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Function: Keep the Last N User Items

Enterprise Edition

Goal: Keep the last N user notifications seen related to a user ID (these could be any documents)

This function keepLastN demonstrates how to keep a user record with the last N activities.
Requires a metadata bucket and a source bucket.
Will operate on any mutation with a key starting with "nu:" of the form "nu:#:#".
The key "nu:#:#" contains two numbers. The first # is an increasing notification number, the second # is the user ID.
Anytime we insert a new record we want to remove the earliest notification record for the user so we only have at most N records for each user.
For our test we will keep just the three (3) most recent notifications per user ID.

keepLastN
Input Data/Mutation
Output Data/Mutation

function OnUpdate(doc, meta) {
    const MAX_RETRY = 16;
    const MAX_ARRAY = 3;

    // will 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};
    // this can be simplified in version 6.6.1 when CAS is added to Eventing
    for (var tries=0; tries < 16; tries++) {
        var user_doc = addToNtfyArray(src_bkt, user_id, insert_json, MAX_ARRAY);
        if (user_doc == null) {
            // do nothing
            return;
        }
        var random_csum = user_doc.random;
        // this is a read write alias to the functions source bucket
        src_bkt["user_plus_ntfys:" + user_id] = user_doc;
        user_doc = src_bkt["user_plus_ntfys:" + user_id];
        if (random_csum !== user_doc.random) {
            // failure need to retry
            tries++;
        } else {
            // success could even delete the input notification doc here
            return;
        }
    }
    log ("FAILED to insert id: " + meta.id, doc)
}

function addToNtfyArray(src_bkt, user_id, insert_json, max_ary) {
    var ntfy_id = insert_json.nid;
    var random_csum;
    var user_doc = src_bkt["user_plus_ntfys:" + user_id];
    if (!user_doc) {
        // generate unique random #
        random_csum = Math.random();
        user_doc = { "type": "user_plus_ntfys", "id": user_id, "notifications" : [], "random": random_csum };
        user_doc.notifications.push(insert_json);
    } else {
        if (user_doc.notifications[0].nid >= ntfy_id && user_doc.notifications.length === max_ary) {
            // do nothing this is older data, we assume that nid always increases
            return null;
        } else {
            // find 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 duplicate data we already have it, assume no updates to notifys
                    return null;
                }
                if (i == user_doc.notifications.length || user_doc.notifications[i].nid > ntfy_id) {
                    // add to array middle or end
                    user_doc.notifications.splice(i, 0, insert_json);
                    random_csum = Math.random();
                    // update unique random #
                    user_doc.random = random_csum;
                    break;
                }
            }
        }
        while (user_doc.notifications.length > max_ary) {
            // ensure proper size
            user_doc.notifications.shift();
        }
    }
    return user_doc;
}

We want to create a test doc set

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"}

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"}

Use the Query Editor to insert the above data items (you do not need an Index)

  INSERT INTO `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"} );

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"
    }
  }],
  "random": 0.5637501636850883
}