doc(record_db): explain how to use record_db

Signed-off-by: Sagar Arora <sagar.arora@openairinterface.org>
Reviewed-by: Nick Hedberg <nhedberg@nvidia.com>
Reviewed-by: Robert Schmidt <robert.schmidt@openairinterface.org>
Assisted-by: gpt-5.4

common/utils/T/DOC/T.md

@@ -14,6 +14,7 @@ It is made of two main parts:
 
 * [Basic usage](./T/basic.md)
 * [Record](./T/record.md)
+* [Record to ClickHouse](./T/record_db.md)
 * [Replay](./T/replay.md)
 * [Multiple tracers](./T/multi.md)
 * [MAC PDUs and wireshark](./T/wireshark.md)

common/utils/T/DOC/T/record_db.md

+<!-- SPDX-License-Identifier: CC-BY-4.0 -->
+
+# Record to ClickHouse
+
+This document describes the database-backed T tracer. Unlike [`record`](./record.md), 
+which writes a `.raw` file, `record_db` writes T tracer events 
+directly into a [ClickHouse database](https://clickhouse.com/docs/intro).
+
+## What `record_db` does
+
+`record_db` connects to a running OAI softmodem instance through 
+the standard T tracer TCP interface and:
+
+- Enables the selected events
+- Creates one ClickHouse table per enabled event
+- Batches received rows and inserts them into ClickHouse
+
+Each table always contains:
+
+- `TsSwNs`: software receive timestamp created by `record_db`
+
+If an event has both `frame` and `slot` (or `subframe`) fields, the
+tool also adds:
+
+- `TsTxUtcNs`: transmission timestamp reconstructed from frame/slot
+
+The remaining columns come from the event definition in
+`T_messages.txt`.
+
+## Build
+
+`record_db` pulls external dependencies (`clickhouse-cpp` and `fmtlog`).
+Also make sure you have `libzstd-dev` version > 1.5.X.
+
+From the repository using build_oai:
+
+```shell
+cd cmake_targets
+./build_oai --gNB --nrUE --cmake-opt -DT_RECORD_DB=ON
+```
+
+Or, from an existing build directory using cmake:
+
+```shell
+cd cmake_targets/ran_build/build
+cmake -DT_RECORD_DB=ON .
+make T_tools
+```
+
+The binary is then available in:
+
+```shell
+cmake_targets/ran_build/build/common/utils/T/tracer/record_db
+```
+
+## Prerequisites
+
+- a running ClickHouse server reachable from `record_db`
+- an OAI softmodem started with T tracer enabled, typically with `--T_stdout 2`
+- the tracer message description file, usually `common/utils/T/T_messages.txt`
+
+By default `record_db` connects to:
+
+- T tracer endpoint: `127.0.0.1:2021`
+- ClickHouse native endpoint: `localhost:9000`
+- ClickHouse database: `oai`
+
+## Start the softmodem
+
+Start the gNB or UE with T tracer enabled as described in
+[basic usage](./basic.md). For example:
+
+```shell
+cd cmake_targets/ran_build/build
+sudo ./nr-softmodem -O <config-file> --T_stdout 2 --T_nowait
+```
+
+Use `--T_port <port>` on the softmodem side if you want a port other
+than `2021`, and pass the same value to `record_db` with `-p`.
+
+## Basic example
+
+To enable all events except the most verbose groups and write them to
+ClickHouse:
+
+```shell
+cd cmake_targets/ran_build/build/common/utils/T/tracer
+./record_db -d ../T_messages.txt \
+  -ON \
+  -off VCD \
+  -off HEAVY \
+  -off LEGACY_GROUP_TRACE \
+  -off LEGACY_GROUP_DEBUG
+```
+
+To stop recording, press `Ctrl-C`. The tool flushes pending buffered
+rows before exiting.
+
+## Select specific events
+
+The event selection rules are the same as for [`record`](./record.md):
+
+- `-ON` enables all events
+- `-OFF` disables all events
+- `-on <GROUP or ID>` enables one group or one event
+- `-off <GROUP or ID>` disables one group or one event
+
+These options are processed in order. For example:
+
+```shell
+./record_db -d ../T_messages.txt -OFF -on GNB_PHY_UL_FD_PUSCH_IQ -on GNB_PHY_UL_PAYLOAD_RX_BITS
+```
+
+This starts from everything disabled and enables only the two listed
+events.
+
+## ClickHouse options
+
+`record_db` accepts the following database-related options:
+
+```shell
+-ch-host <host>      ClickHouse host, default `localhost`
+-ch-port <port>      ClickHouse native port, default `9000`
+-ch-db <database>    database name, default `oai`
+-ch-user <user>      optional ClickHouse user
+-ch-pass <password>  optional ClickHouse password
+-ch-droptables       drop the target database before starting
+-debug               enable debug logging
+```
+
+Example using a remote ClickHouse instance:
+
+```shell
+./record_db -d ../T_messages.txt \
+  -ON -off HEAVY \
+  -ip 192.168.1.10 -p 2021 \
+  -ch-host 192.168.1.20 -ch-port 9000 -ch-db oai_traces
+```
+
+## Inspect the data
+
+Once `record_db` is running, connect to ClickHouse and inspect the
+generated tables:
+
+```shell
+clickhouse-client --host localhost --port 9000 --query "SHOW TABLES FROM oai"
+```
+
+Count rows in one event table:
+
+```shell
+clickhouse-client --host localhost --port 9000 --query "SELECT count(*) FROM oai.GNB_PHY_UL_FD_PUSCH_IQ"
+```
+
+Inspect the latest samples:
+
+```shell
+clickhouse-client --host localhost --port 9000 --query "SELECT * FROM oai.GNB_PHY_UL_FD_PUSCH_IQ ORDER BY TsSwNs DESC LIMIT 5"
+```
+
+## Important behavior
+
+- Tables are created only for enabled events.
+- Table names match the event names from `T_messages.txt`.
+- The current implementation creates tables with ClickHouse
+  `ENGINE = Memory()`.
+- Because of that engine choice, recorded data remains available while
+  ClickHouse is running, but is not persistent across a ClickHouse
+  restart.
+- If `-ch-droptables` is used, the whole target database is dropped and
+  recreated before capture starts.
+
+## Get the full option list
+
+```shell
+./record_db -h
+```