--- title: "Function: Advanced Keep the Last N User Items" description: Keep the last N user notifications related to a user ID. pubDate: 2026-08-17T09:53:44.266Z antora: editUrl: https://github.com/couchbaselabs/docs-devex/edit/release/8.0/modules/eventing/pages/eventing-handler-advanced-keepLastN.adoc xref: xref:server:eventing:eventing-handler-advanced-keepLastN.adoc[] --- [Consult the llms.txt file for a full list of contents](/llms.txt) [View original HTML](/server/current/eventing/eventing-handler-advanced-keepLastN.html) # Function: Advanced Keep the Last N User Items Keep the last N user notifications related to a user ID. The `advancedKeepLastN` function: * Demonstrates how to keep a user record with the last N activities * Requires Eventing Storage (or a metadata collection) and a source collection * Requires a binding of type `bucket alias` * Operates on any mutation where the KEY starts with `nu:` in the form `nu:#:#` * The KEY `nu:#:#` has 2 numbers. The first is an increasing notification number and the second is the user ID. * Only keeps N records for each user * Removes the earliest notification record for a user whenever a new record is inserted for that user The following example assumes that N always increases across time and ignores any duplicates. It keeps only the 3 most recent notifications for each user ID. * advancedKeepLastN * Input data * Output data There are two variants of this function available: a [Couchbase Server version 6.6 that implements userspace CAS](eventing-handler-keepLastN.md), and a Couchbase Server version 6.6.1+/7.0.0+ that uses true CAS. The following example uses true CAS. ```javascript // Configure the settings for the advancedKeepLastN function as follows: // // Version 7.1+ // "Function Scope" // *.* (or try bulk.data if non-privileged) // Version 7.0+ // "Listen to Location" // bulk.data.source // "Eventing Storage" // rr100.eventing.metadata // Binding(s) // 1. "binding type", "alias name...", "bucket.scope.collection", "Access" // "bucket alias", "src_col", "bulk.data.source", "read and write" // // Version 6.6.1 // "Source Bucket" // source // "MetaData Bucket" // metadata // Binding(s) // 1. "binding type", "alias name...", "bucket", "Access" // "bucket alias", "src_col", "source", "read and write" /* * Process all mutations; updateNotifyArrayInKV(...) only processes data with KEYS like nu:#:# */ function OnUpdate(doc, meta) { const MAX_ARRAY = 3; // Keep 'N' items const DEBUG = false; // If true, the debug log can be too long updateNotifyArrayInKV(doc, meta, MAX_ARRAY, DEBUG); } /* * Manipulate the in-memory document to only keep 'MAX_ARRAY' items */ function addToNtfyArray(user_doc, user_id, insert_json, MAX_ARRAY, DEBUG) { var ntfy_id = insert_json.nid; if (user_doc.notifications && user_doc.notifications[0] && user_doc.notifications[0].nid >= ntfy_id && user_doc.notifications.length === MAX_ARRAY) { // Do nothing; this is older data return null; } else { // Find the insert position for (var i = 0; i <= user_doc.notifications.length + 1; i++) { if (i < user_doc.notifications.length && user_doc.notifications[i].nid === ntfy_id) { // Do nothing; this is duplicated data if (DEBUG) log('Ignore DUP ntfy_id', ntfy_id, 'user_id', user_id, 'insert_json', insert_json); return null; } if (i == user_doc.notifications.length || user_doc.notifications[i].nid > ntfy_id) { // Add to middle or end of array user_doc.notifications.splice(i, 0, insert_json); break; } } } while (user_doc.notifications.length > MAX_ARRAY) { // Ensure proper size user_doc.notifications.shift(); } return user_doc; } /* * Creates, gets, and updates (via replace) the KV tracking array document */ function updateNotifyArrayInKV(doc, meta, MAX_ARRAY, DEBUG) { // Process ALL data like nu:#:# var parts = meta.id.split(':'); if (!parts || parts.length != 3 || parts[0] != "nu") return; var ntfy_id = parseInt(parts[1]); var user_id = parseInt(parts[2]); //log("Doc created/updated " + meta.id + " ntfy_id " + ntfy_id + " user_id " + user_id); var insert_json = { "nid": ntfy_id, doc }; // In version 6.6.1, use CAS in Eventing to avoid race conditions var res = null; var req_id = "user_plus_ntfys:" + user_id; var req_meta = { id: req_id }; var user_doc = null; var user_meta = null; while (res === null) { res = couchbase.get(src_col, req_meta); if (DEBUG) log('couchbase.get(src_col,', req_meta, ') success==' + res.success, res); if (res.success) { user_doc = res.doc; // EXAMPLE user_meta // {"id":"user_plus_ntfys:2","cas":"1609965779014844416","data_type":"json"} user_meta = res.meta; } else { if (!res.error.key_not_found) { // Do nothing; this is a big error log("FAILED to insert id: " + meta.id, doc, 'res', res) return; } // Create the document and initialize it user_doc = { "type": "user_plus_ntfys", "id": user_id, "notifications": [] }; res = couchbase.insert(src_col, req_meta, user_doc); if (DEBUG) log('couchbase.insert(src_col,', req_meta, user_doc, ') success==' + res.success, res); // Redo the loop to force couchbase.get res = null; } if (res !== null) { // Successful couchbase.get(...) for both user_doc and user_meta // Manipulate the copy of the user_doc to keep only MAX_ARRAY var new_doc = addToNtfyArray(user_doc, user_id, insert_json, MAX_ARRAY, DEBUG); if (new_doc == null) { // Ignore or skip duplicated data break; } // Try to replace the user_doc with new_doc; pass CAS to test for race conditions res = couchbase.replace(src_col, user_meta, new_doc); if (DEBUG) log('couchbase.replace(src_col,', user_meta, new_doc, ') success==' + res.success, res); if (res.success) { // CAS matches and operation is successful break; } else { // Redo loop and try again res = null; } } } } ``` Create a new test document set using the Query Editor to insert the data items. You do not need an Index. | key | data | | ------- | ----------------------- | | nu:1:1 | {"somekey":"someValue"} | | nu:2:2 | {"somekey":"someValue"} | | nu:3:1 | {"somekey":"someValue"} | | nu:4:1 | {"somekey":"someValue"} | | nu:5:1 | {"somekey":"someValue"} | | nu:6:2 | {"somekey":"someValue"} | | nu:7:2 | {"somekey":"someValue"} | | nu:8:1 | {"somekey":"someValue"} | | nu:9:2 | {"somekey":"someValue"} | | nu:10:2 | {"somekey":"someValue"} | ```sqlpp UPSERT INTO `bulk`.`data`.`source` (KEY,VALUE) VALUES ( "nu:1:1", {"somekey":"someValue"} ), VALUES ( "nu:2:2", {"somekey":"someValue"} ), VALUES ( "nu:3:1", {"somekey":"someValue"} ), VALUES ( "nu:4:1", {"somekey":"someValue"} ), VALUES ( "nu:5:1", {"somekey":"someValue"} ), VALUES ( "nu:6:2", {"somekey":"someValue"} ), VALUES ( "nu:7:2", {"somekey":"someValue"} ), VALUES ( "nu:8:1", {"somekey":"someValue"} ), VALUES ( "nu:9:2", {"somekey":"someValue"} ), VALUES ( "nu:10:2", {"somekey":"someValue"} ); ``` ```json NEW/OUTPUT: KEY user_plus_ntfys:1 { "type": "user_plus_ntfys", "id": 1, "notifications": [{ "nid": 4, "doc": { "somekey": "someValue" } }, { "nid": 5, "doc": { "somekey": "someValue" } }, { "nid": 8, "doc": { "somekey": "someValue" } }], "random": 0.9071605464143964 } NEW/OUTPUT: KEY user_plus_ntfys:2 { "type": "user_plus_ntfys", "id": 2, "notifications": [{ "nid": 7, "doc": { "somekey": "someValue" } }, { "nid": 9, "doc": { "somekey": "someValue" } }, { "nid": 10, "doc": { "somekey": "someValue" } }] } ```