feat(backup_request): add rate-limited backup request policy (#3228) by feng-y · Pull Request #3229 · apache/brpc

feng-y · 2026-02-25T07:46:22Z

Summary

Add ratio-based rate limiting for backup requests to prevent backup request storms under high QPS or downstream latency spikes.

Design

BackupRequestPolicy interface unchanged (ABI stable)
BackupRateLimiter: standalone statistics module tracking backup/total ratio within a sliding time window using bvar::Window<bvar::Adder> + relaxed atomics for lock-free best-effort throttling
RateLimitedBackupPolicy: internal implementation composing BackupRateLimiter, hidden in .cpp
RateLimitedBackupPolicyOptions: public config struct with sensible defaults
CreateRateLimitedBackupPolicy(opts): public factory function (validates inputs, returns NULL on error)
Ownership: caller owns the returned pointer; use unique_ptr or set on ChannelOptions.backup_request_policy

Configuration (`RateLimitedBackupPolicyOptions`)

Field	Default	Description
`backup_request_ms`	-1	Timeout in ms; -1 inherits from `ChannelOptions.backup_request_ms` (only when set via `ChannelOptions`). Must be >= -1.
`max_backup_ratio`	0.1	Max ratio of backup decisions to total RPC legs; range (0, 1]. 1.0 = no effective limit.
`window_size_seconds`	10	Sliding window width; range [1, 3600]
`update_interval_seconds`	5	Cached-ratio refresh interval; must be >= 1

Edge cases handled

Cold start (no traffic yet): ratio defaults to 0.0 → backups allowed freely
Latency spike (backups issued but no completions yet): conservative ratio=1.0 → suppresses further backups until an RPC leg completes
max_backup_ratio > 1.0: rejected by factory (returns NULL with LOG(ERROR))
Factory returns NULL on invalid params; caller must check

Rate-limiting semantics

The denominator counts all completed RPC legs (original + backup), not user-visible RPCs. For max_backup_ratio=R, the effective fraction of user RPCs receiving a backup is approximately R/(1-R) (e.g. ~11% for R=0.1). The cached ratio refreshes at most once per update_interval_seconds, so short bursts within that window may transiently exceed the limit.

Closes #3228

Usage

brpc::RateLimitedBackupPolicyOptions opts;
opts.backup_request_ms = 100;    // or -1 to inherit from ChannelOptions
opts.max_backup_ratio  = 0.1;    // suppress backups when >10% of legs are backups

std::unique_ptr<brpc::BackupRequestPolicy> policy(
    brpc::CreateRateLimitedBackupPolicy(opts));

brpc::ChannelOptions options;
options.backup_request_ms     = 100;
options.backup_request_policy = policy.get();  // channel does NOT take ownership
channel.Init("...", &options);
// policy must outlive channel

Test plan

7 invalid-parameter tests (backup_request_ms, max_backup_ratio, window_size, update_interval)
ValidMinusOneBackupRequestMsInherits: factory succeeds, GetBackupRequestMs returns -1
ValidMaxRatioAtBoundary: max_backup_ratio=1.0 accepted, cold-start allows first backup
ColdStartAllowsBackup: first DoBackup() always returns true
AfterColdStartBackupSuppressedUntilRpcCompletes: after 1.2s interval, DoBackup() returns false (conservative ratio=1.0)

feng-y · 2026-02-25T07:52:20Z

@copilot review this PR

Copilot

Pull request overview

Adds a rate-limiting mechanism for backup requests, aiming to prevent “backup request storms” by suppressing backups when the backup/total ratio exceeds a configurable threshold over a sliding time window.

Changes:

Introduces ChannelOptions.backup_request_max_ratio and creates an internal rate-limited BackupRequestPolicy when enabled.
Adds global gflags for ratio threshold, window size, and ratio cache update interval, backed by bvar::Window + cumulative counters for cold start.
Adds a unit test covering policy creation/precedence plus a timing-based functional check.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 6 comments.

Show a summary per file

File	Description
test/brpc_channel_unittest.cpp	Adds coverage for rate-limited backup request behavior and configuration precedence.
src/brpc/channel.h	Adds per-channel configuration and internal ownership for an auto-created backup policy.
src/brpc/channel.cpp	Creates and wires an internal rate-limited backup policy during channel initialization.
src/brpc/backup_request_policy.h	Refactors `BackupRequestPolicy` into a concrete base with optional rate limiting support.
src/brpc/backup_request_policy.cpp	Implements rate limiter, gflags, and default policy behavior.

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src/brpc/backup_request_policy.cpp

src/brpc/backup_request_policy.h

test/brpc_channel_unittest.cpp

src/brpc/channel.cpp

Copilot

Pull request overview

Copilot reviewed 5 out of 5 changed files in this pull request and generated 1 comment.

Comments suppressed due to low confidence (1)

src/brpc/backup_request_policy.cpp:120

When compare_exchange_strong fails on line 105, the code continues to use the potentially stale ratio value loaded on line 102, even though another thread may have updated _cached_ratio on line 116. While the comment acknowledges this is "best-effort rate limiting," consider reloading _cached_ratio after a failed compare_exchange to reduce the window of staleness, especially in high-concurrency scenarios where the stale value could lead to temporarily incorrect rate limiting decisions.

    bool DoBackup(const Controller* /*controller*/) const override {
        const int64_t now_us = butil::cpuwide_time_us();
        int64_t last_us = _last_update_us.load(butil::memory_order_relaxed);
        double ratio = _cached_ratio.load(butil::memory_order_relaxed);

        if (now_us - last_us >= _update_interval_us) {
            if (_last_update_us.compare_exchange_strong(
                    last_us, now_us, butil::memory_order_relaxed)) {
                int64_t total = _total_window.get_value();
                int64_t backup = _backup_window.get_value();
                // Fall back to cumulative counts when the window has no
                // sampled data yet (cold-start within the first few seconds).
                if (total <= 0) {
                    total = _total_count.get_value();
                    backup = _backup_count.get_value();
                }
                ratio = (total > 0) ? static_cast<double>(backup) / total : 0.0;
                _cached_ratio.store(ratio, butil::memory_order_relaxed);
            }
        }

        return ratio < _max_backup_ratio;

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src/brpc/backup_request_policy.cpp

Copilot

Pull request overview

Copilot reviewed 5 out of 5 changed files in this pull request and generated 4 comments.

Comments suppressed due to low confidence (1)

test/brpc_channel_unittest.cpp:2856

The comment says “ratio refreshes on every DoBackup call” but with FLAGS_backup_request_ratio_update_interval_s = 1 the ratio cache can only refresh at most once per second. This makes the test rationale misleading when debugging failures; please adjust the comment to match the actual behavior.

    // Test 5: Functional — rate limiting reduces backup requests.
    // Use short update interval so ratio refreshes on every DoBackup call.
    // Save/restore gflags outside the inner scope so cleanup always runs.

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test/brpc_channel_unittest.cpp

src/brpc/channel.h

src/brpc/backup_request_policy.cpp

Copilot

Pull request overview

Copilot reviewed 5 out of 5 changed files in this pull request and generated no new comments.

Comments suppressed due to low confidence (4)

src/brpc/channel.cpp:284

Consider checking if CreateRateLimitedBackupPolicy returns NULL before setting _owns_backup_policy to true. While the current guards (line 278 checks max_ratio > 0, validators ensure gflags are valid) should prevent NULL returns in practice, defensive programming suggests verifying the return value and only setting _owns_backup_policy if the policy was successfully created. This would make the code more robust against future changes to CreateRateLimitedBackupPolicy or the validators.

        _options.backup_request_policy = CreateRateLimitedBackupPolicy(
            _options.backup_request_ms, max_ratio,
            FLAGS_backup_request_ratio_window_size_s,
            FLAGS_backup_request_ratio_update_interval_s);
        _owns_backup_policy = true;

test/brpc_channel_unittest.cpp:2868

The gflag FLAGS_backup_request_max_ratio is saved and restored but never actually modified in this test. The test uses opt.backup_request_max_ratio = 0.3 (per-channel config) instead of the global gflag. The save/restore of saved_ratio at lines 2862 and 2868 is unnecessary and could be removed for clarity.

        const double saved_ratio = brpc::FLAGS_backup_request_max_ratio;
        brpc::FLAGS_backup_request_ratio_update_interval_s = 1;
        brpc::FLAGS_backup_request_ratio_window_size_s = 2;
        BRPC_SCOPE_EXIT {
            brpc::FLAGS_backup_request_ratio_update_interval_s = saved_interval;
            brpc::FLAGS_backup_request_ratio_window_size_s = saved_window;
            brpc::FLAGS_backup_request_max_ratio = saved_ratio;

src/brpc/backup_request_policy.h:48

The documentation states "The caller owns the returned pointer" but this is inconsistent with actual usage in channel.cpp where the Channel owns the policy when created via backup_request_max_ratio. Consider clarifying: "The caller owns the returned pointer and must delete it when done. However, when Channel creates this policy internally via backup_request_max_ratio, Channel manages the lifetime automatically."

// The caller owns the returned pointer.

test/brpc_channel_unittest.cpp:2853

Missing test coverage for backup_request_max_ratio = 0 (explicitly disabled). While -1 means "use gflag default", 0 should explicitly disable rate limiting even if the gflag is set to enable it. Consider adding a test case that sets the gflag to a positive value, then creates a channel with opt.backup_request_max_ratio = 0, and verifies that no policy is created.

    // Test 4: No policy when backup_request_ms < 0 (backup disabled)
    {
        brpc::ChannelOptions opt;
        opt.backup_request_ms = -1;
        opt.backup_request_max_ratio = 0.3;
        brpc::Channel channel;
        ASSERT_EQ(0, channel.Init(_ep, &opt));
        ASSERT_TRUE(channel.options().backup_request_policy == NULL);
    }

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Copilot reviewed 5 out of 5 changed files in this pull request and generated 3 comments.

Comments suppressed due to low confidence (3)

test/brpc_channel_unittest.cpp:2831

Missing test coverage: The test doesn't cover the edge case where backup_request_max_ratio = 0. According to the PR description, 0 should explicitly disable rate limiting. A test case similar to Test 2 but with max_ratio = 0 instead of -1 would verify this behavior and ensure both forms of disabling work correctly.

    // Test 2: No policy when ratio is disabled (-1)
    {
        brpc::ChannelOptions opt;
        opt.backup_request_ms = 10;
        opt.backup_request_max_ratio = -1;
        brpc::Channel channel;
        ASSERT_EQ(0, channel.Init(_ep, &opt));
        ASSERT_TRUE(channel.options().backup_request_policy == NULL);
    }

test/brpc_channel_unittest.cpp:2831

Missing test coverage: The test doesn't cover the special case where backup_request_max_ratio = 1.0. According to the implementation (backup_request_policy.cpp:106-107), values >= 1.0 are treated as "no limit" since the ratio cannot exceed 1.0. A test should verify that setting max_ratio to 1.0 allows all backup requests, similar to having no rate limiting.

    // Test 2: No policy when ratio is disabled (-1)
    {
        brpc::ChannelOptions opt;
        opt.backup_request_ms = 10;
        opt.backup_request_max_ratio = -1;
        brpc::Channel channel;
        ASSERT_EQ(0, channel.Init(_ep, &opt));
        ASSERT_TRUE(channel.options().backup_request_policy == NULL);
    }

test/brpc_channel_unittest.cpp:2831

Missing test coverage: The test doesn't cover the case where backup_request_max_ratio > 1.0. According to channel.cpp:273-276, values > 1.0 should be clamped to 1.0 with a warning. A test should verify this clamping behavior works correctly and that the policy is still created with the clamped value.

    // Test 2: No policy when ratio is disabled (-1)
    {
        brpc::ChannelOptions opt;
        opt.backup_request_ms = 10;
        opt.backup_request_max_ratio = -1;
        brpc::Channel channel;
        ASSERT_EQ(0, channel.Init(_ep, &opt));
        ASSERT_TRUE(channel.options().backup_request_policy == NULL);
    }

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src/brpc/backup_request_policy.cpp

src/brpc/channel.cpp

src/brpc/channel.h

Copilot

Pull request overview

Copilot reviewed 5 out of 5 changed files in this pull request and generated 6 comments.

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src/brpc/backup_request_policy.cpp

src/brpc/channel.cpp

test/brpc_channel_unittest.cpp

feng-y · 2026-02-28T08:45:57Z

@wwbmmm Pipeline failed: some packages failed to download.

wwbmmm · 2026-03-01T04:28:16Z

@wwbmmm Pipeline failed: some packages failed to download.

Please merge master branch which fixed CI failure.

src/brpc/backup_request_policy.h

src/brpc/backup_request_policy.cpp

docs/cn/backup_request.md

…3228)

…e guidance - Promote built-in factory function to its own subsection (before custom interface) - Add unique_ptr usage example for policy lifetime management - Add RateLimitedBackupPolicyOptions parameter table with defaults/constraints - Document NULL return on invalid params - Keep cn/en docs in sync

Copilot

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Copilot reviewed 6 out of 6 changed files in this pull request and generated 4 comments.

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src/brpc/backup_request_policy.cpp

src/brpc/backup_request_policy.h

src/brpc/backup_request_policy.cpp

…nts, tests - controller.cpp: When policy returns -1 (inherit sentinel), fall back to _backup_request_ms set from ChannelOptions, so backup timer is actually armed when using a policy with backup_request_ms=-1. - backup_request_policy.cpp: Clarify OnRPCEnd comment to say 'RPC legs' (both original and backup completions counted as denominator). - backup_request_policy.cpp: Warn when update_interval_seconds exceeds window_size_seconds (window would rarely refresh within its period). - backup_request_policy.h: Fix comment typo ('Called when an RPC ends'). - brpc_channel_unittest.cpp: Replace nullptr with NULL to match codebase convention; use ASSERT_TRUE(p != NULL) for unique_ptr null checks. - brpc_channel_unittest.cpp: Add ValidMaxRatioAtBoundary behavioral assert and AfterColdStartBackupSuppressedUntilRpcCompletes test.

…test - docs: fix backup_request_ms default (0→-1) and constraint (>=0→>=-1); add note that -1 inherit only works via ChannelOptions injection path, not Controller::set_backup_request_policy(). - test: replace no-op AfterColdStart test with a real behavioral assertion: after cold-start backup fires, wait 1.2s for ratio refresh, verify DoBackup() returns false (conservative ratio=1.0 path triggers).

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Copilot reviewed 8 out of 8 changed files in this pull request and generated 5 comments.

Comments suppressed due to low confidence (1)

test/brpc_channel_unittest.cpp:3151

The explanation for the cold-start behavior here doesn’t match the current limiter logic: on true cold start (total==0 and backup==0) the computed ratio is 0.0 (allow), and only after backups are counted and the update interval elapses can the conservative ratio=1.0 path kick in (total==0, backup>0). Updating these comments to match the actual transitions would make the test easier to understand/maintain.

    // With max_backup_ratio=1.0, backup is allowed until backup_count/total
    // reaches 1.0. At cold start (total=0) the conservative path applies:
    // ratio is treated as 1.0, so DoBackup() returns false only after the
    // first backup is counted.
    ASSERT_TRUE(p->DoBackup(NULL));   // cold start: no total yet, allow first

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docs/en/backup_request.md

docs/cn/backup_request.md

src/brpc/controller.cpp

src/brpc/backup_request_policy.cpp

…burst caveat

Copilot

Pull request overview

Copilot reviewed 8 out of 8 changed files in this pull request and generated 6 comments.

Comments suppressed due to low confidence (1)

test/brpc_channel_unittest.cpp:3152

The explanatory comment about cold start here doesn’t match the implementation in BackupRateLimiter::ShouldAllow(): when total==0 and backup==0, the code treats this as “true cold start” and sets ratio=0.0 (allow freely). The conservative ratio=1.0 path only applies after backups have been counted but totals are still 0. Consider updating the comment to reflect the actual behavior to avoid confusion when maintaining the test.

    // With max_backup_ratio=1.0, backup is allowed until backup_count/total
    // reaches 1.0. At cold start (total=0) the conservative path applies:
    // ratio is treated as 1.0, so DoBackup() returns false only after the
    // first backup is counted.
    ASSERT_TRUE(p->DoBackup(NULL));   // cold start: no total yet, allow first
}

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src/brpc/backup_request_policy.cpp

src/brpc/controller.cpp

src/brpc/backup_request_policy.cpp

docs/en/backup_request.md

docs/cn/backup_request.md

…CEnd comment, re-allow test, docs - controller.cpp: treat -1 specifically (not all negatives) as the inherit sentinel; other negatives still disable backup, preserving old behavior for custom policies that return negative values to disable backup - backup_request_policy.h: document the -1 sentinel contract on GetBackupRequestMs() so custom implementors know the new interface - backup_request_policy.cpp: fix OnRPCEnd comment — called once per user-level RPC, not once per leg (total_count tracks user RPCs) - test: add OnRPCEndDrivesRatioDownAndReAllows — fires 20 backups to suppress, then completes 50 RPCs via OnRPCEnd, verifies DoBackup re-allows once ratio refreshes below max_backup_ratio - docs (EN+CN): rephrase backup_request_ms=-1 note to clarify the channel-level fallback only applies when set via ChannelOptions

Plain new follows brpc's project-wide OOM convention (abort rather than return NULL). The factory's NULL return already exclusively signals invalid parameters, not allocation failure — adding std::nothrow would conflate the two. Comment added to suppress future linter/AI suggestions.

Copilot

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Copilot reviewed 8 out of 8 changed files in this pull request and generated 4 comments.

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docs/cn/backup_request.md

docs/en/backup_request.md

…oyed before policy The unique_ptr comment was ambiguous: 'released when goes out of scope, as long as it outlives the channel' can be read as contradictory. Reword to make the ordering explicit: destroy channel first, then policy.

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Comments suppressed due to low confidence (1)

test/brpc_channel_unittest.cpp:3150

The comment describing the cold-start behavior is inaccurate: when total==0 and backup==0, BackupRateLimiter::ShouldAllow() treats the ratio as 0.0 (true cold start), not 1.0 (the conservative path only applies when backup>0 && total==0). Please update the comment so it matches the implementation and doesn’t mislead future readers.

    // With max_backup_ratio=1.0, backup is allowed until backup_count/total
    // reaches 1.0. At cold start (total=0) the conservative path applies:
    // ratio is treated as 1.0, so DoBackup() returns false only after the
    // first backup is counted.

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…tioAtBoundary ratio=1.0 conservative path only applies when backup>0 && total==0. True cold start (both zero) sets ratio=0.0 and allows freely.

chenBright

LGTM

feng-y · 2026-03-02T06:28:37Z

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fix(backup_request): clarify comments — negative defer semantics and …

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test(backup_request): fix inaccurate cold-start comment in ValidMaxRa…

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…tioAtBoundary ratio=1.0 conservative path only applies when backup>0 && total==0. True cold start (both zero) sets ratio=0.0 and allows freely.

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feng-y commented Feb 25, 2026 • edited Loading Uh oh! There was an error while loading. Please reload this page.

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Summary

Design

Configuration (RateLimitedBackupPolicyOptions)

Edge cases handled

Rate-limiting semantics

Usage

Test plan

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feng-y commented Feb 28, 2026

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feng-y commented Feb 25, 2026 •

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Configuration (`RateLimitedBackupPolicyOptions`)