0021_how_to_set_application_name_without_extra_queries.md

Originally from: tweet, LinkedIn post.

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How to set application_name without extra queries

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application_name is useful to control what you and others is going to see in pg_stat_activity (it has a column with the same name), and various tools that use this system view. Additionally, it appears in Postgres log (when %a is included in log_line_prefix).

Docs: application_name.

It's good practice to set application_name – for example, it can be very helpful for root-cause analysis during and after incidents.

The approaches below can be used to set up other settings (including regular Postgres parameters such as statement_timeout or work_mem), however here we'll focus on application_name particularly.

Typically, application_name is set via SET (apologies for the tautology):

nik=# show application_name;
 application_name
------------------
 psql
(1 row)

nik=# set application_name = 'human_here';
SET

nik=# select application_name, pid from pg_stat_activity where pid = pg_backend_pid() \gx
-[ RECORD 1 ]----+-----------
application_name | human_here
pid              | 93285

However, having additional query – even a blazing fast one – means an extra RTT (round-trip time), affecting latency, especially when communicating with a distant server.

To avoid it, use libpq's options.

Method 1: via environment variable

❯ PGAPPNAME=myapp1 psql \
  -Xc "show application_name"
 application_name
------------------
 myapp1
(1 row)

(-X means ignore .psqlrc, which is a good practice for automation scripts involving psql.)

Method 2: through connection URI

❯ psql \
  "postgresql://?application_name=myapp2" \
  -Xc "show application_name"
 application_name
------------------
 myapp2
(1 row)

The URI method takes precedence over PGAPPNAME.

In application code

The described methods can be used not only with psql. Node.js example:

❯ node -e "
 const { Client } = require('pg');

 const client = new Client({
   connectionString: 'postgresql://?application_name=mynodeapp'
 });

 client.connect()
       .then(() => client.query('show application_name'))
       .then(res => {
           console.log(res.rows[0].application_name);
           client.end();
       });
 "
 mynodeapp