Decomposing hot-path modules into focused submodules

Signed-off-by: Alexey <247128645+axkurcom@users.noreply.github.com>
This commit is contained in:
Alexey
2026-05-21 13:28:40 +03:00
parent c02c7fbe43
commit 98c985091c
46 changed files with 9297 additions and 8488 deletions

View File

@@ -52,18 +52,15 @@
//! - `SharedCounters` (atomics) let the watchdog read stats without locking
use crate::error::{ProxyError, Result};
use crate::proxy::traffic_limiter::{RateDirection, TrafficLease, next_refill_delay};
use crate::stats::{Stats, UserStats};
use crate::proxy::traffic_limiter::TrafficLease;
use crate::stats::Stats;
use crate::stream::BufferPool;
use std::io;
use std::pin::Pin;
use std::sync::Arc;
use std::sync::atomic::{AtomicBool, AtomicU64, Ordering};
use std::task::{Context, Poll};
use std::sync::atomic::{AtomicBool, Ordering};
use std::time::Duration;
use tokio::io::{AsyncRead, AsyncWrite, AsyncWriteExt, ReadBuf, copy_bidirectional_with_sizes};
use tokio::time::{Instant, Sleep};
use tracing::{debug, trace, warn};
use tokio::io::{AsyncRead, AsyncWrite, AsyncWriteExt, copy_bidirectional_with_sizes};
use tokio::time::Instant;
use tracing::{debug, warn};
// ============= Constants =============
@@ -85,700 +82,11 @@ fn watchdog_delta(current: u64, previous: u64) -> u64 {
current.saturating_sub(previous)
}
// ============= CombinedStream =============
/// Combines separate read and write halves into a single bidirectional stream.
///
/// `copy_bidirectional` requires `AsyncRead + AsyncWrite` on each side,
/// but the handshake layer produces split reader/writer pairs
/// (e.g. `CryptoReader<FakeTlsReader<OwnedReadHalf>>` + `CryptoWriter<...>`).
///
/// This wrapper reunifies them with zero overhead — each trait method
/// delegates directly to the corresponding half. No buffering, no copies.
///
/// Safety: `poll_read` only touches `reader`, `poll_write` only touches `writer`,
/// so there's no aliasing even though both are called on the same `&mut self`.
struct CombinedStream<R, W> {
reader: R,
writer: W,
}
impl<R, W> CombinedStream<R, W> {
fn new(reader: R, writer: W) -> Self {
Self { reader, writer }
}
}
impl<R: AsyncRead + Unpin, W: Unpin> AsyncRead for CombinedStream<R, W> {
#[inline]
fn poll_read(
self: Pin<&mut Self>,
cx: &mut Context<'_>,
buf: &mut ReadBuf<'_>,
) -> Poll<io::Result<()>> {
Pin::new(&mut self.get_mut().reader).poll_read(cx, buf)
}
}
impl<R: Unpin, W: AsyncWrite + Unpin> AsyncWrite for CombinedStream<R, W> {
#[inline]
fn poll_write(
self: Pin<&mut Self>,
cx: &mut Context<'_>,
buf: &[u8],
) -> Poll<io::Result<usize>> {
Pin::new(&mut self.get_mut().writer).poll_write(cx, buf)
}
#[inline]
fn poll_flush(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<io::Result<()>> {
Pin::new(&mut self.get_mut().writer).poll_flush(cx)
}
#[inline]
fn poll_shutdown(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<io::Result<()>> {
Pin::new(&mut self.get_mut().writer).poll_shutdown(cx)
}
}
// ============= SharedCounters =============
/// Atomic counters shared between the relay (via StatsIo) and the watchdog task.
///
/// Using `Relaxed` ordering is sufficient because:
/// - Counters are monotonically increasing (no ABA problem)
/// - Slight staleness in watchdog reads is harmless (±10s check interval anyway)
/// - No ordering dependencies between different counters
struct SharedCounters {
/// Bytes read from client (C→S direction)
c2s_bytes: AtomicU64,
/// Bytes written to client (S→C direction)
s2c_bytes: AtomicU64,
/// Number of poll_read completions (≈ C→S chunks)
c2s_ops: AtomicU64,
/// Number of poll_write completions (≈ S→C chunks)
s2c_ops: AtomicU64,
/// Milliseconds since relay epoch of last I/O activity
last_activity_ms: AtomicU64,
}
impl SharedCounters {
fn new() -> Self {
Self {
c2s_bytes: AtomicU64::new(0),
s2c_bytes: AtomicU64::new(0),
c2s_ops: AtomicU64::new(0),
s2c_ops: AtomicU64::new(0),
last_activity_ms: AtomicU64::new(0),
}
}
/// Record activity at this instant.
#[inline]
fn touch(&self, now: Instant, epoch: Instant) {
let ms = now.duration_since(epoch).as_millis() as u64;
self.last_activity_ms.store(ms, Ordering::Relaxed);
}
/// How long since last recorded activity.
fn idle_duration(&self, now: Instant, epoch: Instant) -> Duration {
let last_ms = self.last_activity_ms.load(Ordering::Relaxed);
let now_ms = now.duration_since(epoch).as_millis() as u64;
Duration::from_millis(now_ms.saturating_sub(last_ms))
}
}
// ============= StatsIo =============
/// Transparent I/O wrapper that tracks per-user statistics and activity.
///
/// Wraps the **client** side of the relay. Direction mapping:
///
/// | poll method | direction | stats updated |
/// |-------------|-----------|--------------------------------------|
/// | `poll_read` | C→S | `octets_from`, `msgs_from`, counters |
/// | `poll_write` | S→C | `octets_to`, `msgs_to`, counters |
///
/// Both update the shared activity timestamp for the watchdog.
///
/// Note on message counts: the original code counted one `read()`/`write_all()`
/// as one "message". Here we count `poll_read`/`poll_write` completions instead.
/// Byte counts are identical; op counts may differ slightly due to different
/// internal buffering in `copy_bidirectional`. This is fine for monitoring.
struct StatsIo<S> {
inner: S,
counters: Arc<SharedCounters>,
stats: Arc<Stats>,
user: String,
user_stats: Arc<UserStats>,
traffic_lease: Option<Arc<TrafficLease>>,
c2s_rate_debt_bytes: u64,
c2s_wait: RateWaitState,
s2c_wait: RateWaitState,
quota_wait: RateWaitState,
quota_limit: Option<u64>,
quota_exceeded: Arc<AtomicBool>,
quota_bytes_since_check: u64,
epoch: Instant,
}
#[derive(Default)]
struct RateWaitState {
sleep: Option<Pin<Box<Sleep>>>,
started_at: Option<Instant>,
blocked_user: bool,
blocked_cidr: bool,
}
impl<S> StatsIo<S> {
#[cfg(test)]
fn new(
inner: S,
counters: Arc<SharedCounters>,
stats: Arc<Stats>,
user: String,
quota_limit: Option<u64>,
quota_exceeded: Arc<AtomicBool>,
epoch: Instant,
) -> Self {
Self::new_with_traffic_lease(
inner,
counters,
stats,
user,
None,
quota_limit,
quota_exceeded,
epoch,
)
}
fn new_with_traffic_lease(
inner: S,
counters: Arc<SharedCounters>,
stats: Arc<Stats>,
user: String,
traffic_lease: Option<Arc<TrafficLease>>,
quota_limit: Option<u64>,
quota_exceeded: Arc<AtomicBool>,
epoch: Instant,
) -> Self {
// Mark initial activity so the watchdog doesn't fire before data flows
counters.touch(Instant::now(), epoch);
let user_stats = stats.get_or_create_user_stats_handle(&user);
Self {
inner,
counters,
stats,
user,
user_stats,
traffic_lease,
c2s_rate_debt_bytes: 0,
c2s_wait: RateWaitState::default(),
s2c_wait: RateWaitState::default(),
quota_wait: RateWaitState::default(),
quota_limit,
quota_exceeded,
quota_bytes_since_check: 0,
epoch,
}
}
fn record_wait(
wait: &mut RateWaitState,
lease: Option<&Arc<TrafficLease>>,
direction: RateDirection,
) {
let Some(started_at) = wait.started_at.take() else {
return;
};
let wait_ms = started_at.elapsed().as_millis().min(u128::from(u64::MAX)) as u64;
if let Some(lease) = lease {
lease.observe_wait_ms(direction, wait.blocked_user, wait.blocked_cidr, wait_ms);
}
wait.blocked_user = false;
wait.blocked_cidr = false;
}
fn arm_wait(wait: &mut RateWaitState, blocked_user: bool, blocked_cidr: bool) {
if wait.sleep.is_none() {
wait.sleep = Some(Box::pin(tokio::time::sleep(next_refill_delay())));
wait.started_at = Some(Instant::now());
}
wait.blocked_user |= blocked_user;
wait.blocked_cidr |= blocked_cidr;
}
fn poll_wait(
wait: &mut RateWaitState,
cx: &mut Context<'_>,
lease: Option<&Arc<TrafficLease>>,
direction: RateDirection,
) -> Poll<()> {
let Some(sleep) = wait.sleep.as_mut() else {
return Poll::Ready(());
};
if sleep.as_mut().poll(cx).is_pending() {
return Poll::Pending;
}
wait.sleep = None;
Self::record_wait(wait, lease, direction);
Poll::Ready(())
}
fn settle_c2s_rate_debt(&mut self, cx: &mut Context<'_>) -> Poll<()> {
let Some(lease) = self.traffic_lease.as_ref() else {
self.c2s_rate_debt_bytes = 0;
return Poll::Ready(());
};
while self.c2s_rate_debt_bytes > 0 {
let consume = lease.try_consume(RateDirection::Up, self.c2s_rate_debt_bytes);
if consume.granted > 0 {
self.c2s_rate_debt_bytes = self.c2s_rate_debt_bytes.saturating_sub(consume.granted);
continue;
}
Self::arm_wait(
&mut self.c2s_wait,
consume.blocked_user,
consume.blocked_cidr,
);
if Self::poll_wait(&mut self.c2s_wait, cx, Some(lease), RateDirection::Up).is_pending()
{
return Poll::Pending;
}
}
if Self::poll_wait(&mut self.c2s_wait, cx, Some(lease), RateDirection::Up).is_pending() {
return Poll::Pending;
}
Poll::Ready(())
}
fn arm_quota_wait(&mut self, cx: &mut Context<'_>) -> Poll<()> {
Self::arm_wait(&mut self.quota_wait, false, false);
Self::poll_wait(&mut self.quota_wait, cx, None, RateDirection::Up)
}
}
#[derive(Debug)]
struct QuotaIoSentinel;
impl std::fmt::Display for QuotaIoSentinel {
fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
f.write_str("user data quota exceeded")
}
}
impl std::error::Error for QuotaIoSentinel {}
fn quota_io_error() -> io::Error {
io::Error::new(io::ErrorKind::PermissionDenied, QuotaIoSentinel)
}
fn is_quota_io_error(err: &io::Error) -> bool {
err.kind() == io::ErrorKind::PermissionDenied
&& err
.get_ref()
.and_then(|source| source.downcast_ref::<QuotaIoSentinel>())
.is_some()
}
const QUOTA_NEAR_LIMIT_BYTES: u64 = 64 * 1024;
const QUOTA_LARGE_CHARGE_BYTES: u64 = 16 * 1024;
const QUOTA_ADAPTIVE_INTERVAL_MIN_BYTES: u64 = 4 * 1024;
const QUOTA_ADAPTIVE_INTERVAL_MAX_BYTES: u64 = 64 * 1024;
const QUOTA_RESERVE_SPIN_RETRIES: usize = 64;
const QUOTA_RESERVE_MAX_ROUNDS: usize = 8;
#[inline]
fn quota_adaptive_interval_bytes(remaining_before: u64) -> u64 {
remaining_before.saturating_div(2).clamp(
QUOTA_ADAPTIVE_INTERVAL_MIN_BYTES,
QUOTA_ADAPTIVE_INTERVAL_MAX_BYTES,
)
}
#[inline]
fn should_immediate_quota_check(remaining_before: u64, charge_bytes: u64) -> bool {
remaining_before <= QUOTA_NEAR_LIMIT_BYTES || charge_bytes >= QUOTA_LARGE_CHARGE_BYTES
}
fn refund_reserved_quota_bytes(user_stats: &UserStats, reserved_bytes: u64) {
if reserved_bytes == 0 {
return;
}
let mut current = user_stats.quota_used.load(Ordering::Relaxed);
loop {
let next = current.saturating_sub(reserved_bytes);
match user_stats.quota_used.compare_exchange_weak(
current,
next,
Ordering::Relaxed,
Ordering::Relaxed,
) {
Ok(_) => return,
Err(observed) => current = observed,
}
}
}
impl<S: AsyncRead + Unpin> AsyncRead for StatsIo<S> {
fn poll_read(
self: Pin<&mut Self>,
cx: &mut Context<'_>,
buf: &mut ReadBuf<'_>,
) -> Poll<io::Result<()>> {
let this = self.get_mut();
if this.quota_exceeded.load(Ordering::Acquire) {
return Poll::Ready(Err(quota_io_error()));
}
if this.settle_c2s_rate_debt(cx).is_pending() {
return Poll::Pending;
}
if buf.remaining() == 0 {
return Pin::new(&mut this.inner).poll_read(cx, buf);
}
let mut remaining_before = None;
let mut reserved_read_bytes = 0u64;
let mut read_limit = buf.remaining();
if let Some(limit) = this.quota_limit {
let used_before = this.user_stats.quota_used();
let remaining = limit.saturating_sub(used_before);
if remaining == 0 {
this.quota_exceeded.store(true, Ordering::Release);
return Poll::Ready(Err(quota_io_error()));
}
remaining_before = Some(remaining);
read_limit = read_limit.min(remaining as usize);
if read_limit == 0 {
this.quota_exceeded.store(true, Ordering::Release);
return Poll::Ready(Err(quota_io_error()));
}
let desired = read_limit as u64;
let mut reserve_rounds = 0usize;
while reserved_read_bytes == 0 {
for _ in 0..QUOTA_RESERVE_SPIN_RETRIES {
match this.user_stats.quota_try_reserve(desired, limit) {
Ok(_) => {
reserved_read_bytes = desired;
break;
}
Err(crate::stats::QuotaReserveError::LimitExceeded) => {
this.quota_exceeded.store(true, Ordering::Release);
return Poll::Ready(Err(quota_io_error()));
}
Err(crate::stats::QuotaReserveError::Contended) => {
this.stats.increment_quota_contention_total();
}
}
}
if reserved_read_bytes == 0 {
reserve_rounds = reserve_rounds.saturating_add(1);
if reserve_rounds >= QUOTA_RESERVE_MAX_ROUNDS {
this.stats.increment_quota_contention_timeout_total();
if this.arm_quota_wait(cx).is_pending() {
return Poll::Pending;
}
reserve_rounds = 0;
}
}
}
}
let limited_read = read_limit < buf.remaining();
let read_result = if limited_read {
let mut limited_buf = ReadBuf::new(buf.initialize_unfilled_to(read_limit));
match Pin::new(&mut this.inner).poll_read(cx, &mut limited_buf) {
Poll::Ready(Ok(())) => {
let n = limited_buf.filled().len();
buf.advance(n);
Poll::Ready(Ok(n))
}
Poll::Ready(Err(err)) => Poll::Ready(Err(err)),
Poll::Pending => Poll::Pending,
}
} else {
let before = buf.filled().len();
match Pin::new(&mut this.inner).poll_read(cx, buf) {
Poll::Ready(Ok(())) => {
let n = buf.filled().len() - before;
Poll::Ready(Ok(n))
}
Poll::Ready(Err(err)) => Poll::Ready(Err(err)),
Poll::Pending => Poll::Pending,
}
};
match read_result {
Poll::Ready(Ok(n)) => {
if reserved_read_bytes > n as u64 {
let refund_bytes = reserved_read_bytes - n as u64;
refund_reserved_quota_bytes(this.user_stats.as_ref(), refund_bytes);
this.stats.add_quota_refund_bytes_total(refund_bytes);
}
if n > 0 {
let n_to_charge = n as u64;
if let Some(remaining) = remaining_before {
if should_immediate_quota_check(remaining, n_to_charge) {
this.quota_bytes_since_check = 0;
} else {
this.quota_bytes_since_check =
this.quota_bytes_since_check.saturating_add(n_to_charge);
let interval = quota_adaptive_interval_bytes(remaining);
if this.quota_bytes_since_check >= interval {
this.quota_bytes_since_check = 0;
}
}
}
if let Some(limit) = this.quota_limit
&& this.user_stats.quota_used() >= limit
{
this.quota_exceeded.store(true, Ordering::Release);
}
// C→S: client sent data
this.counters
.c2s_bytes
.fetch_add(n_to_charge, Ordering::Relaxed);
this.counters.c2s_ops.fetch_add(1, Ordering::Relaxed);
this.counters.touch(Instant::now(), this.epoch);
this.stats
.add_user_traffic_from_handle(this.user_stats.as_ref(), n_to_charge);
if this.traffic_lease.is_some() {
this.c2s_rate_debt_bytes =
this.c2s_rate_debt_bytes.saturating_add(n_to_charge);
let _ = this.settle_c2s_rate_debt(cx);
}
trace!(user = %this.user, bytes = n, "C->S");
}
Poll::Ready(Ok(()))
}
Poll::Pending => {
if reserved_read_bytes > 0 {
refund_reserved_quota_bytes(this.user_stats.as_ref(), reserved_read_bytes);
this.stats.add_quota_refund_bytes_total(reserved_read_bytes);
}
Poll::Pending
}
Poll::Ready(Err(err)) => {
if reserved_read_bytes > 0 {
refund_reserved_quota_bytes(this.user_stats.as_ref(), reserved_read_bytes);
this.stats.add_quota_refund_bytes_total(reserved_read_bytes);
}
Poll::Ready(Err(err))
}
}
}
}
impl<S: AsyncWrite + Unpin> AsyncWrite for StatsIo<S> {
fn poll_write(
self: Pin<&mut Self>,
cx: &mut Context<'_>,
buf: &[u8],
) -> Poll<io::Result<usize>> {
let this = self.get_mut();
if this.quota_exceeded.load(Ordering::Acquire) {
return Poll::Ready(Err(quota_io_error()));
}
let mut shaper_reserved_bytes = 0u64;
let mut write_buf = buf;
if let Some(lease) = this.traffic_lease.as_ref() {
if !buf.is_empty() {
loop {
let consume = lease.try_consume(RateDirection::Down, buf.len() as u64);
if consume.granted > 0 {
shaper_reserved_bytes = consume.granted;
if consume.granted < buf.len() as u64 {
write_buf = &buf[..consume.granted as usize];
}
let _ = Self::poll_wait(
&mut this.s2c_wait,
cx,
Some(lease),
RateDirection::Down,
);
break;
}
Self::arm_wait(
&mut this.s2c_wait,
consume.blocked_user,
consume.blocked_cidr,
);
if Self::poll_wait(&mut this.s2c_wait, cx, Some(lease), RateDirection::Down)
.is_pending()
{
return Poll::Pending;
}
}
} else {
let _ = Self::poll_wait(&mut this.s2c_wait, cx, Some(lease), RateDirection::Down);
}
}
let mut remaining_before = None;
let mut reserved_bytes = 0u64;
if let Some(limit) = this.quota_limit {
if !write_buf.is_empty() {
let mut reserve_rounds = 0usize;
while reserved_bytes == 0 {
let used_before = this.user_stats.quota_used();
let remaining = limit.saturating_sub(used_before);
if remaining == 0 {
if let Some(lease) = this.traffic_lease.as_ref() {
lease.refund(RateDirection::Down, shaper_reserved_bytes);
}
this.quota_exceeded.store(true, Ordering::Release);
return Poll::Ready(Err(quota_io_error()));
}
remaining_before = Some(remaining);
let desired = remaining.min(write_buf.len() as u64);
let mut saw_contention = false;
for _ in 0..QUOTA_RESERVE_SPIN_RETRIES {
match this.user_stats.quota_try_reserve(desired, limit) {
Ok(_) => {
reserved_bytes = desired;
write_buf = &write_buf[..desired as usize];
break;
}
Err(crate::stats::QuotaReserveError::LimitExceeded) => {
break;
}
Err(crate::stats::QuotaReserveError::Contended) => {
this.stats.increment_quota_contention_total();
saw_contention = true;
}
}
}
if reserved_bytes == 0 {
reserve_rounds = reserve_rounds.saturating_add(1);
if reserve_rounds >= QUOTA_RESERVE_MAX_ROUNDS {
this.stats.increment_quota_contention_timeout_total();
if let Some(lease) = this.traffic_lease.as_ref() {
lease.refund(RateDirection::Down, shaper_reserved_bytes);
}
let _ = this.arm_quota_wait(cx);
return Poll::Pending;
} else if saw_contention {
std::hint::spin_loop();
}
}
}
} else {
let used_before = this.user_stats.quota_used();
let remaining = limit.saturating_sub(used_before);
if remaining == 0 {
if let Some(lease) = this.traffic_lease.as_ref() {
lease.refund(RateDirection::Down, shaper_reserved_bytes);
}
this.quota_exceeded.store(true, Ordering::Release);
return Poll::Ready(Err(quota_io_error()));
}
remaining_before = Some(remaining);
}
}
match Pin::new(&mut this.inner).poll_write(cx, write_buf) {
Poll::Ready(Ok(n)) => {
if reserved_bytes > n as u64 {
let refund_bytes = reserved_bytes - n as u64;
refund_reserved_quota_bytes(this.user_stats.as_ref(), refund_bytes);
this.stats.add_quota_refund_bytes_total(refund_bytes);
}
if shaper_reserved_bytes > n as u64
&& let Some(lease) = this.traffic_lease.as_ref()
{
lease.refund(RateDirection::Down, shaper_reserved_bytes - n as u64);
}
if n > 0 {
if let Some(lease) = this.traffic_lease.as_ref() {
Self::record_wait(&mut this.s2c_wait, Some(lease), RateDirection::Down);
}
let n_to_charge = n as u64;
// S→C: data written to client
this.counters
.s2c_bytes
.fetch_add(n_to_charge, Ordering::Relaxed);
this.counters.s2c_ops.fetch_add(1, Ordering::Relaxed);
this.counters.touch(Instant::now(), this.epoch);
this.stats
.add_user_traffic_to_handle(this.user_stats.as_ref(), n_to_charge);
if let (Some(limit), Some(remaining)) = (this.quota_limit, remaining_before) {
if should_immediate_quota_check(remaining, n_to_charge) {
this.quota_bytes_since_check = 0;
if this.user_stats.quota_used() >= limit {
this.quota_exceeded.store(true, Ordering::Release);
}
} else {
this.quota_bytes_since_check =
this.quota_bytes_since_check.saturating_add(n_to_charge);
let interval = quota_adaptive_interval_bytes(remaining);
if this.quota_bytes_since_check >= interval {
this.quota_bytes_since_check = 0;
if this.user_stats.quota_used() >= limit {
this.quota_exceeded.store(true, Ordering::Release);
}
}
}
}
trace!(user = %this.user, bytes = n, "S->C");
}
Poll::Ready(Ok(n))
}
Poll::Ready(Err(err)) => {
if reserved_bytes > 0 {
refund_reserved_quota_bytes(this.user_stats.as_ref(), reserved_bytes);
this.stats.add_quota_refund_bytes_total(reserved_bytes);
}
if shaper_reserved_bytes > 0
&& let Some(lease) = this.traffic_lease.as_ref()
{
lease.refund(RateDirection::Down, shaper_reserved_bytes);
}
Poll::Ready(Err(err))
}
Poll::Pending => {
if reserved_bytes > 0 {
refund_reserved_quota_bytes(this.user_stats.as_ref(), reserved_bytes);
this.stats.add_quota_refund_bytes_total(reserved_bytes);
}
if shaper_reserved_bytes > 0
&& let Some(lease) = this.traffic_lease.as_ref()
{
lease.refund(RateDirection::Down, shaper_reserved_bytes);
}
Poll::Pending
}
}
}
#[inline]
fn poll_flush(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<io::Result<()>> {
Pin::new(&mut self.get_mut().inner).poll_flush(cx)
}
#[inline]
fn poll_shutdown(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<io::Result<()>> {
Pin::new(&mut self.get_mut().inner).poll_shutdown(cx)
}
}
mod io;
use self::io::{CombinedStream, SharedCounters, StatsIo, is_quota_io_error};
#[cfg(test)]
use self::io::{quota_adaptive_interval_bytes, should_immediate_quota_check};
// ============= Relay =============
/// Relay data bidirectionally between client and server.