Обсуждение: How to redirect output from PostgreSQL pg_recvlogical to a file or a pipe?

try use the following syntax (yes, with a 2 before the greater sign)

pg_recvlogical -d postgres --slot test --start -f - 2>> sample.jsonl

Atte

JRBM

El vie, 12 ene 2024 a las 16:35, David Ventimiglia (<davidaventimiglia@hasura.io>) escribió:

Hello! How do I redirect logical decoding output from the PostgreSQL CLI tool pg_recvlogical either to a file or to another command via a pipe? I ask because when I try the obvious, no output is recorded or sent:

pg_recvlogical -d postgres --slot test --start -f - >> sample.jsonl

Lest there be any confusion, I already created the slot in an earlier step. Moreover, I can verify that if I omit the output redirection >> sample then it does work, insofar as it emits the expected change events when I perform DML in another terminal window. When I include the redirection (or alternatively, set up a pipeline), then nothing happens.

Note that I am aware of the option to pass a filename to the -f switch to write to a file.  That works, but it's not what I'm after because it doesn't help update my mental model of how this is supposed to work.  Based on my current (flawed) mental model built up from command line experience with other tools, this should work.  I should be able to send the output to stdout and then redirect it to a file.  It surprises me that I cannot.  

Anyway, thanks!

Best,

David

On 1/12/24 21:23, David Ventimiglia wrote:
> Let me just lay my cards on the table.  What I'm really trying to do is
> capture change events with logical decoding and then send them back into
> the database into a database table.  To do that, I believe I need to
> process the event records into SQL insert statements somehow.  xargs is
> one option.  jq is another.  My idea was to pipe the pg_recvlogical
> output through a jq transform into psql, but that didn't work (neither
> did earlier experiments with xargs).  Redirecting the output to an
> intermediate file via stdout was just an attempt to reduce the problem
> to a simpler problem.  I had /thought/ (incorrectly, as it turns out)
> that I was unable even to redirect it to a file, but evidently that's
> not the case.  I can redirect it to a file.  What I cannot seem to do is
> run it through a jq filter and pipe it back into psql.  I can run it
> through a jq filter and redirect it to a file, no problem.  But the
> minute I change it to pipe to psql, it ceases to produce the desired result.
>
> I tried illustrating this in this screencast:
>
> https://asciinema.org/a/npzgcTN8DDjUdkaZlVyYJhZ5y
> <https://asciinema.org/a/npzgcTN8DDjUdkaZlVyYJhZ5y>
>
> Perhaps another way to put this is, how /does/ one capture output from
> pg_recvlogical and pipe it back into the database (or if you like, some
> other database) with psql.  When I set out to do this I didn't think
> bash pipes and redirection would be the hard part, and yet here I am. 
> Maybe there's some other way, because I'm fresh out of ideas.

This is going to depend a lot on what you define as a change event. Is
that DDL changes or data changes or both?

Some existing solutions that cover the above to a one degree or another:

Event triggers:

https://www.postgresql.org/docs/current/event-triggers.html

PGAudit

https://github.com/pgaudit/pgaudit/blob/master/README.md

Or since you are part of the way there already just using logical
replication entirely:

https://www.postgresql.org/docs/current/logical-replication.html


>
> Best,
> David
>
> On Fri, Jan 12, 2024 at 8:42 PM Juan Rodrigo Alejandro Burgos Mella
> <rodrigoburgosmella@gmail.com <mailto:rodrigoburgosmella@gmail.com>> wrote:
>
>
>     try use the following syntax (yes, with a 2 before the greater sign)
>
>     pg_recvlogical -d postgres --slot test --start -f - 2>> sample.jsonl
>
>     Atte
>     JRBM
>
>     El vie, 12 ene 2024 a las 16:35, David Ventimiglia
>     (<davidaventimiglia@hasura.io <mailto:davidaventimiglia@hasura.io>>)
>     escribió:
>
>         Hello! How do I redirect logical decoding output from the
>         PostgreSQL CLI tool |pg_recvlogical| either to a file or to
>         another command via a pipe? I ask because when I try the
>         obvious, no output is recorded or sent:
>
>         |pg_recvlogical -d postgres --slot test --start -f - >>
>         sample.jsonl |
>
>         Lest there be any confusion, I already created the slot in an
>         earlier step. Moreover, I can verify that if I omit the output
>         redirection |>> sample| then it does work, insofar as it emits
>         the expected change events when I perform DML in another
>         terminal window. When I include the redirection (or
>         alternatively, set up a pipeline), then nothing happens.
>
>         Note that I am aware of the option to pass a filename to the -f
>         switch to write to a file.  That works, but it's not what I'm
>         after because it doesn't help update my mental model of how this
>         is supposed to work.  Based on my current (flawed) mental model
>         built up from command line experience with other tools, this
>         /should/ work.  I should be able to send the output to stdout
>         and then redirect it to a file.  It surprises me that I cannot.
>
>         Anyway, thanks!
>
>         Best,
>
>         David
>

--
Adrian Klaver
adrian.klaver@aklaver.com

I think this might be an A-B problem.  Tell us the "business problem" you are trying to solve, not the problem you're having with your solution to the "business problem".

(If you've already mentioned it, please restate it.)

On Sat, Jan 13, 2024 at 11:49 AM David Ventimiglia <davidaventimiglia@hasura.io> wrote:

Thanks.  I'm aware of all of those other alternatives, but the thing is, I'm not trying to answer this broader question:

"What are some options for capturing change events in PostgreSQL?"

Rather, I'm trying to answer a narrower question:

"How does one capture output from pg_recvlogical and pipe it back into the database with psql?"

Best,

David

On Sat, Jan 13, 2024 at 10:29 AM Adrian Klaver <adrian.klaver@aklaver.com> wrote:

On 1/12/24 21:23, David Ventimiglia wrote:
> Let me just lay my cards on the table.  What I'm really trying to do is
> capture change events with logical decoding and then send them back into
> the database into a database table.  To do that, I believe I need to
> process the event records into SQL insert statements somehow.  xargs is
> one option.  jq is another.  My idea was to pipe the pg_recvlogical
> output through a jq transform into psql, but that didn't work (neither
> did earlier experiments with xargs).  Redirecting the output to an
> intermediate file via stdout was just an attempt to reduce the problem
> to a simpler problem.  I had /thought/ (incorrectly, as it turns out)
> that I was unable even to redirect it to a file, but evidently that's
> not the case.  I can redirect it to a file.  What I cannot seem to do is
> run it through a jq filter and pipe it back into psql.  I can run it
> through a jq filter and redirect it to a file, no problem.  But the
> minute I change it to pipe to psql, it ceases to produce the desired result.
>
> I tried illustrating this in this screencast:
>
> https://asciinema.org/a/npzgcTN8DDjUdkaZlVyYJhZ5y
> <https://asciinema.org/a/npzgcTN8DDjUdkaZlVyYJhZ5y>
>
> Perhaps another way to put this is, how /does/ one capture output from
> pg_recvlogical and pipe it back into the database (or if you like, some
> other database) with psql.  When I set out to do this I didn't think
> bash pipes and redirection would be the hard part, and yet here I am. 
> Maybe there's some other way, because I'm fresh out of ideas.

This is going to depend a lot on what you define as a change event. Is
that DDL changes or data changes or both?

Some existing solutions that cover the above to a one degree or another:

Event triggers:

https://www.postgresql.org/docs/current/event-triggers.html

PGAudit

https://github.com/pgaudit/pgaudit/blob/master/README.md

Or since you are part of the way there already just using logical
replication entirely:

https://www.postgresql.org/docs/current/logical-replication.html


>
> Best,
> David
>
> On Fri, Jan 12, 2024 at 8:42 PM Juan Rodrigo Alejandro Burgos Mella
> <rodrigoburgosmella@gmail.com <mailto:rodrigoburgosmella@gmail.com>> wrote:
>
>
>     try use the following syntax (yes, with a 2 before the greater sign)
>
>     pg_recvlogical -d postgres --slot test --start -f - 2>> sample.jsonl
>
>     Atte
>     JRBM
>
>     El vie, 12 ene 2024 a las 16:35, David Ventimiglia
>     (<davidaventimiglia@hasura.io <mailto:davidaventimiglia@hasura.io>>)
>     escribió:
>
>         Hello! How do I redirect logical decoding output from the
>         PostgreSQL CLI tool |pg_recvlogical| either to a file or to
>         another command via a pipe? I ask because when I try the
>         obvious, no output is recorded or sent:
>
>         |pg_recvlogical -d postgres --slot test --start -f - >>
>         sample.jsonl |
>
>         Lest there be any confusion, I already created the slot in an
>         earlier step. Moreover, I can verify that if I omit the output
>         redirection |>> sample| then it does work, insofar as it emits
>         the expected change events when I perform DML in another
>         terminal window. When I include the redirection (or
>         alternatively, set up a pipeline), then nothing happens.
>
>         Note that I am aware of the option to pass a filename to the -f
>         switch to write to a file.  That works, but it's not what I'm
>         after because it doesn't help update my mental model of how this
>         is supposed to work.  Based on my current (flawed) mental model
>         built up from command line experience with other tools, this
>         /should/ work.  I should be able to send the output to stdout
>         and then redirect it to a file.  It surprises me that I cannot.
>
>         Anyway, thanks!
>
>         Best,
>
>         David
>

--
Adrian Klaver
adrian.klaver@aklaver.com

Am Sat, Jan 13, 2024 at 05:53:14PM -0500 schrieb Ron Johnson:

> *No,* that's a technology problem.  What is the purpose of storing them
> back in the database using psql?

Or even the end goal to be achieved by that ?

Karsten
--
GPG  40BE 5B0E C98E 1713 AFA6  5BC0 3BEA AC80 7D4F C89B

No, that's a technology problem.  What is the purpose of storing them back in the database using psql?

On Sat, Jan 13, 2024 at 4:34 PM David Ventimiglia <davidaventimiglia@hasura.io> wrote:

The business problem I'm trying to solve is:

"How do I capture logical decoding events with the wal2json output encoder, filter them with jq, and pipe them to psql, using pg_recvlogical?"

On 1/13/24 3:34 PM, David Ventimiglia wrote:
> The business problem I'm trying to solve is:
>
> "How do I capture logical decoding events with the wal2json output
> encoder, filter them with jq, and pipe them to psql, using pg_recvlogical?"

I think the missing piece here is that you can't simply pipe JSON into
psql and expect anything useful to happen. Are you using jq to turn the
JSON into actual SQL statements? What does some of your jq output look like?
--
Jim Nasby, Data Architect, Austin TX

Thanks for the reply, Jim.  No, I'm afraid that's not the missing piece.  I knew enough to use jq to transform the JSON output into SQL statements.  What I didn't know enough was about jq.  No, the missing piece turned out not to have anything to do with PostgreSQL or pg_recvlogical (I guessed incorrectly that it might), but rather with jq itself.  I didn't realize that jq buffers its input and it turns out all I had to do was use its --unbuffered switch.  The full chapter-and-verse is described in this Stack Overflow question and answer.  

Cheers,

David

On Tue, Jan 16, 2024 at 12:57 PM Jim Nasby <jim.nasby@gmail.com> wrote:

On 1/13/24 3:34 PM, David Ventimiglia wrote:
> The business problem I'm trying to solve is:
>
> "How do I capture logical decoding events with the wal2json output
> encoder, filter them with jq, and pipe them to psql, using pg_recvlogical?"

I think the missing piece here is that you can't simply pipe JSON into
psql and expect anything useful to happen. Are you using jq to turn the
JSON into actual SQL statements? What does some of your jq output look like?
--
Jim Nasby, Data Architect, Austin TX