connpool: prefer_ro and prefer_rw for call()

This patch introduces "prefer_ro" and "prefer_rw" values for the "mode"
option for the "call()" function.

Closes #9930

@TarantoolBot document
Title: The `mode` option for `call()` and `filter()`

The new `mode` option is now supported by the `call()` and `filter()`
functions from the `experimental.connpool` module. This option allows to
filter candidates based on their `read-only` status.

The `filter()` function supports three values of the `mode` option:
1) `nil` means that the `read_only` status of the instance is not
   checked;
2) `ro` means that only instances with `read_only == true` are
   considered;
3) `rw` means that only instances with `read_only == false` are
   considered.

The `call()` function supports five values of the `mode` option:
1) `nil` means that the `read_only` status of the instance is not
   checked when instance is selected to execute `call()`;
2) `ro` means that only instances with `read_only == true` are
   considered when instance is selected to execute `call()`;
3) `rw` means that only instances with `read_only == false` are
   considered when instance is selected to execute `call()`.
4) `prefer_ro` means that `call()` will only be executed on instances
   with `read_only == false` if it is not possible to execute it on
   instances with `read_only == true`;
5) `prefer_rw` means that `call()` will only be executed on instances
   with `read_only == true` if it is not possible to execute it on
   instances with `read_only == false`.

Note that if this option is not `nil`, a connection will be attempted to
each instance in the config if a connection does not exist. This means
that any of these functions can potentially block for a maximum of
`<number of instances> * 10` seconds.

changelogs/unreleased/gh-9930-mode-option.md

@@ -0,0 +1,4 @@
+## feature/experimental
+
+* Introduced the `mode` option for the `filter()` and `call()` functions
+  in the `experimental.connpool` module (gh-9930).

src/box/lua/experimental/connpool.lua

@@ -207,29 +207,60 @@ local function get_connection(opts)
         return nil, "no candidates are available with these conditions"
     end
 
-    if opts.prefer_local ~= false then
-        local candidate_idx = nil
-        for n, candidate in ipairs(candidates) do
-            if candidate == box.info.name then
-                candidate_idx = n
-                local conn = connect(box.info.name, {wait_connected = false})
-                if conn:wait_connected() then
-                    return conn
-                end
-                break
+    -- Initialize the weight of each candidate.
+    local weights = {}
+    if opts.mode == 'prefer_rw' or opts.mode == 'prefer_ro' then
+        candidates = connect_to_candidates(candidates)
+    end
+    for _, instance_name in pairs(candidates) do
+        weights[instance_name] = 0
+    end
+
+    -- Increase weights of candidates preferred by mode.
+    if opts.mode == 'prefer_rw' or opts.mode == 'prefer_ro' then
+        local mode = opts.mode == 'prefer_ro' and 'ro' or 'rw'
+        local weight_mode = 2
+        for _, instance_name in pairs(candidates) do
+            local conn = connections[instance_name]
+            assert(conn ~= nil)
+            if conn:mode() == mode then
+                weights[instance_name] = weights[instance_name] + weight_mode
             end
         end
-        if candidate_idx ~= nil then
-            table.remove(candidates, candidate_idx)
+    end
+
+    -- Increase weight of local candidate.
+    if opts.prefer_local ~= false then
+        local weight_local = 1
+        if weights[box.info.name] ~= nil then
+            weights[box.info.name] = weights[box.info.name] + weight_local
         end
     end
 
-    while #candidates > 0 do
-        local n = math.random(#candidates)
-        local instance_name = table.remove(candidates, n)
-        local conn = connect(instance_name, {wait_connected = false})
-        if conn:wait_connected() then
-            return conn
+    -- Select candidate by weight.
+    while next(weights) ~= nil do
+        local max_weight = 0
+        for _, weight in pairs(weights) do
+            if weight > max_weight then
+                max_weight = weight
+            end
+        end
+        local preferred_candidates = {}
+        for instance_name, weight in pairs(weights) do
+            if weight == max_weight then
+                table.insert(preferred_candidates, instance_name)
+            end
+        end
+        while #preferred_candidates > 0 do
+            local n = math.random(#preferred_candidates)
+            local instance_name = table.remove(preferred_candidates, n)
+            local conn = connect(instance_name, {wait_connected = false})
+            if conn:wait_connected() then
+                return conn
+            end
+        end
+        for _, instance_name in pairs(preferred_candidates) do
+            weights[instance_name] = nil
         end
     end
     return nil, "connection to candidates failed"
@@ -249,8 +280,10 @@ local function call(func_name, args, opts)
         is_async = '?boolean',
     })
     opts = opts or {}
-    if opts.mode ~= nil and opts.mode ~= 'ro' and opts.mode ~= 'rw' then
-        local msg = 'Expected nil, "ro" or "rw", got "%s"'
+    if opts.mode ~= nil and opts.mode ~= 'ro' and opts.mode ~= 'rw' and
+       opts.mode ~= 'prefer_ro' and opts.mode ~= 'prefer_rw' then
+        local msg = 'Expected nil, "ro", "rw", "prefer_ro" or "prefer_rw", ' ..
+                    'got "%s"'
         error(msg:format(opts.mode), 0)
     end
 

test/config-luatest/rpc_test.lua

@@ -451,7 +451,9 @@ g.test_call_mode = function(g)
                   mode: rw
                 labels:
                   l1: 'first'
-              instance-002: {}
+              instance-002:
+                labels:
+                  l2: 'second'
           replicaset-002:
             replication:
               failover: manual
@@ -461,6 +463,7 @@ g.test_call_mode = function(g)
               instance-004:
                 labels:
                   l1: 'first'
+                  l2: 'second'
     ]]
     treegen.write_script(dir, 'config.yaml', config)
 
@@ -512,6 +515,41 @@ g.test_call_mode = function(g)
         }
         local exp_list = {'instance-002', 'instance-004'}
         t.assert_items_include(exp_list, {connpool.call('f', nil, opts)})
+
+        opts = {
+            labels = {l1 = 'first'},
+            mode = 'prefer_ro',
+        }
+        t.assert_equals(connpool.call('f', nil, opts), 'instance-004')
+
+        opts = {
+            labels = {l1 = 'first'},
+            mode = 'prefer_rw',
+        }
+        t.assert_equals(connpool.call('f', nil, opts), 'instance-001')
+
+        -- Make sure 'prefer_*' mode will execute call on non-preferred
+        --  instance, if there is no preferred instance.
+        opts = {
+            labels = {l2 = 'second'},
+            mode = 'prefer_rw',
+        }
+        exp_list = {'instance-002', 'instance-004'}
+        t.assert_items_include(exp_list, {connpool.call('f', nil, opts)})
+
+        -- Make sure that "prefer_local" has a lower priority than the
+        -- "prefer_*" mode.
+        opts = {
+            labels = {l1 = 'first'},
+            mode = 'prefer_rw',
+            prefer_local = true,
+        }
+        t.assert_equals(connpool.call('f', nil, opts), 'instance-001')
+
+        local exp_err = 'Expected nil, "ro", "rw", "prefer_ro" or ' ..
+                        '"prefer_rw", got "something"'
+        opts = {mode = 'something'}
+        t.assert_error_msg_equals(exp_err, connpool.call, 'f', nil, opts)
     end
 
     g.server_1:exec(check)