connection_pool: implement connection pool with automatic master discovery

Main features:

- Check active connection with configurable time interval and on connection fail switch to next in pool.
- Automatic master discovery by `mode` parameter.

Additional options (configurable via `ConnectWithOpts`):

* `CheckTimeout` - time interval to check for connection timeout and try to switch connection

`Mode` parameter:

* `RW` (writteable instance (master)) - the request can only be executed on master.
* `PREFER_RO` (read only instance (replica)) - if there is one, otherwise fallback to a writteable one (master).
* `PREFER_RW` (write only instance (master)) - if there is one, otherwise fallback to a read only one (replica).

Closes #113

Author: AnaNek

CHANGELOG.md

@@ -16,6 +16,7 @@ Versioning](http://semver.org/spec/v2.0.0.html) except to the first release.
 - Support UUID type in msgpack (#90)
 - Go modules support (#91)
 - queue-utube handling (#85)
+- Master discovery (#113)
 
 ### Fixed
 

README.md

@@ -676,6 +676,38 @@ Additional options (configurable via `ConnectWithOpts`):
 * `ClusterDiscoveryTime` - time interval to ask server for updated address list (works on with `NodesGetFunctionName` set)
 * `NodesGetFunctionName` - server lua function name to call for getting address list
 
+## Connection pool with automatic master discovery
+
+You can use connection pool config with tarantool/connection_pool.
+
+Main features:
+
+- Check active connection with configurable time interval and on connection fail switch to next in pool.
+- Automatic master discovery by `mode` parameter.
+
+Additional options (configurable via `ConnectWithOpts`):
+
+* `CheckTimeout` - time interval to check for connection timeout and try to switch connection
+
+`Mode` parameter:
+
+* `RW` (writteable instance (master)) - the request can only be executed on master.
+* `PREFER_RO` (read only instance (replica)) - if there is one, otherwise fallback to a writteable one (master).
+* `PREFER_RW` (write only instance (master)) - if there is one, otherwise fallback to a read only one (replica).
+
+|Request |  Default mode | User mode support  |
+|---|---|---|
+| call  | no default  | Yes  |
+| eval  | no default  | Yes  |
+| ping | no default  | Yes  |
+| insert | RW  | No  |
+| delete | RW  | No  |
+| replace | RW  | No  |
+| update | RW  | No  |
+| upsert | RW  | No  |
+| select | PreferRO  | Yes  |
+| get | PreferRO  | Yes  |
+
 ## Tests
 
 You need to [install Tarantool](https://www.tarantool.io/en/download/) to run tests.

connection_pool/config.lua

@@ -0,0 +1,29 @@
+-- Do not set listen for now so connector won't be
+-- able to send requests until everything is configured.
+box.cfg{
+    work_dir = os.getenv("TEST_TNT_WORK_DIR"),
+}
+
+box.once("init", function()
+    box.schema.user.create('test', { password = 'test' })
+    box.schema.user.grant('test', 'read,write,execute', 'universe')
+
+    local s = box.schema.space.create('testPool', {
+        id = 520,
+        if_not_exists = true,
+        format = {
+            {name = "key", type = "string"},
+            {name = "value", type = "string"},
+        },
+    })
+    s:create_index('pk', {
+        type = 'tree',
+        parts = {{ field = 1, type = 'string' }},
+        if_not_exists = true
+    })
+end)
+
+-- Set listen only when every other thing is configured.
+box.cfg{
+    listen = os.getenv("TEST_TNT_LISTEN"),
+}