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docs: add an info note documenting known limitation with multiplexed subscriptions

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PR-URL: https://github.com/hasura/graphql-engine-mono/pull/7382
Co-authored-by: Rob Dominguez <24390149+robertjdominguez@users.noreply.github.com>
GitOrigin-RevId: ce1a39d4ddb2f8500a5e2408acf6f3cfddcb44f6
This commit is contained in:
Rakesh Emmadi 2022-12-27 20:03:57 +05:30 committed by hasura-bot
parent 68f4f8a819
commit 9a3488920f
1 changed files with 18 additions and 4 deletions
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22
docs/docs/subscriptions/postgres/livequery/execution-and-performance.mdx
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@ -42,6 +42,20 @@ For example, with the default value, if there are 1000 subscription clients with
subscriptions into 1 batch and make a single SQL query to the DB for that batch. So, in total there will be only 10 SQL
queries to the DB for the 1000 subscriptions.
:::info Limitation
**Subscriptions that rely on SQL functions, such as through [custom root fields](/schema/postgres/custom-functions.mdx)
or [computed fields](/schema/postgres/computed-fields.mdx), may be at risk of multiplexing errors if they throw
"dynamic" exceptions based on data in the underlying tables.**
For example, if the SQL function throws an exception by reading rows and checking an aggregate then it may be prone to
such multiplexing errors. On the other hand, if it throws exception based on static values (e.g. input arguments to the
function), then it is safe from multiplexing errors. To avoid potential issues, it is important to carefully evaluate
the use of subscriptions with such functions. You can set the env variable
`HASURA_GRAPHQL_LIVE_QUERIES_MULTIPLEXED_BATCH_SIZE: 1` to avoid multiplexing errors.
:::
### Example
A practical use of subscriptions can be for tracking the live locations of delivery agents carrying food orders in a
@ -54,10 +68,10 @@ The below figure shows multiplexing in action, where the 3 similar subscriptions
batched into one SQL:
<Thumbnail
src="/img/databases/postgres/subscriptions/subscription-multiplexing.png"
alt="Hasura subscription multiplexing AST"
width="900px"
className="no-shadow"
src='/img/databases/postgres/subscriptions/subscription-multiplexing.png'
alt='Hasura subscription multiplexing AST'
width='900px'
className='no-shadow'
/>
In this case, all 3 subscriptions are multiplexed into 1 batch resulting in 1 SQL query which is run every 1 second. If