Feature Add exponential backoff for failed full syncs - #4742
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Signed-off-by: Su Ko <rhtn1128@gmail.com>
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Included review availability: Your plan provides up to 10 included reviews per hour; 9 remain after this review. 📝 WalkthroughWalkthroughChangesThe replication subsystem adds configurable exponential backoff with jitter for failed full synchronizations. It tracks failures and retry deadlines, updates deadlines when the maximum changes, and resets state after successful synchronization or role changes. Configuration, documentation, and tests cover the behavior. Replication sync backoff
Priority: ➖ Normal Estimated code review effort: 4 (Complex) | ~45 minutes Suggested reviewers: Sequence Diagram(s)sequenceDiagram
participant Replica
participant Primary
participant ReplicationRetry
participant Configuration
Replica->>Primary: Start full synchronization
Primary-->>Replica: Return full-sync result
Replica->>ReplicationRetry: Record failure or clear state
ReplicationRetry-->>Replica: Set or evaluate retry deadline
Configuration->>ReplicationRetry: Update maximum backoff
ReplicationRetry-->>Replica: Adjust retry deadline
Replica->>Primary: Reconnect when deadline is due
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Inline comments:
In `@valkey.conf`:
- Around line 928-930: Update the retry backoff comment for
repl-sync-backoff-base-time and repl-sync-backoff-max-time to explicitly state
that the delay values are measured in seconds, while preserving the existing
description of the capped backoff and jitter.
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src/config.csrc/replication.csrc/server.csrc/server.htests/helpers/fake_primary_fullsync_sequence.tcltests/integration/repl-sync-backoff.tcltests/unit/other.tclvalkey.conf
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Reviewed the backoff state machine, the retry gates, and the new tests. The gating itself is consistent — every connectWithPrimary() call site except the one in replicationSetPrimary() (which resets first) goes through replSyncRetryDue(), and repl_full_sync_in_progress keeps the counter from being charged for plain connection failures. Main concerns are the default-on cap changing replication recovery for existing tests, the jitter being too narrow below the cap to actually break lockstep, and the new test file needing the TLS/valgrind guards other fake-primary suites use.
I ran the backoff math standalone with the default config to confirm the delay sequence: 1-2, 2-3, 4-5, 8-9, 16-17, 32-33, then 30-60 at the cap.
| createIntConfig("cluster-announce-client-tls-port", NULL, MODIFIABLE_CONFIG, 0, 65535, server.cluster_announce_client_tls_port, 0, INTEGER_CONFIG, NULL, updateClusterAnnouncedPort), | ||
| createIntConfig("repl-timeout", NULL, MODIFIABLE_CONFIG, 1, INT_MAX, server.repl_timeout, 60, INTEGER_CONFIG, NULL, NULL), | ||
| createIntConfig("repl-sync-backoff-base-time", NULL, MODIFIABLE_CONFIG, 1, INT_MAX, server.repl_sync_backoff_base_time, 1, INTEGER_CONFIG, NULL, NULL), | ||
| createIntConfig("repl-sync-backoff-max-time", NULL, MODIFIABLE_CONFIG, 0, INT_MAX, server.repl_sync_backoff_max_time, 60, INTEGER_CONFIG, NULL, replicationUpdateSyncBackoff), |
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Defaulting the cap to 60 makes this a default-on change to replication recovery: on consecutive full-sync failures the replica waits 1-2, 2-3, 4-5, 8-9, 16-17, 32-33, then 30-60 seconds (measured by running the function standalone with base 1 / max 60), where today it reconnects immediately via the Reconnecting to PRIMARY ... after failure path.
Two existing tests depend on the old behavior and are not touched by this PR. tests/integration/replication.tcl:888 and tests/unit/moduleapi/testrdb.tcl:100 both loop up to 100 times killing the link while the replica is mid-RDB-load — each iteration a real full-sync failure that increments repl_full_sync_failures — and each iteration only allows wait_for_log_messages -1 {"*Loading DB in memory*"} $loglines 2000 1 for the next attempt to start, i.e. a couple of seconds of polling. Once the count reaches five or six the replica is 16-33s away from retrying and the loop fails.
Either default repl-sync-backoff-max-time to 0 so the feature is opt-in, or add the config override those two tests need (and update the default assertion in tests/unit/other.tcl plus valkey.conf if you change the default).
| if (delay == server.repl_sync_backoff_max_time) { | ||
| /* Keep replicas desynchronized even after exponential growth reaches | ||
| * its cap. */ | ||
| jitter = (delay + 1) / 2; | ||
| delay = jitter + random() % (delay - jitter + 1); | ||
| } else { | ||
| jitter = random() % ((unsigned long)server.repl_sync_backoff_base_time + 1); | ||
| if (jitter > server.repl_sync_backoff_max_time - delay) jitter = server.repl_sync_backoff_max_time - delay; | ||
| delay += jitter; | ||
| } |
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Below the cap the jitter window is base, not a fraction of the current delay. With the defaults (base 1, max 60) the delays come out as 1-2, 2-3, 4-5, 8-9, 16-17, 32-33, then 30-60 — so replicas that failed together stay within one second of each other for the first six retries, and only the capped step is genuinely spread out. That is the lockstep case the PR is meant to solve, and it is the one still unaddressed.
The cap branch already has the right shape; applying it unconditionally spreads every step and collapses the two branches:
| if (delay == server.repl_sync_backoff_max_time) { | |
| /* Keep replicas desynchronized even after exponential growth reaches | |
| * its cap. */ | |
| jitter = (delay + 1) / 2; | |
| delay = jitter + random() % (delay - jitter + 1); | |
| } else { | |
| jitter = random() % ((unsigned long)server.repl_sync_backoff_base_time + 1); | |
| if (jitter > server.repl_sync_backoff_max_time - delay) jitter = server.repl_sync_backoff_max_time - delay; | |
| delay += jitter; | |
| } | |
| /* Equal jitter at every step, not just at the cap, so replicas that | |
| * failed together do not retry together. */ | |
| jitter = (delay + 1) / 2; | |
| delay = jitter + random() % (delay - jitter + 1); |
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| /* Mark the beginning of the full sync */ | ||
| elapsedStart(&server.repl_full_sync_start_time); | ||
| server.repl_full_sync_in_progress = 1; |
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repl_full_sync_failures is only cleared by resetReplFullSyncBackoff(), which runs on a completed full sync, replicaof, or promotion. A successful partial resync returns at the PSYNC_CONTINUE branch a dozen lines above (src/replication.c:4758) without clearing it, so a replica that failed full sync N times, reconnected, and then got a partial resync keeps N indefinitely. When the link later drops and a full sync is needed, the first failure jumps straight to the N+1 delay instead of restarting at repl-sync-backoff-base-time.
Since the counter means "consecutive failures", a successful link establishment should clear it too — a resetReplFullSyncBackoff() on the PSYNC_CONTINUE path covers that.
| return [list $pid $port $count_file] | ||
| } | ||
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| start_server {tags {"repl external:skip"} overrides {save "" enable-debug-command local}} { |
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The fake primary is a plain TCP tclsh listener, so this file cannot run under --tls: ping_server switches to ::tls::socket when $::tls is set (tests/support/server.tcl:152), so start_fullsync_sequence_primary fails with "Failed to start fake primary", and the replica's handshake would not complete either.
The other fake-primary suites guard for exactly this — tests/integration/repl-fullsync-compression.tcl:613 ("The fake primary is plain TCP, so skip under TLS") and tests/integration/repl-compression.tcl:473 both wrap their fake-primary tests in if {!$::tls}. Add tls:skip to these tags, or wrap the file the same way.
| set second_delay [expr {[lindex $times 2] - [lindex $times 1]}] | ||
| set capped_delay [expr {[lindex $times 3] - [lindex $times 2]}] | ||
| set reset_delay [expr {[lindex $times 5] - [lindex $times 4]}] | ||
| assert {$first_delay >= 800 && $first_delay <= 3000} |
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This bound has no headroom. The scheduled delay for the first failure is a whole 1 or 2 seconds, and the retry only fires on the next replicationCron tick, which runs once per second (src/server.c:1769), so the observed gap between PSYNC attempts can reach ~3s before the reconnect and handshake are even counted.
The valgrind integration-type shard runs --single tests/integration (.github/workflows/daily.yml:884), so all four windows here get exercised under valgrind where they are far too tight. Widen the upper bounds to cover the extra cron tick, and add valgrind:skip to the tags of the enclosing start_server for the timing assertions.
Signed-off-by: Su Ko <rhtn1128@gmail.com>
Codecov Report❌ Patch coverage is
Additional details and impacted files@@ Coverage Diff @@
## unstable #4742 +/- ##
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- Coverage 80.74% 80.63% -0.12%
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Files 192 192
Lines 100856 100904 +48
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- Hits 81438 81365 -73
- Misses 19418 19539 +121
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Introduce replica-side exponential backoff with jitter after failed full syncs to avoid retrying PSYNC every second.
This prevents multiple disconnected replicas from repeatedly triggering expensive RDB creation and transfer in lockstep, giving the primary and replicas time to recover from network instability, replication buffer pressure, or snapshot-related resource exhaustion
closes : #4718