Calculate createdLocallyAt using server clock offset #6199
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| /* | ||
| * Copyright (c) 2014-2026 Stream.io Inc. All rights reserved. | ||
| * | ||
| * Licensed under the Stream License; | ||
| * you may not use this file except in compliance with the License. | ||
| * You may obtain a copy of the License at | ||
| * | ||
| * https://github.com/GetStream/stream-chat-android/blob/main/LICENSE | ||
| * | ||
| * Unless required by applicable law or agreed to in writing, software | ||
| * distributed under the License is distributed on an "AS IS" BASIS, | ||
| * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. | ||
| * See the License for the specific language governing permissions and | ||
| * limitations under the License. | ||
| */ | ||
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| package io.getstream.chat.android.client.utils.internal | ||
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| import io.getstream.chat.android.client.events.ConnectedEvent | ||
| import io.getstream.chat.android.client.events.HealthEvent | ||
| import java.util.Date | ||
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| /** | ||
| * Tracks the offset between the local device clock and the server clock using | ||
| * NTP-style estimation from WebSocket health check round-trips. | ||
| * | ||
| * The algorithm keeps only the sample with the lowest observed RTT, since a | ||
| * smaller round-trip means less room for network asymmetry to distort the | ||
| * measurement. Under the assumption that clock skew is constant for the | ||
| * duration of a session, the estimate monotonically improves over time. | ||
| * | ||
| * Thread-safe: single-field writes use [Volatile] for visibility; compound | ||
| * read-modify-write sequences are guarded by [lock] for atomicity. | ||
| * | ||
| * @param localTimeMs Clock source for the local device time (injectable for tests). | ||
| * @param maxRttMs Upper bound on plausible RTT. Samples exceeding this are | ||
| * discarded as stale or mismatched. Defaults to the health check cycle | ||
| * interval (MONITOR_INTERVAL + HEALTH_CHECK_INTERVAL = 11 000 ms). | ||
| */ | ||
| internal class ServerClockOffset( | ||
| private val localTimeMs: () -> Long = { System.currentTimeMillis() }, | ||
| private val maxRttMs: Long = DEFAULT_MAX_RTT_MS, | ||
| ) { | ||
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| private val lock = Any() | ||
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| @Volatile | ||
| private var offsetMs: Long = 0L | ||
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| @Volatile | ||
| private var bestRttMs: Long = Long.MAX_VALUE | ||
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| @Volatile | ||
| private var healthCheckSentAtMs: Long = 0L | ||
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| @Volatile | ||
| private var connectionStartedAtMs: Long = 0L | ||
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| /** | ||
| * Record the local time immediately before starting a WebSocket connection. | ||
| * When the next [ConnectedEvent] arrives, [onConnected] will pair with this | ||
| * timestamp to compute the offset using the NTP midpoint formula. | ||
| */ | ||
| internal fun onConnectionStarted() { | ||
| connectionStartedAtMs = localTimeMs() | ||
| } | ||
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| /** | ||
| * Record the local time immediately before sending a health check echo. | ||
| * The next [onHealthCheck] call will pair with this timestamp to compute RTT. | ||
| */ | ||
| internal fun onHealthCheckSent() { | ||
| healthCheckSentAtMs = localTimeMs() | ||
| } | ||
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| /** | ||
| * Calibration from a [ConnectedEvent]. | ||
| * | ||
| * If [onConnectionStarted] was called before this connection (e.g. right before | ||
| * opening the WebSocket), uses the NTP midpoint of (connectionStartedAt, receivedAt) | ||
| * and serverTime for a more accurate offset. Otherwise falls back to a naive | ||
| * `localTime - serverTime` estimate. | ||
| * | ||
| * Resets health check state, since a new connection means any in-flight health | ||
| * check from the previous connection is stale. | ||
| */ | ||
| internal fun onConnected(serverTime: Date) { | ||
| synchronized(lock) { | ||
| bestRttMs = Long.MAX_VALUE | ||
| healthCheckSentAtMs = 0L | ||
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| val receivedAtMs = localTimeMs() | ||
| val startedAtMs = connectionStartedAtMs | ||
| connectionStartedAtMs = 0L | ||
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| if (startedAtMs > 0L) { | ||
| val rtt = receivedAtMs - startedAtMs | ||
| if (rtt in 1..maxRttMs) { | ||
| offsetMs = (startedAtMs + receivedAtMs) / 2 - serverTime.time | ||
| bestRttMs = rtt | ||
| return | ||
| } | ||
| } | ||
| offsetMs = receivedAtMs - serverTime.time | ||
| } | ||
| } | ||
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| /** | ||
| * Refine the offset using a [HealthEvent] paired with [onHealthCheckSent]. | ||
| * | ||
| * Computes RTT from the stored send time and the current receive time, | ||
| * then applies the NTP midpoint formula: | ||
| * ``` | ||
| * offset = (sentAt + receivedAt) / 2 - serverTime | ||
| * ``` | ||
| * | ||
| * The sample is accepted only if: | ||
| * - There is a pending [onHealthCheckSent] timestamp. | ||
| * - RTT is positive (guards against clock anomalies). | ||
| * - RTT is below [maxRttMs] (rejects stale / mismatched pairs). | ||
| * - RTT is lower than any previous sample (min-RTT selection). | ||
| */ | ||
| internal fun onHealthCheck(serverTime: Date) { | ||
| synchronized(lock) { | ||
| val sentAtMs = healthCheckSentAtMs | ||
| if (sentAtMs <= 0L) return | ||
| healthCheckSentAtMs = 0L | ||
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| val receivedAtMs = localTimeMs() | ||
| val rtt = receivedAtMs - sentAtMs | ||
| if (rtt !in 1..maxRttMs) return | ||
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| if (rtt < bestRttMs) { | ||
| bestRttMs = rtt | ||
| offsetMs = (sentAtMs + receivedAtMs) / 2 - serverTime.time | ||
| } | ||
| } | ||
| } | ||
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| /** | ||
| * Returns the current time adjusted to the server timescale. | ||
| * | ||
| * Before the first [onConnected] call, this returns the raw local time | ||
| * (offset = 0). | ||
| */ | ||
| internal fun estimatedServerTime(): Date = | ||
| Date(localTimeMs() - offsetMs) | ||
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| internal companion object { | ||
| internal const val DEFAULT_MAX_RTT_MS = 11_000L | ||
| } | ||
| } | ||
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