[docs] Describe consumption extensions.

Author: Maksym Kotliar

docs/consumption/extensions.md

@@ -1,3 +1,52 @@
 # Consumption extensions.
 
-[back to index](../index.md)
+
+
+## [LoggerExtension](https://github.com/php-enqueue/enqueue-dev/blob/master/pkg/enqueue/Consumption/Extension/LoggerExtension.php) 
+
+It sets logger to queue consumer context. All log messages will go to it.
+
+## [DoctrineClearIdentityMapExtension](https://github.com/php-enqueue/enqueue-dev/blob/master/pkg/enqueue-bundle/Consumption/Extension/DoctrineClearIdentityMapExtension.php) 
+
+It clears Doctrine's identity map after a message is processed. It reduce memory usage.
+
+## [DoctrinePingConnectionExtension](https://github.com/php-enqueue/enqueue-dev/blob/master/pkg/enqueue-bundle/Consumption/Extension/DoctrinePingConnectionExtension.php)
+
+It test a database connection and if it is lost it does reconnect. Fixes "MySQL has gone away" errors.
+
+## [ReplyExtension](https://github.com/php-enqueue/enqueue-dev/blob/master/pkg/enqueue/Consumption/Extension/ReplyExtension.php)
+
+It comes with RPC code and simplifies reply logic. 
+It takes care of sending a reply message to reply queue.
+
+## [SetupBrokerExtension](https://github.com/php-enqueue/enqueue-dev/blob/master/pkg/enqueue/Client/ConsumptionExtension/SetupBrokerExtension.php)
+
+It responsible for configuring everything at a broker side. queues, topics, bindings and so on. 
+The extension is added at runtime when `--setup-broker` option is used.
+
+## [LimitConsumedMessagesExtension](https://github.com/php-enqueue/enqueue-dev/blob/master/pkg/enqueue/Consumption/Extension/LimitConsumedMessagesExtension.php)
+
+The extension counts processed message and once a limit is reached it interrupts consumption.
+The extension is added at runtime when `--message-limit=10` option is used.
+
+## [LimitConsumerMemoryExtension](https://github.com/php-enqueue/enqueue-dev/blob/master/pkg/enqueue/Consumption/Extension/LimitConsumerMemoryExtension.php)
+
+The extension interrupts consumption once a memory limit is reached.
+The extension is added at runtime when `--memory-limit=512` option is used. 
+The value is Mb. 
+
+## [LimitConsumptionTimeExtension](https://github.com/php-enqueue/enqueue-dev/blob/master/pkg/enqueue/Consumption/Extension/LimitConsumptionTimeExtension.php)
+
+The extension interrupts consumption once time limit is reached.
+The extension is added at runtime when `--time-limit="now + 2 minutes"` option is used.
+
+## [SignalExtension](https://github.com/php-enqueue/enqueue-dev/blob/master/pkg/enqueue/Consumption/Extension/SignalExtension.php)
+
+The extension catch process signals and gracefully stops consumption. Works only on NIX platforms.
+ 
+## [DelayRedeliveredMessageExtension](https://github.com/php-enqueue/enqueue-dev/blob/master/pkg/enqueue/Client/ConsumptionExtension/DelayRedeliveredMessageExtension.php)
+
+The extension checks whether the received message is redelivered (There was attempt to process message but it failed). 
+If so the extension reject the origin message and creates a copy message with a delay. 
+
+[back to index](../index.md)