diff --git a/doc/manual/src/configuration.md b/doc/manual/src/configuration.md
index 02210449..4954040c 100644
--- a/doc/manual/src/configuration.md
+++ b/doc/manual/src/configuration.md
@@ -74,6 +74,30 @@ following:
         }
     }
 
+Populating a Cache
+------------------
+
+A common use for Hydra is to pre-build and cache derivations which
+take a long time to build. While it is possible to direcly access the
+Hydra server's store over SSH, a more scalable option is to upload
+built derivations to a remote store like an [S3-compatible object
+store](https://nixos.org/manual/nix/stable/command-ref/new-cli/nix3-help-stores.html#s3-binary-cache-store). Setting
+the `store_uri` parameter will cause Hydra to sign and upload
+derivations as they are built:
+
+```
+store_uri = s3://cache-bucket-name?compression=zstd&parallel-compression=true&write-nar-listing=1&ls-compression=br&log-compression=br&secret-key=/path/to/cache/private/key
+```
+
+This example uses [Zstandard](https://github.com/facebook/zstd)
+compression on derivations to reduce CPU usage on the server, but
+[Brotli](https://brotli.org/) compression for derivation listings and
+build logs because it has better browser support.
+
+See [`nix help
+stores`](https://nixos.org/manual/nix/stable/command-ref/new-cli/nix3-help-stores.html)
+for a description of the store URI format.
+
 Statsd Configuration
 --------------------
 
diff --git a/src/hydra-queue-runner/queue-monitor.cc b/src/hydra-queue-runner/queue-monitor.cc
index 81678fe1..a3557316 100644
--- a/src/hydra-queue-runner/queue-monitor.cc
+++ b/src/hydra-queue-runner/queue-monitor.cc
@@ -495,19 +495,19 @@ Step::ptr State::createStep(ref<Store> destStore,
     if (!missing.empty()) {
 
         size_t avail = 0;
-        for (auto & [i, maybePath] : missing) {
+        for (auto & [i, pathOpt] : missing) {
             // If we don't know the output path from the destination
             // store, see if the local store can tell us.
-            if (/* localStore != destStore && */ !maybePath && experimentalFeatureSettings.isEnabled(Xp::CaDerivations))
+            if (/* localStore != destStore && */ !pathOpt && experimentalFeatureSettings.isEnabled(Xp::CaDerivations))
                 if (auto maybeRealisation = localStore->queryRealisation(i))
-                    maybePath = maybeRealisation->outPath;
+                    pathOpt = maybeRealisation->outPath;
 
-            if (!maybePath) {
+            if (!pathOpt) {
                 // No hope of getting the store object if we don't know
                 // the path.
                 continue;
             }
-            auto & path = *maybePath;
+            auto & path = *pathOpt;
 
             if (/* localStore != destStore && */ localStore->isValidPath(path))
                 avail++;
@@ -521,11 +521,11 @@ Step::ptr State::createStep(ref<Store> destStore,
 
         if (missing.size() == avail) {
             valid = true;
-            for (auto & [i, maybePath] : missing) {
+            for (auto & [i, pathOpt] : missing) {
                 // If we found everything, then we should know the path
                 // to every missing store object now.
-                assert(maybePath);
-                auto & path = *maybePath;
+                assert(pathOpt);
+                auto & path = *pathOpt;
 
                 try {
                     time_t startTime = time(0);