Issue:
Bug: Deep health check for Hyperswitch Drainer



Components to be verified in this check

- [x] RDS (PostgreSQL)

  - Connection to Database

  - State of Migration

- [x] ElastiCache (Redis)

  - Connection to Redis

Pull request description:
## Type of Change
<!-- Put an `x` in the boxes that apply -->

- [ ] Bugfix
- [x] New feature
- [ ] Enhancement
- [ ] Refactoring
- [ ] Dependency updates
- [ ] Documentation
- [ ] CI/CD

## Description
<!-- Describe your changes in detail -->
This PR adds support for performing a deeper health check of the drainer which includes:

Database connection check with read, write and delete queries (Added support for database transaction too)
Redis connection check with set, get and delete key (Set key with expiry of 30sec)

The deep health check API returns a 200 status code if all the components are in good health, but returns 500 if at least one component's health is down

### Additional Changes

- [ ] This PR modifies the API contract
- [ ] This PR modifies the database schema
- [ ] This PR modifies application configuration/environment variables

<!--
Provide links to the files with corresponding changes.

Following are the paths where you can find config files:
1. `config`
2. `crates/router/src/configs`
3. `loadtest/config`
-->


## Motivation and Context
<!--
Why is this change required? What problem does it solve?
If it fixes an open issue, please link to the issue here.

If you don't have an issue, we'd recommend starting with one first so the PR
can focus on the implementation (unless it is an obvious bug or documentation fix
that will have little conversation).
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## How did you test it?
<!--
Did you write an integration/unit/API test to verify the code changes?
Or did you test this change manually (provide relevant screenshots)?
-->
1. `/health`

![image](https://github.com/juspay/hyperswitch/assets/70657455/d7784ea7-7d21-426f-ba65-69a0f559356e)

2. `/health/deep_check`

![image](https://github.com/juspay/hyperswitch/assets/70657455/aada988f-0058-48af-8654-1070bd810a29)

**THIS CANNOT BE TESTED ON SANBOX AS THE ENDPOINT IS NOT EXPOSED**

## Checklist
<!-- Put an `x` in the boxes that apply -->

- [x] I formatted the code `cargo +nightly fmt --all`
- [x] I addressed lints thrown by `cargo clippy`
- [x] I reviewed the submitted code
- [ ] I added unit tests for my changes where possible
- [ ] I added a [CHANGELOG](/CHANGELOG.md) entry if applicable


Testing:

1. `/health`

![image](https://github.com/juspay/hyperswitch/assets/70657455/d7784ea7-7d21-426f-ba65-69a0f559356e)

2. `/health/deep_check`

![image](https://github.com/juspay/hyperswitch/assets/70657455/aada988f-0058-48af-8654-1070bd810a29)

**THIS CANNOT BE TESTED ON SANBOX AS THE ENDPOINT IS NOT EXPOSED**

Patch:
diff --git a/Cargo.lock b/Cargo.lock
index f0334ce9cfc..49ccfc2c645 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -2331,6 +2331,7 @@ checksum = "1aaf95b3e5c8f23aa320147307562d361db0ae0d51242340f558153b4eb2439b"
 name = "drainer"
 version = "0.1.0"
 dependencies = [
+ "actix-web",
  "async-bb8-diesel",
  "async-trait",
  "bb8",
@@ -2342,8 +2343,10 @@ dependencies = [
  "error-stack",
  "external_services",
  "masking",
+ "mime",
  "once_cell",
  "redis_interface",
+ "reqwest",
  "router_env",
  "serde",
  "serde_json",
diff --git a/crates/api_models/src/health_check.rs b/crates/api_models/src/health_check.rs
index e4611f43bcf..a8f3db11e39 100644
--- a/crates/api_models/src/health_check.rs
+++ b/crates/api_models/src/health_check.rs
@@ -9,6 +9,7 @@ pub struct RouterHealthCheckResponse {
 }
 
 impl common_utils::events::ApiEventMetric for RouterHealthCheckResponse {}
+
 #[derive(Debug, Clone, serde::Serialize, serde::Deserialize)]
 pub struct SchedulerHealthCheckResponse {
     pub database: bool,
diff --git a/crates/drainer/Cargo.toml b/crates/drainer/Cargo.toml
index 67169a15104..0533bd12dab 100644
--- a/crates/drainer/Cargo.toml
+++ b/crates/drainer/Cargo.toml
@@ -14,13 +14,16 @@ hashicorp-vault = ["external_services/hashicorp-vault"]
 vergen = ["router_env/vergen"]
 
 [dependencies]
+actix-web = "4.3.1"
 async-bb8-diesel = { git = "https://github.com/jarnura/async-bb8-diesel", rev = "53b4ab901aab7635c8215fd1c2d542c8db443094" }
 bb8 = "0.8"
 clap = { version = "4.3.2", default-features = false, features = ["std", "derive", "help", "usage"] }
 config = { version = "0.13.3", features = ["toml"] }
 diesel = { version = "2.1.0", features = ["postgres"] }
 error-stack = "0.3.1"
+mime = "0.3.17"
 once_cell = "1.18.0"
+reqwest = { version = "0.11.18" }
 serde = "1.0.193"
 serde_json = "1.0.108"
 serde_path_to_error = "0.1.14"
diff --git a/crates/drainer/src/errors.rs b/crates/drainer/src/errors.rs
index 3034e849f8a..8605ee2ba04 100644
--- a/crates/drainer/src/errors.rs
+++ b/crates/drainer/src/errors.rs
@@ -15,6 +15,22 @@ pub enum DrainerError {
     ParsingError(error_stack::Report<common_utils::errors::ParsingError>),
     #[error("Unexpected error occurred: {0}")]
     UnexpectedError(String),
+    #[error("I/O: {0}")]
+    IoError(std::io::Error),
+}
+
+#[derive(Debug, Error, Clone, serde::Serialize)]
+pub enum HealthCheckError {
+    #[error("Database health check is failing with error: {message}")]
+    DbError { message: String },
+    #[error("Redis health check is failing with error: {message}")]
+    RedisError { message: String },
+}
+
+impl From<std::io::Error> for DrainerError {
+    fn from(err: std::io::Error) -> Self {
+        Self::IoError(err)
+    }
 }
 
 pub type DrainerResult<T> = error_stack::Result<T, DrainerError>;
@@ -30,3 +46,13 @@ impl From<error_stack::Report<redis::errors::RedisError>> for DrainerError {
         Self::RedisError(err)
     }
 }
+
+impl actix_web::ResponseError for HealthCheckError {
+    fn status_code(&self) -> reqwest::StatusCode {
+        use reqwest::StatusCode;
+
+        match self {
+            Self::DbError { .. } | Self::RedisError { .. } => StatusCode::INTERNAL_SERVER_ERROR,
+        }
+    }
+}
diff --git a/crates/drainer/src/health_check.rs b/crates/drainer/src/health_check.rs
new file mode 100644
index 00000000000..33b4a1395a8
--- /dev/null
+++ b/crates/drainer/src/health_check.rs
@@ -0,0 +1,268 @@
+use std::sync::Arc;
+
+use actix_web::{web, Scope};
+use async_bb8_diesel::{AsyncConnection, AsyncRunQueryDsl};
+use common_utils::errors::CustomResult;
+use diesel_models::{Config, ConfigNew};
+use error_stack::ResultExt;
+use router_env::{instrument, logger, tracing};
+
+use crate::{
+    connection::{pg_connection, redis_connection},
+    errors::HealthCheckError,
+    services::{self, Store},
+    settings::Settings,
+};
+
+pub const TEST_STREAM_NAME: &str = "TEST_STREAM_0";
+pub const TEST_STREAM_DATA: &[(&str, &str)] = &[("data", "sample_data")];
+
+pub struct Health;
+
+impl Health {
+    pub fn server(conf: Settings, store: Arc<Store>) -> Scope {
+        web::scope("health")
+            .app_data(web::Data::new(conf))
+            .app_data(web::Data::new(store))
+            .service(web::resource("").route(web::get().to(health)))
+            .service(web::resource("/ready").route(web::get().to(deep_health_check)))
+    }
+}
+
+#[instrument(skip_all)]
+pub async fn health() -> impl actix_web::Responder {
+    logger::info!("Drainer health was called");
+    actix_web::HttpResponse::Ok().body("Drainer health is good")
+}
+
+#[instrument(skip_all)]
+pub async fn deep_health_check(
+    conf: web::Data<Settings>,
+    store: web::Data<Arc<Store>>,
+) -> impl actix_web::Responder {
+    match deep_health_check_func(conf, store).await {
+        Ok(response) => services::http_response_json(
+            serde_json::to_string(&response)
+                .map_err(|err| {
+                    logger::error!(serialization_error=?err);
+                })
+                .unwrap_or_default(),
+        ),
+
+        Err(err) => services::log_and_return_error_response(err),
+    }
+}
+
+#[instrument(skip_all)]
+pub async fn deep_health_check_func(
+    conf: web::Data<Settings>,
+    store: web::Data<Arc<Store>>,
+) -> Result<DrainerHealthCheckResponse, error_stack::Report<HealthCheckError>> {
+    logger::info!("Deep health check was called");
+
+    logger::debug!("Database health check begin");
+
+    let db_status = store.health_check_db().await.map(|_| true).map_err(|err| {
+        error_stack::report!(HealthCheckError::DbError {
+            message: err.to_string()
+        })
+    })?;
+
+    logger::debug!("Database health check end");
+
+    logger::debug!("Redis health check begin");
+
+    let redis_status = store
+        .health_check_redis(&conf.into_inner())
+        .await
+        .map(|_| true)
+        .map_err(|err| {
+            error_stack::report!(HealthCheckError::RedisError {
+                message: err.to_string()
+            })
+        })?;
+
+    logger::debug!("Redis health check end");
+
+    Ok(DrainerHealthCheckResponse {
+        database: db_status,
+        redis: redis_status,
+    })
+}
+
+#[derive(Debug, Clone, serde::Serialize, serde::Deserialize)]
+pub struct DrainerHealthCheckResponse {
+    pub database: bool,
+    pub redis: bool,
+}
+
+#[async_trait::async_trait]
+pub trait HealthCheckInterface {
+    async fn health_check_db(&self) -> CustomResult<(), HealthCheckDBError>;
+    async fn health_check_redis(&self, conf: &Settings) -> CustomResult<(), HealthCheckRedisError>;
+}
+
+#[async_trait::async_trait]
+impl HealthCheckInterface for Store {
+    async fn health_check_db(&self) -> CustomResult<(), HealthCheckDBError> {
+        let conn = pg_connection(&self.master_pool).await;
+
+        conn
+            .transaction_async(|conn| {
+                Box::pin(async move {
+                    let query =
+                        diesel::select(diesel::dsl::sql::<diesel::sql_types::Integer>("1 + 1"));
+                    let _x: i32 = query.get_result_async(&conn).await.map_err(|err| {
+                        logger::error!(read_err=?err,"Error while reading element in the database");
+                        HealthCheckDBError::DbReadError
+                    })?;
+
+                    logger::debug!("Database read was successful");
+
+                    let config = ConfigNew {
+                        key: "test_key".to_string(),
+                        config: "test_value".to_string(),
+                    };
+
+                    config.insert(&conn).await.map_err(|err| {
+                        logger::error!(write_err=?err,"Error while writing to database");
+                        HealthCheckDBError::DbWriteError
+                    })?;
+
+                    logger::debug!("Database write was successful");
+
+                    Config::delete_by_key(&conn, "test_key").await.map_err(|err| {
+                        logger::error!(delete_err=?err,"Error while deleting element in the database");
+                        HealthCheckDBError::DbDeleteError
+                    })?;
+
+                    logger::debug!("Database delete was successful");
+
+                    Ok::<_, HealthCheckDBError>(())
+                })
+            })
+            .await?;
+
+        Ok(())
+    }
+
+    async fn health_check_redis(&self, conf: &Settings) -> CustomResult<(), HealthCheckRedisError> {
+        let redis_conn = redis_connection(conf).await;
+
+        redis_conn
+            .serialize_and_set_key_with_expiry("test_key", "test_value", 30)
+            .await
+            .change_context(HealthCheckRedisError::SetFailed)?;
+
+        logger::debug!("Redis set_key was successful");
+
+        redis_conn
+            .get_key("test_key")
+            .await
+            .change_context(HealthCheckRedisError::GetFailed)?;
+
+        logger::debug!("Redis get_key was successful");
+
+        redis_conn
+            .delete_key("test_key")
+            .await
+            .change_context(HealthCheckRedisError::DeleteFailed)?;
+
+        logger::debug!("Redis delete_key was successful");
+
+        redis_conn
+            .stream_append_entry(
+                TEST_STREAM_NAME,
+                &redis_interface::RedisEntryId::AutoGeneratedID,
+                TEST_STREAM_DATA.to_vec(),
+            )
+            .await
+            .change_context(HealthCheckRedisError::StreamAppendFailed)?;
+
+        logger::debug!("Stream append succeeded");
+
+        let output = self
+            .redis_conn
+            .stream_read_entries(TEST_STREAM_NAME, "0-0", Some(10))
+            .await
+            .change_context(HealthCheckRedisError::StreamReadFailed)?;
+        logger::debug!("Stream read succeeded");
+
+        let (_, id_to_trim) = output
+            .get(TEST_STREAM_NAME)
+            .and_then(|entries| {
+                entries
+                    .last()
+                    .map(|last_entry| (entries, last_entry.0.clone()))
+            })
+            .ok_or(error_stack::report!(
+                HealthCheckRedisError::StreamReadFailed
+            ))?;
+        logger::debug!("Stream parse succeeded");
+
+        redis_conn
+            .stream_trim_entries(
+                TEST_STREAM_NAME,
+                (
+                    redis_interface::StreamCapKind::MinID,
+                    redis_interface::StreamCapTrim::Exact,
+                    id_to_trim,
+                ),
+            )
+            .await
+            .change_context(HealthCheckRedisError::StreamTrimFailed)?;
+        logger::debug!("Stream trim succeeded");
+
+        Ok(())
+    }
+}
+
+#[allow(clippy::enum_variant_names)]
+#[derive(Debug, thiserror::Error)]
+pub enum HealthCheckDBError {
+    #[error("Error while connecting to database")]
+    DbError,
+    #[error("Error while writing to database")]
+    DbWriteError,
+    #[error("Error while reading element in the database")]
+    DbReadError,
+    #[error("Error while deleting element in the database")]
+    DbDeleteError,
+    #[error("Unpredictable error occurred")]
+    UnknownError,
+    #[error("Error in database transaction")]
+    TransactionError,
+}
+
+impl From<diesel::result::Error> for HealthCheckDBError {
+    fn from(error: diesel::result::Error) -> Self {
+        match error {
+            diesel::result::Error::DatabaseError(_, _) => Self::DbError,
+
+            diesel::result::Error::RollbackErrorOnCommit { .. }
+            | diesel::result::Error::RollbackTransaction
+            | diesel::result::Error::AlreadyInTransaction
+            | diesel::result::Error::NotInTransaction
+            | diesel::result::Error::BrokenTransactionManager => Self::TransactionError,
+
+            _ => Self::UnknownError,
+        }
+    }
+}
+
+#[allow(clippy::enum_variant_names)]
+#[derive(Debug, thiserror::Error)]
+pub enum HealthCheckRedisError {
+    #[error("Failed to set key value in Redis")]
+    SetFailed,
+    #[error("Failed to get key value in Redis")]
+    GetFailed,
+    #[error("Failed to delete key value in Redis")]
+    DeleteFailed,
+    #[error("Failed to append data to the stream in Redis")]
+    StreamAppendFailed,
+    #[error("Failed to read data from the stream in Redis")]
+    StreamReadFailed,
+    #[error("Failed to trim data from the stream in Redis")]
+    StreamTrimFailed,
+}
diff --git a/crates/drainer/src/lib.rs b/crates/drainer/src/lib.rs
index abb32c87796..909ae065e26 100644
--- a/crates/drainer/src/lib.rs
+++ b/crates/drainer/src/lib.rs
@@ -1,6 +1,7 @@
 mod connection;
 pub mod errors;
 mod handler;
+mod health_check;
 pub mod logger;
 pub(crate) mod metrics;
 mod query;
@@ -11,6 +12,7 @@ mod types;
 mod utils;
 use std::sync::Arc;
 
+use actix_web::dev::Server;
 use common_utils::signals::get_allowed_signals;
 use diesel_models::kv;
 use error_stack::{IntoReport, ResultExt};
@@ -18,7 +20,10 @@ use router_env::{instrument, tracing};
 use tokio::sync::mpsc;
 
 use crate::{
-    connection::pg_connection, services::Store, settings::DrainerSettings, types::StreamData,
+    connection::pg_connection,
+    services::Store,
+    settings::{DrainerSettings, Settings},
+    types::StreamData,
 };
 
 pub async fn start_drainer(store: Arc<Store>, conf: DrainerSettings) -> errors::DrainerResult<()> {
@@ -49,3 +54,18 @@ pub async fn start_drainer(store: Arc<Store>, conf: DrainerSettings) -> errors::
 
     Ok(())
 }
+
+pub async fn start_web_server(
+    conf: Settings,
+    store: Arc<Store>,
+) -> Result<Server, errors::DrainerError> {
+    let server = conf.server.clone();
+    let web_server = actix_web::HttpServer::new(move || {
+        actix_web::App::new().service(health_check::Health::server(conf.clone(), store.clone()))
+    })
+    .bind((server.host.as_str(), server.port))?
+    .run();
+    let _ = web_server.handle();
+
+    Ok(web_server)
+}
diff --git a/crates/drainer/src/main.rs b/crates/drainer/src/main.rs
index 34c1294d55f..943a66f6791 100644
--- a/crates/drainer/src/main.rs
+++ b/crates/drainer/src/main.rs
@@ -1,4 +1,7 @@
-use drainer::{errors::DrainerResult, logger::logger, services, settings, start_drainer};
+use drainer::{
+    errors::DrainerResult, logger::logger, services, settings, start_drainer, start_web_server,
+};
+use router_env::tracing::Instrument;
 
 #[tokio::main]
 async fn main() -> DrainerResult<()> {
@@ -24,6 +27,19 @@ async fn main() -> DrainerResult<()> {
         [router_env::service_name!()],
     );
 
+    #[allow(clippy::expect_used)]
+    let web_server = Box::pin(start_web_server(conf.clone(), store.clone()))
+        .await
+        .expect("Failed to create the server");
+
+    tokio::spawn(
+        async move {
+            let _ = web_server.await;
+            logger::error!("The health check probe stopped working!");
+        }
+        .in_current_span(),
+    );
+
     logger::debug!(startup_config=?conf);
     logger::info!("Drainer started [{:?}] [{:?}]", conf.drainer, conf.log);
 
diff --git a/crates/drainer/src/services.rs b/crates/drainer/src/services.rs
index 4393ebb9dc9..0d0b78c0cc6 100644
--- a/crates/drainer/src/services.rs
+++ b/crates/drainer/src/services.rs
@@ -1,6 +1,12 @@
 use std::sync::Arc;
 
-use crate::connection::{diesel_make_pg_pool, PgPool};
+use actix_web::{body, HttpResponse, ResponseError};
+use error_stack::Report;
+
+use crate::{
+    connection::{diesel_make_pg_pool, PgPool},
+    logger,
+};
 
 #[derive(Clone)]
 pub struct Store {
@@ -45,3 +51,23 @@ impl Store {
         }
     }
 }
+
+pub fn log_and_return_error_response<T>(error: Report<T>) -> HttpResponse
+where
+    T: error_stack::Context + ResponseError + Clone,
+{
+    logger::error!(?error);
+    let body = serde_json::json!({
+        "message": error.to_string()
+    })
+    .to_string();
+    HttpResponse::InternalServerError()
+        .content_type(mime::APPLICATION_JSON)
+        .body(body)
+}
+
+pub fn http_response_json<T: body::MessageBody + 'static>(response: T) -> HttpResponse {
+    HttpResponse::Ok()
+        .content_type(mime::APPLICATION_JSON)
+        .body(response)
+}
diff --git a/crates/drainer/src/settings.rs b/crates/drainer/src/settings.rs
index 5b80ee375f5..49cb5f4c7c2 100644
--- a/crates/drainer/src/settings.rs
+++ b/crates/drainer/src/settings.rs
@@ -30,6 +30,7 @@ pub struct CmdLineConf {
 #[derive(Debug, Deserialize, Clone, Default)]
 #[serde(default)]
 pub struct Settings {
+    pub server: Server,
     pub master_database: Database,
     pub redis: redis::RedisSettings,
     pub log: Log,
@@ -62,6 +63,24 @@ pub struct DrainerSettings {
     pub loop_interval: u32,     // in milliseconds
 }
 
+#[derive(Debug, Deserialize, Clone)]
+#[serde(default)]
+pub struct Server {
+    pub port: u16,
+    pub workers: usize,
+    pub host: String,
+}
+
+impl Server {
+    pub fn validate(&self) -> Result<(), errors::DrainerError> {
+        common_utils::fp_utils::when(self.host.is_default_or_empty(), || {
+            Err(errors::DrainerError::ConfigParsingError(
+                "server host must not be empty".into(),
+            ))
+        })
+    }
+}
+
 impl Default for Database {
     fn default() -> Self {
         Self {
@@ -88,6 +107,16 @@ impl Default for DrainerSettings {
     }
 }
 
+impl Default for Server {
+    fn default() -> Self {
+        Self {
+            host: "127.0.0.1".to_string(),
+            port: 8080,
+            workers: 1,
+        }
+    }
+}
+
 impl Database {
     fn validate(&self) -> Result<(), errors::DrainerError> {
         use common_utils::fp_utils::when;
@@ -169,6 +198,7 @@ impl Settings {
     }
 
     pub fn validate(&self) -> Result<(), errors::DrainerError> {
+        self.server.validate()?;
         self.master_database.validate()?;
         self.redis.validate().map_err(|error| {
             println!("{error}");