Test Decoding Replication Setup

setup logical replication test decoding test decoding is the longest running change tracking solution for postgresql, and it can capture deletes which enables the soft delete function in datalakehouse io check to see if any logical replication slots already exist by running the command select from pg catalog pg replication slots; there is almost always one default one called pghoard local with a physical slot type if this is the only one that exist then you have no logical replication slots yet execute and record the following settings review current settings select setting from pg settings where name ='wal level'; select setting from pg settings where name ='max replication slots'; select setting from pg settings where name ='wal sender timeout'; select setting from pg settings where name ='max wal senders'; ensure that wal level = 'logical' wal sender timeout = 0 or 60000 (0 = infinity) max replication slots >= 8 max wal senders >= 16 create a user for datalakehouse io replace the placeholders, schema name, etc below with your actual values and record them for later references as you'll need them to setup the user that will have access to the database you will connect with in the datalakehouse io connection info, for example we usually recommend the '' as ' datalakehouse sync svc ' create a user create user \<username> password 'some password'; grant usage on schema "public" to \<username>; grant select on all tables in schema "public" to \<username>; \ #### or for specifically cherry picked selected tables, #### \ alter default privileges in schema "some schema" revoke select on tables from \<username>; \ revoke the tables using this concept \ revoke select on all tables in schema "some schema" from \<username>; alter default privileges in schema "public" grant select on tables to \<username>; \ ………then……for continued individual table access \ grant select on "some schema" "some table" to \<username>; \ alter default privileges in schema "some schema" grant select on tables to \<username>; as an example, to create a user with access to all tables create user datalakehouse sync svc password 'p\@ssword1'; grant usage on schema "public" to datalakehouse sync svc; grant select on all tables in schema "public" to datalakehouse sync svc; create the logical replication creating the logical replication slots require some basic configurations on the postgresql server adjust a few configurations set timeout to 30 mins set statement timeout = '1800'; now create the actual logical replication slot, using specifically the name 'datalakehouseio replication slot' select pg create logical replication slot('datalakehouseio replication slot', 'test decoding'); create a role and grant permissions to the user created previously, create role datalakehouse sync role; alter role datalakehouse sync role with replication; grant datalakehouse sync role to datalakehouse sync svc; other options acceptible creating a logical replication slot has the ability to add options such as "include xids" and "skip empty xacts" as of 12/1/2021, datalakehouse requires the xids (the default), but does not concern over skip empty xacts, but we suggest that leaving the default settings as is unless otherwise communicated by our support team there are other not well known options https //git postgresql org/gitweb/?p=postgresql git;a=blob;f=contrib/test decoding/test decoding c;hb=head for test decoding and wal2json which unless communicated by our support team should not be used analyzing logical replication slots peeking at the logical changes are the easiest way to check if the logical replication is working and if to see if there are any tracked changes flowing through without actually changing the pointerlike when you use get changes command select count( ) from pg logical slot peek changes('datalakehouseio replication slot', null, null); also, retrieve the change inacted as well, as this is the same logic that datalakehouse io uses, select from pg logical slot get changes('datalakehouseio replication slot', null, null); which will move the pointer of the tracking forward and clear out the change records from the wal thus removing the disk space previously consumed so the disk does not grow out of control \[nb]the pg current wal lsn and the lsn distance continue to change as they are based on the log sequence number (lsn), a 64 bit integer used to determine the position of the write ahead log (wal) stream which is there to preserve data integrity acting as a pointer to the xlog the print out is two hexadecimal numbers upt to 8 digits each, separated by a slash (/), for examplte 63/b30000220 if you compare to lsns by using basic operator like =, >, < , −, the result is the number of bytes between the two wal positions test your logical replication in order to confirm that your logical replication is working, you can artificially create, or wait for, a dml activity of insert, update, delete then run the above peek changes command, select from pg logical slot peek changes('datalakehouseio replication slot', null, null); get the last received wal position this will show the last received and last replayed wal positions if there is a delta (in bytes) beteween pg last wal receive lsn and pg last wal replay lsn, then there is a lag then data is available typically select pg last wal receive lsn(), pg last wal replay lsn(), pg last xact replay timestamp(); confirm the logical replication lag/pointer to quickly shown any flush confirmations from using the get changes select slot name, confirmed flush lsn, pg current wal lsn(), (pg current wal lsn() confirmed flush lsn) as lsn distance from pg replication slots \ where slot name in ('datalakehouseio replication slot') ; which will look similar to the following output slot name confirmed flush lsn pg current wal lsn lsn distance pghoard local 63/b00008f0 datalakehouseio replication slot 63/390006b0 63/b00008f0 1996489280 (2 rows) overall big picture replication view select from pg stat replication; get the time lag in human readable format select now() pg last xact replay timestamp() as time lag; general logical replication slot details this table provides the basic information about the logic replication you have created when datalakehouse io is retrieving from the slot, the active colum will typically be set to "t" instead of "f" for true and false repsectively select from pg replication slots; peeking at logical slot changes there are several parameters for working with the pg logical slot peek changes function as further described here, https //pgpedia info/p/pg logical slot peek changes html https //pgpedia info/p/pg logical slot peek changes html , for testing the flow of wal changes as identifying the lsns select from pg logical slot peek changes('datalakehouseio replication slot', null, null, 'include xids', '0'); changes to the pg last wal replay lsn is not leveraged by datalakehouse io as this relates to a streaming replication standby replication instance, where datalakehouse io is on demand frequency sync scheduled query based disk space increase is normal aiven io has a good article on standard operation increase in disk space due to wal, https //developer aiven io/docs/products/postgresql/concepts/pg disk usage html https //developer aiven io/docs/products/postgresql/concepts/pg disk usage html run any of the main postgresql commands to check disk space on the instance \l or \l+ select pg size pretty(pg database size('yourdbname')); logical replication failues in a standard process where the ddl of your postgresql tables do not change much there is very little cause for failure other than not have a sync frequency that is not aligned with the volume of data your database tables produce commensurate with the disk space of the database/server since logical replication does not track ddl changes an error could occur if a ddl change is made but downstream impacts are not considered in the case of datalakehouse io a manual change may be required on your target system in order to reflect a ddl change on your source system when errors, if any occur, please report them immediately by opening up a support ticket methods used on your source database side to clear issues may include things such as using pg replication origin advance, to skip the transaction that is failing other references ( https //www postgresql org/docs/9 4/catalog pg replication slots html)postgresql https //www postgresql org/docs/9 4/catalog pg replication slots html)postgresql replication slots fields/columns available https //techcommunity microsoft com/t5/azure database for postgresql/change data capture in postgres how to use logical decoding and/ba p/1396421 https //techcommunity microsoft com/t5/azure database for postgresql/change data capture in postgres how to use logical decoding and/ba p/1396421