Add docs on configuring kupo

doc/custom-query-layers.md

@@ -1,4 +1,14 @@
-<!-- DOCTOC SKIP -->
+<!-- START doctoc generated TOC please keep comment here to allow auto update -->
+<!-- DON'T EDIT THIS SECTION, INSTEAD RE-RUN doctoc TO UPDATE -->
+
+- [Custom query layers in CTL](#custom-query-layers-in-ctl)
+  - [Adding a new backend](#adding-a-new-backend)
+  - [Replacing some queries in existing backend](#replacing-some-queries-in-existing-backend)
+- [Configuring CTL's default query layer (Kupo/Ogmios)](#configuring-ctls-default-query-layer-kupoogmios)
+  - [Kupo](#kupo)
+    - [Faster sync for Kupo](#faster-sync-for-kupo)
+
+<!-- END doctoc generated TOC please keep comment here to allow auto update -->
 
 # Custom query layers in CTL
 
@@ -15,3 +25,24 @@ A new [backend option](https://github.com/Plutonomicon/cardano-transaction-lib/b
 Substituting the query layer for a few functions only (assuming the original backend remains available for initial connection) can be done without forking CTL.
 
 `ContractEnv` contains a QueryHandle (inside a `Reader`), so a [`local`](https://pursuit.purescript.org/packages/purescript-transformers/6.0.0/docs/Control.Monad.Reader.Class#v:local) call with a function that replaces some `QueryHandle` record entries will just work.
+
+# Configuring CTL's default query layer (Kupo/Ogmios)
+
+Optimising storage requirements and performance of the query layer, unfortunately, depends on your app query needs.
+
+## Kupo
+
+Kupo is used for "utxos-at-address" query in CTL.
+
+This query is mainly used in two contexts:
+
+- manual `utxosAt` calls
+- wallet <-> backend synchronization (see [here](./query-layers.md))
+
+If you don't need either, consider removing Kupo from the runtime and [disabling wallet <-> backend synchronization](./query-layers.md) (**IMPORTANT!**).
+
+If your app works in a way that only requires UTxO lookups on certain addresses (e.g. script addresses), consider using [`--match-pattern`](https://cardanosolutions.github.io/kupo/#section/Getting-started/-match-pattern) to drastically reduce DB size. The downside would be that wallet<->backend sync will have to be disabled, because it requires looking up wallet's addresses via the backend.
+
+### Faster sync for Kupo
+
+Use [`--since` flag](https://cardanosolutions.github.io/kupo/#section/Getting-started/-since-slot-no.header_hash) to perform partial incomplete sync. Find block parameters on [a block explorer](https://cexplorer.io/block). It will let you sync things faster (than in ~4 days for mainnet), for the price of Kupo not seeing UTxOs created after the specified slot. If you disable wallet<->backend sync, and depending on your use case, that might be acceptable for production.

doc/query-layers.md

@@ -6,6 +6,7 @@
     - [Configuring synchronization behavior](#configuring-synchronization-behavior)
     - [Synchronization and wallet UTxO locking](#synchronization-and-wallet-utxo-locking)
     - [Historical notes](#historical-notes)
+  - [See also](#see-also)
 
 <!-- END doctoc generated TOC please keep comment here to allow auto update -->
 
@@ -113,3 +114,7 @@ Initially we underestimated the problem of UTxO set inconsistency between query
 Conceptually, the wallet is responsible for *owning* the UTxOs, so wallet developers may implement behaviors that would prevent us from making general assumptions about the wallet state.
 
 CTL `v5.1.0` introduces better consistency guarantees while not requiring the developer to change any code on their side - for the price of slight delays during the app runtime.
+
+## See also
+
+- [Optimising your app with custom query layers](./custom-query-layers.md)