Added retries after 502

SynonymGroup.ts

@@ -242,6 +242,13 @@ export type JustifiedSynonym = {
   loading: boolean;
 };
 
+async function sleep(ms: number): Promise<void> {
+  const p = new Promise<void>((resolve) => {
+    setTimeout(resolve, ms);
+  });
+  return await p;
+}
+
 export class SparqlEndpoint {
   constructor(private sparqlEnpointUri: string) {
   }
@@ -251,11 +258,34 @@ export class SparqlEndpoint {
   ) {
     fetchOptions.headers = fetchOptions.headers || {};
     fetchOptions.headers["Accept"] = "application/sparql-results+json";
-    const response = await fetch(
-      this.sparqlEnpointUri + "?query=" + encodeURIComponent(query),
-      fetchOptions,
-    );
-    return await response.json();
+    let retryCount = 0;
+    // deno-lint-ignore no-explicit-any
+    const sendRequest = async (): Promise<any> => {
+      try {
+        const response = await fetch(
+          this.sparqlEnpointUri + "?query=" + encodeURIComponent(query),
+          fetchOptions,
+        );
+        if (!response.ok) {
+          throw new Error("Response not ok. Status " + response.status);
+        }
+        return await response.json();
+      } catch (error) {
+        if (error instanceof Error && error.message.endsWith("502")) {
+          if (retryCount < 5) {
+            ++retryCount;
+            console.warn(
+              `!! Fetch Error: 502 Bad Gateway. Retrying in ${retryCount * 50}ms (${retryCount})`,
+            );
+            await sleep(retryCount * 50);
+            return await sendRequest();
+          }
+        }
+        console.warn("!! Fetch Error:", query, "\n---\n", error);
+        return {}; // as not to crash code expecting parsed json
+      }
+    };
+    return await sendRequest();
   }
 }