preFOSDEM MySQL Day 2020 Presentation from Pedro Gomes

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Group Replication: Handling Network Glitches
Best Practices
Pedro Gomes
Software Developer
MySQL Replication Team Censored Dolphins
January 30, 2020
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●@distributedpete
●MySQL Replication Developer
●MySQL Developer since 2012
●Portuguese born and raised 🇵🇹
Pedro Gomes
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What you can expect
Checking the status
Diagnostics
Adapt
Deal with Failures
1
2
3
4
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Checking the status
Diagnostics
Adapt
Deal with Failures
1
2
3
4
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Checking the status
• Where to look?
– The usual suspect: Performance_Schema
• MySQL Shell is your friend
The first step in solving an issue is knowing what the issue is
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Checking the status
Performance Schema
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Checking the status
Performance Schema
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Checking the status
Performance Schema
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Checking the status
MySQL Shell
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Checking the status
MySQL Shell – extended information
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Check the status
Diagnostics
Adapt
Deal with failures
1
2
3
4
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Diagnostics
• Identify the scenario
• Identify the cause
What do I do with this diagnostic data?
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Diagnostics
Easily identified scenarios
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• Loosing a single server

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Diagnostics
Easily identified
scenarios
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• No Quorum

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Diagnostics
• There is a delay or network latency fluctuations on the secondaries
– What stats can I check?
Evaluating delays
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Diagnostics
• From replication_applier_status_by_worker and replication_connection_status
• Replication delay - 'rep delay (sec)':
– SELECT LAST_APPLIED_TRANSACTION_END_APPLY_TIMESTAMP -
LAST_APPLIED_TRANSACTION_ORIGINAL_COMMIT_TIMESTAMP
• Transport time - 'transport time':
– LAST_QUEUED_TRANSACTION_START_QUEUE_TIMESTAMP -
LAST_QUEUED_TRANSACTION_ORIGINAL_COMMIT_TIMESTAMP
• Time to queue on the relay log - 'time RL' :
– LAST_QUEUED_TRANSACTION_END_QUEUE_TIMESTAMP -
LAST_QUEUED_TRANSACTION_START_QUEUE_TIMESTAMP
• Time for application - 'apply time' :
– LAST_APPLIED_TRANSACTION_END_APPLY_TIMESTAMP -
LAST_APPLIED_TRANSACTION_START_APPLY_TIMESTAMP
Evaluating delays
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Diagnostics
Evaluating delays
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Diagnostics
Evaluating delays
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• You can code this as a shell plugin
• Check it at https://github.com/lefred/mysqlshell-plugins

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Diagnostics (non network issues)
• Can your workload be executed in parallel?
– Check if your parallel applier workers are idle
• Can the group handle the load?
– COUNT_TRANSACTIONS_REMOTE_IN_APPLIER_QUEUE
– COUNT_TRANSACTIONS_IN_QUEUE
What if I see no delays?
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Check the status
Diagnostics
Adapt
Deal with the failures
1
2
3
4
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Adapt
• To avoid frequent member expels due to faulty networks make use of:
– group_replication_member_expel_timeout
Adapt to faulty networks
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Adapt
Adapt to higher latencies
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group replication write concurrency

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Adapt
Adapt to higher latencies
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group replication write concurrency

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Adapt
Adapt to higher latencies
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Adapt
Adapt to higher latencies
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Adapt
Adapt to higher latencies
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Adapt
Adapt to higher latencies
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• Also be aware of big transactions:
– group_replication_communication_max_message_size
●
●
●
●
Alternative strategy
– group_replication_transaction_size_limit

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Check the status
Diagnostics
Adapt
Deal with failures
1
2
3
4
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Deal with failures
• Configure your rejoin attempts for each member
– group_replication_autorejoin_tries
• If you don’t want to persist it
Retry retry retry
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Deal with failures
• Configure whom shall the next primary be
Whom shall be next
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Deal with failures
• Configure your timeout when quorum is not achievable
Prevent other members from rising from the dead
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Deal with failures
• What is the behavior when a member goes to error?
– Read only mode : no writes, can still read from it
– Offline mode: no reads or writes, can still access the server with
admin privileges (CONNECTION_ADMIN or SUPER)
– Abort server: the name is self explanatory
Update your exit state action
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Thank you!
Questions?
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