General Operations Tips

Conditional execution date2serial migration

In FactCast v0.7.1, a new UI feature was introduced to allow filtering of events based on their publishing date. For this purpose, the date2serial mapping table was introduced in the schema.

A Liquibase changeset takes care of creating and populating the date2serial table, but it is not executed when the store contains more than 10 million events. This is to prevent the migration from taking too long on larger setups.

As mentioned in the changeset comments, it is suggested to run the changeset manually in such cases. The changeset can be found in the factcast-store module under src/main/resources/db/changelog/factcast/issue2479/date2serial_for_existing_events.sql.

Optimize GIN indexes updates

While GIN Indexes make querying jsonb faster, they are also expensive to update. Especially because a single change can cause the update of multiple index entries. To keep the overhead on write and update statements low, postgres per default enables the fastupdate setting which defers the update of the index and instead gathers changes to execute them all at once. This update happens:

• when the gin_pending_list_limit is reached (default 4MB)
• when the gin_clean_pending_list function is called
• at the end of an autovacuum operation on a table

However, there can be certain disadvantages of fastupdate on GIN indexes

• query performance can suffer significantly when looking through both, the main index and pending list
• when reaching the size limits, in-query cleanups can block other queries

This can cause queries to be a lot slower than usual which we have observed in production setups. In general, if this kind of behavior is observed, it might make sense to consider to:

• reduce the gin_pending_list_limit -> more frequent, smaller flushes
• increase the limit and do manual flushes outside of workload
• turn off fastupdate
• let autovacuum run more often or manually call the clean operation

For now, we have decided to disable the fastupdate setting via src/main/resources/db/changelog/factcast/issue3755/disable_fast_update.sql

Please note that flushing the pending list as part of disabling the fastupdate setting could in theory block any other query. This is why this change set is not executed automatically if the attached condition senses a larger setup (> 10 million events). In this case please execute the change set manually to disable the fastupdate setting.

Autoanalyse & Autovacuum settings

Postgres has a built-in mechanism to keep the statistics up to date and to clean up dead tuples, called Automatic Vacuuming.

In most cases, it might be necessary to adjust the default autovacuum settings to better fit the workload and ensure a more efficient execution of the process:

# disable autovacuum schedule based on scale factor
autovacuum_vacuum_scale_factor:0
autovacuum_analyze_scale_factor:0
# set thresholds based on approximate number of facts inserted
autovacuum_vacuum_threshold:<number of new facts each month>
autovacuum_analyze_threshold:<number of new facts each week>

AWS RDS Configuration

Most of the time, the default RDS configuration of PostgreSQL is sufficient. However, in some cases, it might be necessary to adjust some settings in the RDS Parameter Groups to improve performance. The following settings are recommended for FactCast instances running on production stages:

# hands over concurrency considerations to kernel
effective_io_concurrency:0
# tune accordingly, consider roughly 100mb running on a db.r5.2xlarge RDS instance
work_mem:100000
# the followings might vary, depending on your non-functional requirements
log_statement:'none'
log_min_duration_statement:500
default_statistics_target:100