """
+  def to_mongos(%{mode: :primary}) do
+    nil
   end
 
-  defp transform(config) do
+  # for the others we should use the read preferences
+  def to_mongos(config) do
     mode =
       case config[:mode] do
-        :primary_preferred -> :primaryPreferred
-        :secondary_preferred -> :secondaryPreferred
-        other -> other
+        :primary_preferred ->
+          :primaryPreferred
+
+        :secondary_preferred ->
+          :secondaryPreferred
+
+        other ->
+          other
       end
 
     max_staleness_seconds =
       case config[:max_staleness_ms] do
-        i when is_integer(i) -> div(i, 1000)
-        nil -> nil
+        i when is_integer(i) ->
+          div(i, 1000)
+
+        nil ->
+          nil
+      end
+
+    read_preference =
+      case config[:tags] do
+        [] ->
+          %{mode: mode, maxStalenessSeconds: max_staleness_seconds, hedge: config[:hedge]}
+
+        nil ->
+          %{mode: mode, maxStalenessSeconds: max_staleness_seconds, hedge: config[:hedge]}
+
+        tags ->
+          %{mode: mode, tags: [tags], maxStalenessSeconds: max_staleness_seconds, hedge: config[:hedge]}
       end
 
-    [mode: mode, tag_sets: config[:tag_sets], maxStalenessSeconds: max_staleness_seconds, hedge: config[:hedge]]
-    |> filter_nils()
+    filter_nils(read_preference)
   end
 end

lib/mongo/topology.ex

@@ -373,6 +373,8 @@ defmodule Mongo.Topology do
   # checkout a new session
   #
   def handle_call({:checkout_session, read_write_type, opts}, from, %{:topology => topology, :waiting_pids => waiting} = state) do
+    opts = merge_read_preferences(opts, state.opts)
+
     case TopologyDescription.select_servers(topology, read_write_type, opts) do
       :empty ->
         Mongo.Events.notify(%ServerSelectionEmptyEvent{action: :checkout_session, cmd_type: read_write_type, topology: topology, opts: opts})
@@ -398,6 +400,8 @@ defmodule Mongo.Topology do
   end
 
   def handle_call({:select_server, read_write_type, opts}, from, %{:topology => topology, :waiting_pids => waiting} = state) do
+    opts = merge_read_preferences(opts, state.opts)
+
     case TopologyDescription.select_servers(topology, read_write_type, opts) do
       :empty ->
         Mongo.Events.notify(%ServerSelectionEmptyEvent{action: :select_server, cmd_type: read_write_type, topology: topology, opts: opts})
@@ -579,4 +583,14 @@ defmodule Mongo.Topology do
   defp fetch_arbiters(state) do
     Enum.flat_map(state.topology.servers, fn {_, s} -> s.arbiters end)
   end
+
+  defp merge_read_preferences(opts, url_opts) do
+    case Keyword.get(url_opts, :read_preference) do
+      nil ->
+        opts
+
+      read_preference ->
+        Keyword.put_new(opts, :read_preference, read_preference)
+    end
+  end
 end