Improve ZeroSlot client: add HTTP keep-alive ping and switch to Binary-Tx

- Add HTTP keep-alive ping task (30s interval) using free getHealth method
- Switch from JSON-RPC to faster Binary-Tx endpoint (/txb) for transaction submission
- Send raw binary transaction bytes directly to avoid encoding/decoding overhead
- Update response handling for Binary-Tx plain text status codes (200/403/419/500)
- Follow same keep-alive pattern as BlockRazor, Stellium, and Astralane clients

These changes reduce first-submit cold start latency and overall transaction submission time.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/swqos/zeroslot.rs

@@ -1,12 +1,12 @@
 use crate::swqos::common::{
-    default_http_client_builder, poll_transaction_confirmation, serialize_transaction_and_encode,
+    default_http_client_builder, poll_transaction_confirmation,
 };
 use rand::seq::IndexedRandom;
 use reqwest::Client;
-use serde_json::json;
-use std::{sync::Arc, time::Instant};
-
-use solana_transaction_status::UiTransactionEncoding;
+use std::{sync::Arc, time::Instant, time::Duration};
+use std::sync::atomic::{AtomicBool, Ordering};
+use tokio::task::JoinHandle;
+use bincode;
 
 use crate::swqos::SwqosClientTrait;
 use crate::swqos::{SwqosType, TradeType};
@@ -21,6 +21,8 @@ pub struct ZeroSlotClient {
     pub auth_token: String,
     pub rpc_client: Arc<SolanaRpcClient>,
     pub http_client: Client,
+    pub ping_handle: Arc<tokio::sync::Mutex<Option<JoinHandle<()>>>>,
+    pub stop_ping: Arc<AtomicBool>,
 }
 
 #[async_trait::async_trait]
@@ -60,7 +62,79 @@ impl ZeroSlotClient {
     pub fn new(rpc_url: String, endpoint: String, auth_token: String) -> Self {
         let rpc_client = SolanaRpcClient::new(rpc_url);
         let http_client = default_http_client_builder().build().unwrap();
-        Self { rpc_client: Arc::new(rpc_client), endpoint, auth_token, http_client }
+
+        let client = Self {
+            rpc_client: Arc::new(rpc_client),
+            endpoint,
+            auth_token,
+            http_client,
+            ping_handle: Arc::new(tokio::sync::Mutex::new(None)),
+            stop_ping: Arc::new(AtomicBool::new(false)),
+        };
+
+        // Start ping task
+        let client_clone = client.clone();
+        tokio::spawn(async move {
+            client_clone.start_ping_task().await;
+        });
+
+        client
+    }
+
+    /// Start periodic ping task to keep connections active
+    async fn start_ping_task(&self) {
+        let endpoint = self.endpoint.clone();
+        let auth_token = self.auth_token.clone();
+        let http_client = self.http_client.clone();
+        let stop_ping = self.stop_ping.clone();
+
+        let handle = tokio::spawn(async move {
+            // Immediate first ping to warm connection and reduce first-submit cold start latency
+            if let Err(e) = Self::send_ping_request(&http_client, &endpoint, &auth_token).await {
+                if crate::common::sdk_log::sdk_log_enabled() {
+                    eprintln!("0slot ping request failed: {}", e);
+                }
+            }
+            let mut interval = tokio::time::interval(Duration::from_secs(30)); // 30s keepalive under 65s server timeout
+            loop {
+                interval.tick().await;
+                if stop_ping.load(Ordering::Relaxed) {
+                    break;
+                }
+                if let Err(e) = Self::send_ping_request(&http_client, &endpoint, &auth_token).await {
+                    if crate::common::sdk_log::sdk_log_enabled() {
+                        eprintln!("0slot ping request failed: {}", e);
+                    }
+                }
+            }
+        });
+
+        // Update ping_handle - use Mutex to safely update
+        {
+            let mut ping_guard = self.ping_handle.lock().await;
+            if let Some(old_handle) = ping_guard.as_ref() {
+                old_handle.abort();
+            }
+            *ping_guard = Some(handle);
+        }
+    }
+
+    /// Send ping request: POST with getHealth method (Keep Alive). Free operation, not counted toward TPS.
+    async fn send_ping_request(
+        http_client: &Client,
+        endpoint: &str,
+        auth_token: &str,
+    ) -> Result<()> {
+        let url = format!("{}/?api-key={}", endpoint, auth_token);
+        let response = http_client
+            .post(&url)
+            .header("Content-Type", "application/json")
+            .timeout(Duration::from_millis(1500))
+            .body(r#"{"jsonrpc":"2.0","id":1,"method":"getHealth"}"#)
+            .send()
+            .await?;
+        let _ = response.bytes().await;
+        Ok(())
     }
 
     pub async fn send_transaction(
@@ -70,47 +144,65 @@ impl ZeroSlotClient {
         wait_confirmation: bool,
     ) -> Result<()> {
         let start_time = Instant::now();
-        let (content, signature) =
-            serialize_transaction_and_encode(transaction, UiTransactionEncoding::Base64)?;
 
-        let request_body = serde_json::to_string(&json!({
-            "jsonrpc": "2.0",
-            "id": 1,
-            "method": "sendTransaction",
-            "params": [
-                content,
-                { "encoding": "base64", "skipPreflight": true }
-            ]
-        }))?;
+        // Binary-Tx: Send raw binary transaction bytes directly
+        // This is faster than JSON-RPC as it avoids unnecessary encoding/decoding
+        let tx_bytes = bincode::serialize(transaction)?;
 
+        // Build URL for Binary-Tx endpoint: {endpoint}/txb?api-key={auth_token}
         let mut url = String::with_capacity(self.endpoint.len() + self.auth_token.len() + 20);
         url.push_str(&self.endpoint);
-        url.push_str("/?api-key=");
+        url.push_str("/txb?api-key=");
         url.push_str(&self.auth_token);
 
-        // 4. Use `text().await?` directly, avoiding async JSON parsing from `json().await?`
-        let response_text = self
+        // Send binary transaction directly
+        let response = self
             .http_client
             .post(&url)
-            .body(request_body) // Pass string directly, avoiding `json()` overhead
-            .header("Content-Type", "application/json") // Explicitly specify JSON header
+            .header("User-Agent", "") // Optional: 0slot recommends empty User-Agent
+            .body(tx_bytes)
             .send()
-            .await?
-            .text()
             .await?;
 
-        // 5. Use `serde_json::from_str()` to parse JSON, reducing extra wait from `.json().await?`
-        if let Ok(response_json) = serde_json::from_str::<serde_json::Value>(&response_text) {
-            if response_json.get("result").is_some() {
-                crate::common::sdk_log::log_swqos_submitted("0slot", trade_type, start_time.elapsed());
-            } else if let Some(_error) = response_json.get("error") {
-                eprintln!(" [0slot] {} submission failed after {:?}: {:?}", trade_type, start_time.elapsed(), _error);
+        let status = response.status();
+        let response_text = response.text().await?;
+
+        // Binary-Tx returns plain text responses with specific status codes
+        // 200: ok - transaction submitted successfully
+        // 403: api-key error (null, doesn't exist, or expired)
+        // 419: rate limit exceeded
+        // 500: submission failed
+        match status.as_u16() {
+            200 => {
+                if response_text.trim() == "ok" {
+                    crate::common::sdk_log::log_swqos_submitted("0slot", trade_type, start_time.elapsed());
+                } else {
+                    crate::common::sdk_log::log_swqos_submission_failed("0slot", trade_type, start_time.elapsed(), format!("unexpected response: {}", response_text));
+                    return Err(anyhow::anyhow!("0slot Binary-Tx unexpected response: {}", response_text));
+                }
+            }
+            403 => {
+                crate::common::sdk_log::log_swqos_submission_failed("0slot", trade_type, start_time.elapsed(), response_text.clone());
+                return Err(anyhow::anyhow!("0slot API key error: {}", response_text));
+            }
+            419 => {
+                crate::common::sdk_log::log_swqos_submission_failed("0slot", trade_type, start_time.elapsed(), response_text.clone());
+                return Err(anyhow::anyhow!("0slot rate limit exceeded"));
+            }
+            500 => {
+                crate::common::sdk_log::log_swqos_submission_failed("0slot", trade_type, start_time.elapsed(), "submission failed".to_string());
+                return Err(anyhow::anyhow!("0slot transaction submission failed"));
+            }
+            _ => {
+                crate::common::sdk_log::log_swqos_submission_failed("0slot", trade_type, start_time.elapsed(), format!("status {} body: {}", status, response_text));
+                return Err(anyhow::anyhow!("0slot Binary-Tx failed with status {}: {}", status, response_text));
             }
-        } else {
-            crate::common::sdk_log::log_swqos_submission_failed("0slot", trade_type, start_time.elapsed(), response_text);
         }
 
-        let start_time: Instant = Instant::now();
+        // Get transaction signature from the transaction for confirmation polling
+        let signature = transaction.signatures[0];
+
+        let start_time = Instant::now();
         match poll_transaction_confirmation(&self.rpc_client, signature, wait_confirmation).await {
             Ok(_) => (),
             Err(e) => {
@@ -139,3 +231,21 @@ impl ZeroSlotClient {
         Ok(())
     }
 }
+
+impl Drop for ZeroSlotClient {
+    fn drop(&mut self) {
+        // Ensure ping task stops when client is destroyed
+        self.stop_ping.store(true, Ordering::Relaxed);
+
+        // Try to stop ping task immediately
+        // Use tokio::spawn to avoid blocking Drop
+        let ping_handle = self.ping_handle.clone();
+        tokio::spawn(async move {
+            let mut ping_guard = ping_handle.lock().await;
+            if let Some(handle) = ping_guard.as_ref() {
+                handle.abort();
+            }
+            *ping_guard = None;
+        });
+    }
+}