Added the Troubleshooting section (#10888)

Co-authored-by: Ivan Blinkov <ivan@ydb.tech>

Author: anton-bobkov

ydb/docs/en/core/concepts/glossary.md

@@ -101,6 +101,16 @@ Together, these mechanisms allow {{ ydb-short-name }} to provide [strict consist
 
 The implementation of distributed transactions is covered in a separate article [{#T}](../contributor/datashard-distributed-txs.md), while below there's a list of several [related terms](#distributed-transaction-implementation).
 
+### Interactive transactions {#interactive-transaction}
+
+The term **interactive transactions** refers to transactions that are split into multiple queries and involve data processing by an application between these queries. For example:
+
+1. Select some data.
+1. Process the selected data in the application.
+1. Update some data in the database.
+1. Commit the transaction in a separate query.
+
+
 ### Multi-version concurrency control {#mvcc}
 
 [**Multi-version concurrency control**](https://en.wikipedia.org/wiki/Multiversion_concurrency_control) or **MVCC** is a method {{ ydb-short-name }} used to allow multiple concurrent transactions to access the database simultaneously without interfering with each other. It is described in more detail in a separate article [{#T}](mvcc.md).
@@ -255,6 +265,20 @@ The **actor system interconnect** or **interconnect** is the [cluster's](#cluste
 
 A **Local** is an [actor service](#actor-service) running on each [node](#node). It directly manages the [tablets](#tablet) on its node and interacts with [Hive](#hive). It registers with Hive and receives commands to launch tablets.
 
+#### Actor system pool {#actor-system-pool}
+
+The **actor system pool** is a [thread pool](https://en.wikipedia.org/wiki/Thread_pool) used to run [actors](#actor). Each [node](#node) operates multiple pools to coarsely separate resources between different types of activities. A typical set of pools includes:
+
+- **System**: A pool that handles internal operations within {{ ydb-short-name }} node. It serves system [tablets](#tablet), [state storage](#state-storage), [distributed storage](#distributed-storage) I/O, and so on.
+
+- **User**: A pool dedicated to user-generated load, such as running non-system tablets or queries executed by the [KQP](#kqp).
+
+- **Batch**: A pool for tasks without strict execution deadlines, including heavy queries handled by the [KQP](#kqp) background operations like backups, data compaction, and garbage collection.
+
+- **IO**: A pool for tasks involving blocking operations, such as authentication or writing logs to files.
+
+- **IC**: A pool for [interconnect](#actor-system-interconnect), responsible for system calls related to data transfers across the network, data serialization, message splitting and merging.
+
 ### Tablet implementation {#tablet-implementation}
 
 A [**tablet**](#tablet) is an [actor](#actor) with a persistent state. It includes a set of data for which this tablet is responsible and a finite state machine through which the tablet's data (or state) changes. The tablet is a fault-tolerant entity because tablet data is stored in a [Distributed storage](#distributed-storage) that survives disk and node failures. The tablet is automatically restarted on another [node](#node) if the previous one is down or overloaded. The data in the tablet changes in a consistent manner because the system infrastructure ensures that there is no more than one [tablet leader](#tablet-leader) through which changes to the tablet data are carried out.
@@ -558,7 +582,7 @@ MiniKQL is a low-level language. The system's end users only see queries in the
 
 #### KQP {#kqp}
 
-**KQP** is a {{ ydb-short-name }} component responsible for the orchestration of user query execution and generating the final response.
+**KQP** or **Query Processor** is a {{ ydb-short-name }} component responsible for the orchestration of user query execution and generating the final response.
 
 ### Global schema {#global-schema}
 

ydb/docs/en/core/dev/index.md

@@ -27,4 +27,6 @@ Main resources:
   - [{#T}](../postgresql/intro.md)
   - [{#T}](../reference/kafka-api/index.md)
 
+- [{#T}](troubleshooting/index.md)
+
 If you're interested in developing {{ ydb-short-name }} core or satellite projects, refer to the [documentation for contributors](../contributor/index.md).

ydb/docs/en/core/dev/toc_p.yaml

@@ -18,6 +18,11 @@ items:
     path: primary-key/toc_p.yaml
 - name: Secondary indexes
   href: secondary-indexes.md
+- name: Troubleshooting
+  href: troubleshooting/index.md
+  include:
+    mode: link
+    path: troubleshooting/toc_p.yaml
 - name: Query plans optimization
   href: query-plans-optimization.md
 - name: Batch upload

ydb/docs/en/core/dev/troubleshooting/index.md

@@ -0,0 +1,5 @@
+# Troubleshooting
+
+This section of the {{ ydb-short-name }} documentation provides guidance on troubleshooting issues related to {{ ydb-short-name }} databases and the applications that interact with them.
+
+- [{#T}](performance/index.md)