Keith's Ramblings…

"Fixing a complex system sounds pretty easy when you're an idiot."

PostgreSQL Job Logging & Monitor Extension – The Logger

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Update 2012/06/05: So of course after I go publishing this blog post, I discover a major shortcoming in 0.3.0. It requires super-user privileges to actually run. Teach me to test with only my own account in the database. I’ll be pushing 0.3.1 out shortly with a fix to allow non-superusers to be able to use these logging functions. Just requires a little bit of configuration, which I’ve added to that section below and the README.md file.

Job logging and monitoring in PostgreSQL can be an invaluable tool for tracking down issues when important database tasks are automated. OmniTI has some great job logging systems in place for the databases we support, but there was no consistent monitoring of this logging that was specifically built with it in mind. I thought this would be a great opportunity to take advantage of PostgreSQL 9.1’s new extensions capability to try and bring a consistent logging and monitoring tool into use that would be easily maintained and updatable. This blog post will first concentrate on the logging portion. I’ll do a follow up post about the monitor hopefully next month.

https://github.com/omniti-labs/pg_jobmon

The first, and easier piece to pull into an extension format was the logging. Easier mostly because previous developers here had done a good job of getting logging working. 🙂 Since Postgres’s functions aren’t autonomous, true job logging can be difficult. If a function fails to run, everything that it was doing, including inserts to a logging table, would be rolled back. Using dblink to have Postgres connect back to itself is a neat trick to get autonomous functions working and have intermittent commits actually be persistent despite a job failure. The details of doing this aren’t really the main point of this blog post, though, so if you’d like to see the details on how this works, the source code of the extension is freely available.

Installation

First off, the new extension system makes the install (and future updates) a breeze. I set the extension up to be compatible with the PostgreSQL Extension Network so you can use make/make install to get the files in the correct location. It is available on pgxn, but it is marked testing, so it won’t show up in searches yet. Would like to get some feedback from others before making it v1.0 stable – http://pgxn.org/dist/pg_jobmon. (Update: version 1.0 (and greater) have been available for quite some time in both github & pgxn) After that, it’s just a matter of using the new 9.1 extension management commands. I’d recommend creating a schema instead of installing it to public. I’m assuming the schema jobmon in all examples, but you can choose whichever one you want. Note that the schema cannot be changed after install without reinstalling the extension (it is not relocatable). This extension has table data as well, so re-installation can possibly result in the loss of configuration and logging data without taking precautions.

How to Log Steps

pg_jobmon has some pretty basic functions for doing job logging so getting it up and working isn’t very difficult at all. To start off, some variables to hold job and step ids will need to be defined for any function that requires logging

To start your job logging for a function call the add_job(text) function, storing the ID it generates to a variable. What you enter for JOB NAME will be used in the jobmon.job_log table as the job_name column. All values for the job name are automatically capitalized for consistency and to make searching a little more predictable as to what the values will be. I’ll be using a new, specialized replication extension I’m working on, mimeo, to show examples of how pg_jobmon is used. Specifically, the snap replication which does a full copy of a table from one postgresql instance to another.

This creates an entry in the jobmon.job_log table. pg_jobmon has a set of “show” functions that can make reading the job logs easier. I’ll talk more about them at the end, but I’ll be using them in these examples as well.

From here on out, job logging is primarily done with two functions: add_step(bigint, text) and update_step(bigint, text, text). add_step() is used to start tracking the progress of each step of the function that you’d like to monitor. The first argument is the job_id generated from add_job(). The text should be a short description of the step. Note that this description stays constant and is not changed as part of the update. add_step() returns a step_id that should be stored.

This creates the first step for our job and is logged to the jobmon.job_detail table under the given job_id.

Later in the function, after this step should have been completed, we update that step with the current status using update_step(). The first argument is the step_id that is to be updated. The next argument is a simple status. in this case everything should be ok if it reached this point, so it’s set to ‘OK’. (You can set whatever status message you wish here. Would recommend being consistent with the config table mentioned later.) The final argument is a message that can give more details as to results of this step. This was a fairly simple step, so just saying ‘done’ is good enough. You can update a status or message several times for a single step if needed, showing the current progress of longer running steps.

When you’re done with this step and want to move on to the next, just call the add_step() function again to create a new step ID.

For this step we’ll provide a little more information to the message log. I’ve added a GET DIAGNOSTICS call to the portion where the INSERT is done to get the row count of this snap job. This is added to the message log so we can make sure rows were actually copied into the snap table.

Handling Errors

If your job has a serious problem and needs to fail out completely, you can use the jobmon.fail_job(bigint) function. Provide it the job_id and it will set the status in jobmon.job_log to ‘CRITICAL’. Before calling this function you should do a call to jobmon.update_step() to provide details as to why the failure happened to the log. Note that this function DOES NOT stop your function from running. It only provides the functionality to log the failure. Adding these steps into an EXCEPTION is the common method we use, but it can be done anywhere you need

Speaking of exceptions, unless you handle the QUERY_CANCELED exception by adding logging instructions to it, manually cancelling your normally logged function will leave the final results in the log table as unfinished. If you want to cancel logged functions and properly make note of it in the logs, use the jobmon.cancel_job(bigint) function. Just provide it the job_id of the currently running function and it will do the following:

  • cancel the currently running job using pg_cancel_backend() (pid is stored in job_log table)
  • update the last step to set the status to CRITICAL
  • set the last step message to Manually cancelled via call to jobmon.cancel_job()
  • call the fail_job() function to close out the job as CRITICAL

Configuration

A database superuser can just use these logging functions right away after extension creation. Non-superusers will require a bit of configuration. First, create a role that you can grant the following permissions to:

Set your pg_hba.conf file to allow this user to connect locally to the database then add the rolename and password to the dblink_mapping table. Note the password is stored in the clear, so just give this role the limited permissions it needs and nothing else and control access to this table.

One thing to note about the close_job, fail_job, and cancel_job functions is that they use another configuration table, job_status_text, to set statuses to either OK or CRITICAL (this extension was made for use with Nagios, hence these text values). If you’d like different default values for your jobs, just update the error_text column, keeping the following code meanings in mind.

1 always means the job ran without issue. 2 can be used as an intermediary status (next blog post on monitoring will go into more detail on this). And 3 means a complete failure of the job.
Update 2012/07/17: Version 0.3.4 has changed the column names of this table to make things a little more consistent with the monitoring portion of the extension

I’ll get into more detail on this with my monitoring post, but thought I’d at least mention this here since it involves values inserted to the logging table. I haven’t thought of any use cases for further status codes and these functions won’t use any others you may add to the table. If anyone thinks of any others that they feel should be added and used, I’m open to suggestions.

Reading Logs

Lastly for logging, you’ve seen examples of the show functions throughout this post. I won’t get into too much more detail on each of them since they are fairly self explanatory once a few bits of info are known ahead of time. First, all job names passed as parameters are automatically capitalized to be consistent with add_job(), so you don’t have to. Next, most have a default limit on the number of rows returned and this can be changed by passing an optional integer parameter. Also, all the show functions are set to return full row sets, so you can query and filter them just like a regular table. Of course, you can always query the job_log and job_detail tables directly as well. These were just created to try and make some of the more common queries we were writing against them easier to do. See the docs for more info.

Upcoming Features

A coworker mentioned a handy job logging function could be one that actually accepts a query to run and simply logs the number of rows affected. To quote him

(Update: The above function has been added as well as an “sql_job” function that can log an entire, single step job with a single call)

This covers the logging portion of this extension. Functions similar to these have been in use with several of our clients for quite a while. These have only been changed to be incorporated into an extension or add some new capability. The monitoring tables and function are still fairly new and should be in working order. Their basic use is covered in the readme file, but as I said in the beginning, I will be doing another blog post to cover their use. Would really appreciate any feedback, good and bad, from anyone that starts using it.This is also my first venture into making an extension, so if anyone has any feedback in that area, it would be much appreciated.

Written by Keith

June 4th, 2012 at 4:11 pm

  • Alex

    Thanks for sharing this — I’m looking forward to the follow-up article on monitoring, especially since we’re heave Nagios users ourselves. I’ve seen the extension being updated on PGXN and this post really helps to explain its use and purpose.