futriix/tests/integration/replication-psync.tcl
naglera ff6b780fe6
Dual channel replication (#60)
In this PR we introduce the main benefit of dual channel replication by
continuously steaming the COB (client output buffers) in parallel to the
RDB and thus keeping the primary's side COB small AND accelerating the
overall sync process. By streaming the replication data to the replica
during the full sync, we reduce
1. Memory load from the primary's node.
2. CPU load from the primary's main process. [Latest performance
tests](#data)

## Motivation
* Reduce primary memory load. We do that by moving the COB tracking to
the replica side. This also decrease the chance for COB overruns. Note
that primary's input buffer limits at the replica side are less
restricted then primary's COB as the replica plays less critical part in
the replication group. While increasing the primary’s COB may end up
with primary reaching swap and clients suffering, at replica side we’re
more at ease with it. Larger COB means better chance to sync
successfully.
* Reduce primary main process CPU load. By opening a new, dedicated
connection for the RDB transfer, child processes can have direct access
to the new connection. Due to TLS connection restrictions, this was not
possible using one main connection. We eliminate the need for the child
process to use the primary's child-proc -> main-proc pipeline, thus
freeing up the main process to process clients queries.


 ## Dual Channel Replication high level interface design
- Dual channel replication begins when the replica sends a `REPLCONF
CAPA DUALCHANNEL` to the primary during initial
handshake. This is used to state that the replica is capable of dual
channel sync and that this is the replica's main channel, which is not
used for snapshot transfer.
- When replica lacks sufficient data for PSYNC, the primary will send
`-FULLSYNCNEEDED` response instead
of RDB data. As a next step, the replica creates a new connection
(rdb-channel) and configures it against
the primary with the appropriate capabilities and requirements. The
replica then requests a sync
     using the RDB channel. 
- Prior to forking, the primary sends the replica the snapshot's end
repl-offset, and attaches the replica
to the replication backlog to keep repl data until the replica requests
psync. The replica uses the main
     channel to request a PSYNC starting at the snapshot end offset. 
- The primary main threads sends incremental changes via the main
channel, while the bgsave process
sends the RDB directly to the replica via the rdb-channel. As for the
replica, the incremental
changes are stored on a local buffer, while the RDB is loaded into
memory.
- Once the replica completes loading the rdb, it drops the
rdb-connection and streams the accumulated incremental
     changes into memory. Repl steady state continues normally.

## New replica state machine


![image](https://github.com/user-attachments/assets/38fbfff0-60b9-4066-8b13-becdb87babc3)





## Data <a name="data"></a>

![image](https://github.com/user-attachments/assets/d73631a7-0a58-4958-a494-a7f4add9108f)


![image](https://github.com/user-attachments/assets/f44936ed-c59a-4223-905d-0fe48a6d31a6)


![image](https://github.com/user-attachments/assets/bd333ee2-3c47-47e5-b244-4ea75f77c836)

## Explanation 
These graphs demonstrate performance improvements during full sync
sessions using rdb-channel + streaming rdb directly from the background
process to the replica.

First graph- with at most 50 clients and light weight commands, we saw
5%-7.5% improvement in write latency during sync session.
Two graphs below- full sync was tested during heavy read commands from
the primary (such as sdiff, sunion on large sets). In that case, the
child process writes to the replica without sharing CPU with the loaded
main process. As a result, this not only improves client response time,
but may also shorten sync time by about 50%. The shorter sync time
results in less memory being used to store replication diffs (>60% in
some of the tested cases).

## Test setup 
Both primary and replica in the performance tests ran on the same
machine. RDB size in all tests is 3.7gb. I generated write load using
valkey-benchmark ` ./valkey-benchmark -r 100000 -n 6000000 lpush my_list
__rand_int__`.

---------

Signed-off-by: naglera <anagler123@gmail.com>
Signed-off-by: naglera <58042354+naglera@users.noreply.github.com>
Co-authored-by: Viktor Söderqvist <viktor.soderqvist@est.tech>
Co-authored-by: Ping Xie <pingxie@outlook.com>
Co-authored-by: Madelyn Olson <madelyneolson@gmail.com>
2024-07-17 13:59:33 -07:00

140 lines
5.8 KiB
Tcl

# Creates a master-slave pair and breaks the link continuously to force
# partial resyncs attempts, all this while flooding the master with
# write queries.
#
# You can specify backlog size, ttl, delay before reconnection, test duration
# in seconds, and an additional condition to verify at the end.
#
# If reconnect is > 0, the test actually try to break the connection and
# reconnect with the master, otherwise just the initial synchronization is
# checked for consistency.
proc test_psync {descr duration backlog_size backlog_ttl delay cond mdl sdl dualchannel reconnect} {
start_server {tags {"repl"} overrides {save {}}} {
start_server {overrides {save {}}} {
set master [srv -1 client]
set master_host [srv -1 host]
set master_port [srv -1 port]
set slave [srv 0 client]
$master config set repl-backlog-size $backlog_size
$master config set repl-backlog-ttl $backlog_ttl
$master config set repl-diskless-sync $mdl
$master config set repl-diskless-sync-delay 1
$master config set dual-channel-replication-enabled $dualchannel
$slave config set repl-diskless-load $sdl
$slave config set dual-channel-replication-enabled $dualchannel
set load_handle0 [start_bg_complex_data $master_host $master_port 9 100000]
set load_handle1 [start_bg_complex_data $master_host $master_port 11 100000]
set load_handle2 [start_bg_complex_data $master_host $master_port 12 100000]
test {Slave should be able to synchronize with the master} {
$slave slaveof $master_host $master_port
wait_for_condition 50 100 {
[lindex [r role] 0] eq {slave} &&
[lindex [r role] 3] eq {connected}
} else {
fail "Replication not started."
}
}
# Check that the background clients are actually writing.
test {Detect write load to master} {
wait_for_condition 50 1000 {
[$master dbsize] > 100
} else {
fail "Can't detect write load from background clients."
}
}
test "Test replication partial resync: $descr (diskless: $mdl, $sdl, dual-channel: $dualchannel, reconnect: $reconnect)" {
# Now while the clients are writing data, break the master-slave
# link multiple times.
if ($reconnect) {
for {set j 0} {$j < $duration*10} {incr j} {
after 100
# catch {puts "MASTER [$master dbsize] keys, REPLICA [$slave dbsize] keys"}
if {($j % 20) == 0} {
catch {
if {$delay} {
$slave multi
$slave client kill $master_host:$master_port
$slave debug sleep $delay
$slave exec
} else {
$slave client kill $master_host:$master_port
}
}
}
}
}
stop_bg_complex_data $load_handle0
stop_bg_complex_data $load_handle1
stop_bg_complex_data $load_handle2
# Wait for the slave to reach the "online"
# state from the POV of the master.
verify_replica_online $master 0 5000
# Wait that slave acknowledge it is online so
# we are sure that DBSIZE and DEBUG DIGEST will not
# fail because of timing issues. (-LOADING error)
wait_for_condition 5000 100 {
[lindex [$slave role] 3] eq {connected}
} else {
fail "Slave still not connected after some time"
}
wait_for_condition 100 100 {
[$master debug digest] == [$slave debug digest]
} else {
set csv1 [csvdump r]
set csv2 [csvdump {r -1}]
set fd [open /tmp/repldump1.txt w]
puts -nonewline $fd $csv1
close $fd
set fd [open /tmp/repldump2.txt w]
puts -nonewline $fd $csv2
close $fd
fail "Master - Replica inconsistency, Run diff -u against /tmp/repldump*.txt for more info"
}
assert {[$master dbsize] > 0}
# if {$descr == "no backlog" && $mdl == "yes" && $sdl == "disabled"} {
# puts "Master port: $master_port"
# after 100000000
# }
eval $cond
}
}
}
}
tags {"external:skip"} {
foreach mdl {no yes} {
foreach sdl {disabled swapdb} {
foreach dualchannel {yes no} {
test_psync {no reconnection, just sync} 6 1000000 3600 0 {
} $mdl $sdl $dualchannel 0
test_psync {ok psync} 6 100000000 3600 0 {
assert {[s -1 sync_partial_ok] > 0}
} $mdl $sdl $dualchannel 1
test_psync {no backlog} 6 100 3600 0.5 {
assert {[s -1 sync_partial_err] > 0}
} $mdl $sdl $dualchannel 1
test_psync {ok after delay} 3 100000000 3600 3 {
assert {[s -1 sync_partial_ok] > 0}
} $mdl $sdl $dualchannel 1
test_psync {backlog expired} 3 100000000 1 3 {
assert {[s -1 sync_partial_err] > 0}
} $mdl $sdl $dualchannel 1
}
}
}
}